Browsing by Subject "Turtle grass"

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Abundance, production and carbon dynamics of the seagrass, Thalassia testudinum in Corpus Christi Bay, Texas
(1995) Lee, Kun-seop; Dunton, Kenneth H.
The seasonal production dynamics of the subtropical seagrass, Thalassia testudinum, were examined through measurements of biomass, leaf growth and carbohydrate carbon content from plants collected in Corpus Christi Bay from December 1993 to March 1995. Daily photon flux densities (PFD) showed strong seasonal variations, ranging from 9.6 mol m⁻² d⁻¹ in April to 21.7 mol m⁻² d⁻¹ in July. Shoot density and biomass changed significantly with season; values ranged from 321 shoots m⁻² (454 g dry wt m⁻²) in March to 531 shoots m⁻² (885 g dry wt m⁻²) in September. Rhizome tissues tended to have the highest biomass while root tissue had the lowest. Leaf productivities showed significant seasonal variation that were strongly correlated with temperature, ranging from 0.07 g dry wt m⁻² d⁻¹ in December to 5.6 g dry wt m⁻² d⁻¹ in July. Chlorophyll (chl) concentrations were significantly higher and chl a:b ratios lowest during the spring/summer period of maximum photosynthetic production and growth than during winter. Soluble carbohydrate carbon content was highest in rhizome tissues (111-203 mg C g⁻¹ dry wt) and lowest in leaf tissues (46-70 mg C g⁻¹ dry wt), which is consistent with the rhizome's role as a carbon storage tissue. Soluble rhizome carbohydrate carbon content increased rapidly during June and July, which coincided with high water temperatures, underwater irradiance and blade chlorophyll concentrations. During winter and early spring, rhizome carbohydrate carbon content dropped nearly 50%, suggesting that these reserves were mobilized for maintenance and growth. Estimated annual biomass production of Thalassia testudinum in Corpus Christi Bay over the period of this study was 1320 g dry wt m⁻² yr⁻¹, equivalent to 422 g C m⁻² yr⁻¹. To assess the effects of light reduction on Thalassia testudinum, shade screens were used to reduce underwater light to 1628 mol m⁻² yr⁻¹ (14% of surface irradiance, SI) and 864 mol m⁻² yr⁻¹ (5% SI) starting in April 1993. All plants subjected to 5% SI died after 200 days and over 99% of plants receiving 14% SI died by the end of the experiment (490 days). Blade widths of plants in the controls ranged from 6.4 to 7.0 mm, and decreased to 4.7 mm as a result of light reduction. Leaf production rates were significantly higher in control plants compared to plants within the 14% and 5% SI treatments, with all plants showing a seasonal trend with high productivity in July and low productivity in April. Blade chlorophyll concentrations increased, while the chl a:b ratio decreased with reduced light level. In both light treatments rhizome soluble carbohydrate carbon content was 50% lower and leaf carbohydrate carbon content was about 15% lower than controls, while the root carbohydrate content did not differ significantly between treatments and controls (no decrease in structural carbohydrate carbon content was noted between treatments). Pore water ammonium and sulfide concentrations in the shaded cages were significantly higher than in control cages. Thalassia testudinum in Corpus Christi Bay exhibited a strong seasonal growth cycle in which changes in rhizome carbohydrate reserves and chlorophyll content may be under endogenous control as triggered by a combination of temperature and/or light period. In contrast to the seagrass Halodule, Thalassia maintained a larger carbohydrate reserve and exhibited a stronger physiological response to light reduction, which may contribute to its competitive superiority
Growth and photosynthetic responses of two subtropical seagrasses, Thalassia testudinum and Halodule wrightii to in situ manipulations of irradiance
(1994) Czerny, Andrew Barthel; Dunton, Kenneth H.
The growth and photosynthetic responses of two species of subtropical seagrasses, Thalassia testudinum and Halodule wrightii, were examined in relation to reductions in underwater light. Shade screens reduced irradiance to roughly 30% and 20% of in situ ambient (ISA). Short shoots of T. testudinum had a greater ability to tolerate a ten month period of light reduction as demonstrated by the longer survival of plants exposed to 30% ISA. Evidence for photoadaptation was observed in plants from shaded treatments compared to unshaded controls (plants receiving 100% ISA). Leaf elongation rates were lower in T. testudinum plants exposed to 20% ISA for one month (October 1992 to November 1992) compared to plants receiving 100% ISA, but there were no differences between treatments for the months of January, February, March, and May. However, by July both levels of shaded T. testudinum had significantly lower growth rates compared to plants receiving 100% ISA. Water temperatures above 25°C and the depletion of stored reserves by spring growth most likely contributed to the disappearance of all short shoots of T. testudinum shaded at 20% ISA and both shaded plots of H. wrightii by August 1993. Photosynthesis versus irradiance (P vs. I) parameters revealed significant differences in the rates of light saturated photosynthesis (P [subscript max]), respiration, saturation irradiance (I [subscript k]), and relative quantum efficiency (α) between the two species. In particular, the significantly lower respiration rates of Thalassia testudinum (89.8 ± 8.4 μmol O₂ gdw⁻¹ h⁻¹ at 30°C) compared to Halodule wrightii (186.0 ± 20.0 μmol O₂ gdw⁻¹ h⁻¹ at 30°C ) may contribute to its survival in light-limited environments. Respiration rates and P [subscript max] for both species significantly increased with seasonal increases in temperature. No significant differences were noted in shaded versus unshaded H. wrightii at 20°C or 30°C; however, in T. testudinum, after four months of shading at 20% ISA, P [subscript max] was significantly lower and α significantly higher relative to unshaded controls. The lower growth rates and increased light harvesting capability of T. testudinum in response to reduced irradiance reflects the K-selected strategy of this species compared to H. wrightii
Nitrogen budget of the seagrass Thalassia testudinum in the western Gulf of Mexico
(1998) Lee, Kun-seop, 1961-; Dunton, Kenneth H.
