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 oligotrophic. Nutnents were added to the sediment-water interface using slow-release fertilizer, and the dominant herbivores in the system, the purple sea urchin Lytechlnus vanegatus and herbivorous fishes (primardy the resident parrotfish Sparisoma radians), were partitioned using roofless cages Nutnent enrichment caused an increase in the percent cover of the filamentous, mat-forming macroalga Spyndea 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 column. Enrichment 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 ennchment 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 ( < l m2) patches in aboveground biomass in the seagrass meadow, an occurrence also observed naturally in places at the eutrophic site.

Physiological controls on the pattern of macroalgal distribution were determined along a nutrient gradient in a subtropical bay in Bermuda, where filamentous mat-forming macroalgae, primarily Spyr~dea hypnoides, dominated the eutrophic inner bay but were absent from the lownutrient outer bay. My related work showed that these macroalgae responded favorably to nutrient additions in the inner bay, increasing significantly in abundance after 10 wk. In this study, 1 tested the hypothesis that S. hypnoides was absent from the low-nutrient outer bay because its metabolic nitrogen demands exceeded its nitrogen storage capacity and its ability to acquire nitrogen at low ambient concentrations to replenish depleted internal pools. I transplanted S. hypnoides from the high-nutrient inner bay to the low-nutrient outer bay, and compared these transplants to macroalgae handled and transplanted in situ and to unmanipulated controls. Tissue nitrate concentrations declined by 93 % in the outer-bay transplants after 4 d and remained at near undetectable levels for the duration of the study (14 d). Protein and chlorophyll contents also declined significantly in these transplants, by 55 and 75 % respectively. Nitrogen-deficient macroalgae showed an increase in photosynthetic efficiency per unit chlorophyll; however, dark respiration was 2 X higher than that of nutrient-sufficient macroalgae. Biomass-specific carbon gain was significantly reduced for the nutrient-stressed outer-bay transplants, where maximum rates of photosynthesis were 75 % lower than those for macroalgae from the highnutrient inner bay. This study suggests that the physiological cost of high metabolic nitrogen demands is a low nutrient storage capacity and a greater vulnerability to nutrient stress when ambient nutrient concentrations are chronically low.