Arctic freshwater environments
This is Section 10.2.2 of the Arctic Climate Impact Assessment
Lead Author: Michael B. Usher; Contributing Authors:Terry V. Callaghan, Grant Gilchrist, Bill Heal, Glenn P. Juday, Harald Loeng, Magdalena A. K. Muir, Pål Prestrud
The Arctic has many types of freshwater habitat. There is a wide range of wetlands, including mires, marshes, sedge, and reed beds, floodplain “grasslands”, salt marshes, and coastal lagoons, as well as a large number of rivers, streams, and lakes. In fact, excluding the freshwater locked up in permanent ice in the Antarctic, a large proportion of the earth’s liquid freshwater resources occur in the Arctic.
There is no universally accepted definition of a “wetland”. Wetland has been described as “areas where the water table lies near the surface for much of the year. Shallow water bodies can also be considered as wetlands if they are mainly covered by vegetation. In wetlands at least half of all of the plants should be hydrophytes, which can withstand or may even depend on high water levels.”. With such a loose definition, there can be many gradients from a wetland to some other sort of habitat. For example, as wetlands border onto colder areas, permafrost could become common, whereas near the coast the influence of sea ice will be greater, and toward the taiga there will be an assortment of wet woodland habitats.
Lakes and rivers are abundant in the Arctic. Norway is estimated to have in excess of 200,000 lakes with a surface area greater than 0.01 km2 but less than 1 km2, and 2457 lakes larger than this. Sweden is estimated to have 2908 rivers and the Republic of Karelia 1210 rivers. The 18 largest lakes in Europe are all in northern Europe, although some are located outside the Arctic (located between 60º and 66º N). Such statistics demonstrate the extent of the liquid freshwater resource in the Arctic.
Thus, there is a great range in the type and extent of arctic freshwater environments, and this extent is perhaps proportionally greater than in other geographical areas. For example, the rivers, lakes, and wetlands of Siberia are mainly fed by thaw and summer rains, which account for up to 80% of total annual flow and which do not usually penetrate the impermeable permafrost barrier. Rivers in eastern Siberia typically freeze over in winter, flowing mainly, if not solely, in summer. The larger rivers in western Siberia have greater flows, controlled by discharges from their substantial catchments that extend into more southerly latitudes. The Rivers Ob and Yenisey provide significant contributions to the total freshwater discharge from Asia to the Arctic Ocean. Another example, is the Mackenzie Delta in North America, which is the second largest delta in the Arctic and subarctic, being 200 km long and 65 km wide. The delta has about 50% lake coverage and extensive wetlands. The small coastal rivers in the western Mackenzie Delta freeze over in winter. The spring break-up in the upstream parts of the Mackenzie River catchment causes rapid increases in water and suspended sediment discharges into the delta. These flood low-lying land and can recharge delta lakes.
These examples illustrate two of the special features of arctic freshwater environments. First, that the ecosystems can be frozen for much of the year, meaning water is available for relatively short periods of time. Second, that there is considerable variability, both within and between years, in terms of flooding, drying out, freezing, freeze–thaw cycles, and the periods of time over which these occur.
The dynamics of many of the lotic (river) and lentic (lake) environments in the Arctic are related to permafrost, and freezing can reduce or even halt the flow of rivers. The relationships between river flow, lake depth, and the onset or cessation of freezing conditions are also features of the arctic environment. Sources of water during the summer include, in addition to rain, late or perennial snow patches, glaciers, thawing of permafrost, and groundwater discharges. The projected increases in temperature are likely to result in these water sources becoming greater contributors to the annual water budgets of freshwater ecosystems. Many of the lentic environments are relatively shallow, and so the species within them have to be able to withstand considerable environmental variability, especially when the water bodies freeze.
Arctic freshwater ecosystems are species-poor compared to similar ecosystems in temperate and tropical areas. This makes them particularly suitable for trophic studies, as for example the research using isotopes of nitrogen and carbon. Aquatic food chains in the Arctic are long, which is unusual given the low overall productivity per unit area. This paradox may reflect the pulse-regulated nature of the ecosystems, whereby seasonal resource acquisition and population growth are restricted to short periods. During unfavorable periods for growth and reproduction, low maintenance costs (or migration) enable populations to survive. It is postulated that this “idling” survival strategy allows extended food chains to occur because high-energy demands by organisms do not occur year-round.
