THE LIGHT REQUIREMENTS FOR GROWTH AND PHOTOSYNTHESIS IN 
SEAGRASSES WITH EMPHASIS ON TEXAS ESTUARIES: 
A LITERATURE SURVEY 
J.E. Kaldy and K.H. Dunton 
University of Texas at Austin 

Marine Science Institute 
Port Aransas, Texas 78373 

Submitted to: 
United States Environmental Protection Agency 
Region 6 
1445 Ross Avenue, Suite 1200 
Dallas, Tx 75202-2733 

31 August 1993 
THE LIGHT REQUIREMENTS FOR GROWTH AND PHOTOSYNTHESIS IN 
SEAGRASSES WITH EMPHASIS ON TEXAS ESTUARIES: 
A LITERATURE SURVEY 
J.E. Kaldy and K.H. Dunton 
University of Texas at Austin 
Marine Science Institute 
Port Aransas, Texas 78373 
Submitted to: 
United States Environmental Protection Agency 
Region 6 
1445 Ross Avenue, Suite 1200 
Dallas, Tx 75202-2733 
31 July 1993 
Table of Contents 
Acknowledgements 3 
Executive Summary 4 
Chapter 1. INTRODUCTION 9 
The Texas flora 9
seagrass 
21
Importance of seagrasses 
LIGHT AND WATER TRANSPARENCY 34
Chapter 2. 
Environmental influences on water clarity 34 
light in the aquatic environment 34 
Natural influences 40
on water clarity 
on 43
Anthropogenic influences water quality and water clarity 
THE CHESAPEAKE BAY APPROACH 50
Chapter 3. 
50
Approaches to assessing habitat requirements of seagrasses 
SEAGRASS PHOTOSYNTHETIC PHYSIOLOGY 54
Chapter 4. 
Computer modelling 57 
Effects of in situ light reduction 58 
63
Experimental changes in daily light period 
68
Laboratory studies of reduced light on seagrasses 
70
.Photosynthesis irradiance curves 
85
Seasonal changes in seagrasses 
SYNTHESIS AND RECOMMENDATIONS FOR
Chapter 5. 
FURTHER RESEARCH 97 
literature Cited 101 
Acknowledgements 
Numerous individuals contributed successful of this
greatly to the completion 
literaturereview. WeareverygratefulfortheconstructivecommentsprovidedbySteffanie 
Barnett,Philip Crocker, KenTeague andCarlYoungofthe U.S.Environmental Protection 
Agency (Region 6) and Chris Onuf (U.S. Fish and Wildlife Service) and Warren Pulich 
Parks and
(Texas Wildlife Department). UTMSI graduate students in marine botany, 
Andrew Sharon Herzka and Lee
including Czerny, Kun-seop provided many helpful 
comments as our unofficial editors; Patty Baker and Kim Jackson were instrumental in the 
We also
typing and preparation of the initial drafts that eventually led to the final report. 
of the hard-to-find
thank the UTMSI library staff for their expedient acquisition for many 
books and papers referenced in this review. The cover sketch was prepared by Nancy 
Buskey. This work was supported by the U.S. EPA Region 6 Grant No. X-996025-01-0 to 
Ken Dunton. 
Correct citation of this report is: 
Kaldy, J.E. and K.H. Dunton. 1993. The light requirements for growth and photosynthesis 
in seagrasses with emphasis on Texas estuaries: a literature survey. U.S. EPA 
Region 6, Dallas, Texas. 
Executive Summary 
Seagrasses have been repeatedly demonstrated to be highly valuable components of 
coastal systems. They have high rates of primary productivity and support a diverse 
assemblage ofconsumers as well as rapidly cycling ecologically important elements such as 
carbon,nitrogen,phosphorusandsulphur. Thehighprimaryproductivityofseagrasssystems 
supports many commercially and recreationally important species. In addition, they also act 
and reduce
to modify the deposition of sediments, remove nutrients, attenuatewave energy 
currents. Ingeneral,seagrassecosystemsareacornerstoneofhealthy,productivebaysand 
estuaries. 
the last 20 communities the world have
During years, seagrass throughout 
experienced decreased productivity and distribution. These declines have often been 
as a
attributed to decreased water transparency result of turbidity or shading by epiphytic 
is often an indication of nutrient enrichment caused
algae. Epiphytic shading by 
anthropogenic inputs. Although both epiphytes and turbidity occur as natural phenomena, 
human activities can exacerbate existing natural conditions with adverse effects on seagrass 
communities. 
The objectives of this study were (a) to review the existing literature and data 
available on the effect of natural and anthropogenic factors the underwater light
on 
distribution and
environment; (b) to examine the relationship between light and seagrass 
productivity; and (c) to make recommendations on how to habitats in Texas
protect seagrass 
bays and estuaries. To meet these goals, we have examined the available literature, 
Table 1. list
of all submerged aquatic plant species (in italics) cited in the text. Species 
listed by family and include the author who first described each species (not shown in
are 
italics). 
Seagrasses 
Cymodoceaceae Amphibolis antarctica (Labill.) Sonder ex Aschers. 
Cymodocea nodosa (Ucria) Aschers. 
Halodule wrightii Aschers. 
Syringodium filiforme Kutzing 
Hydrocharitaceae 
Enhalus acoroides (Linnaeus f.) Royle 
HalophUa decipiem Ostenfeld 

Halophila engelmannii Aschers. 
HalophUa johnsonii Eisman HalophUa stipulaceae (Forsk.) Aschers. Thalassia testudinum Banks ex Konig 
Posidoniaceae 
Posidonia angustifolia Cambridge et Kuo Posidonia australis Hook. F. 
Posidonia oceanica (L.) Delile 
Posidonia sinuosa Cambridge et Kuo 
Zosteraceae 
Heterozostera tasmanica (Martens ex Aschers.) den Hartog 
Phyllospadix scouleri Hooker 
Phyllospadix torreyi S. Watson 
Zostera angustifolia (Hornem.) Reichb. Zostera marina L. 
Zostera muelleri Irmisch ex Aschers. 
Freshwater and Estuarine Plants 
Hydrocharitaceae 
Elodea canadensis Michx. 

Vallisneria americana Michx. 
Potamogetonaceae 
Potamogeton pectinatus L. 
Potamogeton perfoliatus L. 
Ruppia maritima L. 

emphasizing the physiological response of seagrasses to light and temperature. By using 
data and observations collected on a variety of species (Table 1) from around the world we 
A
may be better able to define the light requirements of Texas seagrasses. knowledge of 
the minimum annual light requirements for seagrass growth is necessary to maintain the 
current distribution of Texas This information will also be
species. required in the 
development of a management plan that permits the expansion and establishment of new 
habitat.
seagrass 
Five species of seagrasses from three families comprise the submerged 
vascular flora of the Texas some acres
bays and estuaries, encompassing 209,738 (Onuf 
1993). Most of these species have tropical or sub-tropical affinities and are near the 
northern end of their distribution in Texas. One species, Ruppia maritima is considered 
, 
cosmopolitan and occurs as far north as New Hampshire, while Halodule wrightii occurs as 
far north as North Carolina. Most seagrasses are perennial; however, Ruppia has been 
Texas estuaries.
reported as both an annual and a perennial in some Although flowering 
in Thalassia testidinum has been documented only once in Texas (Phillips et aL, 1981), all 
five known to flower and set seed.
are
seagrass species 
Lunar tides seldom exceed 15 cm in Texas bays and estuaries, and therefore 
are heavily influenced by local meteorology. Meterological events (particularly winds and 
storms) also control the turbidity of the bays along the Texas coast. However, 
anthropogenic factors can also influence turbidity. Dredging activities can increase light 
attenuation and prevent the plants from receiving their daily minimum light requirements 
for growth and survival. In Texas, increased turbidity as result of dredging and
a 
construction appears to be the largest threat to seagrasses. Presently, other anthropogenic 
influences such as eutrophication, which results from increased nutrient loading, occurs only 
in a few localized areas (i.e. portions of the Galveston Bay System and Copano Bay). 
increases, does the threat of
as zone so
However, the human population of Texas coastal 
eutrophication. 
Severalrecentpublications haveproposedanovelmethodforassessingwaterquality 
in estuarine systems (Batuik et al, 1992; Dennison et al, 1993). The basic premise of the 
in areas
technique is that plants will not where their habitat requirements (i.e. light,
grow 
nutrients etc.) are not adequatly met. Thus, if the minimum habitat requirements are 
be
known, the parts of the system that are not meeting the minimum requirements can 
inferredusingmappingtechniques. Someaspectsofthistechniquemaybeappropriatefor 
are not.
useinTexassystems,butmany Thisisaresultofthemanagementhistoryofeach 
estuarine system; Chesapeake Bay is highly impacted, and the goals are to restore and 
conserve the remaining resources. Texas systems, with some exceptions, are relatively 
pristine and preservation of the existing seagrass habitats is the major goal. 
In general, lower irradiance results in reduced density and biomass of 
seagrasses. 
Laboratory and field studies show that seagrasses may also respond to a drop in irradiance 
by altering their morphology. The minimum light requirements of seagrasses can be 
quantified through measurement of the daily minimum number of hours of saturating 
irradiance needed to meet their respiratory demands. The H value for some 
ml 
seagrasses (i.e. Zostera marina) may approach 6 h to maintain a positive carbon balance; 
been determined.
however, the requirements for most Texas species have not In Texas, Halodule wrightii has recently been shown to have an H of 3to 5 h (Dunton, submitted).
sat 
A review of the literature suggests that most seagrass species, including those along the 
Texas coast, require 10 to 20% of surface irradiance as an absolute minimum during most 
of the year. 
Although light is a critical factor controlling seagrass growth and distribution, 
A
temperature is also important. light level that is photosynthetically saturating (Ik) at low 
temperatures may be the compensating irradiance (I) at a higher temperatures. As a result 
c 
of seasonal variation in temperature, most seagrasses exhibit seasonal patterns with respect 
to productivity, P vs. I parameters and organic composition. The temporal dynamics of 
are such that the winter to
seagrasses appears beaperiodoflowphysiologicalactivity,while 
the extent of photosynthetic activity in spring, summer and fall determine the long-term 
success of a species. We recommend that human disturbance be kept to a minimum during 
critical periods of 
seagrass growth, which is greatest during spring and early summer. 
CHAPTER 1; INTRODUCTION 
The Texas seagrass flora 
Seagrasses are flowering plants which re-invaded the marine environment. The 
-
date from the Cretaceous about 140 million
earliest fossils of seagrasses years ago (den 
Hartog, 1970). Unlike the algae, which are in the Kingdom Protista, seagrasses are vascular 
plants that have xylem and phloem elements for the transport ofwater and photosynthate 
(Pedersen and Sand-Jensen 1993, Barnabas and Araott 1987, Barnabas 1989,1991). These 
plants also differ from algae in the degree of cellular differentiation and organization; 
have true roots, stems, and leaves. these
seagrasses Algae do not possess organs. 
As marine angiosperms (flowering plants), seagrasses complete their life-history 
underwater. 
They reproduce sexually by flowering and vegetatively by rhizome branching. 
Rhizome formed the initiation of become
branches, by axillary meristems, may 
via
physiologically independent and create new plants. In plants, sexual reproduction occurs 
flowering and pollen exchange. To exchange pollen in the aquatic environment, plants have 
evolved three strategies (Fig. 1). These include (Cox, 1988) the transport of pollen above 
the water surface (category I), pollen transported on the water surface (category II), and 
pollentransportedbeneaththewatersurface(categoryIII). CategoryIpollentransporthas 
beenreportedforonlyone speciesofseagrass(Enhalusacoroides);however,itiscommon 
in freshwater submerged aquatics. Most 
seagrasses exhibit either category II or 111 pollen 
transfer. 
Seed germination occurs underwater and is influenced by both temperature and 
Figure 1. Schematic diagrams of category 2 (top panel) and 3 (bottom panel) hydrophilly. Category2hydrophillyexemplifiedbyHalodulepinifolia(toppanel). A.Erectionofanother at low tide; B. Another dehiscence and assembly of search vehicles; C. Search vehicles 
floating onwatersurface withinsert showing darkfieldsilhouette; D.Pollinationbycollision of search vehicles with filamentous stigmas; E. Close-up of filiforme pollen. 
Category 3 hydrophilly exemplified by Thalassia testudinum (bottom panel). A. Male flowers; B. Mucilage string containing pollen; C. Underwater dispersal of mucilage strand; D. Female flowers with rigid stigmas; E. Pollination by collision. From Cox 1988. 
10 
The details of seed and germling anatomy have been described for Zostera marina
salinity. 
(Taylor 1957a,b) and Thalassia testudinum (Orpurt and Boral 1964). In the early 

development of the seedling, seagrasses exhibit a single cotyledon ("seed leaf'). Thus 
seagrasses aremonocotswhichhavebeenplacedintheclassliliopsida, subclassAlismatidae 
(Cronquist 1981). CJ.G. Petersen and colleagues were the first researchers to realize the importance 
ofcommunities to coastal ecosystems (Petersen 1891,1918; as
seagrass cited in den Hartog 
1980). They suggested that seagrass production (8 million dry tons/year) was the basis of 
the food web for all marine fauna (Petersen and Boysen-Jensen 1911, Petersen 1913,1915, 
1918, Boysen-Jensen 1914; as cited in Rasmussen 1977). However, they underestimated the 
importanceofphytoplankton, asreflectedintheabsenceoffinfisheryfailurewiththeonset 
on
of the wasting disease of the 1930’s (Rasmussen 1977). Much of the other early work 
was
seagrasses related to taxonomy (Ascherson 1907, Hutchinson 1934, Markgraf 1936; as 
citedindenHartog 1970),whilemuchofthephysiologicalandecologicalresearchhasbeen 
conducted since the late 1960’5. 
There are that between the Arctic and Antarctic
occur
about 48 species ofseagrasses 
Circles (Phillips and Menez 1988). Many of these species have a circumglobal distribution 
(e.g.Zosteramarina whichoccursonbothsidesoftheAtlanticandthePacific;denHartog
, 
1970). Almost all seagrass species display continuous areas of distribution (Phillips and 
Menez 1988). However, some species have disjunct distributions, which may be due to the 
movement of the continents during the geological epochs (den Hartog 1970). Because of 
the time scales inferential data and fossil foramanifera
involved, data (i.e. geological 
associated with seagrasses) have been used to account for the present day distribution of 
Like
seagrasses (Phillips and Menez, 1988). many other organisms, the areas of highest 
are tropical and subtropical environments (den Hartog 1970). There are
species diversity 
that are considered characteristic of tropical areas and five that are
seven 
genera genera 
considered temperate in character (Table 2). Although the Pacific coast of South America 
and the Caribbean have relatively high species diversity, they are notas diverse at the Indo­
most areas
West Pacific. The seagrass flora of of the world is well documented, except for 
theAtlanticCoastofSouthAmerica,whichremainsrelativelyunexplored(denHartog 1970, 
Phillips and Menez 1988). 
Strictly speaking, there are four seagrasses (Thalassia testudinum , Syringodium 
Halodule one
filiforme, wrightii and Halophila engelmanni) and euryhaline aquatic plant, 
Ruppia maritima that occur along the Texas coast. Because R. maritima is found in 
, 
freshwater, neither den Hartog (1970) nor Phillips and Menez (1988) consider R maritima 
a seagrass. However, Ruppia does grow and complete its life history in many of the 
of this review
hypersaline bays and estuaries of Texas (Dunton 1990) and for the purpose 
will include Ruppia with the
we seagrasses (Table 3; Figure 2). With the exception of 
the Texas
Ruppia, seagrasses have tropical or subtropical affinities. Ruppia is cosmopolitan 
and occurs as far North as New Hampshire (J. Kaldy, pers. obs.), while Halodule wrightii 
occurs as far north as North Carolina. The seagrasses also tend to be perennial (Table 4); 
annual
although, Ruppia in Texas has been reported as a perennial (Pulich 1985) and as a 
Thalassia
(Dunton 1990). Texas seagrasses, except exhibit category II pollenation; it is 
, 
unclear whether Ruppia exhibits category II or 111 pollen transfer. 
Table 2. The affinities of the major seagrass species. Some of the species with tropical affinities overlap into subtropical or warm areas. From den Hartog 1970. 
Tropical Genera Temperate Genera 
Enhalus Halodule 
Thalassia Heterozostera Halophila Phyllospadix Cymodocea Posidonia 
Zostera
Syringodium 
Thalassodendron 

