European freshwater ecosystems encompass a varied assemblage of systems: lakes of various sizes and depth; streams with different hydrological characteristics; and wetlands, which by definition occupy a spatial continuum between aquatic and terrestrial environments. Wetlands are heterogeneous systems ranging from open-water surfaces to densely vegetated areas. Some wetlands are forested; shrubs, grasses, or mosses dominate others. European freshwater systems have been heavily subjected to and modified by damming, channeling, drainage, and other hydrological alterations; they also are influenced by humans through land and water use, pollution, erosion, and other factors.
The fundamental requirement for the existence of freshwater ecosystems is the spatial and temporal distribution of water in the landscape. The impacts of climate change on the future distribution and extent of these systems are analogous to those discussed in Section 220.127.116.11. However, apart from these impacts, changes in climatic parameters also will impact a range of chemical and biological functions, which combine with physical parameters to create the integrated ecological characteristics of future freshwater ecosystems in Europe. During the past 150 years, the winter ice cover of streams and lakes has declined, and in the northern hemisphere there has been a steady trend toward later freeze and earlier ice breakup (Magnuson et al., 2000). Climate warming is likely to exaggerate this trend, and the timing and duration of freeze and breakup of ice in freshwater systems greatly affect inherent biological and ecological processes.
In the arctic and subarctic, freshwater systems are particularly sensitive to climate change-and most climate change scenarios indicate that the highest and most rapid temperature increases will occur in these regions. Increases in temperature may lead to changes in permafrost distribution (Anisimov and Nelson, 1996, 1997), with concomitant impacts on hydrology. In many wetland areas, permafrost acts as a drainage seal and promotes wetland development. However, Camill and Clark (1998) suggest that high-latitude systems might show lagged and complex dynamics in response to global warming, and local factors may exert more direct control over permafrost than regional ones. The estimated effect of climate change on average runoff in tundra regions is highly uncertain, but if water levels decrease, connections between tundra lakes could be severed. This would result in changes in community structure and possibly elimination of seasonal migrants to shallow, ice-covered winterkill lakes. Climate change impacts on ice breakup timing and intensity also will influence limnological characteristics by regulating the supply and flux of nutrients (Lesack et al., 1991). Furthermore, such changes impact the influx of sunlight, which is a key factor in controlling primary productivity but also can have far-reaching effects on higher trophic levels. The populations of arctic char (and presumably other extreme coldwater fish species) are expected to decrease, especially in low-altitude, shallow lakes (Lehtonen, 1998).
In the boreal areas, scenarios typically display warmer winters (e.g., a shorter season of sub-zero temperatures). This would affect snow-cover conditions and lead to changes in the timing and intensity of snowmelt events and runoff characteristics, which would affect the ecological functions of freshwater systems. Wetland development might benefit if larger fractions of precipitation fell as rain, but the impact would depend on how temperature-induced higher evapotranspiration rates would counteract this effect, as well as topographical characteristics of the landscape. In response to higher temperatures, northern boreal populations of cyprinid and percid fish species are expected to increase at the expense of coldwater, salmonid species (Lehtonen, 1996). Shallow lakes would be most susceptible to these changes because of their lack of thermal stratification. Total freshwater fish production is expected to increase, but with the projected changes in the composition of fish fauna, the recreational and commercial value of catches will decrease (Lehtonen, 1996). A reduction in the spatial and temporal extent of lake and stream ice cover as a result of warmer winters can decrease light attenuation, which is a major limiting factor for production in boreal aquatic systems. Such a change could be expected to cause shifts in the biota of lakes and streams. It also can reduce winter anoxia that typically occurs in shallow lakes. Haapalea and Lepparänta (1997) modeled future ice-cover distribution in the Baltic Sea (which contains freshwater communities in the north). Simulations with a warming of 3.6°C to 2050 reduced the extent of ice cover from 38 to 10%, and a 6.6°C warming to 2100 resulted in no ice cover. The projected increase in biomass productivity in terrestrial systems also would affect lakes and streams because of alterations in the amount and quality of water and solid material inputs. Organic matter inputs are expected to increase when plant productivity increases and would be beneficial for heterotrophic organisms. Increases in organic matter concentrations also result in effects such as reduced light penetration (including damaging UV-B radiation-Schindler and Curtis, 1997) and changes in the vertical distribution of solar heating (Schindler et al., 1996). In lakes, increased summer temperatures could lead to more pronounced thermal stratification, resulting in reduced secondary productivity as well as anoxic conditions in the hypolimnion. Warmer surface water can reduce the nutritional value of edible phytoplankton, but it also may shift primary production toward green algae and cyanobacteria, which are less favored by secondary consumers.
