Hydrologic and Nutrient Fluxes in a Small Watershed with Changing Agricultural Practices

Many watersheds are subject to nonpoint-source inputs of nutrients from human activities, contributing to eutrophication of surface waters. The magnitude of these inputs is in turn dependent on the types of land use within a watershed, and on the specific land management strategies employed. Exact nutrient contributions resulting from particular management actions are difficult to identify, but field studies of nutrient fluxes through a waterway over time can shed light on the net impact of trends in land use and management. I investigated nutrient fluxes through upper Kamm Creek in northwest Washington State, to determine if historical changes in land management, responding to economic shifts and new legislation, had impacted nutrient export from the watershed. I measured streamflow and the concentrations of various forms of nitrogen and phosphorous between October 2015 and October 2016, and compared these measurements to data from a previous water quality study conducted from 1993-1998 on the same watershed. I found significantly higher nitrate fluxes, and significantly lower fluxes of mineral and total phosphorous, compared to the 1993-1998 sampling period. The increased annual nitrate flux resulted primarily from significantly increased summer flux relative to the historical data, while the phosphate and total phosphorous fluxes were significantly lower throughout the year in the current data. Mean nitrate concentration in the current data was high (8.8 +/- 0.17 mg N L-1) and increased relative to the historical mean of 6.96 mg N L-1. Current mean phosphate (0.009 +/- 0.0015 mg P L-1) and total phosphorous (0.033 +/- 0.007 mg P L-1) concentrations decreased relative to historical means by 0.03 and 0.04 mg P L-1, respectively. Concentrations of all nutrient species varied seasonally in both current and historic data, but the trends of increased nitrate and decreased phosphorous concentrations held for most months. The relationships of all nutrient concentrations to streamflow were similar between sampling periods: nitrate concentrations decreased with streamflow, and phosphate and total phosphorous concentrations increased. Annual streamflow did not change compared to the previous sampling period, however streamflow was significantly higher during current summer months than previous summer months. Similar patterns, particularly for nitrate, did not occur on the nearby Nooksack River, which receives water from the study stream. The reduced phosphorous fluxes through Kamm Creek are consistent with the expected impacts of legislation that reduced the application of manure fertilizer between the sampling periods; the increased nitrate flux might result from increased inputs of nitrate-enriched groundwater, potentially in concert with shifts in crop types. Further understanding the relationships between specific land management changes and nutrient fluxes will help land managers trying to both maintain local agricultural productivity and improve water quality.