Seagrass die-off in Florida Bay: Long-term trends in abundance and growth of turtle grass, Thalassia testudinum

Beginning in late 1987 Florida Bay experienced a large and unprecedented di e-off of Thalassia testudinum. The die-off occurred only in stands of dense T. testudinum. We initiated an experimental monitoring effort in 1989 to a ttempt to ascertain the causes of this die-off phenomenon From 1989 to 1995 the abundance and productivity of T. testudinum was measured at five stati ons associated with the seagrass die-off and three stations where no die-of f had occurred (including one on the seaside of Key Large, outside of Flori da Bay). Early in the study the salinity was very high, exceeding 46 psu, b ut it has decreased to 29-38 psu in recent years. Seagrass standing crop an d either short-shoot density or mass per short shoot declined at nearly all stations, including the stations without die-off (unaffected stations). Ov er the course of the study, areal productivity declined at three die-off st ations; but mass-specific productivity increased at all die-off stations an d one unaffected station. Seasonality was pronounced; detrended standardize d residuals showed responses for all of the seagrass parameters to be great er than the yearly mean in spring and summer and less than the mean in fall and winter. Detrended residuals also showed decreased productivity to be c orrelated with increased salinities in the summer despite a long-term recor d of declining salinities. We propose a conceptual model of the seagrass di e-off phenomenon. We document that salinity does contribute to stress on T. testudinum in Florida Bay, but salinity is believed to be only one contrib uting factor to the loss of seagrasses. The documented increase in the mass -specific productivity of T. testudinum over the period 1989-1995 suggests seagrasses are growing rapidly in Florida Bay by 1995; we predict that the loss of T. testudinum may be slowing down and that recovery is possible.