Loss of Coldwater Fish Habitat in Glaciated Lakes of the Midwestern United States after a Century of Land Use and Climate Change
Peter C. Jacobson, Gretchen J. A. Hansen, Leif G. Olmanson, Kevin E. Wehrly, Catherine L. Hein, and Lucinda B. Johnson
Abstract.—Freshwater resources are threatened by multiple stressors, and identifying the relative roles of each is necessary for effective management. The relative contributions of eutrophication and climate change to the loss of coldwater fish habitat was estimated for 5,220 stratified lakes in Michigan, Minnesota, and Wisconsin. A hypolimnetic oxygen model was coupled with a landscape disturbance model to estimate change in oxythermal habitat since European settlement. The general additive model predicted late-summer oxythermal habitat conditions as a function of remotely sensed Secchi depth (mean of 1995–2014 values), geometry ratio (area0.25/maximum depth), and mean July air temperature (1995–2014). The use of the remotely sensed water-clarity variable allowed modeling of lakes that do not have in situ water-quality data. The landscape disturbance model predicted remotely sensed water clarity as a function of levels of catchment land-use disturbance, geometry ratio, and proportion of glacial outwash soils in each catchment. Historic coldwater habitat was estimated for undisturbed landscape conditions and an earlier period of climate and then compared to contemporary conditions. Eutrophication and climate change substantially reduced coldwater fish habitat over the past century in many stratified lakes in Minnesota, Wisconsin, and Michigan. The greatest loss of coldwater habitat occurred in lakes with substantial land-use changes in their catchments, primarily in the Great Plains and Eastern Temperate Forests ecoregions. Oxythermal habitat in many other lakes in the Northern Forests ecoregion remained intact, with only modest changes primarily because of warming climate. To maintain coldwater habitat, deep, clear lakes in the forested ecoregions of Minnesota, Wisconsin, and Michigan should receive high priority for catchment protection efforts. Protective catchment land-use measures will be needed for coldwater fish to survive further climate warming.