|Abstract or Summary
- The anthropogenic introduction of nutrients to water bodies has been shown to alter the structure and function of natural aquatic ecosystems, yet national EPA lake and reservoir nutrient criteria remain too broad for effective regional water quality management. This study uses a reference site approach to propose numeric nutrient criteria for concentrations of total nitrogen (TN) and total phosphorus (TP) specifically applicable to lakes and reservoirs in Oregon. Oregon lake watersheds with existing lakewater nutrient data were characterized using GIS techniques to eliminate lakes with known point and non-point sources of anthropogenic nutrient contributions, resulting in 98 minimally or least-altered Oregon lakes. These minimally altered lakes were used as reference sites for their corresponding Omernik Level III ecoregion based on majority ecoregion composition of the lake watershed. CART regression and other statistical procedures were used to relate existing data on summer nutrient concentrations to reference site factors, including: water residence time, soil properties, elevation, lake depth, precipitation, and watershed deciduous forests and wetlands. Lake classifications from this analysis were only significantly related to precipitation and supported aggregate ecoregion groupings of the Western Forested Mountains (Coast Range, Willamette Valley, Cascades, Klamath Mountains, and Blue Mountains), Xeric West (Northern Basin and Range, Columbia Plateau, and Snake River Plain), and Eastern Cascades were used to set ecoregional criteria for TP. For TN, the Eastern Cascades were not significantly different from the Xeric West and aggregated into the Xeric West ecoregion. As per EPA guidelines, the 75th percentile of reference site nutrient concentrations in an ecoregion were used to set ecoregional nutrient criteria with the following results; Western Forested Mountains: 11 μg TP/L and 277 μg TN/L; Eastern Cascades: 78.5 μg TP/L; and Xeric West: 490 μg TP/L and 760 μgTN/L. Nutrient concentrations in a random sample of Oregon lakes taken from EPA's 2007 and 2012 National
Lake Assessment surveys were assessed using the proposed criteria. Statewide, 21- 37% of lakes had TP below criteria and 40 - 64% of lakes had TN below criteria in the two surveys. These results contribute to nutrient management efforts and inform agency scientists and decision makers regarding the complexity of nutrient – land use relationships in lakes and their watersheds.