Thalassia testudinum belowground biomass weights, leaf weights, leaf growth rates, areal shoot densities (m (2 ), and leaf C:N:P ratios were compared to a set of biogeochemical parameters to gain information on seagrassÁsediment interactions that may influence seagrass growth. Data were compiled from three surveys conducted in Santa Rosa Sound, located in northwest Florida, at three different meadows in sequential years. Biomass measurements and leaf growth rates decreased between stations along transects from shallow to deeper water. Belowground biomass weights decreased and leaf C:P ratios increased with temperature reflecting a seasonal growth pattern. The T. testudinum parameters were highly correlated with each other. Sulfate reduction rates (at times exceeding 1000 nmol ml (1 day (1 ) were among the highest recorded for seagrass beds with temperature accounting for 79% of the variation. Even though sulfate reduction rates were high, total Fe:reduced S ratios indicated sufficient Fe to account for all reduced S as pyrite. Sediment Fe, C, N, and organic P concentrations increased with sediment depth, whereas inorganic P decreased with depth, suggesting burial of organic P and root uptake of inorganic P. Leaf C:N:P ratios indicated P-limited growth for two surveys. NH 4 ' was detected in water above the sediment surface during some surveys demonstrating T. testudinum meadows at times may serve as sources of inorganic N to the water column. Plant parameters correlated with concentrations of sediment organic C and N, Fe, S, and porewater NH 4 ' . These results highlight the importance of the organic matter and Fe contents of sediments to seagrass growth.
Macrobenthic animal communities that colonized uncontaminated and fenvalerate‐contaminated sand (0.1, 1 and 10 μg/g dry weight, nominal) in boxes placed for 8 weeks in an estuary were compared to assess effects of fenvalerate on community structure. As much as 27% of initial concentrations of this synthetic pyrethrin persisted in sediment at the end of the test. The average number of species (35.6) in communities in five replicates exposed to 10 μg/g was significantly less than that in the control (47.8) and lower concentrations (45.0 and 46.2). Of the dominant phyla collected (Annelida, Mollusca, Chordata and Arthropoda), abundance of chordates only (primarily lancelets, Branchiostoma caribaeum) was reduced by 10 μg fenvalerate/g. Biological indices applied to the data showed the greatest structural differences for communities exposed to the highest concentration, but these did not differ substantially from those for the control. Effective concentration for exposure via the sediment was five orders of magnitude greater than that for waterborne exposure determined in earlier benthic community studies.
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