Table 3. 
A listing of Texas seagrass species. 
Family and Species Common Name 
Cymodoceaceae Halodule wrightii Aschers. Shoal 
grass Manatee
Syringodium filiforme Kiitzing grass 
Hydrocharitaceae Halophila engelmanni Aschers. Clover grass Thalassia testudinum Banks ex Konig Turtle grass 
Potamogetonaceae Ruppia maritima L. Widgeon grass 
Figure 2. Schematic drawings of the seagrass flora of the Texas coast. Modified from Zieman 1989. 
Table 4. Some of the characteristics of Texas seagrasses. 
Common name Pollen category Biogeographic affinity Longevity 
Clover grass II tropical perennial Manatee II
grass tropical perennial Shoal grass II subtrop-trop. perennial Turtle grass m tropical perennial Widgeon grass ii, in cosmopolitan annual/perennial 
Mostseagrasses, includingTexas species,growinprotectedbays andestuaries,where 
andrhizomestructureoftheplantspermitsthepenetrationandcolonization ofsoft
the root 
bottom substrate (i.e. sand and mud). However, there is at least one that inhabits the
genus 
rocky intertidal along the high energy coasts of the Pacific (i.e. Phyllospadix). The species 
hair
ofPhyllospadix have special adaptations, such as extensive hypodermal fibers and root 
rhizomes and small lacunae to colonize the crevices of rocks
development, thickened 
(Cooper and Mcßoy, 1988). With the exception of Phyllospadixmost seagrasses require
, 
sediment depths between 5 and 25 cm for adequate anchoring (Zieman 1972). Sediments 
are classified as either terrigenous (i.e., derived from terrestrial sources) or carbonate. 
Carbonate sediments are usually biogenic in origin (Scoffin 1970). Seagrasses also obtain 
atleastpartoftheirinorganic nutrientsfromthesediments(Thursby andHarlin 1982;Short 
and Mcßoy 1984). 
Seagrasses colonize a relatively broad range of depths (Figure 3), depending upon 
available light and substrate type as well as the physiological requirements of the species. 
IntherelativelyclearwatersoftheMediterraneanSea,Posidonia hasbeenreportedto 
grow 
below about 1.5
at depths of 50 m (Gessner 1961). However, Zostera marine does not grow 
mintheturbidwatersofSanFranciscoBay,(Zimmermanetal.,1991). Meanwhile,Ruppia 
maritima along the Texas Coast has been reported growing at 0.3 m above mean low water 
(C. Belaire, 1993, pers. comm). Thus, the depth distribution of a species is variable and 
several factors.
depends upon 
Although neither Posidonia nor Zostera occur along the Texas coast, they serve as 
examples to show the extreme range of depths that these plants are able to colonize. 
Figure 3, The reported depth limits of 31 marine angiosperm species. Bars represent the of values encountered, while the solid value for each
range square represents the average 
species. Number in right column indiciates the number of estimates. From Duarte 1991. 

Several authors have examined the depth distribution of Thalassia testudinum within the 
Gulf of Mexico. Buesa (1974) examined the population and biological parameters of 
Thalassia testudinum on the Cuban shelf. He found that Thalassia had a biomass of 
m'nr
approximately 200 g 2 to a depth of 1 m, while the lowest biomass, less than 50 g 2, 
occurred at 14 m depth (Figure 4). The depth distribution of other species varies 
considerably. For example, Halophila decipiens is found to a depth of 25 m (Figure 4). 
Buesa (1974) also suggested that Thalassia would not grow in areas where irradiance at the 
seabed was less than 25% of surface irradiance. Vicente and Rivera (1982) examined the 
depth limits of Thalassia growing in Puerto Rico and found a statistically significant positive 
mean Secchi depth and the lower limits of Thalassia (see chapter 2).
correlation between 
Thus, they suggest that where the water is clearer (deeper secchi depth) the plants will 
colonize at lower depths. However, they also suggest that herbivory may limit the depth 
distribution of Thalassia. Dawes and Tomasko (1988) investigated the depth distribution 
of Thalassia in Florida. They found that plants collected from the deep edge of the bed had 
lowershoot density, greaterleafarea,andgreaterabove:belowgroundbiomass than plants 
collectedfromtheshallowareas,suggestingthatdeepplantsweremorelight stressedthan 
shallow plants. 
It is generally accepted that light availability is the factor that controls the depth 
distribution of seagrasses. For example, Buesa (1974) concluded that light energy and 
temperature were the factors that control the depth distribution of Thalassia. Vincente and 
Rivera (1982) also suggested that Thalassia was limited to areas where irradiance at the 
plant depth was adequate. Iverson and Bittaker (1986) proposed that the depth 
Figure 4. The depth distribution of four seagrass species on the northwest coast of Cuba. 
1 = Thalassia testudinum 2 = 3 = 4 = H.
Syringodium filiformeHalophila decipiens
,
,, 
From Buesa 1974.
engelmanni. 
distribution of Thalassia was light limited. Orth and Moore (1988) suggested that species 
zonation was a physiological response to the interaction of light and temperature. 
Importance of seagrasses 
We are interested in defining the light requirements of seagrasses because they are 
an important component of estuarine and coastal ecosystems (Zieman 1982; Phillips 1984; 
Thayer et al., 1984; Zieman and Zieman 1989). There is a large body of literature 
documenting the beneficial qualities of seagrass beds. The three-dimensional habitat that 
beds create is important to species of fish and shellfish including
seagrass many 
commercially important species (Figure 5). There were two studies during the 1970’s that 
beds.
documented the importance of seagrass Thayer et aL (1975) found that eelgrass beds 
support numerous types of macrofauna and epifauna, which may consume more than half 
ofthe net 
productionoftheeelgrass-plankton-algalsystem. Rassmussen(1977)showedthat 
the species composition and abundance of invertebrate organisms changed with the loss of 
eelgrass (Zostera marina) due to the 1930’s outbreak of the wasting disease. 
During the 1980’s several other investigators documented the importanceof seagrass 
habitat. 
Orth et al (1984) compared unvegetated areas with seagrass meadows and found 
that seagrass beds contained a dense and rich assemblage of vertebrates and invertebrates. 
Species abundance was positively correlated with two aspects of plant morphology: 1) the 
root-rhizome mat, and 2) the plant canopy. The increased species diversity and abundance 
suggests that the three-dimensional habitat was beneficial to the 
on
Figure 5. Proposed food web for a seagrass bed, based dominant organisms of Indian 
River Lagoon, Fla. The grazing amphipod (Cymadusa comptd) is about to be preyed upon by the shrimp (Palamonetes intermedins) which is to be preyed upon by the fish (Bairdiella chrysoura ). From Virnstein 1987. 
secondary productivity of the system. Vimstein and Curran (1986) studied the colonization 
ofartificial habitat.Theyfoundrapidcolonizationofsubstratewithmaximum
seagrass new 
diversity and abundance within 4-8 days, suggesting that new seagrass substrate may enhance 
Eckman
secondary productivity within an estuary. (1987) found that eelgrass (Zostera 
marina) changed the hydrodynamics ofwatercurrents, facilitating the recruitmentofscallops 
(Argopecten irradians) and the commonjingle (Anomia simplex). Eckman also suggested that 
the hydrodynamic influence of seagrass was more important than predation in determining 
the abundance of recruits to the system. Short (1988) found that scallops actively migrated 
beds.
into transplanted seagrass 
During the 1990’s there have been several more investigations ofthe importance of 
seagrass habitat. Pohle et aL (1991) found that juvenile scallops actively attached themselves 
to eelgrass blades above the substrate as a refuge from predation. Sogard and Able (1991) 
demonstrated that vegetated substrate (Zostera or Ulva) was superior in quality (based on 
fish and decapod densities) to adjacent unvegetated substrate. Sites with Zostera as the 
dominant plant had higher densities of most fish species than areas dominated by Ulva; 
however, they concluded that Ulva was important in areas lacking seagrass Hoven
cover. 
(1992) suggested that eelgrass (Zostera marina) meadows are of considerable importance as 
sites for the settlement of blue mussel (Mytilus edulis) larvae. of these
Although many 
studieswereconcerned withcommerciallyimportantspecies,otherspeciesalsobenefitfrom 
habitat.
seagrass In general, healthy seagrass systems help to create and foster secondary 
production. 
Seagrassesact as asubstrate forthe growthofalgal epiphytes which contribute large 
13
amounts of fixed organic carbon to both the and detrital food webs. C
grazer Using <S 
are source to
values, Fry (1984) suggested that algal epiphytes a more important carbon 
Florida estuarine communities 
than Syringodium filiforme. Kitting et al (1984) also found 
that epiphytic algae may have an important trophic role. Stable carbon isotope evidence 
showed that invertebrates were assimilating algal epiphyte carbon rather than seagrass
many 
carbon (Figure 6; Fry 1984), reflecting the high refractory nature of vascular plant material 
(Fenchel 1977; Opsahl and Benner 1993). 
Carbon derived from both to coastal
seagrasses and their epiphytes is important 
ecosystems. Rapid export of seagrass leaf material was suggested as a reason for the 
relatively low importance of Syringodium to estuarine food webs in Florida (Fry 1984). 
Recent work from Australia (Thresher et al, 1992) using a variety of techniques including 
stable isotopes and gut analyses suggests that seagrass detritus advected offshore during 
storms may be an important resource for larval fish. 
Only a few marine animals (turtles and sea urchins) directly consume seagrasses; 
as a food resource. Along the Atlantic
however, water fowl rely heavily upon seagrasses 
Coast the dramatic loss of waterfowl during the 1930’s attributed to the decline of
was 
eelgrass(Zosteramarina),thebird'sfoodsource, causedbythewastingdiseaseandpollution 
(Milne and Milne 1951). Widgeons (Anas americana) have also been observed to consume 
Zostera marina and Ruppia maritima as well as some freshwater macrophytes (Bellrose, 
1976). McMahan (1970) found that rhizomes of Halodule wrightii comprised 84 and 88% 
by volume of the diets of redhead (Aythya americana) and pintail ducks (Anas acuta) 
respectively. Up to 78% of the redhead duck population winters on the Figure 6. A <S 13C histogram of the plants, sediments and fauna collected in Syringodiwn filiforme seagrass meadows in the Indian River Lagoon (Florida). From Fry 1984. 
Texas 
coast, while 80% of pintail ducks that winter in the central flyway occur along the 
Texas coast. McMahan (1970) found that the rhizomes of Halodule wrightii were the 
preferred food, making up the bulk of the diet for these birds. In another study, Cornelius 
(1977) found that 71% of the diet of redhead ducks {Aythya americana) consisted of 
Halodule wrightii rhizomes. Using stable isotope techniques (<S 13C), the carbon signature of 
redhead ducks was shown to vary with their use of habitat. Parker et al. (1992) found that 
13
the <S C values of muscle tissue collected from ducks in South Dakota were closer to that 
ofC-3plants (-22to-28%o)onwhich theduckswerefeeding(Figure7);insouthTexas the 
6 13 C values of the tissues changed to reflect a seagrass diet (-8 to -14 %o). Thus, seagrass 
beds are an important food source for a variety of water fowl species, especially along the 
Texas coast. 
In addition to providing habitat and food to other organisms, seagrasses can 
alter their immediate environment. Numerous authors have shown that
dramatically 
actasathree-dimensionalbaffle,reducingcurrentsandattenuatingwaves. Some
seagrasses 
was
of the earliest work examining seagrass-sediment interactions done by Ginsburg and 
Lowenstam (1958) in Florida Bay. They found that can modify sedimentation
seagrasses 
in two ways: (1) by stabilizing the sediments and (2) by producing a layer of semi-
motionless water that allows fine particles to settle out. Ginsburg and Lowenstam (1958) 
a alter the pattern of sediment
suggest that as result of binding and trapping, seagrasses 
deposition from what would be expected based on physical processes only. Similarly, it is 
reported that kelp beds act as traps for sand and mud particles in southern California 
(Ginsburg and Lowenstam 1958). Working in Bimini Lagoon in the 
13
Figure 7. Scatter plot of the <S C values of leg muscle of redhead ducks. South Texas ducks 
are isotopically heavier (closer to seagrass carbon) than ducks from South Dakota. From Parker et al., 1992. 
Scoffin and Thalassia the
Bahamas, (1970) found that Rhizophora (mangroves) were 
strongest binders of sediments. As a result these areas formed deeper sediments (relative 
to bedrock) than areas without plants. Scoffin (1970) also found that the distribution of 
plants throughout the lagoon was related to the depth of sediment overlying the bedrock. 
Other research has suggested that seagrass meadows preferentially concentrate small 
sized particles (Burrell and Schubel 1977). Rasmussen (1977) documented the changes in 
beachsedimenttexturefromafinesand-siltcomplexto coarsecobbleandassociated
a very 
these changes with the loss of Zostera to the wasting disease. Christiansen et aL (1981) 
suggested that changes in the coastline at Kyhom, Denmark were related to the dieback of 
eelgrass (.Zostera marina) presumably due to the wasting disease. The loss of seagrasses in 
the area allowed the mobilization and redistribution of sediments in the harbor. 
a
Scoffin (1970) presented hierarchy of plants which attenuate current strength, 
roots > Thalassia >
where Rhizophora prop Thalassia with heavy epiphytes > Laurencia > 
Polysiphonia. Mangroves (Rhizophora) and seagrass ( Thalassia ) were most efficient at 
reducing current flow. The ability of Thalassia to reduce current velocity was directly 
proportional to the density of seagrass blades; current velocities > 150 cm s' 1 were required 
to erode sediments from dense Thalassia beds (Scoffin 1970). To experimentally assess the 
impact of seagrass beds on currents, Fonseca et al (1982) used a salt water flume. They 
foundapredictablereductionintheapparentcurrentvelocitybyseagrass andsuggestedthat 
current reduction properties may vary between species and sites, due to morphological and 
Ward et
hydrographical variation (Fonseca et al, 1982). al (1984), working in Chesapeake 
Bay found that attenuated wave As
seagrasses energy and inhibited sediment resuspension. 
a result, sedimentation rates were substantially higher in seagrass communities than in 
unvegetated areas. Fonseca and Cahalan (1992) evaluated wave attenuation by four 
seagrass species andfoundthatbroad,shallowseagrass meadowssubstantiallyattenuateup 
of bed.
to 40% of wave energy per meter seagrass 
Recent research has also shown that communities act as
seagrass a "biological 
scrubber." Short and Short (1984) found that seagrasses, growing in mesocosms, rapidly 
filtered out sediments that were added to the water column, resulting in increased light 
penetration. Additionally, they found that seagrasses rapidly removed nutrients added to 
the filtered
water column (Figure 8). Thus, the seagrasses out both sediments and nutrients. 
In general, research has shown that seagrasses are a cornerstone to the health and 
productivity of estuarine communities (Zieman 1982; Phillips 1984; Thayer et at, 1984; 
Zieman and Zieman 1989). They provide habitat and food to a wide variety of organisms. 
Seagrasses also physically alter the environment they inhabit, by influencing water currents 
and sedimentation processes. However, the long-term future of these communities is in 
jeopardy. 
world wide decline in For
Recently, there has been a seagrass habitat (Figure 9). 
example,OrthandMoore(1984)showthatbeforethe 1960’5,submergedaquaticvegetation 
(SAV) was a widespread feature of the Chesapeake Bay System. However, 
Percent of surface light extinction and coefficients in a
Figure Ba. Halodule wrightii tank, 
an
a Syringodium filiforme tank and unvegetated tank following addition of Indian River From Short and
sand-silt sediment (a), clay sediment (b), and organic-silt sediment (c). Short 1984. 
Figure Bb. Nutrient removal from the water column by the 
seagrass community. Changes in phosphate (a) and ammonium (b) concentration for Halodule and a 
nonseagrass control tank over time. From Short and Short 1984. 
30 
time in Australia. From Cambridge and McComb
Figure 9. Changes in seagrass cover over 1984. 
since the 1960’s there has been a dramatic loss of SAY communities. Livingston (1987) 
have been virtually eliminated from Pensacola and Tampa Bays.
reported that seagrasses 
Eleuterius (1989) has documented the loss of about 70% of the seagrass habitat from 
1989.
Mississippi Sound between 1969 and During the 1930’s up to 90% of the eelgrass 
populations occurring along the Atlantic Coast were lost (Costa 1988). The decline of 
eelgrass was due to infection by the "wasting disease" caused by a marine slime-mold 
Labyrinthula zosterae (Muehlstein et al, 1991). The wasting disease was a natural 
caused Disease
catastrophe, by the parasitic organism. still plays a role in controlling 
declines in
seagrass distribution (Short et al., 1991); however, the more recent seagrass 
habitat are due primarily to anthropogenic influences. 
Aerial photography and ground truthing have been used to document changes in the 
distribution and biomass of seagrasses. Merkord (1978) documented changes in 
seagrass 
distribution of the Laguna Madre system between 1965 and 1976. In the Upper Laguna 
Madre, Halophila and Halodule cover increased, while Ruppia cover decreased. In Lower 
Laguna Madre, Syringodium cover increased, displacing Halodule in some areas while 
Thalassiaexpandeditsrangenorthward. Additionally,significantportionsofPortIsabelBay 
have become bare devoid of cover. that the
areas, seagrass Merkord (1978) suggested 
changes were a result of decreased salinity (over the long term) and increased turbidity. 
Costa (1988) documented changes in the abundance of eelgrass related to anthropogenic 
and natural disturbance using aerial photographs, charts, writtenreports, local residents and 
sediment He suggests that the wasting disease took a massive toll on eelgrass
cores. 
populations, eliminating about 99% of the eelgrass in Buzzards Bay. 
Pulich and White (1991) have documented the decline of seagrass habitat in the Galveston 
For
Bay System. They attribute various processes to the loss of specific seagrass habitat. 
example, the loss of Ruppia maritima was related to Hurricane Carla and a rise in relative 

