Nonpoint-source inputs of nutrients from agriculture can contribute to eutrophication of surface waters, with the magnitude of these inputs dependent on the specific agricultural management strategies employed in the surrounding landscape. We investigated the impact of changes in crop type and management on nutrient fluxes through the watershed of upper Kamm Creek, in northwest Washington State. We measured streamflow and the concentrations of various forms of nitrogen and phosphorous between October 2015 and October 2016, and compared these measurements against data from a previous water quality study conducted from 1993-1998 on the same watershed.
Results/Conclusions
We found significantly higher nitrate fluxes, and significantly lower fluxes for both forms of phosphorous, compared to the 1993-1998 sampling period. The increased annual nitrate flux was due primarily to significantly increased summer flux relative to the historical data, while the phosphate and total phosphorous fluxes were significantly decreased throughout the year. 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. Similar patterns did not occur on the nearby Nooksack River, which receives water from the study stream. The reduced phosphorous fluxes on Kamm Creek are consistent with the expected impacts of legislation that reduced manure use between the sampling periods, while the increased nitrate flux may result from a combination of changing crop types and nitrate-enriched groundwater inputs. Understanding the relationships between nutrient fluxes and specific land management changes is essential for planners trying to balance local agricultural and environmental interests, and deserves further investigation.