Identifying the Influence of Anthropogenic Landuse on Phosphorus Levels in Minnesota Fishing Lakes for Prioritizing Conservation Efforts

Lake total phosphorus (TP) concentrations are known to be a significant factor influencing fish populations in Minnesota lakes.  However, phosphorus concentrations in Minnesota lakes vary widely corresponding to differences in geomorphology leading the Minnesota Pollution Control Agency to use ecoregion criteria to differentiate goals for acceptable TP levels in lakes.  We compiled data for 1330 natural Minnesota lakes managed for fishing to identify levels of anthropogenic landuse disturbances likely to result in excessive TP.  Regression tree, random forest, and generalized additive models were used to model spatial variation in lake phosphorus concentrations across Minnesota lakes.  Key landscape variables known to regulate TP concentrations in lakes, including lake depth, geomorphology, and watershed size, were used as explanatory variables in these models along with landuse quantified for individual lake watersheds from the National Land Cover Dataset.  These models explain up to 60 percent of the variation in TP among lakes across the state and identify a critical threshold of anthropogenic landuse disturbance that occurs once approximately 40 percent of the watershed is altered.  Once this threshold is reached there is a high risk of fish habitat being altered by excessive phosphorus.  This information should be useful to fish managers for prioritizing conservation efforts at statewide and regional scales.