Interactions between semi-infaunal suspension feeding bivalves (Modiolus americanus) and seagrass assemblages (Thalassia testudinum)

Suspension feeding bivalves are commonly associated with seagrass habitats in the Gulf of Mexico and Caribbean Sea. Field experiments estimated mean annual biodeposition rates of Modiolus americanus to be 215 g N/m{dollar}sp2{dollar} and 7.1 g P/m{dollar}sp2{dollar} and that the biodeposits were capable of increasing the pore water nutrient content. Two simultaneously conducted field experiments using live mussel density manipulations and a 2 x 3 factorial incomplete randomized design utilizing mussel mimics and nutrient enrichment of the sediments were conducted to examine the effect of mussels on Thalassia seagrass assemblages. The live mussel density manipulations resulted in significantly increased nutrient concentration of the sediments, leaf widths and leaf lengths. They also produced significantly reduced leaf tissue C:N, N:P and C:P ratios and reduced epiphytic loads. All of these parameters resulted in increased Thalassia production when mussels were present. The 2 x 3 factorial design tested the separate factors of increased habitat complexity and increased nutrient enrichment resulting from the presence of the mussels. Structure had a significantly positive effect on epiphytic grazer densities and a significantly negative effect on epiphytic biomass. Nutrient had a significantly positive effect on sediment nutrient concentrations and a significantly negative effect on leaf tissue N:P and C:P ratios. This study showed that the direct effect of nutrient enrichment produced the greatest positive response in Thalassia productivity during the early months of the growing season. As the season continued, the indirect effect of structure on epiphytic biomass and subsequent % light reduction had an increasingly positive effect on productivity. Finally, a field experiment was designed to evaluate the effects of a seagrass on the survivorship of Modiolus. The purpose of this experiment was to test the reciprocal effect of the presence of the seagrasses on Modiolus and appraise the possible mutualistic association between these two organisms. Significantly greater mortality occurred in the unvegetated sand than in vegetated habitats. These results illustrate that seagrass assemblages significantly effect the survival of M. americanus and that a facultative mutualistic association is present.