PHOSPHORUS BUDGET-BASIN RELATIONSHIPS FOR LAKE OKEECHOBEE TRIBUTARY BASINS

Accelerated cultural eutrophication of Lake Okeechobee, FL by excessiv e phosphorus (P) loading has generated a need to determine P sources a nd sinks in the watershed. Average annual P budgets were estimated for each of the 19 tributary basins in the northern Lake Okeechobee water shed for the period 1985-1989. Net P imports were estimated based on P usage by land use type, and land use area with a geographic informati on system (GIS) model. Off-site P losses, from uplands to wetlands, we re estimated from literature values of P runoff concentrations for sel ected land use types. The P loads from each basin were measured at the discharge to the lake. For the north Okeechobee watershed, total net P imports were estimated at 2380 t P yr(-1); P = 300 t P yr(-1) off-si te P load = 760 t P yr(-1); and lake P load = 300 t P yr(-1). An estim ated 90% of imported P was retained in the basin. Of the P loaded to t he streams and wetlands, 60% was retained. The intensity of phosphorus use, indicated by net P imports, explained 70% of the variability in basin P loads to the lake. Basin characteristics extracted from the GI S database, including: area of emergent wetlands; area of selected soi l associations; area of developed land; and total length of canals wer e correlated significantly with tributary P loads. Other physical char acteristics, such as basin shape, length, size, and distance to the la ke, had no significant explanatory power.