POTENTIAL EFFECTS OF CLIMATE CHANGES ON AQUATIC SYSTEMS - LAURENTIAN GREAT-LAKES AND PRECAMBRIAN SHIELD REGION

The region studied includes the Laurentian Great Lakes and a diversity of smaller glacial lakes, streams and wetlands south of permanent per mafrost and towards the southern extent of Wisconsin glaciation. We em phasize lakes and quantitative implications. The region is warmer and wetter than it has been over most of the last 12000 years. Since 1911 observed air temperatures have increased by about 0.11 degrees C per d ecade in spring and 0.06 degrees C in winter; annual precipitation has increased by about 2.1% per decade. Ice thaw phenologies since the 18 50s indicate a late winter warming of about 2.5 degrees C. In future s cenarios for a doubled CO2 climate, air temperature increases in summe r and winter and precipitation decreases (summer) in western Ontario b ut increases (winter) in western Ontario, northern Minnesota, Wisconsi n and Michigan. Such changes in climate have altered and would further alter hydrological and other physical features of lakes. Warmer clima tes, i.e. 2 x CO2 climates, would lower net basin water supplies, stre am flows and water levels owing to increased evaporation in excess of precipitation. Water levels have been responsive to drought and future scenarios for the Great Lakes simulate levels 0.2 to 2.5 m lower. Hum an adaptation to such changes is expensive. Warmer climates would decr ease the spatial extent of ice cover on the Great Lakes; small lakes, especially to the south, would no longer freeze over every year. Tempe rature simulations for stratified lakes are 1-7 degrees C warmer for s urface waters, and 6 degrees C cooler to 8 degrees C warmer for deep w aters. Thermocline depth would change (4 m shallower to 3.5 m deeper) with warmer climates alone; deepening owing to increases in light pene tration would occur with reduced input of dissolved organic carbon (DO G) from dryer catchments. Dissolved oxygen would decrease below the th ermocline. These physical changes would in turn affect the phytoplankt on, zooplankton, benthos and fishes. Annual phytoplankton production m ay increase but many complex reactions of the phytoplankton community to altered temperatures, thermocline depths, light penetrations and nu trient inputs would be expected. Zooplankton biomass would increase, b ut, again, many complex interactions are expected. Generally, the ther mal habitat for warm-, cool- and even cold-water fishes would increase in size in deep stratified lakes, but would decrease in shallow unstr atified lakes and in streams. Less dissolved oxygen below the thermocl ine of lakes would further degrade stratified lakes for cold water fis hes. Growth and production would increase for fishes that are now in t hermal environments cooler than their optimum but decrease for those t hat are at or above their optimum, provided they cannot move to a deep er or headwater thermal refuge. The zoogeographical boundary for fish species could move north by 500-600 km; invasions of warmer water fish es and extirpations of colder water fishes should increase. Aquatic ec osystems across the region do not necessarily exhibit coherent respons es to climate changes and variability, even if they are in close proxi mity. Lakes, wetlands and streams respond differently, as do fakes of different depth or productivity. Differences in hydrology and the posi tion in the hydrological flow system, in terrestrial vegetation and la nd use, in base climates and in the aquatic biota can all cause differ ent responses. Climate change effects interact strongly with effects o f other human-caused stresses such as eutrophication, acid precipitati on, toxic chemicals and the spread of exotic organisms. Aquatic ecolog ical systems in the region are sensitive to climate change and variati on. Assessments of these potential effects are in an early stage and c ontain many uncertainties in the models and properties of aquatic ecol ogical systems and of the climate system. (C) 1997 by John Wiley B Son s, Ltd.