THE EFFECTS OF NUTRIENT ENRICHMENT ON ALGAE AND MACROINVERTEBRATES IN THE EVERGLADES - A REVIEW

Over 400 metric tons of phosphorus and 12,000 metric tons of nitrogen flow annually into the northern part of the Everglades. Data describing the effects of nutrient enrichment, especially phosphorus, on algae and macroinvertebrates in the Everglades are reviewed and compared to our preliminary results. The Everglades is a harsh aquatic environment characterized by high summer temperatures (>35 C) and fluctuating, low oxygen conditions. In unenriched habitats, oxygen concentrations at the algae-water interface can vary from 200% supersaturation (approximately 30 mg/1) during the day to less than 3.0 mg/l at night. Although surface water carbon dioxide concentrations from nighttime respiration are depleted by noon of the following day, pH remains circumneutral because of high bicarbonate concentrations. Growths of cyanobacteria, often exceeding 6 cm in thickness, characterize unenriched open water habitats. Despite long-standing suggestions to the contrary, there is no evidence that the cyanobacterial mat is consumed by higher trophic levels. Annual net daily metabolism in unenriched habitats ranges from -0.065 to 2.01 g C m 2 d-1 with an average of 0. 9 2 g C m-2 d-1. Previous reports concerning the effects of P enrichment on oxygen and macroinvertebrates are confounded by comparisons between habitats that differ with respect to hydroperiod, water depth, and macrophyte species composition in addition to nutrient loading. Nutrient enrichment causes disintegration of the abundant cyanobacterial mat and a shift in algal species composition and relative abundance. Annual primary production and algal biomass were 2.9 and 3.0 x greater, respectively, in phosphorus enriched conditions compared to unenriched areas. Algal diversity, however, was not significantly different in enriched compared to unenriched habitats. Our preliminary analyses indicate that macroinvertebrate diversity and density are higher within enriched compared to unenriched areas if similar habitat types (submerged macrophyte beds) are sampled. Also, there is no evidence that "fish kills" caused by anaerobiosis occur with greater frequency in enriched compared to unenriched areas.