Commercial and recreational freshwater fisheries are important to the economy of many regions, as well as the well-being of native populations. In many aquatic ecosystems, freshwater fish also are important in maintaining a balance in other aquatic populations lower in the food web (via predatory and other effects). In broader terms, aquatic ecosystems are important as recreational areas, as sources of water for domestic and industrial use, and as habitat for a rich assemblage of species, including some that are threatened or endangered.
Several studies have indicated that projected climate change will have important impacts on North American freshwater fisheries and aquatic ecosystems. It must be noted, however, that most studies to date have used results from earlier climate model simulations that gave air temperature increases under a 2xCO2 climate that were as much as twice as large for the same time period as more recent estimates that include aerosol forcing-thus overestimating the effects of temperature increases, particularly in the summer.
Changes in survival, reproductive capacity, and growth of freshwater fish and the organisms and habitats on which they depend result from changes in water temperature, mixing regimes, and water quality.
In North America, freshwater fish have been grouped into three broad thermal groups (cold-water, cool-water, and warm-water guilds) based on differences in the temperature optima of physiological and behavioral processes. In simulations of deep, thermally stratified lakes in the mid- and high latitudes, including the Laurentian Great Lakes, winter survival, growth rates, and thermal habitat generally increase for fish in all three thermal guilds under the 2xCO2 climate (DeStasio et al., 1996; IPCC 1996, WG II, Sections 10.6.1.2 and 10.6.3.2; Magnuson and DeStasio, 1996). However, in smaller mid-latitude lakes, particularly those that do not stratify or are more eutrophic, warming may reduce habitat for many cool-water and cold-water fish because deep-water thermal refuges are not present or become unavailable as a consequence of declines in dissolved oxygen concentrations (IPCC 1996, WG II, Section 10.5.4). For example, Stefan et al. (1996) examined the effect of temperature and dissolved oxygen changes in lakes in Minnesota; they projected that under a 2xCO2 climate (from a GISS GCM that projected a 3.8�C air temperature increase in northern Minnesota), cold-water fish species would be eliminated from lakes in southern Minnesota, and cold-water habitat would decline by 40% in lakes in northern Minnesota.
Changes in the productivity and species composition of food resources also may accompany climatic warming and, in turn, influence fish productivity. Production rates of plankton and benthic invertebrates increase logarithmically with temperature; rates increase generally by a factor of 2-4 with each 10�C increase in water temperature, up to 30�C or more for many organisms (Regier et al., 1990; Benke, 1993; IPCC 1996, WG II, Section 10.6.1.1). Although this effect generally should increase fish productivity, shifts in species composition of fish prey with warming might prevent or reduce productivity gains. Biogeographic distributions of aquatic insects are centered around species thermal optima, and climate warming may alter species composition by shifting these thermal optima northward by about 160 km per 1�C increase in temperature (Sweeney et al., 1992; IPCC 1996, WG II, Section 10.6.3.1). If species range shifts lag changes in thermal regimes because of poor dispersal abilities or a lack of north-south migration routes (e.g., rivers draining northward or southward) or if species adaptation is hindered by limited genetic variability, climatic warming might result initially in reductions in the preferred prey organisms of some fish (IPCC 1996, WG II, Section 10.6.3.3).
Climatic warming may result in substantial changes in the thermal regimes and mixing properties of many mid- and high-latitude lakes. In the mid-latitudes, some lakes that presently are dimictic (mixing in spring and autumn) may no longer develop winter ice cover and may become monomictic (mixing during fall, winter, and spring), with a longer summer stratification period. At high latitudes, some lakes that presently are monomictic and mix during summer may stratify in summer and mix twice a year, in autumn and spring (IPCC 1996, WG II, Section 10.5.4). Changes in lake mixing properties may have large effects on hypolimnetic dissolved oxygen concentrations (affecting available fish habitat) and on epilimnetic primary productivity, although these effects are likely to depend greatly on the morphometric characteristics of individual lakes and are difficult to predict (IPCC 1996, WG II, Section 10.5.4). For example, longer summer stratification and higher water temperature result in more severe hypolimnetic oxygen depletion in lakes in Minnesota under a 2xCO2 climate simulation (Stefan et al., 1993). In other lakes, reduction in the duration or lack of winter ice cover might reduce the likelihood of winter anoxia (IPCC 1996, WG II, Section 10.6.1.4). At high latitudes, development of summer stratification under a warmer climate might increase lake primary productivity by maintaining algae for longer periods within the euphotic zone. Climate changes that result in decline in runoff also may have substantial effects on the mixing properties of smaller lakes that are heavily influenced by fluxes of chemicals from their catchments. For example, the surface mixed layer of boreal lakes at the Experimental Lakes Area in northWest Ontario has deepened over the past 20 years as a result of a long-term drought that reduced inputs of DOC from the catchment and thus increased water clarity (IPCC 1996, WG II, Section 10.5.3 and Box 10-2; Schindler et al., 1996).
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