Phosphorus budget - land use relationships for the northern Lake Okeechobee watershed, Florida

Eutrophication of Lake Okeechobee, Florida, has been accelerated by excessive phosphorus (P) loading to the lake from land use practices involving phosphorus-containing materials. Average annual P budgets were estimated for each of the 25 tributary basins in the northern Lake Okeechobee watershed for current land uses and land use practices. Phosphorus import, export, and net import coefficients in terms of kg P ha(-1) per year were determined for each land use being practiced in the watershed based on landowner surveys and literature data. The net P coefficient for each land use was applied to the appropriate land use area with a Geographic Information System (GIS) to obtain a basin-wide P budget. Phosphorus runoff load was estimated based on measured data and literature values of P concentrations and runoff estimates for each land use. The P loads to the lake were measured at discharge structures from each basin. On-site P storage and wetland assimilation values were estimated using a mass balance approach for each basin. For the northern Lake Okeechobee watershed, total net P imports from land use activities were estimated at 1717 metric tonnes (t) per year, which is about 28% lower than the previous P budget conducted in 1991. Other P import, export, and storage components were obtained, and results were compared with the previous budget data. Linear regression was performed to determine which basin/landscape characteristics influence lake loading. Runoff phosphorus, the amount of developed land, and net P imports had the strongest correlation when related to lake loading. Using multiple linear regression, the annual changes in P retention correlated well with net P import intensity and stream/canal density, and the net P import and length of streams/canals correlated well with lake loading. (C) 2003 Elsevier B.V. All rights reserved.