level due to subsidence. However, in the lower bay both Halodule wrightii and Ruppia
sea 
maritima disappeared between the 1950’s and the 1980’s. These declines may be due to 
increased human activities such as urban development, wastewater discharges, chemical 
spills and dredging activities. Quammen and Onuf (1993) documented increase of 140
an 
km2 of bare bottom in Laguna Madre, Texas. They suspect that light reduction from 
maintenance dredging has caused the loss of cover. Thus, historical data bases,
seagrass 
nautical charts, and aerial photography have proven useful in documenting changes in the 
distribution of 
seagrasses. 
Presently, declining water quality (both transparency and nutrients) is adversely 
as a
affecting seagrasses. In general, reduced light availability result of anthropogenic 
influence is the greatest threat to seagrasses worldwide (Merkord 1978; Cambridge and 
McComb, 1984; Costa 1988; Giesen et al, 1990; Pulich and White 1991; Short et aL, 1991; 
Quammen and Onuf 1993). Thus, before seagrass can be expected to recolonize (either 
with or without human intervention), water quality problems must be addressed. 
CHAPTER 2; LIGHT ANDWATER TRANSPARENCY 
Environmental influences on water clarity 
Estuarine biologists and managers often refer to the concepts of "water quality" and 
"water somewhat This is unfortunate, because there is a
clarity" interchangeably. 
fundamental difference between the two expressions. Water quality refers to the chemical 
and physical parameters (i.e. nutrient concentrations, dissolved oxygen, salinity, temperature, 
etc.) that characterize 
a parcel of water with respect to the effect of these parameters on 
the
the health of aquatic organisms, in this case plants. Water clarity or transparency, on 
other hand, is a specific character of water quality. It is defined by the amount of light 
transmitted through a body of water. Decreased water quality (e.g. increased nutrient 
can
concentration) stimulate phytoplankton blooms which reduce light transmittance (e.g. 
water clarity). Thus, water clarity is related to and influenced by water quality, but these 
will
terms are not the same. Throughout this document we use the terms water quality and 
consistent with the above definitions.
water clarity in a manner 
Light in the aquatic environment 
Photosynthetically active radiation (PAR) is that portion of the electromagnetic 
PAR extends from about 350 nm to about
spectrum utilized by plants for photosynthesis. 
700 nm wavelength and is roughly equivalent to the range of wavelengths to which the 
human eye is sensitive (i.e. visible light). Seagrasses require light energy for the process 
ofphotosynthesis. However,thereareseveralfactorsthatinfluencetheamountoflightthey 
The
receive; three of these major factors are: (1) albedo; (2) scattering and (3) absorption. 
portionoflightreflectedbackinto isreferredtoasthealbedoofthatsurface(Figure
space 
10). The albedo of the Atlantic Ocean at 30° North latitude is approximately 0.068 (Payne, 
surface at this
1972). Thus, approximately6.8%ofthelight energyimpingingontheocean 
latitude is reflected back to space and is unavailable to marine plants, light is also 
scattered within the water 
column by suspended particles (Figure 11). In addition to albedo 
as it
and scattering, light is selectively absorbed passes through the water column (Figure 
12). 
used to measure the light field underwater because
Spherical quantum sensors are 
they measureboth downwelling and scattered light. Flat cosine sensors underestimate light 
availabilitybecausetheyaccountonlyfordownwellingirradiance. Becauseofscatteringand 
selective modelled with
wavelength attenuation, light penetration through water is an 
exponential decay function (Kirk 1983): 
(b)
I* I»e­
= 
where \is the irradiance at depth z, I is the incident irradiance at the water-atmosphere
Q 
interface, and k is the light attenuation coeffecient. 
Simulation models and field data have been used in the 
investigation of light 
column of estuaries. simulation model
penetration through the water Hogan (1983) used a 
nm in
of light attenuation and found that maximum transmissivity occurred at about 465 
clear water and at 550 nm in turbid water (Figure 12b). He concluded that the shift was 
Figure 10. Schematic diagram showing the different origins of light received from a remote sensor above the water. The reflection of the direct solar beam at the surface is termed albedo. From Kirk 1983. 
11.
Figure light availability underwater is determined by attenuating processes. Light attenuationresultsfromtheabsorption andscatteringoflightbyparticlesinthewater(i.e., suspended solids, phytoplankton, etc.) as well as the absorption of light by the water itself. From Dennison et al 1993. 
., 
37 
Figure 12a. The spectral range of underwater irradiance decreases with increasing depth in clear oceanic water. At 10 m about 70% of
the spectrum is fairly broad while at 90 m the quanta is in the 450-500 run band. From Saffo 1987. 
Figure12b. Sectralchangesofunderwaterirradiancewithincreasingwaterturbidityat10 in
m depth. Moderate turbidity (3) results in maximal transmittance green portion of spectrum while under conditions of maximal turbidity (type 9) transmitted light is mostly yellow. From Saffo 1987. 
38 
due to suspended matter in the water, which absorbed and scattered shorter wavelengths 
more The ultimate result was that the maximum transmittance of
than longer wavelengths. 
occurred in the in turbid coastal while total
light longer wavelengths environments, 
transmittance decreased. Pierce et al (1986) investigated how light transmittance 
respondedtochangesintheturbidityoftheRhodeRiver Estuary. lighttransmittance,both 
spectral quality and intensity, varied with changes in the amount of dissolved and suspended 
High concentrations of suspended solids and dissolved materials were correlated
matter. 
with increased attenuation in the upper water column; attenuation varied with wavelength 
depending on the materials present. Regression analysis indicated that the concentration 
ofchlorophyllaandc,aswellasmineralmatter,accountedformostofthevariation. These 
investigations suggest that light transmittance in turbid estuarine waters is more complex 
than clear oceanic water. 
Recently, many authors have estimated attenuation coefficients from Secchi depths 
as a means 
of calculating light availability (Chambers and Kalff 1985; Duarte 1991; Batuik 
et al, 1992; Dennison et al, 1993 and others). Preisendorfer (1986) examined the theory 
and mathematics behind the use of the Secchi disk. He suggested that use of the Secchi 
disk to evaluate the attenuation coefficient, k, obliterates and abuses the primary function 
of the Secchi disk. He concluded that transparency measurements made with a Secchi disk 
do not yield valid information on the availability of light at depth. Therefore, long term in 
situ electronic light measurements are required to estimate underwater light availability. 
However, other investigators (Megard and Berman 1989) suggest that Secchi depth 
(transparency measurements) is proportional to the attenuation coeffecient for the most 
conducted
penetrating waveband. However, the work of Megard and Berman (1989) was 
in oceanic waters and was not subject to the same scattering problems endemic to nearshore 
and estuarine systems. Historically the Secchi depth has been taken as the 18% light level; 
however, new calculations suggest that Secchi depth is the 22% light level (Megard and 
Berman Due
1989). to the ongoing literature debate, the applicability of Secchi depth 
environments As a researchers
measurements to estuarine is questionable. result, are 
starting to measure in situ light availability directly using photoelectric cells, such as 
spherical quantum sensors (Tomasko and Dunton, 1991). A long term (four-year) data set 
now 
exists for Upper Laguna Madre, Texas (Dunton, submitted). 
Natural influences on water clarity 
As mentioned previously, three factors control how much light reaches submerged 
Albedo varies with latitude and season due
vegetation: albedo, scattering and absorption. 
to the declination of the earth with respect to the sun. However, scattering and absorption 
of light within the water column are affected by a number of natural and anthropogenic 
influences. tides and floods increase
Currents, may the suspended solids, reducing the 
amount of light reaching the plants. In a rather extreme case, the onshore movement of 
coastal sand smothered a seagrass bed in Australia (Kirkman, 1978). Wind events have also 
waves can cause 
beencitedasinfluencinglight;wind theresuspensionofsedimentsresulting 
in high light attenuation (Ward et aL, 1984). Severe wind events, in the form of hurricanes, 
havebeencitedashavingadverse effects onseagrasses and lightinFlorida(Zieman 1975a) 
and in Mississippi Sound (Eleuterius 1989). In a recent manuscript Onuf (1993 in review). 
had to incorporate wind speed and direction into his statistical analysis to accurately model 
the propagation of dredging effects in Laguna Madre, Texas. 
and
Meteorological forcingintheformofwindwaves, large-scalewind-driven gyres 
flushing due to frontal passage have been shown to strongly influence the hydrography of 
Gulf estuaries (Ward and Armstrong 1980). The influence of meteorological forcing is a 
consequence of large surface area to volume ratios as well as the intensity and variability 
Wind driven 
waves are a
of meteorological events (Ward 1979). result of meteorological 
forcing; light to moderate winds over long fetches allow the development of intense surface 
waves. The mixing action of these wind waves results in waters that are usually vertically 
homogenous, except in the deeper dredged channels (Ward 1979; Ward and Armstrong 
1980). The seasonality of winds may also play a role in controlling light attenuation; for 
example,Riceetal. (1983)found thatsediment resuspension occursyear-roundbutthatit 
may be most active during winter storms. The seasonality of winds along the Texas coast 
is governed by the intensity of the Bermuda High (Ward and Armstrong 1980). As a result, 
light transmittance during certain times of the be affected more by sediment
year may 
suspensionthanduringotherpartsoftheyear(RiceetaL, 1983;Wardetaly1984). 
Water clarity in bays and estuaries along the Texas coast is mostly meteorologically 
driven, but also be influenced by biological substances dissolved in the water, such as
may 
tannins, humic acids and chlorophylls, which increase light attenuation through the water 
column. 
Gelbstoff (gilvin or ’yellow substance’) is a result of decomposition of organic 
matter into a complex group of compounds called "humic substances". In general, humic 
substances are large molecular weight compounds; for example, the average atomic 
compositionofhumic substancefromtheOkefenokee SwampinGeorgiawas 
These substances from a molecular of a few hundred to insoluble
vary weight 
macromolecular aggregates. Humic substances absorb light, especially at the blue end of 
a
thespectrum,resultingin shiftinthemaximumpenetrationofspecificwavelengthsoflight 
Pierce and mineral
(Figure 12b; Kirk 1983). et al (1986) found that chlorophyll a,c 
suspensate accounted for most of the attenuation of light through the water column in the 
Rhode River Estuary. They suggested that the high attenuation of selected wavelengths in 
the column
upperpartofthewater mayreducetheavailabilityofPARbelowthatnecessary 
forbenthic plantsorshiftcommunitystructuretofavor speciescapableofusingwavelengths 
greater than 525 nm. Carter and Rybicki (1990) document a dramatic shift in the quality 
of light reaching 1 m depth as a result of suspended solids. 
Natural plankton blooms may also reduce the amount of light transmitted through 
column.
the water Cosper et al (1987) documented blooms of the chrysophyte Aureococcus 
anophagefferens with cell counts greater than 109 cells L l The bloom of this alga reduced 
. 
light penetration through the water column and resulted in the loss of 55% of the eelgrass 
habitat in Long Island bays (Cosper et al, 1987; Dennison et al, 1989). 
Texas has recently 
been experiencing a similar "brown tide" that has been persistent in some portion of Laguna 
Madre since July 1990. The brown tide organism (BTO) appears to be an undescribed type 
HI aberrant chrysophyte related toAureococcus anophagefferens and Pelagococcus subviridis. 
The BTO is a 4-5 pm in diameter; maximum chlorophyll concentrations of 70 Mg L' 1 and 
densities The BTO has also
up to 109 cells L 1 have been reported (Stockwell et al, 1993). 
had dramatic effects on both the micro and mesozooplankton populations (Buskey and 
Stockwell 
1993). As a result of the high cell densities, light has been dramatically reduced 
(by up to 60%) in Laguna Madre (Dunton submitted) and could contribute to the loss of 
habitat.
seagrass 
Banks of drift also influence communities (Cowper 1978; Benz
algae may seagrass 
et al, 1979; Gilbert and Clark 1981; Kulczycki et al, 1981; Zimmerman and Montgomery 
1984; Vimstein and Carbonara 1985). The development of the drift algal communities 
reduce light availability to the seagrasses (Cowper 1978). Additionally, the decomposition 
of drift algal banks may influence nutrient dynamics (Zimmerman and Montgomery 1984; 
Vimstein and Carbonara 1985). 
Although microalgal epiphytes do not decrease water clarity per se, they can 
influence the amount of light that a macrophyte receives. Numerous studies have 
documented decreased light availability to macrophytes as a result of epiphyte growth. 
Some of the earliest work (Sand-Jensen 1977) suggested that diatomaceous epiphytes act as 
a barrier to carbon uptake and reduce light availability. Bothwell (1989) found that the 
nutrient to the inner was diffusion limited.
supply periphyton layers Additionally, 
Meulemans (1987) found that light is strongly and selectively absorbed in the upper layers 
of periphyton communities. Thus, recent studies have confirmed the findings of Sand-
Jensen. 
Other studies have shown that microalgal epiphytes can attenuate 58-94% of light 
incident on the leaf surface (Batuik et al 1992; Staver 1985; Twilley et al, 1985).
., 
Anthropogenic influences on water quality and clarity 
Human activities tend to cause a decrease in overall water quality. 
For example, 
enrichment water and estuaries often results in
nutrient (decreased quality) in bays 
that has been used and abused by estuarine
eutrophication. Eutrophication is another term 
and of environmental
biologists managers. Eutrophication is a process and community 
change caused by the interaction of three components: (1) excessive nutrient availability; (2) 
reduced illumination and (3) a shift in the species composition as a result of altered light 
and nutrient regimes (Kaldy, 1992). There are numerous investigations from around the 
world that have shown or suggested that increased nutrient loading stimulate the growth of 
algal competitors (epiphytes or phytoplankton) which shade out seagrasses (Zieman 1975a; 
Sand-Jensen 
1977; Zieman 1982; Cambridge and McComb 1984; Cambridge et aL, 1986; 
Zieman and Zieman 1989; Giesen et aL, 1990; Short et aL, 1991; Kaldy 1992; Short et aL, 
inreview). Thissequenceofeventshasalsobeenshowntoaffectperennialmacroalgaelike 
and
Focus (Kautsky 1991) and freshwater submerged macrophytes like Elodea canadensis 
Potamogeton pectinatus (Ozimek et aL, 1991). Several conceptual models (Phillips et aL, 
1978, Short et aL, 1991, Vogt and Schramm 1991) have been developed to examine the 
in
process of eutrophication (Figure 13; Kaldy 1992, Short et aL, submitted, Kaldy et aL, 
prep.). 
Presently, eutrophication problems appear to be confined to a few localized areas 
of the Galveston and White
in Texas, including portions Bay System (Pulich 1991). 
However, other areas are not immune to eutrophication. Most of the watershed that drains 
intoCopanoBayissubjectedtoagriculturalfertilizerapplicationthatinfluences thenutrient 
regimeofthebay(Shormann 1992). Additionally, asignificantamountofthenutrientinput 
to Copano Bay is from point source sewage outfalls from urban areas within the watershed. 
Figure 13. Chemical loading hypothesis suggests that under low nutrient levels seagrasses are dominant with some algae present As nutrient loading increases seagrass density 
decreases and phytoplankton and epiphytes become more prevalent Under conditions of excessive nutrientloadingseagrass densityandbiomassbecomeslow,andoneofthreealgal 
forms (phytoplankton, microalgal epiphytes or macroalgal epiphytes) becomes dominant. From Short et al 1991. 
., 
Thus, Shormann (1992) suggests that Copano Bay can be considered eutrophic. In addition, 
recent a considerable amount
researchsuggeststhat ofthenitrogeninputtoTexasbaysand 
estuaries comes from rain falling directly into the bays (Shormann 1992, T. Whitledge pers. 
comm.). The potential for eutrophication problems along the Texas coast cannot be 
one of
ignored. Reports on population growth rates indicate that the Texas Gulf coast is 
the most rapidly developing areas of the country (NOAA 1988, 1990a,b). As a result, most 
of the bays and estuaries along the coast are highly susceptible to pollution, including 
excessive nutrient loading (NOAA 1990b). 
Decreased water clarity, due to increased suspended solids, also occurs as a result 
disturbance Zieman
of dredging, construction, erosion, runoff and (Zieman and 1989). 
Odum (1963) showed that silt from dredging activities in the Gulf Intracoastal Waterway 
have caused imbalance
may an in respiration and photosynthesis resulting in decreased 
productivity. Zieman (1975a) suggested that dredging was responsible for the destruction 
of seagrass as a result of direct physical damage, reduced light and hypoxia associated with 
the high oxygen demand of decomposing material. He also suggested that clam dredges 
were 
justasdamagingasdredgingnavigationalchannels. PulichandWhite(1991)suggested 
that construction, dredging and suspended solids all contributed to the decline of 
seagrasses 
in the Galveston Bay system. Giesen et al (1990) suggested that recent large losses of 
eelgrass from the Dutch Wadden Sea are the result of increased turbidity from both 
progressive eutrophication and dredging activities. Onuf (1993, in review) found that light 
attenuation near the dredge spoil site remained elevated 15 months after dredging had 
occurred. Increased light attenuation near the edge of a seagrass beds was evident for 10 
months after dredging. Thus, human construction activities often have a destructive 
influence on seagrass habitat. 
also exacerbate water
Anthropogenic physical disruption of the environment may 
Bulthuis et aL (1984) found that the concentrations of suspended solids,
quality problems. 
phosphorus and silicate were higher in water ebbing from denuded mudflats than from 
covered mudflats. The efflux of nitrogen from the sediments was light mediated
seagrass 
as a result of demand by photosynthetic organisms, and was not different between denuded 
and covered areas. Bulthuis et al. (1984) suggested that denudation ofseagrass-covered tidal 
mudflats would lead to increased efflux of suspended solids and nutrients from the 
sediments 
to the overlying water. 
While not generally decreasing water quality, boating activities have deleterious 
There are in Thalassia
effects on seagrasses. several reports of motorboat propeller scars 
beds (Zieman 1975a, 1976; Dunton pers. comm. 1993). These scars do not recover rapidly 
after disturbance, and persist for 2-5 years even in healthy, thriving beds (Zieman 1976). 
In addition to physical damage to the plants the sediment microhabitat is impacted. For 
example, changes in the grain size, pH and eH of the disturbed sediments have been 
observed (Zieman 1976). Walker et aL (1989) used aerial and underwater photography to 
assess the adverse effects of boat moorings on seagrass beds. Moorings scoured circular 
was 
2.
patches ranging in size from 3to 300 m Although less than 2% of the seagrass area 
lost to moorings the increased edge effect makes more of the beds susceptible to erosion 
and "blowouts". In general, the mechanical disturbance of beds is a worldwide
seagrass 
problem typified by localized impacts associated with construction activity (Short et al.. 
1991). 
Thermalandtoxicpollutionalsohaveadverseeffectsonwaterquality. Ziemanand 
Wood (1975) have shown that thermal pollution reduces the diversity and abundance of 
algae and animals near effluent canals. They also suggested that several estuarine plant 
groupswerelikelytorespondtopollution; seagrasses,macroalgae,phytoplankton,epiphytic 
microalgae and benthic microalgae (Wood and Zieman 1969). However, seagrasses were 
relatively more resistant to thermal stress than algae (Zieman 1975a). Tropical seagrasses 
live close to their thermal tolerance, (e.g. Thalassia testudinum has an optimal temperature 
rangeof28-30°C);therefore,raisingthetemperatureregimecanbe deleterioustotropical 
and subtropical estuaries (Zieman 1975a). 
habitat. Pulich and
Toxic pollution has also been implicated in the loss of seagrass 
White (1991) suggest that pollution, including chemical spills, may have contributed to the 
decline in the Galveston that
of seagrasses Bay System. Livingston (1987) suggested 
losses in Florida estuaries were mainly the result of decreased water quality from
seagrass 
avarietyofurbanandindustrialsources. Eleuterius(1989)alsosuggestedthatspillsoftoxic 
substances have contributed to the decline of seagrasses in Mississippi Sound. 
The consensus researchers is that light is the environmental factor that has
among 
the greatest influence on the depth distribution of Albedo affects the amount
seagrasses. 
of light that actually enters the water and is influenced by latitude and declination of the 
Earth with respect to the sun. Natural and anthropogenic factors can dramatically alter the 
amountHumans have
oflight reaching seagrasses by increasing scattering and absorption. 
little, ifcontrol over
very any, natural weather phenomena; however, we have the capability 
to minimize the adverse effects of human activity (Table 5). 
48 
Table 5. meadows in Florida.
list of data concerning historic anthropogenic impacts on seagrass 
From Livingston 1987. 
StudyArea 
Indian River 
Biscayne Bay 
Florida Keys 
Florida Bay 
Tampa Bay system 
Charlotte Harbor 
Pensacola Bay system 
Choctawhatchee Bay 
St.Andrews Bay 
St. Joseph Bay 
Apalachicola Bay system 
4 
Apalachee Bay 
Location 
Southeast Florida Atlantic Ocean 
Southeast Florida Atlantic Ocean 
South Florida Atlantic Ocean 
South Florida 
Southwest Florida Gulf of Mexico 
Southwest Florida Gulf of Mexico 
Northwest Florida Gulf of Mexico 
Northwest Florida Gulf of Mexico 
Northwest Florida Gulf of Mexico 
Northwest Florida Gulf of Mexico 
North Florida Gulf of Mexico 
North Florida Gulfof Mexico Status of SeaMeadows
grass 
Historic declines in number and of
coverage 
meadows. Declines in Veto Beach
seagrass area, 
Fort Pierce Inlet (25%) and Sebastian Inlet 
(38%) from 1951 through 1984. 
Undetermined deterioration in northern Biscayne 
Bay. Some damagetoThalassia-Halodule beds 
near 
power plant (heatedeffluents) in south 
Biscayne Bay. Card Sound unaffected by power 
plant discharge. 

