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Linking landscape characteristics and stream nitrogen in the Oregon Coast Range: Empirical modeling of water quality monitoring data
Citation:
COMPTON, J. E., E. Greathouse, AND J. VAN SICKLE. Linking landscape characteristics and stream nitrogen in the Oregon Coast Range: Empirical modeling of water quality monitoring data. Presented at National Water Quality Monitoring Council Conference: Monitoring from the Summit to the Sea, Denver, CO, April 25 - 29, 2010.
Impact/Purpose:
Background sources of nitrogen (N) provide a challenge for setting stream nutrient criteria in the Pacific Northwest US. Red alder (Alnus rubra), an early successional nitrogen fixing tree, and sea salt inputs can strongly influence stream N concentrations observed in individual studies in western Oregon and Washington.
Description:
Background sources of nitrogen (N) provide a challenge for setting stream nutrient criteria in the Pacific Northwest US. Red alder (Alnus rubra), an early successional nitrogen fixing tree, and sea salt inputs can strongly influence stream N concentrations observed in individual studies in western Oregon and Washington. We used data from various monitoring and GIS efforts (e.g., US EPA, Oregon DEQ, LEMMA GNN vegetation maps) to compile a database of stream N and landscape characteristics in the Oregon Coast Range. Dissolved nitrate was the most commonly measured N parameter. Nitrate was measured at 386 non-tidal stream sites that we considered independent (sites were > 5 km apart, if nested) and with watersheds entirely contained within the Coast Range; 336 sites had measurements or estimates of total nitrogen (TN). We focused on nitrate and TN because ammonium concentrations were generally below a high detection limit (20 ug N/L), and total dissolved N was not well represented in the database. Basal area of alder as a percent of watershed area accounted for 37% and 38% of the variation in summer nitrate and TN, respectively. Stream nitrate and TN concentrations from watersheds with high cover of alder were above nutrient criteria proposed by US EPA. Distance to the coast and latitude, which are likely surrogates for sea salt inputs, were also related to nitrate values. Nitrate levels were lowest in the summer, highest in winter and at an intermediate level in fall and spring, but there was no interaction between alder and season in our models of log (x+l) –transformed nitrate. Although outliers in our model of nitrate against season and alder consistently had high values of urban land cover, few sites had substantial human land use in the watershed (324 sites had natural land cover >90%), and thus, best fit multiple regression models did not include land use (e.g., urban land cover, agricultural land cover, number of confined animal feeding operations). Our results provide evidence, at a regional scale, that background sources (alder) and processes (sea salt interactions) cause many Coast Range streams to exceed proposed nutrient criteria.