Production and carbon reserve dynamics of the seagrass Thalassia testudinum in Corpus Christi Bay, Texas, USA

Abstract

The seasonal production dynamics of Thalassia testudinum in Corpus Christi Bay, Texas (USA) were evaluated through measurements of biomass, leaf growth and carbohydrate carbon content from December 1993 to March 1995. Shoot density and blomass changed significantly with season; values ranged from 321 shoots m-' (454 g dry wt m-2) in March to 531 shoots m-' (885 g dry wt m-?) in September Biomass of individual plant parts was significantly different over the sampling period; rhizome tissues tended to have the highest biornass while root tissues had the lowest. However, leaf biomass was higher than that of rhizome tissues during summer. Leaf productivities showed significant seasonal variation that was strongly correlated with temperature, ranging from 0.07 g dry wt m-2 d-' in December to 5.6 g dry wt d-' in July. Chlorophyll [chl) concentrations were significantly higher and chl a:0 ratios lowest during the spnng/summer period of maximum photosynthetic production and growth than during winter. Non-structural carbohydrate carbon content was highest in rhizome tissues (111 to 203 mg C g-' dry wt) and lowest in leaf tissues (46 to 70 mg C g-l dry wt), which is consistent with the rhizome's role as a carbon storage tissue. Rhizome carbohydrate carbon content increased rap~dly during June and July, which coincided with h ~g h water temperatures, underwater irradiance and blade chl concentrations. During winter and early spring, rhizome carbohydrate carbon content dropped nearly 50%, suggesting that these reserves were mobilized for tissue maintenance and new growth in early spring. Estimated annual blade production of T testudinum in Corpus Christi Bay over the period of this study was 792 g dry wt rn-"r-l, equivalent to 253 g C m-2 yr-' Overall, annual productivity appears to be primarily regulated by temperature and secondarily by ~rradiance, as reflected in the timing and magnitude of the strong seasonal variations in leaf product~v~ty, total chl content, chl a:b ratios and rhizome carbohydrate carbon content in T testudinum.