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We examined the response of demographic, morphological, and chemical parameters of turtle grass (Thalassia testudinum), to much-higher-than-normal rainfall associated with an El Niño event in the winter of 1997-1998. Up to 20 inches of added rain fell between December 1997 and March 1998. triggering widespread and persistent phytoplankton blooms along the west coast of Florida. Water-column chlorophyll concentrations estimated from serial Sea WiFS imagery were much higher during the El Niño event than in the previous or following years, although the timing and magnitude of phytoplankton blooms varied among sites. Seagrass samples collected in 1997, 1998, and 1999 provided an excellent opportunity to test the responsiveness of Thalassia to decline and subsequent improvement of water quality and clarity in four estuaries. Using a scoring technique based on temporal responsiveness, spatial consistency, and statistical strength of indicators, we found that several morphological parameters (Thalassia shoot density, blade width, blade number, and shoot-specific leaf area) were responsive and consistent measures of light stress. Some morphological parameters, such as rhizome apex density, responded to declines and subsequent improvement in water clarity, but lacked the statistical discriminating power necessary to be useful indicators. However, rhizome sugar, starch, and total carbohydrate concentrations also exhibited spatially and temporally consistent variation as well as statistical strength. Because changes in shoot density, as well as water clarity, affect rhizome carbohydrate levels, a composite metric based on Thalassia shoot density and rhizome carbohydrate levels together is probably more useful than either parameter alone as an indicator of seagrass health.

An experimental investigation of drift macroalgal accumulation in seagrass beds was conducted to determine if the relationship between passively dispersed plant structure and the spatial arrangement of rooted macrophytes differed when examined across two spatial scales. Experiments were performed from December 1992 to April 1993 at four different sites in Tampa Bay, Florida, utilizing artificial seagrass units (ASUs) of uniform shoot length and density but with different areal dimensions [1 m 2 (S) versus 4 m 2 (L)]. Drift macroalgae were also collected from 1 m×1 m plots of natural seagrass at each of the experimental sites from November 1990 to May 1992 to determine the relationship between macroalgal abundance and structural characteristics of natural seagrass. Disproportionately higher amounts of macroalgae were captured in L compared to S plots suggesting that macroalgal accumulation does not scale up directly with the areal dimensions of ASU patches. Higher amounts of algae recovered in L plots is in accordance with patterns expected if algae accumulate in zones of attenuated water flow. Neither seagrass shoot density nor blade length could adequately describe the patterns of algal accumulation. These combined results suggest that explanations for trapping/retention of passively dispersed particles should extend beyond traditional measures of vegetation complexity.