Coldwater fish oxythermal habitat in Minnesota lakes: influence of total phosphorus, July air temperature, and relative depth

An empirical model was developed that describes the influence of lake productivity, climate, and morphometry on coldwater fish oxythermal habitat. An oxythermal habitat variable called temperature at 3 mg.L-1 of dissolved oxygen (T-DO3) was developed by interpolating the water temperature at a benchmark oxygen concentration (3 mg.L-1) from a temperature-oxygen profile. Coldwater habitat was most available in the least productive lakes (total P < 25 mu g.L-1) with the greatest relative depths (geometry ratios < 2 m(-0.5)) and where mean July air temperatures were less than 17 degrees C. Species response curves were developed from values of T-DO3 measured during the greatest period of oxythermal stress in late summer (maxT(DO3)). Lake trout was present in lakes with the lowest values of maxT(DO3), while cisco was present in lakes with the highest and broadest range of maxT(DO3). Projections for a scenario where climate warming (+4 degrees C in mean July air temperature) was accompanied by eutrophication (doubling of total P) indicated that coldwater fish oxythermal habitat would be devastated in a subset of lakes typical for Minnesota. Protecting deep, unproductive lakes from eutrophication will be a necessary management strategy to ensure that coldwater fish persist in at least some Minnesota lakes after climate warming.