Development of eutrophication criteria for Minnesota streams and rivers using multiple lines of evidence

Development of water-quality criteria to address eutrophication impacts on aquatic life in streams and rivers lags behind development of lake eutrophication criteria, in part, because of multiple stressor pathways, natural variability in nutrient responses between water bodies, and a lack of methods that can address this complexity. Several investigators have demonstrated significant and predictable relationships among summer nutrients, sestonic chlorophyll a, and 5-d biochemical O2 demand (BOD5) in medium to large Minnesota rivers. Diel dissolved O2 (DO) flux also is positively correlated with total P (TP) and chlorophyll a concentrations. Our findings demonstrate significant relationships among several sensitive macroinvertebrate and fish metrics and TP, chlorophyll a, BOD5, and diel DO flux. Criteria were developed using multiple lines of evidence including quantile regression and regression tree analysis of macroinvertebrate and fish data and reference-condition analysis. A regional approach was used to develop eutrophication criteria and demonstrated that different criteria should be applied across regions. Draft river eutrophication criteria based on these lines of evidence are presented.