Phosphorus budget-basin relationships for Lake Okeechobee tributary basins

Accelerated cultural eutrophication of Lake Okeechobee, FL by excessive phosphorus (P) loading has generated a need to determine P sources and sinks in the watershed. Average annual P budgets were estimated for each of the 19 tributary basins in the northern Lake Okeechobee watershed 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 information system (GIS) model. Off-site P losses, from uplands to wetlands, were estimated from literature values of P runoff concentrations for selected 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-site P load = 760 t P yr(-1); and lake P load = 300 t P yr(-1). An estimated 90% of imported P was retained in the basin. Of the P loaded to the 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 GIS database, including: area of emergent wetlands; area of selected soil associations; area of developed land; and total length of canals were correlated significantly with tributary P loads. Other physical characteristics, such as basin shape, length, size, and distance to the lake, had no significant explanatory power.