Few data found. Little effect of Key West 
desalination plant. 
Postulated altered species relationships due 
to increased salinity caused by redirection 
of freshwater runoff. 
Almost forty percent reduction in Boca Cicga 
Bay due to dredging, filling, and associated 
activity from 1950 through 1968. Multiple 
sources (urbanization, storm water runoff, 
sewage discharge, industrialization,toxic 
substances). Reduction of seagrass meadows 
in Tampa Bay system from 30,970 ha to5,750 ha. 
Decline of 29 percent of seagrass beds from 
1943 through 1984. 
Complete loss of seagrass beds in Escambia Bay, 
East Bay, and Pensacola Bay from 1949-1979. 
Some fresh-brackish water species extant in delta 
areas. Some Thalassia-Halodule beds still alive 
in Santa Rosa Sound. Losses due to urbanization, industrial waste discharge, dredgingand filling, cultural eutrophication. 
Historical deterioration of 
seagrass beds from 
1949 through 1983. Causes unknown. 

to
No data found: Presumed impact cue 
urbanization, industrialization 
Extensive 
coverage unchangedfrom 1972 through 1983. Relatively unpopulatedarea. 
Generally healthyassemblages ofseagrasses. 
Localimpact duetodredgedopeningin associated barrier island. Introduced species spreading in delta areaswith as yet undetermined impact. Area under increased pressure from urganization. 
Impactsduetodisposalofpulpmill wastes (Fcnhollowayestuary)from 1954tothepresent. Slow 
recovery noted inouterportionsofimpact area (associated with pollution abatement program). Area now threatened by proposed 
inshore navigation channel and possible off­shore oil drillingoperations. 
Information Source 
Goodwin and Goodwin, 1976; 
Florida Department of Natural Resources, unpub­lished data. 
McNulty, 1961; Roessler and 
Zieman, 1969; Thorhaug 
et al., 1973; Zieman, 1970, 
1982. 
Chcshcr, 1971 
Zieman, 1982 
Lewis and Phillips, 1980; Simon, 1974; Lewis et al., 1985; Taylor and Saloman, 1968 
Harris etal., 1983 
Livingston, 1979; Livingston 
etal., 1972;Olingcretal., 1975 
Burch, 1983 
McNulty et al., 1972; Savastano et al., 1984. 
Livingston, 1980c, 1983 
Heck, 1976; Hooks et al., 
1976; Livingston, 1975, 
1982a, 1984a; Zimmerman and Livingston, 1976a,b. 
49 
CHAPTER 3: THE CHESAPEAKE BAY APPROACH 
Approaches to assessing habitat requirements of seagrasses 
Recently, several documents have been published which present a novel approach 
to aL, 1992; Dennison et aL, 1993). Dennison
the management ofseagrass habitat (Batuik et
et aL (1993) presents the major findings of a comprehensive technical synthesis conducted 
by Batuik et aL (1992). Both Batuik et al (1992) and Dennison et al. (1993) advocate the 
measurement of physical and chemical water column parameters in existing seagrass beds 
as a means of identifying impacted systems. The premise is that by knowing the conditions 
required for plant growth, we can determine the water quality of a specific area through 
examination of or absence. Thus, areas indicate
seagrass presence where plants do not grow 
that some water quality parameter(s) do not meet the minimum requirements of the plant. 
Waterqualityparameterrequirements wouldbespecificforeachplant speciesandestuarine 
habitat. Chesapeake Bay parameter requirements were developed by monitoring water 
quality gradients within the system over time (Batuik et aL, 1992). The habitat requirements 
developed in this manner represent the absolute minimal water quality characteristics 
necessary to sustain plants in shallow water (Dennison 1993). 
The growth, survival and depth distribution ofsubmerged aquatic vegetation (SAY), 
including seagrasses, is related to underwater light availabilty (Chapter 4). The parameters 
examined were a
to determine habitat requirements total suspended solids, chlorophyll 
levels, dissolved inorganic nitrogen and phosphorus and the light attenuation coefficient 
(Batuik et aL, 1992; Dennison et aL, 1993). 
These parameters affect light availability in a 
variety ofways, either by directly absorbing and scattering light or by stimulating the growth 
ofphytoplankton(seeChapter2forfurtherdiscussion). Thehabitatrequirementapproach 
does on
not rely on understanding the interactions of water quality and light but relies 
empirical water quality data and SAY survival (Dennison et aL, 1993). 
Measurements were
of the water quality parameters made monthly through the 
growing season, although longer databases (up to 10 years) do exist for some portions of the 
Chesapeake Bay System. Minimum habitat requirements for SAY in polyhaline areas 
(salinitygreaterthan 18%o)wasalightattenuationcoefficientof1.5(m'1),totalsuspended 
solids of 15 mg L l, chlorophyll aof 15 /xg L 1 and dissolved inorganic nitrogen and 
phosphorousof10and0.67/xM,respectively. Otherminimumrequirementsweredeveloped 
for other salinity regimes (Table 6). In areas where the water quality parameters do not 
exceed these values one would expect to find SAY. 
Although the habitat requirement approach is unique and potentially very useful, 
there are some drawbacks. Measurements were carried out monthly during the growing 
season. 
Due to the immense spatial and temporal variability of marine systems, monthly 
sampling of water quality parameters is not adequate to characterize a system. Monthly 
sampling is also likely to underestimate parameter levels experienced by SAY due to the 
fact that most field avoid sampling during inclement weather. while
programs Also, 
sampling during the growing season may be appropriate for plant species that overwinter 
as a tuber or seed, it may not be appropriate for species that grow throughout the year (i.e. 
Zostera marina). In addition, estimates of the light attenuation coefficient are derived from 
Secchi depths (Batuik et aL, 1992; Dennison et aL, 1993). In view of the problems related 
Table 6. For each
Chesapeake Bay submersed aquatic vegetation habitat requirements. parameter, the maximal growing season median value that correlated with plant survival is 
given for each salinity regime. Growing season defined as April-October, except for tidal fresh =
polyhaline (March-November). Salinity regime defined as 0.05%0, oligohaline = 0.0-5%0, mesohaline = 5-18%0, polyhaline more than 18%0. From Dennison et al 1993. 
., 
Light Total Dissolved Dissolved attenuation 
suspended inorganic inorganic 
coefficient solids 
Chlorophyll nitrogen phosphorus Salinity regime (mg/l) a (Mg/l) (mM) (mM) 
Tidal freshwater 2.0 15 15 0.67
_ 
—
2.0 15 15 0.67
Oligohaline 
Mesohaline 1.5 15 15 10 0.33 Polyhaline 1.5 15 15 10 0.67 
to the use of Secchi depth to measure attenuation (see Chapter 2) it seems that direct 
measurement of light availability is more appropriate. 
The habitat requirement approach may be useful tool to estuarine managers in
a 
It
developing water quality standards to prevent the loss of SAV, including seagrasses. 
should be pointed out that there is a fundamental difference between the Chesapeake Bay 
and the bays and estuaries in Texas. The Chesapeake Bay program is using water quality 
criteria for the reestablishment and restoration of 
a highly perturbed system. In Texas, the 
goal is to establish water quality criteria to prevent the destruction of habitats.
seagrass 
Consequently, a different approach and strategy may be appropriate for preserving seagrass 
habitats in Texas. 
CHAPTER 4; SEAGRASS PHOTOSYNTHETIC PHYSIOLOGY 
Historically, muchoftheworkonthephotosyntheticphysiologyofmarineplantshas 
been done with phytoplankton, as they are at the base of the oceanic food web. Based 
on 
these studies, the euphotic zone was defined the depth to which 1% of surface PAR
as 
penetrates (Bougis 1976; Raymont 1980). The 1% light level is not appropriate for defining 
the depth limits of seagrasses due to the higher respiratory demands of the below ground 
tissues (Kenworthy and Haunert 1991). Until recently, it was generally accepted that 10% 
of surface PAR was required to sustain seagrass populations. More recent studies suggest 
that most seagrass species require 15-25% of surface irradiance (Table 7; Kenworthy and 
Haunert1991;DennisonetaL, 1993).Numericalmodelsof
seagrass depth limits have been 
developed and used to predict the depth distribution of submerged macrophytes. Many of 
Chambers
the models are regression models of field data using least squares methodology. 
andKalff(1985)developedregression modelsforavarietyoffreshwatermacrophytesfrom 
Canada (Table 8). They used attenuation coefficients developed from mean summer Secchi 
depth to estimate light availability. In general, approximately 20% of surface PAR was 
required to sustain freshwater macrophyte species. Duarte (1991) developed a regression 
model (Table 8) that predicts depth distribution for a variety ofseagrass species. According 
to his model, most seagrasses require about 11% of surface PAR (Duarte 1991). 
Many of the species used in Duarte’s (1991) model occur in clear waters and
very 
thus may underestimate the minimum light requirement for In addition to this
seagrasses. 
problem,bothChambers andKalff(1985) andDuarte (1991)relyonattenuationcoefficients 
Table 7. were calculated
Maximaldepthlimitsandminimallightrequirementsofvariousseagrassspecies. Minimallightrequirementsaspercentlightatmaximaldepth. Ranteofmaximaldepthlimit andmeantSEofminimallightrequirementsgivenforlocationswith multiple data points. From Dennison etal., 1993. 
Maximal Minimal light Genus and species Location depth limit (m) requirement (%) 
7.0
Amphibolisantarctica* WaterlooBay(Australia 24.7 
4.0 10.2
Cymodocea nodosa Ebro Delta (Spain) 
Malta 38.5 
+ 
Halodule wrighdi Florida (US) 1.9 17.2 

Halophila decipiens^ St. Croix (US) 4.4 
C. nodosa 7.3 
40.0 
Northwest Cuba 24.3 8.8 Northwest Duba 
H. decipiens 
14.4 23.7 
Heterozostera tasmanica 3.8-9.S 5.0±0.6 

Halophila engelmanni Victoria (Australia) 
Chile 7.0
H. tasmanica 17.4 
H. tasmanica 39.0 4.4
Spencer Gulf (Australia) 
H. tasmanica 8.0 20.2
Waterloo Bay (Australia) Poisdonia angustifolia Waterloo Bay (Australia) 
7.0 24.7 Island (Spain)
Poisdonia oceanica Medas 15.0 7.8 
P. oceanica Malta 35.0 9.2 Poisdonia ostenfeldii Waterloo Bay (Australia) 
7.0 24.7 Poisdonia sinuosa 7.0 24.7
Waterloo Bay (Australia) Ruppia maritima Brazil 0.7 8.2 Syringodium filiforme 
Northwest Cuba 16.5 19.2 
S. filiforme* Florida (US) 18.3
6.8 
+ 

Florida (US) 1.9 
17.2 
Thalassia testudinum Northwest Cuba 14.5 

S. filiforme 
233 
T. testudinum Puerto Rico 1.0-5.0 24.4±4.2 
T. testudinum Florida (US) 73 15.3 Zostera marina Kattegat (Denmark) 20.1±2.1
3.7-10.1 Z marina 2.0-5.0 19.4±1.3 
* 
Roskildc (Denmark) 

Zmarina Denmark 13-9.0 
20.6H3.0 Z marina Woods Hole (US) 6.0 18.6 Z marina Netherlands 23 29.4 
Z marina 2.0-5.0
Japan 18.2±43 
*Duartc 1991 
+ 
WJ. Kenworthy, personal communication, 1990 and Dennison 1990 S Ostenfcld 1908 
#Borum 1983 
55 
Table 8. Regression equations used to predict the depth distribution of different aquatic plants. 
Group Equation 
Freshwater 
=
Angiosperms (ZJ-3 1.33 log(D) + 1.40 
Bryophytes (Zys = -0.48 log(D) + 0.81 
c
=
Charophytes log(Z) 0.87 log(D) + 0.31 
c 
Marine Seagrasses 
= 
-
general log (Z) 0.26 1.07 log(k)
c
-
Thalassia Z = 0.27 
0.93 log(k)
c 
-
Zostera Z = 0.27 
0.84 log(k)
c 
Chambers and Kalff 1985 2From Duarte 1991 Z = colonization depth
c 
D = Secchi depth
mean summer k = diffuse light attenuation coefficient 
from Secchi measurements. Priesendorfer that it is
developed depth (1986) suggests 
notappropriate to derive attenuation coefficients from Secchi depth (see Chapter 2). 
Computer modelling 
Computer simulation models of seagrass growth and productivity in relation to light 
