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Threats to Water Availability in Canada

13. Climate Variability and Change - Wetlands

Garth van der Kamp and Philip Marsh

Environment Canada, National Water Research Institute, Saskatoon, SK

 


Current Status

Extent and Nature of Canada’s Wetlands

The National Wetlands Working Group (NWWG) (1988) defined the term wetland as “land that has the water table at, near, or above the land surface” (National Wetlands Working Group, 1988). Common words for wetlands include swamp, marsh, bog, muskeg, and slough. Typically, wetlands are occupied for the most part by water-loving vegetation such as willows, sedges, cattails, bulrushes and mosses. Where there is open water it is generally less than 2 m deep. In contrast to lakes (see Chapter 12), the open water of wetlands is either very shallow or occupies at most only a small portion of the total area of wetland.

Wetlands occupy 14% of Canada’s land surface, about 1,300,000 km2, an area slightly larger than the entire province of Ontario. The major wetland regions are arctic, sub-arctic, boreal, prairie, temperate, and mountain (Fig. 1). Peatlands, which occur primarily in the boreal and sub-arctic regions, are by far the most common type of wetland, occupying about 1,100,000 km2, 85% of the total area of wetlands in Canada.

Figure 1: Major wetland regions of Canada.

Fig. 1. Major wetland regions of Canada. Boundaries after NWWG (1988). Coloured background represents percentage of area covered by wetlands (National Atlas of Canada).

Canadian wetlands can be grouped into five major classes with distinctive properties: bogs, fens, marshes, swamps and shallow water (National Wetlands Working Group, 1997). Bogs receive water from precipitation only and are dominated by sphagnum mosses. Fens receive groundwater as well as precipitation and have sedges as well as mosses and other vegetation. Bogs and fens tend to accumulate peat. Swamps have little standing water, are dominated by trees and shrubs, and may accumulate peat. Marshes have persistent standing water, are rich in emergent aquatic vegetation, but have little moss. Shallow water wetlands have open standing water over much of their area and their vegetation consists mostly of submerged or floating aquatic plants. Most marshes and shallow water wetlands do not form peat.

Wetlands, by their very nature, occur wherever the ground surface is wet or covered with shallow water throughout most of the year. Most Canadian wetlands occur in flat, poorly drained terrain or in depressions in the landscape. Wetlands can also occur on slopes and high land if they are continuously fed by water from rain, melting snow or groundwater seepage. Beavers have created numerous wetlands, especially in the boreal and sub-arctic regions, through their dam-building activities.

For many Canadians the most common acquaintance with wetlands comes about through wetlands within the parks and nature reserves of urbanized areas. These more-or-less artificial marshes and shallow water wetlands commonly also serve as stormwater retention ponds. Relative to other wetland types, the total area of urban wetlands is minute, but their importance is high because they affect the day-to-day lives and recreation of many Canadians.

Values and Functions of Wetlands

Wetlands used to be viewed as unproductive wastelands that can be improved by filling or drainage. That perspective is changing, and wetlands are recognized as having many important functions and values (Government of Canada, 1991). Through storage and slow release of water, wetlands can recharge groundwater, reduce peak flows during floods, and help maintain flow in rivers during dry periods. In regions where they occupy a large proportion of the landscape, such as the Mackenzie River basin, the moist surface of wetlands may also have a moderating influence on climate by maintaining regional evapotranspiration, even during extended dry periods (Rouse et al., 2003).

Wetlands represent critical and highly productive habitat for fish and wildlife and for many unique types of plants. They provide an important resource base for hunting and fishing, and are valued highly for recreational opportunities such as bird watching. The North American Waterfowl Management Plan puts strong emphasis on wetland preservation and habitat enhancement.

Wetlands absorb and store contaminants, such as heavy metals and sulphur from acid rain that enter them via precipitation, surface water flow, and groundwater seepage. Wetlands can also serve an important remediation function because many contaminants such as nitrate are permanently broken down within wetlands.