The nitrogen (N) budget of the seagrass Thalassia testudinum was examined with respect to inorganic-N acquisition and the effects of sediment NH₄⁺ enrichment on two distinct populations in south Texas. The two populations exhibit different biomass allocation patterns at Corpus Christi Bay (CCB) and lower Laguna Madre (LLM): plants at CCB have a higher above-ground biomass while plants at LLM have a higher below-ground biomass. Ambient sediment pore water NH₄⁺ concentrations at CCB (ca. 100 μM) were significantly higher than at LLM (ca. 30 μM). Therefore, it was hypothesized that 1) differences in biomass allocation are a result of the differential sediment N availability, 2) sediment NH₄⁺ enrichment will affect growth, leaf morphology and tissue nutritional content of T. testudinum to a greater degree at low sediment N conditions, and 3) the relative contributions by leaf and root tissues to total N acquisition will differ between the two study sites. To examine the effects of sediment NH₄⁺ enrichment, the seagrass bed sediments were fertilized with commercial N fertilizer, and changes in production, biomass, leaf morphology, tissue nutritional content and carbon (C) reserves were monitored. Additionally, N uptake by leaves and roots of T. testudinum from the two sites were measured seasonally. After fertilization, leaf production rates and shoot height at LLM increased to reach levels equivalent to CCB. However, sediment NH₄⁺ enrichment had little effect on production and leaf size of T. testudinum at CCB. These results suggest that sediment N availability at LLM limits seagrass production. Rhizome non-structural carbohydrates (NSC) decreased in response to sediment NH₄⁺ enrichment during the early periods of the experiment which suggests that C was reallocated from rhizome to leaf tissues to support the stimulated leaf growth. Thus, the NH₄⁺ enrichment affected concentration and allocation of C as well as N. Root NH₄⁺ uptake accounted for about 52 % of total N acquisition, while leaf NH₄⁺ uptake contributed about 38 % and leaf NO₃⁻ uptake accounted for the remaining 10 % at both sites. The high biomass, chlorophyll, and C content in leaf tissues at CCB and the high biomass, C and NSC content in rhizome tissues at LLM demonstrated that plants responded to high sediment N conditions by enhancing leaf function, and to low N conditions by enhancing function of below-ground tissues
Understanding factors that control seagrass reproductive success in sub-tropical ecosystems
(2014-08) Darnell, Kelly Marie; Dunton, Kenneth H.
Seagrasses are submerged marine plants that provide essential ecosystem functions, but are declining in abundance worldwide. As angiosperms, seagrasses are capable of sexual reproduction, but also propagate asexually through clonal rhizome growth. Clonal growth was traditionally considered the primary means for seagrass propagation. Recent developments in genetic techniques and an increasing number of studies examining seagrass population genetics, however, indicate that sexual reproduction is important for bed establishment and maintenance. Few studies have investigated the reproductive biology and ecology of sub-tropical seagrass species, although this information is necessary for effective management and restoration. This work investigates the influence of pore-water nutrients on flowering, water flow on seed dispersal, consumption on seed survival, and describes the reproductive phenology in Texas for the two dominant seagrass species in the Gulf of Mexico: turtle grass (Thalassia testudinum) and shoal grass (Halodule wrightii). These species exhibit distinctive reproductive seasons that span summertime months, but reproductive output varies spatially and temporally. Results of an in situ nutrient enrichment experiment indicate that turtle grass produces fewer flowers (but more somatic tissue) when exposed to high pore-water ammonium than when exposed to low pore-water ammonium, suggesting that nutrient loading has the potential to reduce seagrass reproductive output. Seed consumption may also limit reproduction and recruitment in some areas, as laboratory feeding experiments show that several local crustaceans consume shoal grass and turtle grass seeds and seedlings, which do not survive consumption. Dispersal experiments indicate that seed movement along the substrate depends on local water flow conditions, is greater for turtle grass than shoal grass, and is related to seed morphology. Under normal water flow conditions in Texas, turtle grass secondary seedling dispersal is relatively minimal (< 2.1 m d⁻¹) compared to primary dispersal, which can be on the order of kilometers, and shoal grass secondary seed dispersal can be up to 1.1 m d⁻¹, but seeds are likely retained in the parent meadow. Results from this work can be used when developing seagrass management, conservation and restoration actions and provide necessary information concerning a life history stage whose importance was historically under-recognized.