NITROGEN AND PHOSPHORUS IN THE ST. JOHNS RIVER, FLORIDA, 1991-2002

摘要

The water quality of the 90-mile-long middle reach of the St. Johns River from Cocoa to Deland, Florida, wascharacterized using existing and recently-collected data. Agricultural land is concentrated in the southern part of the studyarea. Retrospective data at 15 sites (1991-99) and biweekly data at 4 sites (2000-02) were used to characterize nutrientconcentrations. Retrospective data showed median nitrate concentrations in the river ranged from 0.02-0.10 milligrams perliter (mg/L) as nitrogen. Median nitrate concentrations were lowest at the most upstream sites on the river and greatest at themost downstream sites. Lower median concentrations at upstream sites may be due to plant uptake or denitrification inwetlands. Biweekly data showed that nitrate concentrations increased from about October through February; conditionsduring this period may have been more conducive for nitrification. Median total phosphorus concentrations in the riverranged from 0.03 to 0.09 mg/L during 1991-99. During high-flow periods, phosphorus concentrations increased to values asgreat as 0.44 mg/L, possibly due to manure or wetland inputs. During low-flow conditions in 2000, phosphorusconcentrations as great as 0.25 mg/L were measured at the most upstream site, possibly due to sediment releases. Increasednitrate and phosphorus concentrations in the ground water and near the mouth of the Wekiva River indicate that ground-waterinflow also affects the downstream sites on the St. Johns River. Wetland and in-stream processes and contributions by groundwater need to be considered to assess the water quality of streams that receive substantial amounts of wetland and ground water inflow.