Conference Report on Water Ecology Workshop – Morning Talks

Peter Sousounis, University of Michigan and Brent Lofgren, Great Lakes Environmental Research Lab (GLERL), NOAA

Evaluating changes in synoptic patterns is tantamount to understanding regional climate change. To date, the synoptic evaluations that have been done regarding climate change output from General Circulation Models have been restricted mainly to examining changes in storm tracks across large areas (e.g., the Atlantic Ocean). In this presentation, we looked at output from the Canadian Coupled Climate Model (CGCM1) and the Hadley Coupled Climate Model (HadCM2), which were used as part of the U.S. National Assessment of Climate Change. We examined potential changes, relative to present conditions, in synoptic patterns, as well as changes in temperature, precipitation, and lake levels over the Great Lakes region toward the end of the 21st century.

Both models show a decrease in the number of extremely cold days, an increase in the number of extremely hot days, and an increase in precipitation for the—future—particularly for heavy precipitation (e.g., ›12.5 mm) events. Both models show a decrease in surface windspeed and an increase in the number of days with an easterly wind component. Both models exhibit decreases in cyclone numbers for the future. The Canadian Model shows a general decrease in the number of moderately strong cyclones and decreases in each month. The Hadley Model shows a slight increase in the number of strong cyclones but a greater decrease in the number of weak—cyclones—especially during the spring. The Canadian Model exhibits significant decreases in the number of anticyclones in summer and significant increases in fall, but the model does not exhibit any systematic changes in terms of intensity. The Hadley Model shows a slight increase in the number of weak anticyclones but a greater decrease in the number of strong anticyclones. Most of the decreases occur during the—summer—so that the seasonal distribution is more uniform.

In addition, the net effects on lake levels of the Great Lakes because of future changes in temperature and precipitation as simulated by the Canadian Model and the Hadley Model are quite different. The CGCM1 yields a drop in the level of Lakes Michigan and Huron of 0.72 m (2.4 feet) by 2030 and 1.38 m (4.5 feet) by 2090. On the other hand, using the results of the HadCM2, the same lakes rise by 0.05 m (0.2 feet) by 2030 and 0.35 m (1.1 feet) by 2090.

All of the changes are consistent with changes in the general large-scale flow patterns. An understanding of all these synoptic changes provides richness and a more conceptual understanding of how climate change may affect the Great Lakes region.

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Forecasting future conditions of the Great Lakes in response to climate change puts scientists in an uncomfortable role. The most confident prediction is that ecological surprises will emerge. The living communities of our lakes have intimate and complex two-way linkages to the physical and chemical world, and many of the linkages still await discovery.

The challenges of projecting future ecological conditions arise mainly from our incomplete understanding of the present state and the ways that biota can respond. Different visions of future climate generated by alternative climate models lead us to anticipate fundamental changes in the physical environment of the lakes. Water temperatures will be higher, the lakes will not mix deeply for as long as they do now, and more ultraviolet light will strike the water surfaces. We understand that deep mixing resets important elements of the biological and chemical clockwork of the lakes, and that temperature changes the speed of these clocks. To date, the direct responses of organisms to climate variables have received most or all of scientistsÕ attention. We need to remain alert to the far-reaching consequences of ecological complexity. We do not yet know enough, for example, to project how temperature, mixing, UV light, and biological processes will interact to affect toxic metals like mercury that become concentrated up a food chain. We do not yet know exactly which new species will establish themselves in the lakes and which existing species will be eliminated, or what new or invigorated parasites will emerge. Our ecological knowledge does warn us about the types of surprises that will occur. But current theory is no substitute for a strong program of observation and interpretation of Great Lakes ecology as an insurance policy and early warning system for future environmental problems.

Projected average temperature of lake bottom at average lake depth under HadCM2

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Projected maximum temperature of the mixed layer under HadCM2

John Magnuson, University of Wisconsin-Madison

All aspects of a fish´s life such as survival, growth, and habitat choice are dependent on water and water temperature, both of which are directly affected by climate warming. Warming alters the amount of thermal habitat suitable for coldwater fishes (trout), coolwater species (perch and walleye), and warmwater fishes (bass and bluegill). In streams, ponds, and shallow lakes in the Great Lakes region, warming scenarios reduce habitat for coldwater and coolwater fishes but increase habitat for warmwater species. In deeper lakes that thermally stratify in summer, such as Lake Mendota and Lake Michigan, warming increases the amount of thermal habitat for all three thermal groups of fishes. However, coldwater habitat suitable for coldwater fishes is degraded by loss of oxygen in deeper water; this loss is severe in the shallower or more productive lakes such as Mendota and could be severe in the larger lakes such as Lake Michigan if thermal stratification became more permanent. Warming also is expected to increase the invasion of warmwater fishes into the Great Lakes and the streams and inland lakes of the region. Invasions of warmer water fishes would move progressively northward, and extirpations of coldwater and coolwater fishes in the streams and inland lakes would become progressively more common initially in the southern part of the region. The invasions would result in species interactions that can accelerate the rate of extirpations. For example, in Ontario´s inland lakes, the arrival of warmwater basses (usually by stocking) results in the loss of minnow species. Fishes are an excellent indicator of the expected changes from global warming because they are sensitive to water temperature and interactions with the northward migration of fishes; they also are highly valued by people.

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The African Great Lakes are similar in size to the Laurentian Great Lakes, but they experience a much warmer climate. Thus, they can provide clues as to what changes might be expected in the Laurentian Great Lakes if the regional climate warms. Two notable aspects in which tropical large lakes differ from temperate large lakes are hydrology and lake circulations, which in turn affect nutrient cycles, algal production, and fish production.

Although the African Great Lakes experience annual rainfall similar to that of the Laurentian Great Lakes, greater evaporation rates under warm conditions result in reduced outflow. This reduction in outflow has implications for lake levels, contaminant retention, and hydroelectricity generation. Deep tropical lakes also tend to be permanently stratified. Observations in the African Great Lakes suggest that permanent stratification of the Laurentian Great Lakes would result in lower deep-water dissolved oxygen concentrations, large changes in the cycling of nitrogen and phosphorus, and greater inter-annual variability of plankton and fish production.

Lake Michigan Lake Level (m asl) compared with Water Level Lake Malawi (m asl) (see H. Bootsma presentation for full image)