and nutrients have been developed. The development of mathematical models provide a 
mechanism for synthesizing the information available in the literature (Short 1980). Models 
have become predictive tools for management, but can also provide insight to the dynamic 
of One of the first mathematical models of
response seagrass ecosystems (Short 1980). 
how nutrients affected
seagrass ecosystems investigated eelgrass growth; however, to 
realistically model the system it was necessary to incorporate light limitation (Short 1980). 
ShortutilizedSteele’sequationforphytoplanktonphotosynthesis, whichdescribesproduction 
as 
increasing with increased light up to an optimum light intensity. Beyond the optimum, 
as a
production decreases result of photoinhibition (Short 1980). The model was run using 
data from Charlestown Pond, R.I. The simulation yielded a good representation of the 
seasonal trends and a reasonable fitto the observed data (Short 1980). Wetzel and Neckles 
a
(1986) developed model of photosynthesis and growth for Zostera marina in relation to 
selected physical-chemical variables. They found that physical parameters such as light and 
temperature controlled growth and photosynthesis. Small changes in submarine irradiance 
or temperature resulted in decreased plant productivity and the eventual loss of the seagrass 
community. These simulations suggest small changes in light (or temperature) may cause 
the complete loss ofseagrass communities on the edge of their physiological tolerance (e.g., 
Wetzel and Heckles (1986) concluded that ambient
the deep edge of the seagrass bed). 
light was a principle factor controlling the longevity and survival of seagrass beds. 
Zimmerman et al (1987) modelled nitrogen budgets and light availability using field and 
were tested
experimental data from numerous investigations, and the model predictions 
against other field data. They found that light had a significant effect on the rate and site 
marina. The to H
of nitrogen uptake in Z. model predicts that for eelgrass exposed 
sat 
greater than 6 h (i.e., "normal" conditions) most of the nitrogen uptake will occur through 
the roots. However, in low light environments (short there is an increase in the 
importance of nitrogen uptake and assimilation by the leaves. Thus, the site of nutrient 
uptake appears to be partially dependent on the light environment the plant experiences. 
As a general approach to seagrass ecology, simulation models are invaluable. They 
can be used to synthesize the available data and point out new directions for research. 
that mathematical simulations could be used
Short (1980) showed to adequately predict 
seagrass productivity as a function of several parameters including nutrients, light and 
Wetzel and Heckles that small in the diffuse
temperature. (1986) suggested changes 
attenuation coefficient could result in the elimination of habitat. Zimmerman et
seagrass 
the
al (1987) suggest that the site of nutrient acquisition is dependent, at least in part, on 
light environment the plant experiences. In general, simulation models can be very 
interesting and provide useful information to seagrass biologists and estuarine managers. 
Effects of in situ light reduction 
Although computer simulations are informative, researchers also need to 
quantitatively evaluate the effect of model parameters on in situ populations. During the 
late 1960’s and studies were conducted to examine the effects of reduced
1970’s numerous 
light on seagrass growth, production and morphology. Some of the earliest work involving 
was done by Burkholder and Doheny (1968), who used to reduce
seagrass shading cages 
available 10 and 1.6 % of surface irradiance (SI). Eelgrass (Zostera
light to 100, 60, 20, 
marina) growing in these cages became noticeably stunted and did not survive at light levels 
less than 20% SI. In an attempt to validate a model, Short et al (1974) investigated how 
the hydrodynamics of the Charlestown Pond R.I. were influenced by eelgrass. They 
examined the influence of light on seagrasses (and in turn the hydrodynamic model) by 
which resulted in shorter plants as well as reduced biomass and
shading eelgrass with cages, 
density. Backman and Barlotti (1976) reduced 63% of the light reaching Z. marina in a 
coastal lagoon in California for a period of nine months. They found reduced biomass, 
shoot density and incidence of flowering. Congdon and McComb (1979) examined the 
reduced light in Australian estuary. They
an
productivityofRuppiamaritimainresponse to 
used 
seven light levels in the study: 100, 60, 41, 28, 19, 15, and 7.5% SI. However, they 
made no in situ measurements of irradiance. They reported a general seasonal pattern of 
As the duration of
low standing crop during winter with a rapid increase in the spring. 
shading increased, the plants required higher light levels to persist. At least 20% SI was 
for
requiredtomaintain50%ofinitialstandingcrop upto100days,whilegreaterthan60% 
SIwasrequiredtomaintain50%ofinitialbiomassformorethan200days. Theyconcluded 
thatareductioninlightintensitymayresultinthelossofconsiderable quantitiesofRuppia. 
Duringthe 1980’sseveralotherinvestigationsexaminedtheinfluenceofreducedlight 
various Bulthuis (1983a) examined the of
on seagrass species. For example, response 
Australia.
Heterozostera tasmanica to in situ light reduction in Victoria, The light levels 
below the screens were approximately 18, 13, 4.7 and 1% SI. Light levels less than 4.7% 
SI resulted in the death of all shoots within 2to 10 months. Light levels of 18 and 13% SI 
caused reduced shoot density relative to the controls, suggesting the plants were unable to 
survive indefinitely at these light levels. Some changes in shoot morphology were also 
noted;however,leafgrowthrateandleafwidthremainedthesame. Bulthuis(1983a)data 
indicate that H. tasmanica have
may a higher light requirement at summer temperatures 
than at winter temperatures and thus may be more sensitive to reduced irradiance during 
summerthan in winter (Bulthuis 1983a). Carter and Rybicki (1985), found that both light 
penetration and grazing pressure effected the survival of transplanted Vallisneria americana 
Odum (1985) examined the of Thalassia
in the tidal Potomac River, Maryland. response 
testudinum to shading by epiphytes. She did an in situ shading experiment during the winter 
where light intensity was reduced to 2-10% SI. Shoot density decreased under the reduced 
light treatments and after eight months all the plants were dead. It was suggested that 
demise of the would have occurred more
during summer the seagrasses quickly. 
Neverauskus (1988) examined the response of Posidonia sinuosa and P. angustifolia in 
Australiatochroniclong-termlightreduction(Figure 14). Heconstructed acanopyof50% 
shade cloth, which was placed over the community. During the first six months of treatment, 
shoot density was unchanged but leaf density and standing crop had declined; during the 
second six months shoot density decreased dramatically. However, although Neverauskus 
(1988) documented changes in plant canopy structure, the study itself was very poorly 
Figure14. Changesinstandingcrop,leafdensityanddryweightofepiphytesofPosidonia in response to 50% reduction in ambient light. Error bars represent standard deviation, n 
= 4. From Neverauskas 1988. 
designed. There was no control plot, no statistical analysis nor was there any estimate of 
the actual amount of light received by the plants. 
Stable carbon have been to examine the effects of
isotope ratios used recently 
13
reduced light on The <S C values of shaded Thalassia
seagrasses (Durako and Hall 1992). 
leaves were significantly lower than those of unshaded plants. Changes in the £I3C values 
were correlated with the relative amount of light reaching the plants. The increasing 
isotopic fractionation with light reduction reflect decreasing carbon demand associated
may 
with lower photosynthetic rates. Thus, at low irradiances there be greater relative
may 
carbon availability. As light is reduced to levels that limit photosynthetic rates, carbon 
appears to become non-limiting. 
There have been many studies describing the effects of reduced irradiance on 
different parts of the
seagrasses. Although the experiments have been carried out in many 
world on a variety of are some common observations that can be
seagrass species, there 
made. and of shoots decreases
First, the standing crop density with decreasing light. 
Second, there are often changes in the morphology of seagrass shoots (i.e., longer and 
thinner blades) with reduced irradiance. Third, several studies suggest there are seasonal 
differences in the light requirements ofseagrasses. Fourth, it appears that light availability 
13
may influence the <S C signature of seagrasses. Fifth, there is a lack of consistency in the 
experimental design of these studies with respect to light reduction techniques or 
measurements of light availability. 
Experimental changes In daily light period 
Several field studies have examined the effects of changes in daily light period length 
on seagrasses. However, before examining the findings of these studies it is necessary to 
define H and H (Figure 15). H refers to the daily number of hours at which PAR 
sat
comp comp 
is at or above a plant’s compensation irradiance. H refers to the daily number of hours 
sat 
at which PAR equals or exceeds a plant’s saturation irradiance. Dennison and Alberte 
(1982) found that both H and H may be manipulated in situ by using light reflectors 
sat 
comp 
and shading screens to study the effects of changing the daily light period (Table 9). They 
concluded 
that during short-term experiments, eelgrass responded to changes in the light 
environmentbychangingleafproductionrates. Dennison andAlberte(1985)examinedthe 
roleofthedailylightperiodonthedepthdistributionofZosteramarina. Thelengthofthe 
photoperiod was increased by suspending underwater lights over an eelgrass bed and 
shortened using shade screens (Table 10). Based on these manipulations, Dennison and 
Albert (1985) determined that eelgrass required a minimum of about 6 hrs to maintain 
balance.
a positive carbon They concluded that was a better predictor of plant 
absolute PPFD Dennison and
productivity than (photosynthetic photon flux density). 
Alberte (1986) examined photoadaptation and growth along a depth gradient for Z. marina. 
Zostera transplantedfromdeepwatertoshallowwatersurvived well,while transplantsfrom 
shallow water to deep water did not survive. 
Daily quantum flux varied with depth from 
38.8 to 4.4 E m'2 1 at 0.8 and 7 In addition varied
day'm water depths, respectively. 
from 12.7 to 5.8 h for 0.8 and These studies that in
7 m depths, respectively. suggest 
addition to the actual PFFD that plants experience, the photoperiod and particularly H^ 
Figure 15. Generalized diurnal light curve in which the light saturation point (Ik) and light compensation point (I) for photosynthesis are used to determine periods of saturating
c 
and compensating (H) quantum irradiance. From Dennison 1987. 
comp
Table 9. and H result of reflectors and
Changes in the H parameters for eelgrass as a 
sat
comp 
shading. From Dennison and Alberte 1982. 
Station percent change 
Shallow (1.3 m) Reflectors 
35% increase of light 
H +2% 
comp 
H^+1% 
55% decrease of light H -4% Shades 
comp 
H^-1% 
Deep (5.5 m) Reflectors 40% increase of light 
Hemp*4% 
H +14% 
sat 
Shades 55% decrease of light Vn* H„-52% 
Table 10. 1985.  Changes in  as  a result of lights and shading.  From Dennison and Alberte,  
Station  Change in H M  
Shallow Lights Shades  + 4 hours -4 hours  
Deep  Lights Shades  + 5 hours -3 hours  

