Landscape factors influencing lake phosphorus concentrations across Minnesota

Total phosphorus (TP) concentrations are known to be a significant factor influencing fish populations in Minnesota lakes. Consequently, a primary focus of the Minnesota Department of Natural Resources to address fish habitat in lakes across the state has been to determine relationships between TP concentrations and watershed conditions in Minnesota lakes. Because phosphorus concentrations in Minnesota lakes vary widely corresponding to differences in geomorphology, nutrient criteria were established by the Minnesota Pollution Control Agency for specific ecoregions. To refine these relationships in Minnesota lakes managed for fishing, we gathered mean summer epilimnetic TP concentrations on 1330 natural lakes to identify where agricultural and urban development have elevated phosphorus levels. Random forest, regression tree, and generalized additive models were used to model spatial variation in lake phosphorus concentrations across Minnesota. Key landscape variables known to influence TP concentrations in lakes, including lake depth and watershed size, were used as explanatory variables in these models, along with agricultural and urban development quantified for lake watersheds from the National Land Cover Dataset. These models explained up to 60% of the variation in TP in lakes across the state and showed a critical benchmark of anthropogenic land use disturbance at 40%, that once exceeded could significantly alter TP levels and consequently fish populations. This information should be useful for fish managers to prioritize conservation efforts and to set appropriate fish population goals.