The dominating wetland types in the boreal regions are peatlands. Typically, the vegetation pattern and composition of boreal peatlands are governed by moisture regime rather than temperature and show high spatial variability in plant communities caused by variation in topography. It follows that a change in water balance could affect the function of boreal wetlands, including their carbon sequestering and carbon storage functions. A study in Finland suggests that very nutrient-poor peatlands can increase their long-term soil carbon accumulation after drainage (Minkkinen and Laine, 1998). In more nutrient-rich peatlands, however, soil carbon sequestering rates decrease and could shift to potential sources of atmospheric CO2. Cao et al. (1998) have suggested that a temperature increase of less than 2°C could enhance methane (CH4) emission rates from boreal wetlands, but greater warming might lead to reduction of fluxes because of decreasing soil moisture. Furthermore, field manipulation and laboratory experiments in Finland have shown that enhanced CO2 concentrations (560 ppm) can lead to a 10-20% increase in CH4 efflux from oligotrophic mire lawn communities (Saarnio and Silvola, 1999; Saarnio et al., 2000). It is likely that boreal peatlands will expand further north into subarctic/arctic areas where the topography after permafrost disintegration still supports wetland formation.
In temperate Europe, the potential for precipitation decreases that result in lower flow rates could have major implications for lakes and streams. This could lead to changes in habitat and breeding locations of aquatic flora and fauna. These hydrological changes have the potential to be more significant for freshwater organisms than a temperature increase. The effect of warmer winters that lead to less extensive ice cover of lakes is expected to affect Europe's temperate lakes and streams as discussed above. Wetlands in the temperate regions of CEE are regarded as vulnerable to climate change (in combination with other anthropogenic threats-Best et al., 1993; Hartig et al., 1997). In the past, wetlands have been extensive in this area-for example, in the basins of the Pechora, Severnaya Dvina, and Upper Dnieper Rivers and in Karelia they have occupied 10-30% of the area. Now, many of them have been converted to agriculture, are affected by agricultural drainage, or are used in other ways, such as growing reed for thatch and livestock feed or collecting peat as a fuel for heating and cooking.
In the Mediterranean, the risk of acute water shortage in response to global warming would have severe impacts on freshwater ecosystems in the region. Hydrologically isolated systems, such as wetlands in topographical depressions, would be the most vulnerable, whereas those situated along larger rivers and lake shores might be less sensitive (Mortsch, 1998), although the extent of the latter may decrease as a result of lower flow rates. Increased competition for diminishing water resources also poses a potential threat to freshwater ecosystems. Although precipitation may increase during the winter-which is the main season for the seasonal wetlands in this area-this probably will be accompanied by comparably large increases in temperature, thus affecting net water availability. Summers are predicted to become warmer and drier, which would lead to deterioration of freshwater ecosystems (Haslam, 1997). Seasonal systems that presently can cope with occasional or periodic drought will experience additional stress that some species might not be able to survive (Brock and van Vierssen, 1992). The fact that wetlands in many parts of southern and central Europe are scattered in their location may prevent species migration to suitable climate conditions. In general, wetland plants with short life cycles are better adapted for geographical migration, indicating that this response is likely to occur faster in nonforested wetlands than in forested ones.
The risk of increased fire disturbance of terrestrial biota also will have consequences for lakes and streams in southern Europe. Freshwater systems adjacent to burned areas will receive an initial increase in solute input after fire; if the fire generates canopy gaps, the water bodies will be more influenced by wind mixing, inducing changes in thermal and chemical stratification characteristics.
