Productivity and biomass of the seagrass Thalassia testudinum along a gradient of freshwater influence in Charlotte Harbor, Florida

Eight meadows of the seagrass Thalassia testudinum Banks err, Konig represe nting a gradient of freshwater influence in Charlotte Harbor, Florida (Unit ed States), were sampled on it bimonthly basis from April 1995 to August 19 96. Spatial and temporal variation in the density, biomass, productivity, a nd epiphyte loads of short shoots were determined. Physical factors such as water temperature, salinity, and light extinction coefficients were also m easured. Areal blade production (g dw m(-2) d(-1)) of T, testudinum was not strongly associated with water temperature, salinity, or the amount of sub surface irradiance reaching the bottom at each station. Variation in produc tion could be described by a linear combination of the independent variable s water temperature and salinity. Water clarity (expressed as the percent o f subsurface irradiance reaching the bottom) was positively related to sali nity The lack of a clear relationship between water clarity and areal produ ction was probably due to water clarity being highest during times of the y ear when water temperatures were too cold to support growth of T. testudinu m. Our results suggest that seagrass light requirements determined by avera ging irradiance levels measured during the growing season might be more rel evant than those established by averaging light measurements collected thro ughout the year. The use of field studies for estimating lower salinity tol erances of seagrasses might be inappropriate for those systems where water clarity is positively associated with salinity.