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Many of the ecological impacts on freshwater systems depend on hydrological responses to climatic change, which are relatively uncertain given the current status of GCMs. The problem is one of scale: Many of the most significant impacts on freshwater ecosytems will result from hydrological changes at the scales of small catchments and drainage basins (which are unresolved by GCMs) and changes in hydrological variability rather than mean conditions, which is poorly taken into account by climate models.
Although the biodiversity of freshwater species may increase in a warmer climate-particularly in middle and high latitudes-there may be an initial reduction in species diversity in cool temperate and boreal regions (essentially Fennoscandia and northern Russia) if the northward migration of warm-water species cannot keep pace with the rate of loss of cool-water species because of physiological limitations, lack of north-south migration corridors, and limitations to genetic adaptation.
It is likely that the impacts of climate change will be more pronounced in the littoral zones of lakes than in the pelagic zones. Because aquatic macrophytes have access to the nutrients contained in sediments, they will be able to exploit higher temperatures; emergent vegetation is likely to benefit from higher CO2 concentrations (Kankaala et al., 1996).
Water levels in lakes and reservoirs are highly sensitive to weather conditions; in some regions of Europe, small lakes and reservoirs may fluctuate rapidly in response to changes in precipitation and evapotranspiration. Where water levels are likely to decline, inshore areas will change significantly. In shallow lakes and reservoirs in particular, inshore aquatic vegetation and surrounding wetlands would decrease in area. This decrease may result in changed habitats for aquatic biota, reduction of productivity, and even extinction of fish and invertebrate species that are dependent on these types of biomes. Where lakes have extensive bordering wetlands, declining water levels would reduce productivity and impact negatively on populations of fish and invertebrates that are dependent on these types of wetlands for their survival. Even if lake levels remain constant, decreasing throughflow may change the water quality of lakes.
Decreasing lake volumes and areas also would result in increased loading of nutrients (nitrogen, phosphorus, and others) from catchments per unit lake area or volume. Correspondingly, increased eutrophication can be expected-with high production of aquatic biomass, decreasing species diversity, deterioration of oxygen conditions, and adverse effects on water quality. In regions where most drinking water comes from surface sources, decreasing lake volumes and lower water quality may cause serious problems for human use.
In areas where increased snowfall and rainfall is expected, loading by acidifying pollutants from the atmosphere may increase. This effect would be critical for lakes located on bedrock with low neutralizing capacity (as in parts of the Alps, the Pyrenees, the Carpathians, and Fennoscandia). With further acidification of lakes, reductions in species and trophic-level diversity might occur. In the past decade, however, emission reduction measures in Europe have resulted in a decrease in atmospheric sulfur deposition of up to 50%; nitrogen deposition levels have remained unchanged. As a result, sulfate concentrations in freshwaters are decreasing at most European sites, and alkalinity values are increasing (particularly in the 1990s). Nitrate concentrations increased in European freshwaters during the 1980s but have remained stable or decreased in the 1990s. It has been postulated that the changes in the 1990s may reflect a climatic influence rather than changes in nitrogen deposition rates (UNECE, 1997).
In many parts of Europe, hydrological systems are heavily impacted by human activities (e.g., water use, sewage effluents, channel modification, removal of vegetation that acts as refugia for numerous plant and animal species, drainage, land-use changes, fertilization in catchment areas, soil erosion following deforestation). Such direct human interference, along with other environmental stress factors such as air pollution, is likely to exacerbate the impacts of climate change, with a consequent reduction of biodiversity in most parts of the continent.
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