Effects of nutrient enrichment and grazing on shoalgrass Halodule wrightii and its epiphytes: results of a field experiment

Abstract

We assessed the individual and combined effects of removing large predators and enriching water column nutrients on shoalgrass Halodule wrightii meadows in Big Lagoon, Florida, USA. To simulate the first-order effects of large predator reductions, we stocked 2.0 m 2 enclosures with elevated (~3 to 4× ambient) densities of the omnivorous pinfish Lagodon rhomboides, the domi- nant fish in local seagrass habitats, and we supplemented N and P in the water column to nearly 3× ambient levels. Monthly determinations of water column nutrients and chlorophyll a (chl a), coupled with bimonthly measurements of leaf epiphyte biomass, seagrass growth and biomass, and begin- ning and ending comparisons of mesograzer abundance, were used to evaluate the effects of increas- ing nutrient supply and changing food web structure. Results showed significant predator and nutri- ent effects, although there were fewer consumer effects and more negative nutrient effects on seagrasses than in our previous experiments, which had shown that mesograzers ameliorated the harmful effects of elevated nutrients on seagrasses. Epiphyte proliferation in enrichment treatments did not occur; thus, algal overgrowth could not explain the negative effects of nutrient loading on sea- grass biomass. Instead, nutrient loading resulted in nitrogen-rich shoalgrass, and it appears that this high-quality food stimulated pinfish herbivory. Elevated pinfish consumption of the enriched shoal- grass then resulted in the decline of seagrass biomass in enrichment enclosures. These results add additional complexity to understanding and predicting the effects of eutrophication in coastal waters.