Nutrient budgets for watersheds on the southeastern atlantic coast of the United States: temporal and spatial variation

Nitrogen budgets were constructed for the watersheds of twelve rivers in the southeastern U.S. following the methodology of the SCOPE project. Total inputs ranged 2from 2,762-6,232 kg/km/yr, with the largest contributions from fertilizer and net food/feed import. Export to the estuaries of these rivers was positively related to inputs and averaged 9%. In contrast, reported export to estuaries further north averaged 25%. These differences are consistent with temperature-driven changes in denitrifier activity and suggest a possible control for watershed nitrogen export. In order to evaluate changes in nutrient input over time, nitrogen and phosphorus budgets were constructed for the watershed of the Altamaha River estuary in Georgia for 1954, 1974, and 1992. Nutrient inputs increased between 1954 and 1974 and decreased in 1992 (from 2,007 to 3,553 to 222,977 kg N/km/yr and 408 to 532 to 340 kg P/km/yr). These changes were largely driven by trends in fertilizer use.