An experimental test of the mechanism by which suspension feeding bivalves elevate seagrass productivity

A field experiment utilizing mussel mimics and sediment nutrient enrichment was conducted to examine the effect of the mussel Modilous americanus on meadows of the seagrass Thalassia testudinum. The experimental design tested the separate factors of increased habitat structure and increased nutrient enrichment resulting from the presence of the mussels. Nutrient enrichment had a significantly positive effect on sediment porewater nutrient concentrations and a significantly negative effect on leaf tissue C:N, C:P and N:P ratios. Increased habitat structure had a significantly positive effect on epiphytic grazer densities and a significantly negative effect on epiphytic biomass. In addition, calculated % light reduction by epiphytes was significantly reduced by structure and significantly increased by nutrient enrichment. This study showed that the direct effect of nutrient enrichment by mussel biodeposition produced the greatest positive response in productivity of T. testudinum in the seagrass meadows of St. Joseph Bay, Florida. In other systems experiencing eutrophication, however, it is possible that increased habitat complexity may have the greater effect on seagrass productivity.