Nutrient concentrations and discharges in a midwestern agricultural catchment

The amounts of nitrogen and phosphorus in river water are significantly influenced by anthropogenic inputs associated with land cover, land use, and point sources. We quantified seasonal and spatial patterns in the concentrations of nitrate N and soluble reactive phosphorus (SRP) at 17 sites distributed among tributaries and along the mainstem of the River Raisin, Southeastern Michigan, and also determined land cover, surficial geology, and discharges from waste water treatment plants (WWTPs). Annual flow-weighted mean concentrations (FWMCs) ranged from nondetectable le, els to 18.1 mg L-1 for nitrate and to 184 mu g L-1 for SRP. Nitrate concentrations increased from headwaters to river mouth, and were highest in spring and in association with high runoff episodes. Mean nitrate concentration correlated strongly with the ratio of agricultural to forested land upstream of the sampling location. Concentrations of SRP also increased markedly from headwaters downstream, and exhibited little seasonal variation other than a modest increase during the summer period of low flow. Geology and phosphorus loading from WWTPs best predicted SRP concentrations. The annual yield of nitrate was consistent with expectations based on land use. Point source loading contributed a negligible fraction of the river's discharge of nitrate and at most one-fourth of its SRP discharge, Compared with findings from headwater sites and other, less altered rivers, nutrient concentrations in this catchment appear strongly influenced by human activity.