EVERGLADES WATER QUALITY ISSUES: I. PHOSPHORUS CONTAMINATION

The south Florida wetlands ecosystem (the Everglades and Florida Bay) has historically been an oligotrophic environment. Anthropogenic activities (especially agriculture) during the past 50-70 years, however, have delivered excess phosphorus (P) to some areas of the ecosystem, contributing to changes in water quality and ecology. This presentation will review the major water quality and biological issues related to P contamination of the Everglades, including the distribution of P in the ecosystem, its impact on biological resources, the fate and recycling of P in Everglades°¦ sediments, historical levels of P within the Everglades, and management strategies aimed at reducing P contamination to the Everglades.

Concentrations of P at pristine sites in the freshwater Everglades range from 1-20 ppb in surface water, 10-100 ppb in sediment porewater, and 300-500 ƒÝg/g dry wt. in sediments. At contaminated marsh sites, however, P concentrations often exceed 100 ppb in surface water, 3 ppm in porewater, and 2,000 ƒÝg/g dry wt. in sediment. Uranium is also enriched in sediments that contain excess P, and the 234U/238U alpha activity ratio of readily-exchangeable U in sediments is consistent with derivation of U (and by inference, some P) from fertilizer sources. This fertilizer P from agricultural fields washes into canals draining the EAA, and the canal water is discharged into the marshes. The excess P is quickly incorporated by aquatic plants, and accumulates in peat following plant senescense. At some contaminated sites, eutrophic aquatic plants (e.g. cattails) have displaced the natural Everglades vegetation (e.g. sawgrass). The cattails effectively accumulate P, but also appear to decompose much more quickly than sawgrass. Hence P, while accumulated rapidly at contaminated areas is also rapidly recycled. Accumulation rates of P in peat at contaminated sites are typically 100x higher compared to pristine areas, but the ratio of accumulation rate to the flux of recycled P from porewater varies by only a factor of 2. Areas contaminated with excess P have been observed in the northern Everglades, northeastern Everglades National Park, and surface sediments in the northeastern portions of Florida Bay. Stormwater treatment areas (STA°¦s) in combination with best management practices (BMP°¦s) in agricultural areas are being used to mitigate P inputs to the Everglades.