|EVANS, DANIEL - Virginia Polytechnic Institution & State University|
|SCHOENHOLTZ, STEPHEN - Virginia Polytechnic Institution & State University|
|WIGINGTON, PARKER - Environmental Protection Agency (EPA)|
|FLOYD, WILLIAM - Agriculture Canada|
Submitted to: Environmental Monitoring and Assessment
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/15/2013
Publication Date: 2/28/2014
Citation: Evans, D.M., Schoenholtz, S.H., Wigington, P.J., Griffith, S.M., Floyd, W.C. 2014. Spatial and temporal patterns of dissolved nitrogen and phosphorus in surface waters of a multi-land use basin. Environmental Monitoring and Assessment. 186(2):873-887.
Interpretive Summary: Surface water quality is a growing concern in Oregon’s Willamette River Basin because 70% of Oregon’s expanding population lives within this basin and relies on surface waters for market and non-market amenities. Research addressing nutrients in surface waters and their relationship with land use is a high priority, particularly in basins with a predominance of agriculture and forestry, which have the potential to increase surface water nutrient levels and phosphorus (P) often limit productivity of terrestrial and aquatic ecosystems. Consequently, increases of the inorganic, more bio-available forms of N and P above background levels can lead to growth of nuisance algae and aquatic plant growth, often leading to negative impacts on surface waters such as eutrophication. We measured dissolved nitrogen (DN) and ortho-P along 130 km of the Calapooia River (Oregon, USA) and 44 of its sub-basins for three years to test for associations with land use. Nutrient concentrations were analyzed for spatial and seasonal patterns and for relationships with land-use and stream discharge. During our study period many lower sub-basins had concentrations of nitrate-N that were higher than the EPA standard for drinking water and ortho-P concentrations were above EPA guidelines for avoiding nuisance aquatic plant growth. However, DN and ortho-P concentrations were lower than accepted EPA guidelines in the main stem of the river, likely due to dilution of the lower sub-basin water with water from the forest upper part of the basin. We expect that comparable patterns exist in basins with similar climatic regimes and land-use/land-cover signatures. Strong, consistent, positive associations were evident between the amount of agriculture in a sub-basin and dissolved N and P concentrations. However, agriculture in the Calapooia Basin occurs predominately on soils that tend to be poorly drained. These soils enhance the tendency to deliver nutrients to aquatic systems due to stream channel expansion and overland flow during wet periods and may contribute to the strong relationships that we found between dissolved nutrients and %AG.
Technical Abstract: Research on relationships between dissolved nutrients and land use at the watershed scale is a high priority for mitigating or protecting surface water quality. We measured dissolved nitrogen (DN) and ortho-P along 130 km of the Calapooia River (Oregon, USA) and 44 of its sub-basins for three years to test for associations with land use. Nutrient concentrations were analyzed for spatial and seasonal patterns and for relationships with land-use and stream discharge. Ortho-P and DN were higher in lower-elevation sub-basins dominated by poorly-drained soils and agricultural production when compared to higher-elevation sub-basins that are dominated by well-drained soils and forests. Eight lower basins had at least one sample period with nitrate-N >10 mg L-1. The main stem of the Calapooia River had lower dissolved nutrient concentrations compared to the lower sub-basins, often by an order of magnitude. Dissolved organic N represented a greater proportion of DN in the upper forested sub-basins. Seasonal nitrate-N, ammonium-N, dissolved organic N, and ortho-P concentrations had strong positive correlations to the percent of a sub-basin that was managed for agriculture (%AG) in all seasons (p-values=0.019) except summer, when the association between nitrate-N and %AG was not significant. Stream discharge was a significant predictor of nitrate-N for all sub-basins (p-values<0.0001) and was also a significant predictor of ortho-P in the agriculture-dominated sub-basins (p-value=0.005). Results suggest that agricultural lands are contributing to stream nutrient concentrations. However, poorly drained soils in agricultural areas may also contribute to the strong relationships that we found between dissolved nutrients and agriculture.