Europe is predominantly a region of fragmented natural or semi-natural habitats in a highly urbanized, agricultural landscape. A significant proportion of surviving semi-natural habitats of high conservation value is enclosed within protected sites, which are especially important as refuges for threatened species (Plowman, 1995). Nature reserves form a similarly important conservation investment across the whole of Europe. However, species distributions are projected to change in response to climate change (Huntley and Webb, 1989), and valued communities within reserves may disassociate, leaving species with nowhere to go (Peters and Darling, 1985; Peters and Lovejoy, 1992),
The impact of climate change on a particular reserve will depend on its location in relation to the climatic requirements of the species it accommodates. Sites that lie near the current maximum temperature limits of particular species could expect that if climate warms beyond those limits, species would become extinct at that site. Conversely, sites that lie close to the minimum temperature limits of species may assume greater importance for such species as the climate warms (Huntley, 1999). In Europe, nature reserves tend to form habitat "islands" for species in landscapes that are dominated by other land uses. The possibility of species colonizing other habitat islands could be limited. As a result of climate change, reserve communities may lose species at a faster rate than potential new species can colonize, leading to a long period of impoverishment for many reserves.
The requirements of a future conservation strategy in the advent of climate change have been considered by Huntley et al. (1997). They suggest that for Europe, where large-scale range changes are projected, a network of habitats and habitat corridors will be required to facilitate migration.
Questions that urgently must be asked are as follows: To what extent do rare and vulnerable species in Europe rely on protected areas for their survival in the present day? Do current policy measures being implemented throughout Europe under the Biodiversity Convention take into account the potential impacts of climate change? It will become increasingly important for conservation strategies to be developed on a pan-European scale to protect species in parts of their ranges that are least likely to be negatively impacted by climate change. Reevaluation of conservation priorities and the role of reserves is required for individual sites and in relation to national and international conservation strategies (Hendry and Grime, 1990; Parsons, 1991).
Insects: Parmesan et al. (1999) analyzed data for 35 nonmigratory butterflies with northern range limits in Great Britain, Sweden, Finland, or Estonia and southern boundaries in southeastern France, Catalonia (Spain), Algeria, Tunisia, or Morocco. More than 60% were found to have shifted north by 35-240 km in the 20th century, consistent with the 120-km northward shift of climatic isotherms reported in the SAR. This finding is contrary to the trend that might have been expected as a result of land-use change: Habitat loss has been greater in northern European countries over this period than southern ones. Scientific knowledge of butterfly biology supports the inference that this shift is in response to increased temperatures. Population eruptions of several species of forest lepidoptera in central Europe in the early 1990s, including the gypsy moth Lymantria dispar, have been linked to increased temperatures (Wulf and Graser, 1996), as have northward range expansions of several species of Odonata and Orthoptera (Kleukers et al., 1996).
Insect pests: Most studies concur that insect pests are likely to become more abundant in Europe as temperature rises, as a result of increased rates of population development, growth, migration, and overwintering (Cannon, 1998). There has been little or no reported research at the level of pest population dynamics, however, about potential responses of insect pests to increased CO2 (Cannon, 1998). Although changes in rainfall also could have a substantial effect (Lawton, 1995), this is difficult to quantify, particularly given uncertainties with regional precipitation scenarios. Migratory species may be able to extend their ranges as crop distributions change. For example, a 3°C rise in temperature would advance the limit for grain maize across much of Europe, which could be followed by a northward range expansion by the European corn borer Ostrinia nubilalis of as much as 1,220 km (Parry et al., 1990; Porter et al., 1991; Porter, 1995).
Birds: Climate change in Europe already has been demonstrated to be affecting migratory wild bird populations. In the UK, 20 of 65 species, including long-distance migrants, significantly advanced their egg-laying dates by 8 days, on average, between 1971 and 1995 (Crick et al., 1997; Crick and Sparks, 1999). In general, species show advancement in average arrival and laying dates of about 3-5 days per 1°C. It is quite likely that birds will be able to adapt faster than most taxa to such changes, given their mobility and genetic variability (e.g., Berthold and Helbig, 1992). However, increased aridity in the Mediterranean region may be detrimental to the trans-Saharan migrants that use the area for foraging en route. Potentially great problems face the internationally important populations of waterfowl that use a relatively limited number of sites for wintering or while on passage in Europe. Where sea-level rise causes coastal squeeze on the availability of intertidal feeding areas (because sea defenses prohibit encroachment onto currently dry land), feeding resources available to wintering waterbirds may become limited and lead to population declines (Norris and Buisson, 1994). Arctic-breeding shorebirds are predicted to benefit in the short term as warmer temperatures increase the numbers of their insect food supplies, but in the long term they may suffer from the disappearance of their habitat as vegetation zones move northward toward the limit of any land (Lindström and Agrell, 1999).
Other reports in this collection