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Sources of Nitrogen and Phosphorus in an Estuary of the Chesapeake Bay

USDA, Agric. Res. Service, Environ. Chem. Lab., Beltsville, MD 20705.

^* Corresponding author.

ABSTRACT

This study examined the contribution of agriculture to the nutrient (N and P) concentrations in the Wye River, an estuarine tributary the Chesapeake Bay. The river drains a watershed that is devoted mainly to farming and is devoid of any sources of municipal and industrial wastes. Water samples were analyzed for NO₃-N + NO₂-N and molybdate-reactive P (MRP). The concentrations of these forms nutrients were then examined in relation to sampling site, sample salinity, and time of year. Freshwater entering the river (brackish) from the watershed contained consistently high concentrations of NO₃-N (about 4 mg L^–1) and low concentrations of PO₄-P (about 40 µg L^–1). Once these nutrients entered the estuarine part of the river, however, the concentrations of NO₃-N decreased to low levels within a short distance (500–700 m) from the point of riverine discharge, through both dilution with brackish water and apparent microbial action. The concentrations of MRP increased markedly through release from within estuary sources. Concomitantly, the estuary released large quantities of NH₄. The estuary itself, therefore, was the immediate major source of N (as NH₄) and P.

Key Words: Tide • Ammonium-N • Nitrate-N • Molybdate-reactive phosphate • Salinity