Browsing by Subject "Seagrasses"

Now showing 1 - 3 of 3

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
Impacts of human disturbances on seagrass communities in the Padre Island National Seashore
(2004-12-18) Fellows, Kelly Ann; Dunton, Kenneth H.
Human trampling effects, such as those from wade fishermen, on seagrasses and surrounding sediments were examined using experimental trampling lanes in the Padre Island National Seashore (PAIS) in South Texas. Six sites were established throughout three regions in PAIS (Bird Island, Yarborough Pass, and Nine-Mile Hole). At each site control, low intensity, and high intensity treatments were randomly assigned to one of three 2.5-m by 5-m trampling lanes. The plots were trampled monthly from April to June 2003 and then repeated at six different sites from September to December 2003. Water column, sediment, and seagrass characteristics were measured prior to initial trampling, and two, four, and 10 months after trampling initiation. Sites trampled in spring were also sampled 16 months after initial sampling event to assess long-term seagrass recovery. After four months of spring and fall trampling, reductions in Halodule wrightii percent cover were evident in all low and high intensity lanes except for one low intensity lane during the fall trampling. By the end of the spring and fall experiments, two low intensity and six high intensity lanes remained lower than the control lanes. Resistance (condition immediately after trampling), tolerance (condition 6-12 months after trampling) and resilience (recovery over a defined period after trampling) indices were calculated to compare trampling responses in the three regions. Seagrasses in Bird Island, where higher shoot densities were consistently measured, were the most resistant to trampling in the spring (59.0%) and fall (77.5%) experiments. Yarborough Pass seagrass beds were more resilient to trampling by the end of the spring (76.3%, month 16) and fall (98.8%, month 10). The lower resilience index in the spring combined with extremely high root:shoot ratios (10.0 to 79.2) at the start of the spring experiment suggest a seasonal response to trampling. However, no long-term effects were detected as the low and high intensity lanes in this region all returned to 100% H. wrightii cover by the end of the experiment. Seagrass response in Nine-Mile Hole was extremely variable and a complete seagrass die-off in all lanes (control, low intensity, high intensity) at one site in that region occurred during the spring experiment. This die-off could possibly be a result of ammonium toxicity. Excessive porewater ammonium concentrations were measured at this site throughout the entire experiment (352 to 1000 μM) and appear to contribute to the instability of this region. Despite the general trends detected in each region, variability in the data makes it difficult to conclude with certainty whether H. wrightii was or was not affected by trampling in PAIS. Natural variability inherent to the system can be more important in determining seagrass distribution than localized disturbances
Seasonal variations and trophic relationships among concentrated populations of small fishes in seagrass meadows
(1983) Huh, Sung-Hoi; Kitting, Christopher L.; Wohlschlag, Donald E. (Donald Eugene), 1918-2007
Temporal and experimental changes in populations of potential competitors, their resources used, and their available resources can illustrate periods of competition. Dense populations and foods of small fishes were analyzed during 1982-1983 for day-night and monthly changes in an assemblage concentrated naturally in seagrass meadows of Redfish Bay, Texas. Samples totalled 10,223 fishes, in 23 families and 40 species. Fish densities averaged 15.1/m² in shoalgrass and 6.2 individuals/m² in turtlegrass meadows, which were somewhat deeper. The darter goby, pinfish, code goby, and Gulf pipefish were the most common fish species in both shoalgrass and turtlegrass meadows throughout the year. The darter goby predominated in shoalgrass, while the pinfish and code goby predominated in turtlegrass meadows. No clear day-night differences in densities of each common fish were detected during most of the year. Peak abundance of total fishes occurred during spring, with a secondary peak in fall and a minimum in winter. Each of the four common species showed its own seasonal abundance pattern, and had a different larval recruitment and peak abundance separated 1-3 months from other species, with some overlap. Seasonal feeding data for the four most abundant fish species were compared with respect to prey availability to illustrate how resource partitioning of food could mediate possible competition among these abundant consumers. When prey (mainly amphipods) were abundant, during spring, many fish species showed high overlap in food use. Regardless of food availability, the code goby and Gulf pipefish fed mainly on amphipods and copepods. The more common darter goby and pinfish were carnivorous during spring, but they showed herbivorous feeding habits during summer, when they consumed mainly epiphytic algae during periods of lower prey availability. These changes in resource use also resulted during depletion of major foods by fishes concentrated experimentally in cages. These shifts in diets reflect a temporary partitioning of available foods. Different seasonal abundance patterns with different times of peak recruitment among seagrass fish species thus seem to permit use of the seagrass meadow habitats with reduced, seasonal competition for major foods among these concentrated fishes