Projected Impacts of Climatic Changes on Cisco Oxythermal Habitat in Minnesota Lakes and Management Strategies

Abstract

Water quality and fish habitat models were developed and applied to investigate impacts of future climate change on cisco oxythermal habitat in Minnesota lakes. Long-term daily water temperature (T) and dissolved oxygen (DO) profiles were simulated for different types of representative lakes (surface area from 0.05 to 50 km²) in Minnesota under the past climate conditions (1961–2008) and projected future climate scenarios. A process-oriented, dynamic, and one-dimensional year-round lake water quality model was developed and applied for the temperature and DO simulations, which were run in daily time steps over a 48-year simulation period. The lake parameters required as model input were surface area (A _s ), maximum depth (H _max), and Secchi depth (as a measure of radiation attenuation and trophic state). Weather records from eight stations in Minnesota and North Dakota were used for model simulations. Two projected future climate scenarios were based on the output of the third-generation Canadian Centre for Climate Modeling and Analysis coupled general circulation model (CCCma CGCM 3.0) and the Model for Interdisciplinary Research on Climate (MIROC 3.2). The climate scenarios lead to a longer period of hypoxic hypolimnetic conditions in stratified lakes that will result in various negative environmental and ecological impacts in lakes. The study has identified potential refuge lakes important for sustaining cisco habitat under climate warming scenarios. Cisco Coregonus artedi is the most common cold-water stenothermal fish species in lakes over the several northern states in the USA such as Minnesota. To project its chances of survival under future warmer climate conditions, using simulated daily T and DO profiles in 44 representative and 30 virtual lake types, three oxythermal habitat modeling options were used: (1) constant lethal T and DO limits, (2) lethal-niche-boundary curve, and (3) an oxythermal habitat variable, TDO3, i.e., water temperature at DO = 3 mg/L. The fish habitat models using constant and variable lethal limits were validated in the 23 Minnesota lakes of which 18 had cisco mortality while five had no cisco mortality in the unusually warm summer of 2006. Cisco lethal and habitable conditions in the 23 lakes simulated by the models had overall good agreement with observations in 2006. Cisco lethal days were simulated in the 44 representative lake types. Polymictic shallow lakes with lake geometry ratio A _s ^0.25 /H _max > 5.2 m^−0.5 (A _s in m² and H _max in m) were simulated to typically not support cisco oxythermal habitat under past climate conditions and the future climate scenario (MIROC 3.2). Medium-depth lakes are projected to be most vulnerable to climate warming with most increase in the number of years with cisco kill. The mean daily TDO3 values over a 31-day fixed and variable benchmark periods were calculated for each of simulated years and then averaged over the simulation period for each lake type. Projected increases of the multiyear average TDO3 (called AvgATD3) under the two future climate scenarios and relative to the 47-year simulation period from 1962 to 2008 had averages from 2.6 °C to 3.4 °C. Isopleths of AvgATD3 were interpolated for the 30 simulated virtual lakes on a plot of Secchi depth versus lake geometry ratio used as indicators of trophic state and summer mixing conditions, respectively. Marking the 620 Minnesota lakes with identified cisco populations on the plot of AvgATD3 allowed to partition the 620 lakes into three tiers depending on where they fell between the isopleths: lakes with AvgATD3 ≤ 11 °C (tier 1 lakes) were selected to be most suitable for cisco; lakes with 11 °C < AvgATD3 ≤ 17 °C (tier 2 lakes) had suitable habitat for cisco; and non-refuge lakes with AvgATD3 > 17 °C (tier 3 lakes) would support cisco only at a reduced probability of occurrence or not at all. About 208 (one third) and 160 (one fourth) of the 620 lakes that are known to have cisco populations are projected to maintain viable cisco habitat under the two projected future climate scenarios using the fixed and variable benchmark periods, respectively. These selective lakes have a Secchi depth greater than 2.3 m (mesotrophic and oligotrophic lakes) and are seasonally stratified (geometry ratio less than 2.7 m^−0.5). Management strategies were developed and implemented for some of the refuge lakes.