ANTHROPOGENIC ENRICHMENT ALTERS A MARINE BENTHIC FOOD WEB

The identification and understanding of shifts in resource availability and community structure caused by a variety of anthropogenic perturbations is essential to future rehabilitation efforts and recovery of essential ecosystem processes. We focus here on the contribution of nutrient enrichment to the overgrazing of macrophyte-dominated systems, which has often been linked to predator and competition release of urchins due to overfishing. However, the contribution of nutrient loading to the progression and per- sistence of this phenomenon should also be considered. In an effort to understand the contribution of nutrient loading to the overgrazing phenomenon and associated simplified community in a Caribbean seagrass bed, a detailed isotopic (d 13 C, d 15 N) assessment of the food web structure was conducted at both a nutrient-enriched and a control site. The general pattern at the enriched site indicated significantly lighter d 13 C signatures (i.e., phytoplankton influenced) for non-generalist primary consumers (i.e., specialist grazers, subsurface deposit feeders) and for the sediment organic material (SOM) when compared to the control site. The contribution of phytoplankton and associated particulate organic material to the SOM was also different, 7% vs. 44% at the enriched and control site, respectively. The loss of the autochthonous seagrass detritus pool, in the wake of high densities of generalist urchins (.66 000 individuals/ha) and low overall consumer diversity, appears to have been partially replaced by opportunistic ''alternatives,'' epiphytes but mainly phytoplankton, that benefit directly from elevated input of anthropogenic, allochthonous nutrients. The availability of such alternative, allochthonous resources to generalist urchins could potentially allow for the persistence of simplified seagrass communities. Here, elevated densities of urchins enable a persistent suppression of autochthonous benthic-macrophyte production through grazing and the consumption of newly recruited competitors and predators.