Understanding Effects of Climate Change and Eutrophication on Fish Habitat in Glacial Lakes of the Midwest States and Management Strategies

Climate change and eutrophication are potential environmental stressors that are altering water quality conditions and then influence indigenous fish habitat in glacial lakes across the Midwest states. Several approaches were used to study the effects of climate changes with or without eutrophication on fish habitat, which was quantified using a single oxythermal habitat parameter TDO3, i.e., water temperature at DO = 3 mg/L. The first approach used deterministic lake water quality and fish habitat models to investigate the impacts of future climate change on cisco habitat in Minnesota lakes. Cisco Coregonus artedi is the most common coldwater stenothermal fish species in lakes across the Midwest states (Minnesota, Wisconsin, and Michigan). Long-term daily water temperature (T) and dissolved oxygen (DO) profiles were simulated for different types of representative lakes (the surface area from 0.05 to 50 km2) in Minnesota under the past climate conditions (1961–2008) and two projected future climate scenarios. The lake parameters required as input for the process-oriented, dynamic, and one-dimensional year-round lake model MINLAKE2012 were surface area, maximum depth, and Secchi depth (as a measure of radiation attenuation and trophic state). 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 using mean TDO3 over 31-day variable benchmark periods over 47 years (called AvgATD3VB). Isopleths of AvgATD3VB were interpolated for the four types of 82 virtual lakes on plots 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 AvgATD3VB allowed to partition the 620 lakes into the three tiers (Tier 1 and 2 refuge lakes and Tier 3 non-refuge lakes) depending on where they fell between the isopleths. About 120 (~20%) of the 620 lakes that are known to have cisco populations are projected to maintain viable cisco habitat under the projected future climate scenarios. The second approach developed a number of generalized additive models (GAMs) to relate the response variable TDO3 for coldwater fish species or the relative abundance for coolwater and warmwater fish species to predictor variables including total phosphorus or Secchi depth for productivity, relative depth (geometry ratio), mean July air temperature or mean annual air temperature for the climate. GAMs were developed for Minnesota lakes or lakes in three Midwest states to project future climate impacts using a 4 °C air temperature increase or temperature increases from various global circulation models or hindcast the climate warming from a pre-disturbance period (1896–1925) to the contemporary period (1981–2010) (over 100 years) on fish habitat. GAMs were also used to study the impact of eutrophication on fish habitat in lakes when eutrophication is linked with total phosphorus or Secchi depth that are estimated with GAMs or a regression model connecting with land use. The influences of climate and eutrophication on fish habitat or relative abundance for different species have been estimated and quantified by different methods in these studies. Management strategies were developed and implemented for some of the cisco refuge lakes. A conservation framework was developed to guide protection and restoration efforts for lakes in Minnesota by considering two major disturbance drivers (the shoreline disturbance from development and watershed disturbance from urbanization and agriculture). Various future study ideas were also proposed.