The important role of Canada’s wetlands in the global carbon balance is receiving increased attention. All wetlands store organic carbon, but Canada’s peatlands are overwhelmingly important in this respect: they contain about 150 billion tonnes of carbon in the form of peat, 25 times the amount of fossil fuel carbon released each year by the entire world (Roulet, 2000). This peat carbon store has built up over thousands of years and is probably still increasing very slowly, year by year. As long as the peatlands remain saturated to near their surface the carbon will remain in stable storage. Thus, the continued availability of sufficient water to maintain the peatlands is a major concern in the context of climate change. Loss of carbon from the peatlands by fire and decay as a result of increasingly dry conditions would increase emissions of carbon dioxide to the atmosphere, whereas flooding of the peatlands can lead to increased emissions of methane--a potent greenhouse gas. Either way, climatically induced destabilization of Canada’s peatlands could have significant repercussions for global climate change.

Vulnerability to Climate Change

Wetlands gain water from precipitation on the wetland itself and from the surrounding uplands by overland runoff, drifting snow, and groundwater inflow. They lose water by evaporation from open water, transpiration from growing plants, surface outflow, and groundwater outflow; hence, the water balance of most wetlands is influenced by the vegetation in the wetland and by land use and vegetation cover on the surrounding uplands.

Due to their large wet area and shallow depths, wetlands are particularly vulnerable to water losses by evapotranspiration. Any variation in climate that increases the relative importance of evaporation compared to precipitation is likely to result in drying out of wetlands. Thus, the shorter warmer winters and longer summers predicted under most climate change scenarios (and that are already occurring in western Canada) imply that wetlands in Canada will be under increasing stress due to water shortage, unless increases in precipitation offset the increased losses by evapotranspiration.

Many Canadian wetlands owe their existence, at least in part, to cold Canadian winters and the resulting permafrost, frozen soil, snow drifting, and river ice jams. Such cold-climate wetlands include those that exist by virtue of impeded drainage due to underlying permafrost, as is the case for many sub-arctic and arctic wetlands (Rouse et al., 1997). In the mountain and arctic regions, small wetlands that depend on meltwater from long-lasting snowbanks are also very sensitive to climatic warming. Millions of small prairie marshes, commonly referred to as potholes or sloughs, owe their existence to snow that blows into them and snowmelt water that flows into them in early spring over the frozen soil of the surrounding land. Some shoreline and delta wetlands owe their existence to inundation by annual or near annual flooding events (e.g., Marsh and Hey, 1994) and will dry out if peak water levels in rivers rise less high due to reduced spring snowmelt freshets or lessening of ice-jam events. All these cold-climate wetlands will be affected by warmer, shorter winters, regardless of how precipitation patterns change.

Some types of wetlands are less likely to be affected by climate change. These include wetlands fed by large deep groundwater systems (Chapter 10) which tend to maintain a steady flow even under large climatic variations (Winter, 2000). Many fens may be in this category if the groundwater flowing in to them constitutes an important part of the total water input. Through-flow wetlands maintained in a balance between large surface water inflows and outflows may be little affected by climate change. Marshes along the margins of lakes and rivers with stable water levels are likely to be insensitive to climate change.

Most peatlands appear to be relatively stable, having persisted and grown for thousands of years, through long wet and dry periods. Recent field studies suggest that the present-day rates of peat growth may be similar to the rates of growth over the past thousand years. However, their future stability under changing climate is uncertain. Bogs are vulnerable to changes in precipitation because that is their only water input. Many peatlands in the sub-arctic region and the northern part of the boreal region are wholly or partly underlain by thin and discontinuous permafrost (Fig. 1), and climate warming will affect these wetlands through thawing and retreat of the permafrost.

Climate warming scenarios suggest the southern boundary of the boreal region may move northwards by hundreds of kilometres over the next century. The reality of this possibility is attested to by the fact that 6000 years ago the southern boundary of the boreal forest and peatlands was located 200 to 400 km north of the present boundary (Vitt et al., 2000). At that time, the climate of northern Canada was warmer than at present because the area received more solar energy due to slow changes in the tilt of the earth’s axis. If a large northward movement of the southern limit of the boreal forest does indeed occur, the peatlands may degrade as well and perhaps disappear. The water resources of the area would change drastically and of course there would be very large releases of carbon to the atmosphere.

