Trophic implications of nutrient enrichment in seagrass beds

The capacity of epifauna to control algal proliferation following nutrient input depends on responses of both grazers and upper trophic level consumers to enrichment. We examined the responses of Thalassia testudinum (turtle grass) epifaunal assemblages to nutrient enrichment at two sites in Florida Bay with varying levels of phosphorus (P) limitation. At the severely P-limited site, caridean shrimp, grazing isopod, and gammarid amphipod densities were higher in enriched than in unenriched plots. At the less P-limited site, epifaunal density was not affected by nutrient addition. At both sites, some variation in epifaunal density was explained by features of the macrophyte canopy, such as T. testudinum and Halodule wrightii percent cover, suggesting that enrichment may change the refuge value of the macrophyte canopy for epifauna. Additional variation in epifaunal density was explained by epiphyte pigment concentrations, suggesting that enrichment may change microalgal food resources. Stable isotopic signatures (δ¹⁵N) revealed increased importance of H. wrightii in the diet of benthic consumers, primarily benthic grazing snails at the P-limited site. The diet of epiphyte grazers did not change in enriched plots, but increased density suggests that grazers may be able to control epiphytic algal proliferation following moderate nutrient input to Florida Bay.