NONPOINT SOURCE LOADINGS OF NUTRIENTS AND DISSOLVED ORGANIC-CARBON FROM AN AGRICULTURAL-SUBURBAN WATERSHED IN EAST CENTRAL FLORIDA

Unit area loadings for nitrogen, phosphorus and dissolved organic carbon were measured from four sub-watersheds and the entire 256 km2 catchment area tributary to the Indian River Lagoon. The catchment area is a rapidly growing suburban area drained by 322 km of canals in east central Florida. One-third of the annual total nitrogen, one-half of the total phosphorus and one-fourth of the annual dissolved organic carbon were exported in the drainage canal water at the water control structure to downstream waters during a 6-week period when three major storms occurred. High annual loadings of dissolved inorganic (1.3 kg N/ha-yr) and organic (4.4 kg N/ha-yr) nitrogen were recorded for the rotational grazing sub-watershed; during storm events, the entire dissolved inorganic nitrogen load leaving the catchment basin originated from the agricultural sub-watershed. Annual total nitrogen and total phosphorus export coefficients from the agricultural sub-basin deviated considerably from published "national" averages, but were very similar to "regional" coefficients published for Wisconsin. Dissolved inorganic nitrogen and phosphorus had wide variances to the published "regional" coefficients, emphasizing the need for site-specific studies.