Present Status

At present there is little region-wide monitoring of wetland status (water levels, area, vegetation, etc.) in the boreal, mountain, sub-arctic and arctic regions, but specific wetlands or wetland complexes are being monitored as part of detailed investigations (Cihlar and Tarnocai, 2000). In the prairie region and the southern margin of the western boreal forest, annual pond counts, carried out in the context of waterfowl management, provide a detailed inventory of wetlands starting in 1955 (Conly and van der Kamp, 2001). It may be surmised that wetlands in the temperate region are closely watched by local agencies, due to their location in densely populated areas.

Boreal and sub-arctic bogs and fens are relatively pristine in terms of human disturbance, although the impacts of clear-cut logging and road construction may be important in the southern boreal forest. In the boreal region, permafrost occurs almost exclusively within the peatlands. The present-day southern distribution of permafrost is in part a relict of the Little Ice Age (approx. 1400–1850) and is not in equilibrium with the present-day climate (Vitt et al., 1999). Through a wide east-west zone of the western boreal region, bordering the southern limit of discontinuous permafrost (Fig. 1), peatlands are affected by warming of the climate over the last 100 years. In this zone, thawing of permafrost causes subsidence of the peat surface and wetter conditions in the peat.

Temperate wetlands (southern Ontario and Quebec, southwest British Columbia) are relatively small in area but lie in the area of main population concentration and are heavily affected by drainage for agriculture and urbanization. Other impacts include peat harvesting for horticulture, changes in water runoff from surrounding areas, changes in vegetation (e.g., cultivation, introduction of purple loosestrife), and changes in wildlife populations (beavers and muskrats).

Shoreline marshes of the Great Lakes are relatively stable but dependent on multi-year wet-dry cycles, which control the water-level changes in the lakes (Mortsch, 1998). Post-glacial uplift, centred on Hudson’s Bay, has imposed slow, long-term change of these wetlands through continued tilting of the earth’s surface. This glacial isostacy has a controlling influence on the development of the extensive peatlands south and west of Hudson’s Bay (Fig. 1).

Delta and floodplain wetlands are primarily dependent on peak water levels in the rivers, caused by extreme flows and ice jams (Marsh and Hey, 1994). Many of these have been affected by changes of flow regime caused by dams and reservoir operations, and change will continue as river beds slowly adjust to the new flow regimes (cf. Chapter 2 and Chapter 4).

Prairie wetlands lie in a semi-arid region where potential evaporation exceeds precipitation. They nearly all occur within small closed drainage basins and go through multi-year wet-dry cycles so that the number of wetlands that contain standing water can change by a factor of 10 over a few years (Conly and van der Kamp, 2001). The great majority of these wetlands lie within privately owned cultivated fields, with little regulatory control. Many prairie wetlands have been drained and drainage is continuing, especially along the boreal fringe where water excess is a hindrance to farm operations in most years.

A prairie wetland in the Allan Hills of Saskatchewan.

Trends

In the boreal region of western Canada, permafrost degradation is changing the character of many peatlands. The southern boundary of discontinuous permafrost occurrence has moved northward by tens of kilometres during recent decades and will continue to migrate (Vitt et al., 1999). Extensive peatland plateaus in the northern part of the boreal region, presently underlain by permafrost, will be increasingly affected. The water storage and release characteristics of the peatlands change with thawing of underlying permafrost, and these changes can be expected to affect streamflow and climate feedbacks. The disappearance of permafrost from beneath peatlands can also lead to increased long-term rates of carbon storage in the form of peat because peatlands underlain by permafrost tend to be relatively dry and vulnerable to fire (Robinson and Moore, 2000; Vitt et al., 2000). This retreat of permafrost can be considered the largest present-day impact of climatic change on Canadian wetlands.

