Terrestrial herbivores often preferentially consume plants that have higher nitrogen concentrations. In these circumstances, low nitrogen concentrations in exposed tissues would be an effective defense against herbivory. In marine environments, the relationship between nitrogen concentration in plants and herbivore preferences has only recently been explored, and results are inconsistent. We examined preferences of the bucktooth parrotfish Sparisoma radians for the turtlegrass Thalassia testudinum with high or low nitrogen concentrations in both field and laboratory experiments. These species were chosen for study because (1) turtlegrass, the dominant seagrass in the Caribbean, is the primary food resource of the bucktooth parrotfish and (2) S. radians is known to be undeterred by the chemical defenses (i.e. phenolic compounds) and physical defenses (i.e. tough tissues) of T. testudinum. In field choice experiments, we found that S. radians consumed a greater proportion of T. testudinum shoots with high nitrogen concentrations than shoots with low nitrogen concentrations (68 vs 5%). Similarly, in laboratory experiments, S. radians consumed more T. testudinum with high nitrogen than with low nitrogen concentrations (25 vs 5%). To determine whether S. radians used the color or shape of shoots as a basis for its preferences, we repeated laboratory experiments using an agar mixture with powdered T. testudinum leaves of either high or low nitrogen concentration. S. radians again showed a significant preference for the high nitrogen food source. We, therefore, conclude that mechanisms for choice of high nitrogen food were most likely related to gustation and/or olfaction of chemical cues (i.e. nitrogen or phenols).

The removal of fish biomass by extensive commercial and recreational fishing has been hypothesized to drastically alter the strength of trophic linkages among adjacent habitats. We evaluated the effects of removing predatory fishes on trophic transfers between coral reefs and adjacent seagrass meadows by comparing fish community structure, grazing intensity, and invertebrate predation potential in predator-rich no-take sites and nearby predator-poor fished sites in the Florida Keys (USA). Exploited fishes were more abundant at the no-take sites than at the fished sites. Most of the exploited fishes were either omnivores or invertivores. More piscivores were recorded at no-take sites, but most (approximately 95%) were moderately fished and unexploited species (barracuda and bar jacks, respectively). Impacts of these consumers on lower trophic levels were modest. Herbivorous and smaller prey fish (< 10 cm total length) densities and seagrass grazing diminished with distance from reefs and were not negatively impacted by the elevated densities of exploited fishes at no-take sites. Predation by reef fishes on most tethered invertebrates was high, but exploited species impacts varied with prey type. The results of the study show that, even though abundances of reef-associated fishes have been reduced at fished sites, there is little evidence that this has produced cascading trophic effects or interrupted cross-habitat energy exchanges between coral reefs and seagrasses.

Although early studies qualitatively documented the importance of cross-habitat energy transfers from seagrasses to coral reefs, such exchanges have yet to be quantified. Empirical evidence suggests that grazing by reef-associated herbivores along the coral reef -seagrass interface can be intense (e.g. conspicuous presence of bare-sand 'halos' surrounding coral reefs). This evidence must be interpreted with caution, however, as most of it comes from areas that have experienced sustained, intense overfishing. To quantify the impacts of piscivore removal on cross-habitat energy exchange at the coral reef -seagrass interface, we compared grazing intensity along fished and notake reefs in the upper and lower Florida Keys. Using visual census techniques and direct measures of seagrass grazing, we documented the impacts of piscivore density on herbivory along the seagrass-coral reef interface. Grazing rates were greater than observed seagrass (Thalassia testudinum) production near reefs in the upper Keys, but less than 48% of production in the lower Keys. Analyses showed that these large differences were not related to regional differences in either herbivore density or species composition. Seagrass biomass was also lower near reefs in the upper Keys, where estimates of grazing were highest. Piscivores were dominated by transient predators (jacks and barracudas) whose densities varied with region and distance from reef, but not with protection from fishing. A nonsignificant negative correlation between great barracuda density and leaf losses from tethered shoots, coupled with the greater abundances of larger grazers near reefs, suggests that predation risk, rather than direct reductions in density, may limit grazers to intense feeding on seagrasses adjacent to reefs in the upper Keys. The large-scale variation in grazing intensity illustrates the need for more detailed quantifications of energy exchanges along the seagrass-coral reef boundary.

Alterations of hydrology are known to trigger changes in coastal ecosystems, such as the composition and abundances of local flora and fauna. It is less well known how these alterations lead to changes in nutrient and energy transfers in these systems. We used comparisons of stable isotope signatures (δ 13 C, δ 15 N, and δ 34 S) in the tissues of conspecific plants and animals collected on each side of the Mobile Bay Causeway to determine the extent to which the causeway may have altered energy and nutrient exchange between the Mobile-Tensaw Delta and upper Mobile Bay. While the δ 13 C signatures of most plants and animals varied irrespective of their location relative with causeway location, their δ 15 N and δ 34 S signatures were almost always more enriched south of the causeway, indicating significant alterations of trophic linkages within this estuarine food web. Dual isotope plots and mixing model analyses indicated that while terrestrial and floating plants were trophically important to consumers north of the causeway, submerged aquatic vegetation was more important to consumers south of the causeway. Although limited in spatial and temporal scale, our results preliminarily show that there are noteworthy differences in stable isotope signatures most likely due to the Mobile Bay Causeway altering energy and nutrient transference.

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