A crucial feature of the biodiversity of the Arctic’s freshwater environment is the fish, generally occurring at high trophic levels and providing an important resource for the human population. Given the slow growth rates and low overall productivity, these fish populations can easily be over-exploited. There are numerous possible effects of climate change on a number of fish stocks, both those resident in freshwater and those that are diadromous (migrating between freshwater and sea water). Anadromous behavior (migrating from salt to freshwater, as in the case of a fish moving from the sea into a river to spawn) is most prevalent in northern latitudes because the ocean is more productive than the freshwater environments.
Climate change will affect arctic freshwater habitats by causing local extinctions and by changing the distribution ranges of species. Changes in the amount of precipitation and the length of snow lie will be important. The effects of increased precipitation for freshwater habitats will be primarily geomorphological, especially in the increased sediment loads in rivers and the increased deposition of sediments in lakes, at hydroelectric dams, and in estuaries. Such changes will affect habitats and the species they support, and so are likely to impact adversely on the biodiversity of the Arctic. The effects of decreased precipitation could be even more severe, resulting in the drying of wetlands, oxidation of organic compounds in sediments, and so a further release of CO2 to the atmosphere. Changes in temperature are likely to affect the physiology of individuals, altering population dynamics and interactions between species. Temperature effects are very likely to be most pronounced in relation to fish, potentially opening up arctic freshwater ecosystems to fish species that currently have a more southern distribution.
Conservation of the biodiversity of freshwater habitats in the Arctic has been hampered by the lack of a common classification of habitats, especially for the wetlands. With each country using different definitions, it is difficult to determine trans-Arctic trends and to compare differences between regions. Classification schemes can be contentious, but it is vital that schemes are adopted as soon as possible. For conservation, classification of habitats or species provides a framework for communication, management and, where necessary, legislation or regulation. This is important because of the many threats to arctic freshwater biodiversity. An analysis of environmental trends in the Nordic countries viewed threats to the freshwater environment from a two-dimensional perspective (Figure 1). The vertical axis shows the area over which the threat operates and the horizontal axis represents the perceived seriousness of the threat. The illustration includes 14 current threats to biodiversity and ten longterm threats to the natural resources of the Nordic countries. The position of the ellipses on each diagram is therefore analogous to a risk assessment for that particular threat. The diagram does not show how these threats will change as the climate changes, but it is likely that many of the ellipses will move to the right.
Such predictions contain many uncertainties. Nevertheless, a series of nine predictions about the effects of climate change on freshwater environments and their biodiversity are seen here:
- microbial decomposition rates are likely to increase;
- increased production is very likely to result from a greater supply of organic matter and nutrients;
- shifts in invertebrate species’ ranges and community compositions are likely to occur;
- shifts in fish species’ ranges, composition, and trophic relations will very probably occur;
- spawning grounds for cold-water fish species are likely to diminish;
- an increased incidence of mortality and decreased growth and productivity from disease/parasites are likely to occur in fish species, and will possibly occur in aquatic mammals and waterfowl;
- subsistence, sport, and commercial fisheries will possibly be negatively affected;
- probable changes in habitat are likely to result in altered migration routes and timing of migration for aquatic mammals and waterfowl; and
- probable changes in timing of habitat availability, quality, and suitability are very likely to alter reproductive success in aquatic mammals and waterfowl.
These issues pose many challenges, and neither traditional knowledge nor scientific knowledge are able to meet these challenges completely. In addition to the need for more research, the development of generic models is essential if research in one area, on one species, or on one habitat, is to be applied to other areas, to other species, or to other habitats.
Chapter 10: Principles of Conserving the Arctic’s Biodiversity
10.2 Conservation of arctic ecosystems and species
10.2.1 Marine environments
10.2.2 Arctic freshwater environments
10.2.3 Environments north of the treeline
10.2.4 Arctic boreal forest environments
10.2.5 Human-modified habitats
10.2.6 Conservation of arctic species
10.2.7 Incorporating traditional knowledge
10.2.8 Implications for biodiversity conservation
10.3 Human impacts on the biodiversity of the Arctic
10.4 Effects of climate change on the biodiversity of the Arctic
10.5 Managing biodiversity conservation in a changing environment
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