also influence the depth distribution of Dennison (1987) examined the effects
seagrasses. 
of light on seagrass photosynthesis, growth and depth distribution. The longest periods of 
lightcenteredaroundthesummersolstice; increasedrespiration duetohighertemperatures 
and
effectively decreased both H There was an exponential relationship between 
comp 
light extinction coeffecients and maximum depth limits; thus, Dennison (1987) suggests that 
Secchi disc depths averaged throughout the year can be used to predict the maximum depth 
ofZ.marina(seechapter2andZimmermanetaL, 1991).Heconcludedthattheperennial 
of a year-round integration of environmental factors that
nature 
most seagrasses represents 
influence the compensation depth. 
Although they did not manipulate H Zimmerman et al (1991) found the length of 
sat, 
had a pronounced impact on the survival and depth distribution of Zostera marina in 
San Francisco Bay. They estimated that an H of between 3 and 5 h was required to meet 
sat 
the respiratory demands of the plants. H requirements at the more turbid sites were 
sat 
longer, limiting plants to shallower depths than those predicted from estimates of carbon 
budgets and mean H availability. Zimmerman et al (1991) suggested that the mean 
sat 
diffuseattenuationofthewatercolumn isnotagoodpredictorofthedepthlimitofeelgrass 
be
particularly in locations where transient and/or seasonal periods of high turbidity may 
common. These findings may be applicable to the meteorologically driven systems ofTexas. 
Recent work in Texas (Dunton and Tomasko, submitted; Dunton submitted) have 
examined the of Halodule Madre. have
light requirements wrightii in Laguna They 
determined that H. wrightii growing near the lower limit of depth penetration had an 
of Bto9 h. These and H
requirementof3to5handanH valuesrepresentan 
comp comp 
annual quantum budget of 5100-5700 mol m 1 (Dunton, submitted). 
The relationship between mean light availability and depth distribution of eelgrass 
in San Francisco Bay appears to be more complicated than simple models based on mean 
diffuse attenuation coefficients would suggest (Zimmerman et al> 1991). Carbohydrate 
reserves may act as a buffer during periods when H is less than that required by the 
ml 
plants; however, the reserves are limited in nature. Plants at the lower edge of the beds 
may not be able to build up adequate reserves to sustain them through periods of low light. 
Thus, brief periods of extreme turbidity may be more critical than the mean turbidity in 
controlling the depth distribution of existing populations and the establishment ofseedlings 
or as well as the
propagules. The average length of the daily H period is important,
sat 
number of "critical days" per month or season when H requirements are not met. The 
sat 
number of extreme attenuation days (BAD) when diffuse attenuation values prevent net 
of habitat
carbon gain or adequate root oxygenation may provide a quantitative measure 
tobe in
suitability that may prove a sensitive predictor of eelgrass growth and survival 
different habitats (Zimmerman et aL, 1991). 
Laboratoiy studies of reduced light on seagrasses 
In addition to in situ studies of 
reduced irradiance and photoperiod there have been 
two in vivo studies the to reduced
examining response of submerged aquatics light. 
and examined the
Goldsborough Kemp (1988) response of Potamogeton perfoliatus to 
changes in total irradiance. Plant populations were grown in aquaria and the light 
environment was manipulated using neutral density screens which alter light intensity but 
The 100% SI.
not spectral quality. treatment levels were 11, 32 and After three days, 
shaded plants showed an increase in photosynthetic efficiency and chlorophyll a 
concentrations. Following ten days of treatment there were significant changes in the 
morphological characteristics of the plants including elongation of stems, thinning of lower 
leaves, and canopy formation at the water surface. Golsborough and Kemp (1988) 
concluded and conferred
that the physiological morphological responses of the plants 
improvements in plant fitness under the treatment conditions. Tomasko (1992) examined 
changesinthemorphologyofHalodulewrightii duetochangesinthespectralcomposition 
of light. Halodule was grown under a canopy of Thalassia testudinum that changed the ratio 
of red;far red light. Other plants were grown under neutral density screens of equivalent 
light reduction that did not alter the redifar red ratio. Plants under Thalassia
grown 
testudinum had longer internode lengths, while plants under neutral density screens showed 
reduced growth rates compared to controls. Tomasko (1992) suggested that Halodule 
minimizes competitive interaction with Thalassia by varying its morphology. 
Studiesofreduced irradiance and dailylight periodsuggest thatwhen 
seagrasses are 
light stressed they change their morphology (within limits) to optimize photon capture. 
Thus, it appears that morphological plasticity may confer some advantage to submerged 
reduced irradiance.
macrophytes, allowing them to survive periods of The work examining 
of eelgrass and the laboratory studies of Potamogeton also suggest these plants may 
physiologically adapt to reduced light. 
curves
Photosynthesis irradiance 
Photosynthesis irradiance (P vs. I) curves are a means of quantifying the 
Blade tissue is
photosynthetic rate on an area, weight or chlorophyll basis (Kirk 1983). 
placed in a chamber with an oxygen electrode and a light sensor. Oxygen evolution or 
a In
consumption is measured as function of light intensity striking the plant (Figure 16). 
as a result of
the dark there is no photosynthesis and plants exhibit net consumption of0 
2 
respiration. As light intensity is increased, some O production occurs; however, this shows 
z 
up as a diminution of the 0 2 consumed by the blade tissue (respiration exceeds 
irradiance 0 the 0
photosynthesis). The at which photosynthetic 2 production equals 2 
consumed in respiration is the compensation irradiance (/). At light intensities greater than 
c 
I 
the plants exhibit net photosynthetic production. Maximum photosynthetic production
c 
(P) is achieved when increases in PAR no longer result in an increase in oxygen
max
be estimated from the intersection of the initial
evolution;thelightsaturationpoint(Ik) can 
slope with Pmax or is more accurately calculated as P /0.. Alpha (a) is represented by the 
max
initial slope of the P vs. I curve and is a measure of the efficiency with which the plant 
biomass utilized light (Kirk 1983). It represents the efficiency with which light quanta are 
11. reflects the
absorbed and used to transfer electrons through photosystems I and 
dark reactions of the various metabolic
of photosynthesis and is regulated by recycling 
intermediates (ATP and NADPH). 
Although there have been numerous studies of the P vs. I parameters of seagrasses, 
few have been carried out in situ. Wetzel and Penhale, (1983) examined in situ the P vs. I 
characters that allowpopulations ofSAY inChesapeake Bay tosurvive inavery stochastic 
Figure 16. light saturation curve for photosynthesis. P, photosynthesis; P maximum 
max, 
incident
photosynthesis; gross photosynthesis; P net photosynthesis; R, respiration;
n, 
I PAR. From South and Whittick 1987.
PAR; saturating PAR; compensation
k, 
environment. 
They found that submarine PAR and temperature act both singularly and 
control the biomass and distribution of For
interactively to seagrasses in Chesapeake Bay. 
maritima P correlated with and of
Ruppia was temperature there was no sign
, max 
photoinhibition. P for Zostera marina was not correlated with temperature, although
max 
Zostera appears to have a temperature optima below 28 °C. Wetzel and Penhale (1983) 
found that the two species were therefore physiologically distinct, allowing them to coexist 
in the same niche. The authors suggested that changes in the distribution of in
seagrasses 
Chesapeake Bay may be related to changes in light availability. Libes (1986) examined the 
P of Posidonia oceanica in situ
vs. I relationship using Cl4 techniques. Photosynthetic 
summer.
efficiencywasgreatestinwinterandleastin Additionally,productivitywasgreatest 
in the morning and least in the evening. Libes also found evidence of seasonal 
photoinhibition and suggestedthatPosidonia mayhaveaseasonalendogenousrhythmwith 
regard to photosynthetic capacity. Ic andIk were similar for both the seagrass and its 
epiphytes. Dunton and Tomasko (submitted) made in situ measurements of the 
photosynthetic performance of Halodule wrightii in Texas. They documented seasonal 
variation in all of the parameters that describe the P vs. I curve. 
Laboratory studies of P vs. I characters are easier to perform than field studies, and 
as a result, numerous laboratory studies of seagrass P vs. I characteristics have been 
completed. Drew (1978) investigated the factors affecting photosynthesis and its seasonal 
variation in Cymodocea nodosa and Posidonia oceanica. He measured photosynthesis, dark 
respiration and leaf chlorophyll content on plants from shallow (1-5 m) and deep (25-33 m) 
sites. 
Cymodocea had similar spring and summer light saturated photosynthetic rates, while 
Posidonia had higher photosynthetic rates in spring than in summer (Figure 17). The 
for photosynthesis was about 30 °C for both species and dark
temperature optima respirationwas similar during both the spring and summer. Drew (1979) examined the 
physiological aspects of primary production in Cymodocea nodosa, Posidonia oceanica, 
PI
Halophila stipulaceaPhyllospadix torreyi, Zostera angustifolia and Zostera marina. vs. 
, 
curves were developed for each species (Figure 18) as were plots of photosynthesis versus 
temperature. All species showed similar P vs. I curves that usually shows light saturation 
around 2-3 mW/cm2 (~138 pmol m'2 s'1). Halophila was the only species that showed 
evidenceofphotoinhibition. DespiteawiderangeofP (8.1to26.0/igCcm'2h'1)allof 
max 
the Ik values were about 10% of full sunlight and all of the Ic values were about 1% SI 
(Table 11). Williams and Mcßoy (1982) examined the effects of light on carbon uptake in six 
seagrass species: Thalassia testudinum , Syringodium filiforme, Halodule wrightii, Halophila 
engelmanniPhyllospadix scouleri and Ruppia maritima. Seagrasses from Texas became light
, 
saturated between The five had similar half-
at high irradiances, 64-85% SI. species 
saturation constants ranging from 49-56% SI. 
Williams and Mcßoy (1982) suggested that 
Thalassia and Syringodium are "climax species" and that the other species are "colonizers". 
to
Kerr and Strother (1985) examined how photosynthesis in Zostera muelleri responded 
irradiance, temperature and salinity (Figure 19). Photosynthesis increased with increasing 
light (from 17 to 185 fimol m'2 s* 1) at 16 °C, with no indication of photoinhibition at the 
highest light levels used in the experiment. 
Because thedarkreactionsofthephotosynthetic process areenzymaticallymediated. Figure 17. Effect of temperature on net photosynthesis and dark respiration in shallow-growing leaves ofCymodocea and Posidonia in spring and summer; photosynthesis measured 
2 
at 20 mW cm' From Drew 1978. 
. 
Figure 18. P vs. I curves for six seagrass species at ambient environmental temperatures From Drew 1979. 
Table 11. 
Photosynthetic rates, respiration rates, saturation and compensation irradiance at ambient environmental temperatures in six seagrass species. From Drew 1979. 
RR
Ymax \ Ic Temp. (*C)
ic 
Phyllospadix torreyi 26.0 3.6 0.5 -3.8 -5.3 15 
21.8 3.8 0.4 -2.5 -3.5 
25 Zostera angustifolia 18.i 3.2 0.3 -1.8 -2.1 10 Zostera marina 14.2 5.0 0.6 -1.7 -1.4 15 
Cymodocea nodosa 
9.0 2.0 0.2 -1.2 -1.1 25
Halophila stipulacea 
Posidonia oceanica 8.1 2.6 0.4 -1.3 -1.5 17 
22 2 
*yMgCcm'h1;Iand7mWcm'PAR;R}-andR /xgCcm' ]h-1.
max* tt C, d, 
Figure 19a. Relationshipbetweenapparentphotosynthesisandirradiance(PAR)inZostera muelleri at 16 °C. From Kerr and Strother 1985. 
19b.
Figure Relationship between apparent photosynthesis and temperature in Zostera muelleri at an irradiance of 47 ± 2 /xmol m‘2 s'1 From Kerr and Strother 1985. 
. 
effect the Several
temperature can have a pronounced on (Bulthuis 1987). 
ofP vs.Icharacters
investigations have examined the response to changes in temperature. 
examined
As part of his study, Drew (1979) the effects of temperature on the rates of 
photosynthesisandrespiration. Measurementsofphotosynthesisandrespirationweremade 
at six temperatures between 10 and 40 °C (Figure 20). Photosynthesis increased linearly 
with increased temperature {r2= 0.99 to 1.00). For some species (Posidonia and 
Cymodocea) temperatures greater than 30 °C caused thermal damage and resulted in 
decreasedphotosynthesis. Attemperatureslowerthannormallyencounteredintheirnatural 
a
habitat, all species maintained moderately low rate of respiration. Bulthuis (1983b) 
on the P vs. I curve of Heterozostera tasmamca. He
examined the effects of temperature 
At
measured the P vs. I parameters at 8 temperatures ranging from 5 to 40° C (Figure 21). 
°
25 and 30 
C photosynthesis was not light saturated even at the highest intensities (955 /xmol 
m*2 s'1). P increased by a factor of 2.5 between 5 and 30 °C and decreased sharply
nmx 
between 30 and 35 °C. factor of 2 between 5
The rate of dark respiration increased by a 
and 20 °C with small increases above 20 °C. Evans examined
(1984) the physiological 
response of Chesapeake Bay populations of Zostera marina and Ruppia maritima to 
temperature. For both species the lowest P was at 8° C. The highest occurred for 
max 
°°
Zostera at 19 C and for Ruppia at 26 and 30 °C. At temperatures between 8 and 19 C 
2°
Zostera had a higher P than Ruppia. I for Zostera ranged from 46 pmol m' s'1 at 26 C 
max k 
21 to s’
