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 nitrogen from agricultural run-off. During 1990-91, invertebrates were collected from replicate sweep and core samples at eight sites along the nutrient enrichment gradient in Water Conservation Area 2A (WCA-2A). Species richness, Shannon's diversity, the number of unique species, and the density of invertebrates and small fish were all greater within enriched and intermediately enriched open water habitats than unenriched sloughs. Sorenson's taxonomic similarity index was significantly different between enriched and unenriched areas. Ostracods in particular were 14 times more abundant in the enriched area than at unenriched sites. The freshwater shrimp (Palaemonetes paludosus) was the only common species with lower densities in enriched than unenriched areas. However, the trophic structure or percent composition of grazers, predators, and collector-gatherers and the number of species within taxonomic orders and functional feeding groups was very similar among sites along the nutrient enrichment gradient. Higher invertebrate and small fish diversity and density within enriched sites indicates that nutrient enrichment has not caused direct harmful foodweb effects that may adversely influence higher trophic levels (e.g., wading birds). Assuming, however, that nutrients can cause cattails to overgrow and eliminate sloughs, the centers of biological diversity in the Everglades, then nutrient enrichment may have harmful indirect effects.