In the Arctic and sub-Arctic, longer, warmer summers will lead to a deeper active (thawed) layer in summer, drainage of some wetlands, and creation of others. Areas of ice-rich permafrost, which occur mostly in soft unconsolidated sediments, are especially vulnerable (Beilman et al., 2001).

There do not appear to be any reports of general changes in the southern or northern limits of peatlands over the last few decades, suggesting that if such changes are occurring they are, for now at least, small and not easily detected amidst changes of land use and effects of climatic variability. Drainage and mining of peatlands have reduced the extent of many peatlands in the temperate region and of some peatlands in the southern boreal region. The local impacts of such changes are large, but, in comparison to the total area of peatlands, the impact on a Canada-wide basis is small.

Prairie wetland numbers and water levels show large year-to-year variations since 1955 when monitoring started (Conly and van der Kamp, 2001). Drainage of wetlands is certainly having a large impact on wetland occurrence in some areas, but overall the percentage of wetlands lost by drainage appears to be in the range of 2 to 4% per decade over the last half century (Watmough et al., 2002). On the surrounding uplands, there has been a marked decline of the area in summer fallow, from 30% in 1985 to 10% in 1999, and a small increase in the proportion of non-cultivated land (Watmough et al., 2002). There are other ongoing gradual and pervasive changes in agricultural practices and land use, such as the current shift to tall stubble and minimum tillage. These changes of land use on the uplands may have a region-wide impact by conserving moisture on the uplands and decreasing runoff to the wetlands (van der Kamp et al., 1999). The effects of climate change will likely continue to be masked by large year-to-year variations of precipitation and runoff, accumulating impacts of wetland drainage, and continuing changes in farming practices and land use.

Delta and floodplain wetlands in southern Canada will become more dependent on peak runoff from summer rainfall and less dependent on spring freshets and ice jams (Prowse and Beltaos, 2002). The corresponding decrease of peak flows, together with longer summertime evaporation periods, will likely lead to drying out of some floodplain wetlands.

Temperate wetlands may be affected by climate change; however, due to their occurrence in the densely populated areas of Canada the main concern with their health will continue to be direct human influences including drainage, urbanization, road salts, and runoff from roads.

Emerging Issues

Impacts of climate change on Canada’s boreal and sub-arctic peatlands may be large and carry significant implications for the global atmospheric carbon balance. Similar extensive peatlands exist in northern Russia with a total area twice that of Canada’s peatlands (Zhulidov et al., 1997), and it is precisely in these boreal and sub-arctic regions where climatic change will be largest and is already well underway. However, little is as yet known of how Canada’s peatlands may react to climate warming or prolonged dry conditions, nor is much known as to whether management of peatlands and of surrounding uplands represents a practical option for maintaining the water balance of the peatlands and protecting the carbon stored in them.

Existing monitoring programs may not detect long-term changes in status of Canadian wetlands due to climate change and other impacts (Cihlar and Tarnocai, 2000). For example, the southern boundary of the boreal wetland region may retreat northwards 200 to 400 km by the year 2050, roughly back to its extent during the warmer climate that prevailed over much of Canada during the early Holocene period (between about 8000 to 5000 years ago). It is highly uncertain whether such a retreat of the boreal wetlands will happen, but if it does indeed occur the resulting loss of peat by burning and by decay would represent a major input of carbon to the earth’s atmosphere. The corresponding northward expansion of peatlands would do little to offset the carbon losses in the south, because peat can accumulate only very slowly--at rates of less than one millimetre per year (Vitt et al., 2000). Without adequate data on past and present trends, early detection of gradual but critical changes in the peatlands will not be possible, and evaluation of potential management strategies for maintaining the peatlands will be seriously compromised.

