A lo-yr record of the thermal characteristics of four lakes at the North-Temperate Lakes Long-Term Ecological Research site was analyzed and used to validate simulations of lake physics with the dynamic reservoir simulation model. Simulations of cool, warm, and intermediate years were rerun with meteorology from four general circulation models with a doubling of CO,. In all simulations with doubled COZ there is an earlier onset of stratification, increased summer epilimnetic temperature (l-7"C), and an increased intensity and longer duration of stratification. Maximum surface temperatures at times may exceed upper lethal limits of warm and cool water fish in some scenarios. Suitable thermal habitat for cold water, cool water, and warm water fish generally increases in all scenarios after climate change.Changes in the vertical migration of Daphnia, however, are expected to vary depending on the interaction of thermal stratification and fish habitat use. In northern Wisconsin lakes with cold water planktivores, habitat overlap between fish and zooplankton is expected to decrease, while in southern Wisconsin lakes habitat overlap is expected to increase. Although most physical responses of lakes to climate change are consistent among all climate scenarios, biological responses will likely be more variable owing to the complex nature of factors determining ecological interactions in lakes.

We estimated potential changes in the size of thermal habitat of representative cold-, cool-, and warmwater fish for southern Lake Michigan and the central basin of Lake Erie before and after simulated global climate warming. Observed midlake thermal structures were modeled (BASE) and then manipulated with three general circulation climate models (OSU, GISS, GFDL) that projected warmer climates when atmospheric carbon dioxide concentrations were doubled. Under BASE conditions, on an annual basis, lake trout Salve/inns natnaycush had the largest thermal habitat in southern Lake Michigan, coho salmon Oncorhynchus kisutch and yellow perch Perca flavescens had smaller thermal habitats, and largemouth bass Micropterus salmoides had none. Even for lake trout, the suitable thermal habitat was only 5-20% of the upper 200 m through the year. With rare exceptions, thermal habitat increased for species in all thermal guilds for all climate-warming scenarios. No thermal habitat was estimated for coldwater fish in the central basin of Lake Erie because the hypolimnion becomes anoxic in summer. The median increase in thermal habitat was 2.5 x BASE condition both for southern Lake Michigan and the central basin of Lake Erie. After climate warming, minimum thermal habitat during summer decreased for walleye Stizostedion vitreum. increased for yellow perch, and remained the same for lake whitefish Coregonus clupeaformis and lake trout. Fish yields estimated from published empirical models that relate thermal habitat in summer to maximum sustained yields remained about the same for lake trout and lake whitefish but increased for walleye. The general conclusions were that the sizes of the habitat favorable for cold-, cool-, and warmwater fish would increase in Lake Michigan, whereas the habitats favorable only for cool-, and warmwater fish would increase in Lake Erie.

We used fish bioenergetics models to assess the effect of global climate warming on the growth and prey consumption of warm-, cool-, and coldwater fishes at three sites spanning the range of thermal environments in the Great Lakes. Historical air and water temperature data and projected air temperature changes from three global climate models were used as input to regression models, which generated projections of water temperature changes before and after climate warming that would result from a doubling in atmospheric CO 2 concentration. The bioenergetics simulations indicated that annual growth by yearling fish would increase with climate warming if prey consumption increased, but would decrease if prey consumption was constant. Changes in growth would be most pronounced in spring and autumn owing to a lengthening of the period during which fishes may behaviorally thermoregulate to find their optimal temperature for growth. Fish unable to thermoregulate (e.g., due to hypolimnetic oxygen depletion) would undergo decreased growth or weight loss in summer in warmer areas of the Great Lakes, where near-surface water temperatures would increase above the fishes* optimum. We conclude that food web dynamics and the potential for thermoregulation will greatly influence the direction and magnitude of changes in fish growth as the climate warms.

No abstract

No abstract