Intensification in direct uses of water will negatively influence "natural" waters and could seriously reduce services provided by these ecosystems. Some values and services of lakes and rivers would be degraded by human responses to their greater demands for water for direct human use through dam construction, dyke and levee construction, water diversions, and wetland drainage (Postel and Carpenter, 1997). Such activities can alter water temperatures, recreation, pollution dilution, hydropower, transportation, and the timing and quantity of river flows; lower water levels; destroy the hydrologic connection between the river and the river floodplain; reduce natural flood control, nutrient and sediment transport, and delta replenishment; eliminate key components of aquatic environments; and block fish migration. At risk are aquatic habitat, biodiversity habitat, sport and commercial fisheries, waterfowl, natural water filtration, natural floodplain fertility, natural flood control, and maintenance of deltas and their economies (Ewel, 1997).
Water-level increases as well as decreases can have negative influences on lakes and streams. A recent example is from Lake Baikal, where the combined influences of a dam on the outlet and increased precipitation has increased water level by 1.5 m and decreased biodiversity and fishery production (Izrael et al., 1997; ICRF, 1998).
Figure 5-7: Simulated changes in thermal habitat for fish in the continental United States. Left panel is modified from Keleher and Rahel (1996). Center panel is modified from Mohseni and Stefan (2000) and Fang et al.(1998). Right panel is modified from McCauley and Beitinger (1992). Simulation is for a 2xCO2 climate from the Canadian Climate Model and represents air temperature increases of 3-6.5ºC in different parts of the United States. Coldwater fish include trout and salmon; coolwater fish include yellow perch, walleye, northern pike, and white sucker; and warmwater fish include sunfish (black basses, bluegill, pumpkinseed) and common carp.
Potential effects of climate change on fisheries are documented in the SAR by Everett et al. (1996); freshwater fisheries in small rivers and lakes are considered to be more sensitive to changes in temperature and precipitation than those in large lakes and rivers. Changes in temperature and precipitation resulting from climate change are likely to have direct impacts on freshwater fisheries through changes in abundance, distribution, and species composition. Because current exploitation rates tend to be high or excessive, any impact that concentrates fish will increase their catchability and further stress the population.
Assessing the potential impacts of climate change on aquaculture is uncertain, in part because the aquaculture industry is mobile and in a period of rapid expansion. There is no question that large abundances relative to traditional wild harvests can be produced in small areas and that in many cases these sites can be moved to more favorable locations. Changes in groundwater may be especially significant for aquaculture. In tropical areas, crustaceans are cultured in ponds (Thia-Eng and Paw, 1989); fish frequently are cultured in cages. Commonly cultured species such as carp and tilapia may grow faster at elevated temperatures, but more food is required and there is an increased risk of disease. Temperature is a key factor affecting growth, but other factors relating to water quality and food availability can be important. Modeling studies suggest that for every 1°C average increase, the rate of growth of channel catfish would increase by about 7%, and the most favorable areas for culture would move 240 km northward in North America (McCauley and Beitinger, 1992; see Figure 5-7, right panel). The southern boundary also would move northward as surface water temperatures increase, perhaps exceeding lethal ranges on occasion. Growth would increase from about 13 to 30°C and then fall off rapidly as the upper lethal limit of about 35°C is reached (McCauley and Beitinger, 1992). Thus, aquaculture for traditional species at a specific location may have to switch to warmer water species.
Warmer conditions are more suitable for warmer loving flora and fauna and less suitable for cold-loving flora and fauna. Warmer temperatures, however, lead to higher metabolic rates, and if productivity of prey species does not increase, reductions in growth would occur at warmer temperatures (Arnell et al., 1996; Magnuson et al., 1997; Rouse et al., 1997). Rates of natural dispersal across land barriers of less mobile species poleward or to higher altitudes are not likely to keep up with rates of change in freshwater habitats. Species most affected would include fish and mollusks; in contrast, almost all aquatic insects have an aerial life history stage, thus are less likely to be restricted. Some streams have a limited extent and would facilitate only limited poleward or altitudinal dispersal. This could be especially problematic as impoundments restricting movements of organisms increase in number. Coldwater species and many coolwater species would be expected to be extirpated or go extinct in reaches where temperatures are at the warmer limits of a species range. Many lakes do not have surface water connections to adjacent waters, especially in headwater regions where interlake movement would be limited without human transport of organisms across watershed boundaries.
Exotics will become a more serious problem for lake and stream ecosystems with warming. In the northern hemisphere, for example, range extensions occur along the northern boundaries of species ranges and extinctions occur along the southern boundaries, in natural waters and in aquaculture operations (Arnell et al., 1996; Magnuson et al., 1997). In addition, loss of habitat for biodiversity will result from warmer, drier conditions interacting with increases in impoundment construction.
Distributions of fish are simulated to move poleward across North America and northern Europe. In northern Europe, Lehtonen (1996) forecasts a shrinking range for 11 coldwater species and an expanding range for 16 cool- and warmwater fish. In simulations for Finland by Lappalainen and Lehtonen (1995), lake whitefish lost habitats progressively toward the north, whereas brown trout did better, at least in the north. Coolwater fish were forecast to spread northward through the country. In simulation studies, boundaries of individual warmwater species ranges in were projected to move northward by 400-500 km in Ontario, Canada (Minns and Moore, 1995), and southern boundaries of coldwater fishes were projected to move 500-600 km northward in the southeastern United States (estimated from Fang et al., 1998). In simulations based on an elevated CO2 scenario using the Canadian Climate Centre model, warmwater fish were projected to benefit in shallow eutrophic and mesotrophic lakes around the United States, owing to reduction in winterkill, but habitable lakes and streams for coolwater fish and especially coldwater fish were projected to decline, owing to summer kill (Fang et al., 1998; Mohseni and Stefan, 2000). Habitat changes from various studies over large regions (see Figure 5-7, center panel) projected 0-43% reductions for coldwater species, 50% reductions to 12% increases for coolwater species, and 14% reductions to 31% increases for warmwater fish. Changes differ among ecosystem types and areas, depending on latitude and altitude.
A dramatic picture of the regional decline in trout habitat in the Rocky Mountain region of the western United States is provided by Keleher and Rahel (1996) (see Figure 5-7, left panel). Even a 1°C increase in mean July air temperatures is simulated to decrease the length of streams inhabitable by salmonid fish by 8%; a 2°C increase causes a reduction of 14%, a 3°C increase causes a 21% decline, a 4°C increase causes a 31% reduction, and a 5°C increase causes a 43% reduction. There also is likely to be a increased fragmentation of inhabitable areas for the North Platte River Drainage in Wyoming (Rahel et al., 1996).
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