Abundance, production and carbon dynamics of the seagrass, Thalassia testudinum in Corpus Christi Bay, Texas



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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