Nutrient and grazing influences on a subtropical seagrass community

Nutrient and grazing levels were manipulated in a 10 wk field experiment at 2 sites in a subtropical lagoon in Bermuda to evaluate their relative effects on the abundance and growth of the dominant seagrass and macroalgae. One site was eutrophic, the other mesotrophic to oligotrophc. Nutrients were added to the sediment-water interface using slow-release fertilizer, and the dominant herbivores in the system, the purple sea urchin Lytechinus variegatus and herbivorous fishes (primarily the resident parrotfish Sparisoma radians), were partitioned using roofless cages. Nutrient enrichment caused an increase in the percent cover of the filamentous, mat-forming macroalga Spyridea hypnoides. This response is consistent with the dominance of this macroalga in the eutrophic portion of the bay and with previous work showing that this species has rapid nutrient uptake and growth potentials, enabling it to take advantage of elevated nutrient concentrations in the water colu-. ~nrichment also caused a decline in both the percent cover and aboveground biomass of the dominant seagrass Thalassia testudinum at the eutrophic site but not at the mesotrophic site. Estimates of productivity and grazing losses during the experiment indicated that the decline was primarily a result of enhanced grazing by herbivorous fishes and not of either increased urchin grazing or reduced seagrass growth from shading. Nutrient enrichment caused an increase in the nitrogen content of the seagrass, particularly at the eutrophic site. Fertilization may have been less effective at the mesotrophic site due to the dilution of fertilizer nutrients in greater current flow. Herbivorous fishes preferentially chose the highnitrogen epiphyte-covered T. testudinum at both sites, but particularly selected the higher-nitrogen seagrass at the eutrophic site. Intense grazing on this nitrogen-enriched T. testudinum was responsible for the creation and maintenance of small-scale (<1 mZ) patches in aboveground biomass in the seagrass meadow, an occurrence also observed naturally in places at the eutrophic site.