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.