RESPONSE OF MACROINVERTEBRATES AND SMALL FISH TO NUTRIENT ENRICHMENT IN THE NORTHERN EVERGLADES

The northern Everglades (Water Conservation Area 2A) annually receives an excess addition of 60 M tons of phosphorus and 1814 M tons of nitr ogen from agricultural run-off. During 1990-98, invertebrates were col lected from replicate sweep and core samples at eight sites along the nutrient enrichment gradient in Water Conservation Area 2A (WCA-2A). S pecies richness, Shannon's diversity, the number of unique species, an d the density of invertebrates and small fish were all greater within enriched and intermediately enriched open water habitats than unenrich ed sloughs. Sorenson's taxonomic similarity index was significantly di fferent between enriched and unenriched areas. Ostracods in particular were 14 times more abundant in the enriched area than at unenriched s ites. The freshwater shrimp (Palaemonetes paludosus) was the only comm on species with lower densities in enriched than unenriched areas. How ever, the trophic structure or percent composition of grazers, predato rs, and collector-gatherers and the number of species within taxonomic orders and functional feeding groups was very similar among sites alo ng the nutrient enrichment gradient. Higher invertebrate and small fis h diversity and density within enriched sites indicates that nutrient enrichment has not caused direct harmful foodweb effects that may adve rsely influence higher trophic levels (e.g., wading birds). Assuming, however, that nutrients can cause cattails to overgrow and eliminate s loughs, the centers of biological diversity in the Everglades, then nu trient enrichment may have harmful indirect effects.