28pmolm’at30°C,whileforRuppiatherangeatthesametemperatureswasgreater 
(39-72pmol m2s'1). Ingeneral,RuppiahadahigherI thanZosteraatalltemperatures.
k 
Evans (1984) suggested that at lower temperatures Zostera has the competitive edge over 
Figure 20. Effect of temperatures both above and below ambient on the rates of gross in four Horizontal bar is the of environmental
photosynthesis seagrasses. range temperatures normally encountered by the plant; open 
circles reflect the incubation. From 
Drew 1979. 
Figure 21. Temperature vs. apparent photosynthesis in Heterozostera tasmanica at three irradiance levels. Mean ± 1 SE (n = 3-9) for plants collected from Crib Point, Australia. From Bulthuis 1987. 
Ruppiawhile at higher temperatures Ruppia has the advantage. Thus temporal resource 
, 
partitioning may allow the coexistence of these species. Kerr and Strother (1985) examined 
the photosynthetic rate of Z. muelleri at light levels of 47 fimol m 2 s'1 and a range 
to 30 °C and showed a
oftemperatures. Photosynthesis increased with temperature from 5 
marked decrease at temperatures greater than 30 °C (Figure 19). 
Marsh et aL (1986) examined the effects of temperature on photosynthesis and 
at 20-22 °C
respiration in Zostera marina. They developed P vs. I curves for plants grown 
and subjected to various temperatures between 0° C and 35 ° C (Figure 22). Light saturated 
net photosynthesis increased with temperature to an optimum around 25-30 °C and 
decreased at 35 °C. The initial slopes of the P vs. I curve (a) were greatest at 0° C and least 
at 35 °C. Kerr and Strother (1985) suggest that in winter, low temperatures and low light 
conditions enable the plants to maintain a positive carbon balance; however, high 
Bulthuis
temperatures (>3O°C) and low light would result in a negative carbon balance. 
(1987) reviewed the effects of temperature on seagrasses. He found compensation 
irradiance increases with temperature; therefore, to maintain a positive carbon balance, 
seagrass plants require a greater irradiance during summer than during winter. The 
literature indicates temperature optima for light saturation of photosynthesis is generally 
between 25 and 35 C; however, there tends to 
° be a rapid loss of photosynthetic capacity at 
I
temperatures above the optimum. Bulthuis (1987) suggests that the seasonality of 
may
c 
at near
have important implications for seagrasses living or their minimum light 
requirements. Madsen and Adams (1989) found that Potamogeton pectinatus exhibited 
°
optimalphotosyntheticproductionat30 °Candthatphotosynthesisat10 Cwas63%lower 
Figure 22. P vs. I relationships of Zostera marina leaf segments measured at different 
temperatures. Leaf tissue was grown at 20-22 °C in the field was incubated at the given temperatures for 15 minutes prior to measurement; a, photosynthesis expressed per mg Chlor.'1min'1;b,photosynthesisexpressedperdm*2min'1 FromMarshetal.,1986. 
. 
of
than that at 30 °C. Perez and Romero (1992) examined the photosynthetic response 
Cymodoceanodosatolightandtemperature. ThePvs.Icurvesshowedseasonalvariation, 
with and Ik higher during summer than during winter(Figure 23). I did not exhibit any
c 
seasonality. Cymodocea showed no indication of photoinhibition at light levels up to 2500 
nmo\ m'2 s'1 Rates of light saturated photosynthesis and dark respiration increased 
. 
significantly with increasing temperature. Perez and Romero (1992) also suggested the 
control of be
seagrass seasonality may more complex than previously thought. 
As with most other fields of science there are some controversies with regard to 
seagrassesandphotosynthesis. Oneofthebiggestdifficultiesisstandardizingunitsoflight 
energy. Through the years a bewildering variety of units have been used to measure light 
energy, everything from Langleys (Williams and Mcßoy 1982) to milliWatts cm'2 (Drew 
1979) to /xEinsteins (Kerr and Strother 1985, Perez and Romero 1992). Although there are 
numerous conversion factors in the literature, most do not take into account the selective 
absorption of specific wavelengths of light by water (Liming 1990; Megard and Berman, 
1989). Thus, most calculations that employ conversion factors between units are relatively 
poor estimates of light energy at depth. The best method for estimating underwater light 
availability is to measure itin situ (Dunton, submitted). 
One of the other controversies in the literature is the validity and/or usefulness of 
carbon budgets based on P vs. I curves developed from leaf segments (Fourqurean and 
Zieman 1991a). TheI valuedevelopedforleaftissuedoesnottakeintoaccountthelarge
c 
respiratory demands of the below-ground tissue. Thus, whole plant carbon budgets based 
on leaf tissue probably underestimate the true carbon budget of the whole plant 
Figure 23a. Seasonal trends in P vs. I curves in Cymodocea nodosa for November (boxes), From Perez and Romero 1992.
June (triangles), and August (circles). 
Figure 23b. Response of both net photosynthesis (open symbols) and dark respiration (black symbols) to temperature in August (triangles) and February (circles). From Perez and Romero 1992. 
84 
(Fourqurean and Zieman 1991 b). Fourqurean and Zieman (1991a) used a P vs. I chamber 
that allowed them to develop P vs. I curves for whole plants in a more natural orientation 
to the light field. They found that leaves account for less than 50% of the total plant 
respiration. Knowledge of the above-to below-ground apportionment and the factors that 
control this ratio are critical to modelling carbon budgets of seagrasses. In addition, the 
leaf tissue P I data in models of plant carbon
vs versus
question of using whole plant 
budgets must still be addressed. 
Measurementsofphotosynthesisinseagrasses suggestthatlightandtemperatureare 
thefactorsthatcontroltherateofphotosynthesisandrespiration. Ingeneral,photosynthesis 
at a given light level increases until the optimum temperature is reached. At temperatures 
above the optimum, photosynthesis decreases. Compensation irradiance also varies with 
as a
increased temperature result of increased respiration. Thus, at elevated temperatures 
plants require greater irradiance to maintain a net carbon balance. The observed seasonal 
result of seasonal
pattern in P vs. I parameters is probably a temperature changes. 
Seasonal changes in seagrasses 
Seagrasses, like most plants, exhibit seasonal changes in production, P vs. I 
organic constituents. Odum (1963) examined
parameters and the productivity of Texas 
Thalassiatestudinum andreportedamarkedseasonalcycleinproductivitywithconsiderable 
interannual variation (Figure 24). Zieman (1975b) examined the seasonal variation of 
Florida Thalassia with reference and Maximum
to temperature salinity. productivity, 
standing crop, leaf length and blade density occurred by early summer. He found that 
1957-1961 for
Figure 24. Record of salinity, gross photosynthesis and total respiration Thalassia testidinum in Texas. From Odum 1963. 
although Thalassia undergoes marked seasonal variation, blade density during winter was 
about 50% of maximum blade density during summer. Maximum blade density occurred 
during late April and May and leaves were carried until October. Minimum blade densities 
occurred from December through March. Zieman (1975b) concluded that temperature and 
in Florida.
salinity were the major factors controlling the seasonal variation of Thalassia 
Walker and McComb (1988) examined the seasonal variation oiAmphibolis antarctica and 
Posidonia australis in Shark Bay, Western Australia. Posidonia showed no clear seasonal 
or stock for the duration of the
pattern of production standing study (1982-1983). coincident with highest light and
Amphibolis had maximum production during summer, 
temperatures. Although seasonal variation was evident for Amphibolis, it was not very 
pronounced,whilePosidonia didnotexhibitaseasonalpattern. Ingeneral,itappearsthat 
seagrass biomass and density increase with increasing light and temperature during late 
spring and early summer (Figure 25). Dunton (1990) examined the production ecology of 
Ruppia maritima and Halodule wrightii in two estuaries along the Texas coast. The species 
time and of
differed with respect to seasonality of growth, of flowering persistence 
was a
overwintering populations. In both estuaries Ruppia strict opportunist colonizing bare 
no
areas yearly (i.e. overwintering populations) and completing its growth cycle in four 
months. Halodule was 
absent from the Guadalupe Estuary (San Antonio Bay), but in the 
Nueces Estuary (Corpus Christi Bay) itproduced overwintering populations with year-round growth. Dunton (1990) concluded that in some Texas estuaries Ruppia grows as an annual 
weed, whereas Halodule grows as a perennial (Dunton 1990). However, other investigators 
have documented perennial populations of Ruppia maritima in Upper Laguna Madre and 
Figure 25a. Some seasonal environmental features of an Australian study site: total irradiance (squares, Wm’2), PAR photon flux density (triangles, fimol m'2 s’1) and 
° 
temperature (C, circles). Values are monthly averages. From Perez and Romero 1992. 
Figure 25b. Seasonal pattern of above-ground biomass. Standard errors are indicated by vertical bars. From Perez and Romero 1992. 
88 
Redfish Bay (Pulich 1985). Thus, both annual and perennial stocks ofRuppia occur along 
the Texas coast. 
have noted that the P vs. exhibit a
Many investigators I parameters of seagrasses 
seasonal examined factors affecting the seasonal variation in
pattern. Drew (1978) 
photosynthesis of Posidonia and Cymodocea in the Mediterranean. Cymodocea had 
while Posidonia had higher
comparable photosynthetic rates during both spring and summer, 
summer(Figure17). Calculationofacarbon
photosynthetic rates during spring than during 
balance based on the experimental P vs. I numbers shows that shallow Posidonia
a 
community may have a positive carbon balance during spring and a negative carbon balance 
(due to higher respiratory demands) during summer (Drew 1978). However, it should be 
noted that the carbon budget calculations based on leaf P vs. I curves do not take into 
accounttherespiratorydemandsofthebelow-groundtissues. CongdonandMcComb(1979) 
found thatRuppia exhibits seasonal changes in above and below-ground biomass, with low 
crops occurring during late autumn and winter. Although they did not investigate seasonal 
changes in the P vs. I curves per se they inferred changes in the light requirements based 
, 
on changes in standing crop (Congdon and McComb 1979). Ott (1979) presented evidence 
foranannualrhythminthegrowthcycleofPosidoniaoceanica. Aftertwoyearsofgrowth 
in vivo under conditions of constant light and P. oceanica still
temperature, the seagrass 
exhibited a seasonal
pronounced growth rhythm. However, changes in photosynthetic 
capabilities were not examined in this study. Bulthuis (1983b) reported that temperature 
dramaticallyinfluencedthePvs.IcurveofHeterozosteratasmanica, Bulthuissuggestedthat 
Heterozostera has higher light requirements during summer than during winter due to 
increased respiration associated with high summer temperatures. Thus, this species exhibits 
It was
seasonal changes in its light requirements due to changes in the rate of respiration. 
also suggested that these plants are more susceptible to decreased light availability during 
summer seasonal
than during winter (Bulthuis 1983b). Macauley et al (1988) examined 
changes in standing crop and chlorophyll content of Thalassia testudinum in the Northern 
Gulf of Mexico. Thalassia exhibited seasonal 
variation in standing crop and chlorophyll 
content; standing crop was strongly correlated with temperature and moderately correlated 
with incident irradiance. 
They suggested that temperature was the controlling factor for 
ThalassiaproductivityalongtheNorthernGulfofMexico(Macauleyetal, 1988).Perezand 
Romero (1992) found that Cymodocea nodosa exhibits some photoadaptation to the yearly 
and P
lightcycle. Duringsummer,whenthereishighirradiance,theplantshavehighI 
k max 
values and do 
not exhibit photoinhibition; however, during the winter these plants shift 
towardshade-adaptedcharacters(Table 12).Itissuggestedthatwinterphotosyntheticrates 
may be limited by low temperatures rather than low light. The mechanisms causing these 
adaptations are not well understood and may involve short-term light adaptation and 
internal rhythms keyed to changes of the seasons. 
Researchers have also investigated seasonal changes in the organic constituents of 
seagrasses. Dawes and Lawrence (1979) investigated how blade removal influenced the 
proximate composition of the rhizome of Thalassia testudinum. They concluded that the 
rhizome acts as a storage organ which supports blade regeneration as well as seasonal 
growth. Soluble carbohydrate was the primary reserve and was actively mobilized through 
the rhizome. Rhizomes seasonal rise
had the largest carbohydrate reserves exhibiting a 
Table 12. Seasonal variation in the P-I parameters of Cymodocea nodosa. P and R 
max 
expressed in mg 02 per d dry h'1; I and expressed as fimo\ m'2 s'1 From Perez and 
Mt 
. 
Romero 1992. 
Month P R 
max 
February 2.31 0.41 69 10 April 2.64 0.14 238 12 June 5.48 0.47 398 31 August 4.37 0.56 88 10 November 2.51 0.73 160 36 December 3.22 1.00 127 30 
during late spring and summer and a decrease during winter. Carbohydrate levels in the 
Dawes and Lawrence made seasonal
rhizome were lowest in early spring. (1980) a 
examination of the proximate constituents of Thalassia testudinum Halodule wrightii and 
, 
Syringodium filiforme (Table 13). They found that in the rhizomes soluble carbohydrate 
levels were highest during the fall and lowest during spring. They suggest that soluble 
low. Calorific
carbohydrates sustained the plants through the winter when productivity was 
