15.2.2.2 Freshwater systems

Arctic

Climate variability/change has historically had, and will continue to have, impacts on Arctic freshwater resources. First-order impacts (e.g., changes to the snow/ice/water budget) play a significant role in important global climate processes, through feedbacks (e.g., changes to radiative feedbacks, stability of the oceanic stratification and thermohaline circulation, and carbon/methane source-sink status). Cascading effects have important consequences for the vulnerability of freshwater systems, as measured by their ecological or human resource value.

From an ecological perspective, the degree of vulnerability to many higher-order impacts (e.g., changes in aquatic geochemistry, habitat availability/quality, biodiversity) are related to gradual and/or abrupt threshold transitions such as those associated with water-phase changes (e.g., loss of freshwater ice cover) or coupled bio-chemical responses (e.g., precipitous declines in dissolved oxygen related to lake productivity) (Wrona et al., 2005). Historically, Arctic freshwater ecosystems have adapted to large variations in climate over long transitional periods (e.g., Ruhland and Smol, 2002; Ruhland et al., 2003), but in the next 100 years the combination of high-magnitude events and rapid rates of change will probably exceed the ability of the biota and their associated ecosystems to adapt (Wrona et al., 2006a). This will result in significant changes and both positive and negative impacts. It is projected, however, that overall the negative effects will very probably outweigh the positive, implying that freshwater systems are vulnerable to climate change (Wrona et al., 2005).

From a human-use perspective, potential adaptation measures are extremely diverse, ranging from measures to facilitate modified use of the resource (e.g., changes in ice-road construction practices, increased open-water transportation, flow regulation for hydroelectric production, harvesting strategies, and methods of drinking-water access), to adaptation strategies to deal with increased/decreased freshwater hazards (e.g., protective structures to reduce flood risks or increase floods for aquatic systems (Prowse and Beltaos, 2002); changes to more land-based travel to avoid increasingly hazardous ice). Difficulties in pursuing adaptation strategies may be greatest for those who place strong cultural and social importance on traditional uses of freshwater resources (McBean et al., 2005; Nuttall et al., 2005).

Antarctic

Antarctic freshwater systems are fewer and smaller than those in the Arctic, but are no less vulnerable to climate change. The microbial communities inhabiting these systems are likely to be modified by changing nutrient regimes, contaminants and introductions of species better able to cope with the changing conditions. A drop in air temperature of 0.7°C per decade late in the 20th century in the Dry Valleys led to a 6 to 9% drop in primary production in the lakes of the area (Doran et al., 2002). In marked contrast, summer air temperature on the maritime sub-Antarctic Signy Island increased by 1°C over the last 50 years and, over the period 1980 to 1995, water temperature in the lakes rose several times faster than the air temperature; this is one of the fastest responses to regional climate change in the Southern Hemisphere yet documented (Quayle et al., 2002). As a consequence, the annual ice-free period has lengthened by up to 4 weeks. In addition, the area of perennial snow cover on Signy Island has decreased by about 45% since 1951, and the associated change in microbial and geochemical processes has led to increased amounts of organic and inorganic nutrients entering the lakes. There has also been an explosion in the population of fur seals (Arctocephalus gazella) on the island due to decreased ice cover and increased area available for resting and moulting. Together these changes are leading to disruption of the ecosystem due to increased concentrations of nutrients (eutrophication) (Quayle et al., 2003). Similar ecological vulnerabilities are expected to exist in other Antarctic freshwater systems.