(South Florida Water Management District, 3301 Gun Club Rd., West Palm Beach, FL 33406, USA)

Abstract: Lake Okeechobee watershed in South Florida comprises roughly 5,160 square kilometers and contains 25 drainage basins that contribute nutrient enriched runoff to the lake. Phosphorus is of particular concern because it has been identified as the key element that contributes to the eutrophication of the lake.

Phosphorus import, export, and net import coefficients were determined for each land use based on data collected from 1998 to 2001.  These coefficients are unit phosphorus import/export values in terms of kg P/ha per year and are obtained based on landowner surveys, scientific data from the previous research, and literature values.  Major agricultural land uses in the northern Lake Okeechobee watershed include improved pasture (36%), rangeland/unimproved pasture (16%), citrus (5%), sugarcane field (2%), dairy farm (2%), sod farm (0.9%), ornamentals (0.6%), and row crops (0.6%).  The remaining land uses (37%) are wetlands, water, forest, and urban/residential.

Phosphorus import/export coefficients were applied to the appropriate land use area with a geographic information system (GIS) to obtain watershed scale phosphorus budget.   Phosphorus load in runoff was estimated based on literature values of phosphorus runoff concentrations and runoff estimates for each land use summed by basin.  The phosphorus loads to the lake were measured at basin discharge structures.  On-site phosphorus storage and wetland assimilation values were estimated using a mass balance approach for each basin.

 Phosphorus Imports.  The total phosphorus import to the northern Lake Okeechobee watershed is 2,961 metric tonnes per year.  The four most noteworthy land uses with regard to percentage of phosphorus imports, listed in order of magnitude, are improved pasture (29%), dairy (22%), row crops (20%), and citrus (9%).  Approximately 69% of phosphorus imports are from fertilizer usage by improved pasture, row crops, and citrus and 31% of phosphorus imports are from feed imports by dairy, improved pasture, and residential land use.

Phosphorus Exports. The total phosphorus exported from the northern Lake Okeechobee watershed is 1,244 tonnes per year.  The four most important land uses with regard to percentage of phosphorus exports are sod (26%), improved pasture (24%), dairy (15%), and sugarcane (12%).

Net phosphorus Import. The amount of net phosphorus import is the difference between phosphorus import and export. The overall net phosphorus import to the watershed based on land use is 1,717 tonnes per year.  The land uses with the most influence (positive or negative) within the northern Lake Okeechobee watershed in terms of net phosphorus import are improved pasture (33%), row crop (32%), dairy (27%), sod farm (-14%), and citrus (11%).

Phosphorus in Runoff.  The total amount of phosphorus in runoff is 488 tonnes per year.  The three primary land uses contributing to phosphorus runoff are improved pasture (58%), citrus (9%), and dairy (8%).

Phosphorus Storage.  Phosphorus input from rainfall is 184 tonnes per year.  Therefore, the total net phosphorus imports including rainfall to the northern Lake Okeechobee watershed is 1,901 tonnes per year.  The onsite phosphorus storage was calculated as the sum of net phosphorus imports, including rainfall, minus phosphorus in surface runoff.  Wetland storage was calculated as the amount of upland phosphorus in runoff minus the amount of phosphorus loads to the lake.  Approximately 74% (1,413 tonnes) of the total net phosphorus import is stored on site in upland soils and vegetation, while 26% (488 tonnes) is lost in runoff.  Approximately 32% (156 tonnes) of that runoff phosphorus is stored in wetlands, while approximately 68% (332 tonnes) is loaded to the lake. 

In summary, anthropogenic land use activities contributed 90% of the total net phosphorus import.  With regards to phosphorus management, improved pasture, row crop, and dairy land uses should receive increased attention. To effectively reduce phosphorus loads to Lake Okeechobee from its watershed, Best Management Practices and technologies are being implemented to these land uses.  The results provided above should be reevaluated or updated once new information available. 

Key words: nutrient management; phosphorus budget; agricultural land use; Lake Okeechobee watershed