Temporary storage of flood water in wetlands can clearly contribute to moderating floods, at least on a local scale, as demonstrated by the widespread use of stormwater retention ponds in urban areas (Anderson et al., 2002). Wetlands could help reduce floods even in large watersheds, if the total storage capacity of the wetlands is large enough. Such a function of wetlands would also contribute to wildlife habitat, carbon sequestration, erosion control, and water quality improvements. Extensive areas of wetland have been drained for agriculture. Effects of this drainage on peak flows in Canadian rivers and the potential moderating effects of wetland restoration are not well understood and may not be important for extreme flood events (Juliano and Simonovic, 1999). Current hydrological models for wetland water balance and river flow do not incorporate storage and release effects of wetlands very well (Price and Waddington, 2000). Considering the hydrological and ecological benefits of reducing flooding through maintenance and restoration of wetlands on a landscape scale, it is apparent that the role of wetlands in this regard should be taken more seriously and requires more rigorous, multidisciplinary evaluation.

Floodplain, delta and lakeshore wetlands have been affected by changes of river and water-level regime caused by reservoir operations. These wetlands will also be increasingly affected by climate change, notably earlier and smaller spring runoff and increased importance of summertime flood events (Marsh and Lesack, 1996). The nature of these impacts is not well understood at present and possible management options for protecting wetlands are only beginning to be identified.

Prairie wetlands are dependent for their existence on springtime runoff of snowmelt water over frozen cultivated ground. There is an ongoing shift to farming practices that trap snow on the uplands and conserve soil moisture such as tall stubble, minimum tillage, continuous cropping, and conversion to grassland. These are likely having a region-wide impact by decreasing runoff to the wetlands (see Chapter 7). Climate change may also lead to an increased dominance of evaporation over precipitation and runoff. Thus it may be concluded that prairie wetlands are under serious threat. At present there is little information as to the effectiveness of various land management practices for wetland conservation.

Wetlands may be significantly affected by changes in biota due to human interference or to climate change. Changes in beaver populations have had widespread impacts in the past in some areas of Canada. Changes in vegetation such as loss of elms due to Dutch elm disease, or invasion by purple loosestrife may affect the water balance and ecological balance of wetlands. Climate warming is likely to have other impacts on wetland biota: for instance, by reducing the depth and duration of frozen ground or by causing peak water levels and subsequent drying out to occur earlier during the growing season.

Many of the threats to wetlands in Canada have social, economic and environmental aspects. Government policies and regulations with implications for wetlands are sometimes conflicting or inconsistent. There is still a widely held opinion, embedded deep in our culture, that wetlands are “unimproved” land, which should be brought into production for agriculture and forestry or drained and filled for housing and industry. The climate of opinion is changing slowly, but wetlands continue to be inaccessible to public view and to political support.

Knowledge and Program Needs

A nationwide program for wetland monitoring should be set up (Cihlar and Tarnocai, 2000). In addition to direct measurements of water levels, such a program could include remote sensing, air photos, vegetation and wildlife inventories, stable benchmarks for detecting changes of peat thickness, and fixed points for recurrent photos. Resulting data would serve to track changes in the occurrence and status of Canada’s wetlands.

In view of the major national and global implications of peatland dynamics under changing climate, there is an urgent need to understand Canada’s northern peatlands more fully. The impact of climate change on the southern limit of the boreal wetlands must be better understood. Will the wetlands degrade and retreat? Will they persist and even expand? It depends on a complex interaction among temperature regimes, changes in snowfall and rainfall, water chemistry and vegetation. The role of seasonal freezing and of permafrost in peatland dynamics should be better understood because warming is a clear and ongoing impact of climate change and its effects on peatland are, as yet, poorly understood and predictable.

The lack of adequate wetlands hydrology in hydrological models should be remedied. With reliable and practical models, the impacts of climate change on wetlands can be better predicted, and consequences of wetland dynamics for floods and for low flow can be analyzed and predicted. Such hydrological models would allow evaluation of the potential for using wetlands as a means of moderating floods, and would allow assessment of the potential impacts of climate change on wetland ecology.

Potential impacts of changing farming practices on prairie wetlands should be evaluated so that policies and management practices can be adapted for optimizing the balance between farming and wetland conservation. Similarly, impacts of forestry and other disturbances on boreal wetlands should be better understood so that practices can be adapted to benefit the wetlands.

References

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