levels were similar between species with the highest levels occurring in the rhizomes. Of 
the three species, the rhizomes of Thalassia had the highest level of organic material, 
be more tolerant of adverse conditions. Dawes and
suggesting that this species may 
Lawrence (1983) suggest that Thalassia is better adapted to year round growth, while the 
other species (Halodule and Syringodium) are more opportunistic. 
Pirc(1985)examinedthegrowthdynamicsofPosidoniaoceanicawithrespect tothe 
seasonal changes of soluble carbohydrates, starch and other organic compounds in different 
parts of the plant. He concluded that winter leaf growth was supported by the mobilization 
of starch from the rhizomes. 
Pirc (1985) hypothesized that subsidized winter leaf growth 
enabled the plant to take maximal advantage of increased light during the spring. During 
summer and autumn large quantities of carbohydrates found in both the leaves and
were 
the rhizomes. Plants utilize summer and fall influx to
energy compensate for lowproduction 
during winter and spring. Ralph et al. (1992) examined the distribution of extractable 
carbohydrate reserves in the rhizome of Posidonia australis. Carbohydrate levels were 
reserves were
significantly higher in the stele tissue than in the cortex. Lower carbohydrate 
near
found in juvenile tissue and the apical meristem, while unexpanded internodes had Table 13. Seasonal variation of protein, soluble carbohydrate and kilocalories for blades 
From Dawes 1987.
and rhizomes of three seagrass species. 
January April July October 
Thalassia testudinum Blades Protein 8 9 22 13 Carbohydrate 6 9 9 7 Kilocalories 2.4 3.0 3.1 2.6 Rhizome Protein 9 8 16 7 Carbohydrate 12 21 24 36 Kilocalories 3.2 3.4 3.0 2.8 
v
Syringodium filiforme 
Blades Protein 9 8 13 13 Carbohydrate 22 16 18 20 Kilocalories 3.1 2.4 3.2 3.1 Rhozome Protein 9 5 12 16 Carbohydrate 36 38 50 46 Kilocalories 3.6 3.7 3.6 3.5 
Halodule wrightii Blades Protein 19 18 19 14 1419 15 13
Carbohydrate Kilocalories 3.1 3.5 3.3 3.3 Rhizome Protein 9 7 8 8 Carbohydrate 43 40 43 54 Kilocalories 3.7 3.7 3.4 3.6 
Percent dry weight 2Per gram dry weight 
It was are used
relatively large carbohydrate reserves. suggested that stored carbohydrates 
for winter maintenance and support early spring blade growth. 
Seagrasses can also adjust the concentration and distribution of chlorophyll within 
their tissues. Mazzella et aL (1979) concluded that the photosynthetic activity of Zostera 
marina leaves was and
regulated by four factors: age of tissue, light intensity, exposure 
For
of epiphytes. young parts of leaves, light intensity and tissue maturity were
presence 
most important. For older leaves, the presence of epiphytes was more important than the 
other factors. Wiginton and McMillan (1979) examined the chlorophyll composition of 
seagrasses under controlled light conditions. Chlorophyll concentrations in both Thalassia 
and Halodule were 
correlated with chlorophyll content increased with decreasing light in 
both the field and lab. 
Chlorophyll a:b ratios were correlated with different depth ranges 
and affect depth distribution of seagrasses. The similarities in light levels at the
may 
in both Croix and
maximum depth of seagrasses St. Texas suggests that the seagrass 
were restricted at each locale. In
populations by similar light relationships general, 
are
seagrasses respondingmainlytochangesinlightquantity,notlightquality(Wigintonand 
McMillan 1979). Mazzella et al (1980) found that Zostera had a gradient of leaf 
pigmentation and photosynthetic activity. The initial slope of the P vs. I curve increased 
fromleafbasetoleaftip. Lightsaturationoccurredat100-150/nmolm2s'1forallleaf 
types. Czeczuga (1986) examined the effect of light quality on the photosynthetic pigments 
of the green alga Chora. In freshwater Chora forms meadows that have the same
may 
functions as seagrasses in estuarine waters. Highest chlorophyll a,b and carotenoids were 
found when green and yellow filters were placed over the light source, while the lowest 
pigmentconcentrations occurredwhenblueandredfilterswereused. Enriquezetal(1992) 
examined how light was absorbed byPosidonia oceanica. Their results indicate the amount 
of light absorbed increases linearly with increased pigment packaging in the leaves. Thus, 
in increased The
increasing chlorophyll results light absorption per unit leaf weight. 
increasing absorption per unit weight should increase the photosynthetic and growth rates 
of a light-limited plant. 
in the concentration allow a to survive in a
Changes chlorophyll may species 
particularhabitat. Severalstudieshaveinvestigatedthephysiologicalecologyofseagrasses 
in relation to Dennison et
light and photosynthesis. aL (1981) concluded, based on shading 
and reflecting studies, that Zostera marina adjusts to light conditions by changes in leaf area 
production. Under low light the plants increase their leaf area to intercept more photons. 
Jimenez et aL (1987) examined the of Zostera noltii and Z. marina to high light
response 
under stressed conditions. 
Zostera marina had higher chlorophyll levels at low light, with 
maximumchlorophyllconcentrationsoccurringat150pmolm'2 1 Zosteranoltiichlorophyll
s’
. 
increased with light intensity and were constant at light levels above 150 pmol m’2 s’1 
. 
21
Zostera noltii was light saturated at 3600 pmol m’ s’ showing no evidence of 
photoinhibition,while Z.marinawaslightsaturatedat1100nmolm’2s’1withsignificant 
was
photoinhibition above this light intensity. For both speciesI 30-35 /imol m’2 s’1 They
c. 
concluded that Z. noltii 
was more photosynthetically efficient, especially at high irradiances. 
Dawes et al (1989) compared the physiological ecology of Halophila dedpiens and H. 
from Florida.
johnsonii With respect to P vs. I characters, H. dedpiens was strongly 
photoinhibitedatirradiances above 300 /xmolm’2s’1while H.johnsonii wasphotoinhibited 
21 2 
atirradiancesabove 1000pmolm' s'HalophiladecipienshadanI valueof29pmolm'
.c 
1 21 
s'compared to about 50 /imol m' s' for H. johnsonii. 
Although interannual variation may be large, most seagrass species exhibit very 
pronounced seasonal trends. In general, plant biomass increases rapidly during spring and 
earlysummerandismaintainedthroughthesummer. Duringfall,biomass decreasesasthe 
blades senesce and slough off of the plant. The P vs. I characters of most seagrass species 
followsimilartrend. and "shade
a The plants appear to be "sun adapted" during the summer 
adapted" during the winter. Generally, P is lower during the winter than during the 
max 
The
summer; presumably, this is due to reduced enzymatic activity at lower temperatures. 
concentration of organic constituents also varies seasonally. During summer, when 
production is greatest, the plants store large amounts of carbohydrate in the rhizomes. 
During the fall and winter, when light levels are low, the plants draw upon these reserves. 
During late winter (very early spring) the plants put forth a new crop of leaves, supported 
at least in part by stored carbohydrate. During this time the plants are probably most 
susceptible to prolonged periods of reduced irradiance. 
CHAPTER 5: RECOMMENDATIONS FOR FURTHER RESEARCH 
Texas estuarine systems are remarkably diverse, as examplified from the hypersaline 
Laguna Madre to the freshwater dominated estuaries of the northern Texas coast. As a 
to water
consequence, the potential problems faced by resource managers with respect 
quality and water clarity (transparency) is unique to each estuary. Estuarine systems 
dominated by freshwater inflow are more likely to experience the effects of eutrophication 
than estuaries where evaporation exceeds inflow (e.g., Laguna Madre). Conversely, systems 
that are are
flushed frequently less likely to experience the long-term effects of chronic 
phytoplankton blooms, as is now occurring in Laguna Madre (the brown tide). Setting water 
quality criteria to maintain the current level of productivity in these systems will therefore 
be estuary specific. The immediate challenge is to thoroughly define hydrographic 
characteristics common to each before to to
system they begin respond negatively 
anthropogenic inputs. The goal will then be to use this database to establish water quality 
standards to maintain the health of each system. With very few exceptions, Texas is 
fundamentally different than most other East Coast and Gulf Coast states which have lost 
much of their valuable coastal habitat; in Texas 
the goal is still to preserve the natural 
habitat, not to restore or create what has been lost. 
The extensive development ofsubmerged aquatic vegetation in south Texas bays and 
estuaries is primarily related to reduced freshwater inflows and lower levels of inorganic 
nutrients. light attenuation is the major problem that must be addressed in these systems. 
Although increasing urban and agricultural development do present an alarming potential for eutrophication, dredging and other construction activities clearly present the greatest 
immediate danger to seagrass beds. These activities significantly increase turbidity which 
reduces the amountof light reaching the bottom. 
of beds now has
Our failure to largely prevent the loss seagrass and in the past 
largely resulted from an inadequate knowledge of: 
(1) the minimum light requirements needed to maintain a positive carbon balance 
and net growth for the various species, 
(2) the amount of light actually reaching the plants on the seabed. 
Although some effort has been made to address the photosynthetic light 
these
requirements of seagrasses through laboratory experiments, measurements often 
cannot be extrapolated into the field. In situ measurements using entire plants thus
are 
highly recommended and potentially the most useful for management purposes. 
Currently, we have information on the light requirements and underwater light fields 
foronlyoneoffiveTexasseagrasses (Halodulewrightii). Insituphotosyntheticparameters 
need to be measured for both Syringodium filiforme and Thalassia testudinum which along
, 
with Halodule of meadows in
constitute the majority of the approximate 850 km2 
, seagrass 
Texas. Without the knowledge of the minimum light requirements for growth and 
photosynthesis of these plants, the development of water quality and water transparency 
standards will be extremely difficult. 
Compilation of published nutrient and water chemistry data for each estuary with 
information on the in situ light requirements and underwater light fields ofTexas seagrasses 
can be used to develop water quality criteria to in Texas estuaries. The
preserve seagrasses 
contributions of suspended solids, chlorophyll a and dissolved inorganic nitrogen to the 
diffuse light attenuation coefficient (k) for each estuary can then be used in carbon budget 
models for seagrasses within each estuary to maintain the current productivity and 
distribution ofseagrasses. This approach is similar to that used in Chesapeake Bay (Batuik 
et al 1992; Dennison et al 1993) but incorporates quantitative data on underwater light
., ., 
fields and the light requirements of the plants. Recent studies suggest that the physiological 
Halodule is similar estuaries
light requirements of the seagrass wrightii in Texas among 
(Dunton, submitted and unpub. data); if this is true for other species then the strategy 
described above is not unnecessarily complex, and our goal of establishing water quality 
standards in Texas coastal environments is attainable.
to protect seagrasses 
Based on the arguments presented above, our recommendations for further work to 
achieve this goal are: 
1. 
Determine the in situ photosynthetic requirements ofTexas seagrasses, including 
(by priority): Thalassia testudinum, Syringodium filiforme, Halophila engelmanni 
and Ruppia maritima (this work on Halodule wrightii is complete). 
2. CollectinsitucontinuousmeasurementsofunderwaterPARinconjunctionwith 
nutrient and chlorophyll measurements in Texas estuaries containing seagrasses 
for calculation of diffuse light attenuation coefficients (k). (This work has been 
in progress in both Upper Laguna Madre and Corpus Christi Bay since 
1990/1991; see Dunton, submitted). 
3. Evaluate the contributions of suspended solids, water column chlorophyll a, 
inorganic nitrogen and dissolved matter to the diffuse light attenuation 
coefficient (k) for specific Texas estuaries based on available data and field 
sampling. 
4, Develop seasonal and annual carbon budgets for predominant Texas seagrasses. 
This information can be used to: (a) assess the impacts of chronic light 
reduction, (b) minimize loss of habitat by temporal or seasonal restriction of 
construction activities and, (c) determine the feasibility of mitigation (creation 
or 
restoration) projects. 
Literature Cited 
Ascherson, P. and P. Graebner. 1907. Potamogetonaceae. Pflanzenreich Heft 31:1-184, f. 
1-36. (as in den Hartog 1970). 
Backman, T.W. and D.C. Barlotii. 1976. Irradiance reduction: effects on standing crop of 
Mar. Biol. 34: 33-40.
the eelgrass Zostera marina in a coastal lagoon. 
Barnabas, A.D. 1989. Apoplastic tracer studies in the leaves of a seagrass. 11. Pathway into 
leaf veins. Aquat. Bot. 35: 375-386. 
Barnabas, A.D. 1991. Thalassodendron ciliatum (Forssk.) den Hartog: root structure and 
129-143.
histochemistry in relation to apoplastic transport. Aquat. Bot. 40: 
Barnabas, A.D. and H.J. Arnott. 1987. Zostera capensis Setchell: root structure in relation 
to function. Aquat. Bot. 27: 309-322. 
Batuik, R.A., R.J. Orth, K.A. Moore, W.C. Dennison, J.C. Stevenson, L.W. Staver, V. 
Carter, N.B. Rybicki, R.E. Hickman, S. Kollar, S. Bieber, P. Heasley. 1992. 
Chesapeake Bay submerged aquatic vegetation habitat requirements and restoration 
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