Nonpoint-source nutrients contribute to eutrophication of surface waters. While effects of particular management actions are difficult to identify at the watershed scale, assessing nutrient fluxes over time can illuminate the net impact of trends in land use and management. We investigated nutrient fluxes in upper Kamm Creek in northwest Washington State to determine if historical changes in land management, responding to economic and regulatory shifts, had impacted nutrient export. We compared current (October 2015 to September 2018) nitrogen and phosphorus fluxes in Kamm Creek to data from a previous study (1993 to 1998). We found significantly higher current fluxes of nitrate, and significantly lower current fluxes of orthophosphate (P) and total phosphorus (TP), compared to the historical sampling period. The increased annual nitrate flux resulted from slightly higher average discharge and significantly higher nitrate concentrations throughout the year. In contrast, current P and TP concentrations were significantly lower throughout the year than previously. The Nooksack River, which receives water from the study stream, showed similar patterns for P and TP, but not nitrate. Kamm Creek had reduced phosphorus fluxes and increased nitrate fluxes between the 1993 to1998 and 2015 to 2018 time periods. The watershed experienced several concurrent land-use changes, including reductions in hay and corn acreage, increases in berry crop acreage, legislation to reduce manure application, and altered use of nitrate-enriched groundwater. We cannot currently distinguish among these. Further understanding relationships between specific management changes and nutrient fluxes will help to maintain local agricultural productivity and improve water quality.
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Vol. 95 • No. 1