NUTRIENT FRACTIONATION IN SOUTH FLORIDA AND IMPLICATONS FOR COASTAL WATERS AND ECOSYSTEM RESTORATION

摘要

South Florida coastal waters are mostly shallow carbonate ecosystems. Because of the chemical scavenging of P by calcium carbonate, one would expect P to be the primary limiting nutrient. This is what one observes along the east side of South Florida in Biscayne Bay and eastern Florida Bay. Anthropogenic P from land tends to be scavenged by limestone before it reaches the coastal waters, as observed along the Everglades-Taylor Slough flow pathway. Direct injection of runoff through canals, however allows for more P to reach coastal waters, as observed in Biscayne Bay during the rainy season. The western side of South Florida has large deposits of phosphorite that were transported from the north during the Miocene. It is hypothesized that these deposits are the source of the higher P concentrations observed on the western side of South Florida. The P concentrations are high enough that the coastal ecosystems are shifted to N limitation. Because inorganic N compounds (unlike P) flow more readily through soil and limestone without chemical scavenging, the western side of South Florida is more susceptible to anthropogenic N inputs. This is clearly observed downstream of the N-rich Everglades Agricultural Area where freshwater runoff meets P-rich waters from western Florida Bay. The flux of N to Florida Bay increased dramatically in the 1980s as runoff from the Everglades Agricultural Area began being pumped south into the Everglades instead of north into Lake Okeechobee, and the South Dade Conveyance System was expanded to allow more water to be pumped into Florida Bay. This was followed by algal blooms, seagrass dieoff, increased turbidity, and sponge dieoff. The algal blooms increased further as N-rich water from the Everglades-agricultural system was pumped further west, closer to the P source. Some of this nutrient enriched water from Florida Bay and the Shark River is transported to the middle and lower Florida Keys, where it may be adversely affecting the coral reefs and other oligotrophic ecosystems there. South Biscayne Bay is relatively oligotrophic, but increased direct runoff through canals could add more P to this P-limited ecosystem, causing more eutrophication and a possible shift from a seagrass dominated ecosystem to a phytoplankton dominated ecosystem. North Biscayne Bay is already eutrophic, having approximately ten times higher algal concentrations and relatively few seagrasses. The Ten Thousand Islands area appears to be naturally somewhat eutrophic because of the high P concentrations, hypothesized to come from phosphorite deposits. As this results in N limitation, it is predicted that eutrophication will increase in the Ten Thousand Islands area as urban and agricultural areas upstream expand and result in larger N loads downstream. It is hypothesized that the increase in freshwater flow into Florida Bay proposed by the Comprehensive Everglades Restoration Plan will increase the flux of N into eastern Florida Bay, which will then mix with the P from the west and increase the algal blooms observed in central Florida Bay. Increased runoff into Biscayne Bay could increase the flux of P normally associated with detritus and soil and promote algal blooms along the western side of the bay.