Introduction

Patches of wetland covering 101-103 m2 are commonin the otherwise arid polar deserts. They providehydrological and ecological conditions important to theplants, insects and rodents, and sometimes they are animpediment to the preparation of road beds and buil-ding sites. In the Canadian High Arctic, hydrologicalstudies of wetlands have been made by Buttle andFraser (1992), Glenn and Woo (1997), Ryd�n (1977), andWoo and Xia (1995), but there is no comprehensive dis-cussion on the interactions of moisture and the frozensoil in a wetland. The objective of this paper is to relatethe occurrence of patchy wetlands in the polar desert totheir frost and hydrological characteristics. The resultsshould enhance our understanding of the preservationor deterioration of patchy wetlands in the High Arctic.

Study sites and methods

The area around Resolute, Cornwallis Island,Northwest Territories, Canada (74¡43'N, 93¡45'W) istypical of the polar desert. The mean daily temperaturebetween June and September is above 0¡C but tempera-tures often fall below -30¡C in February and March.Annual precipitation, when corrected for snow gaugeunderestimation, is about 200 mm (Woo et al., 1983).The undulating terrain is underlain by Silurian lime-stone of the Read Bay Formation (Thorsteinsson, 1958).Cruickshank (1971) mapped the surficial deposits into

lithosol (coarse gravelly and bouldery materials), polardesert (a pebbly loam) and wetland soils (with anorganic layer overlying the original lithosol or polardesert soil).

The field study was carried out in 1996 which had acold, wet summer. Mean daily air temperature did notrise above 0¡C until June 26 and winter arrived early,with temperatures hovering around 0¡ to 2¡C betweenJuly 18 and August 6. Snowfall on August 8 and there-after did not undergo significant melting until the sub-sequent spring. Many late-lying snowbanks remainedthroughout the summer. Four patchy wetlands and asite with strongly humified peat were studied (Figure1). Site 1 is a wetland below a late-lying snowbank; Site2 occupies a slight topographic depression; Site 3 islocated below a slope concavity and is the same wet-land reported in Woo and Xia (1995); and Site 4 is a wet-land along a rill that underwent drainage modificationabout three decades ago, leaving the higher zones ofthis wetland in a partially dry state. A comparison ofthe vegetation cover on the study wetlands and theiradjacent non-wetland zones shows that the latter areashave greater percentages of gravelly surfaces (20-50%)than the wetlands (3-13%). The gravels may have analgal cover but that is mainly of the Nostoc sp. There isusually no surface water on the non-wetlands except atSite 1 below a melting snowbank. The wetlands havelarger coverage of mosses (25-46%) and vascular plants(23-43%) than the non-wetlands.

Abstract

The occurrence of patchy wetlands in the polar desert is predicated upon focused water supply and a shallowfrost table to inhibit deep percolation. Several such wetlands near Resolute, Cornwallis Island, Canada, arecompared. Meltwater from a late-lying snowbank or suprapermafrost groundwater seepage creates high watertables in these wetlands. The wetlands have thin thawed zones compared with their adjacent non-wetlandlocations due to the insulation properties of the peat layer and because much heat is needed to thaw the abun-dant ground ice in the peaty soil. Internal processes and external disturbances can be deleterious for the satu-rated environment and modify the characteristics of these patchy wetlands.

Ming-ko Woo, Kathy L. Young 1141

CHARACTERISTICS OF PATCHY WETLANDS IN A POLAR DESERTENVIRONMENT, ARCTIC CANADA

Ming-ko Woo1, Kathy L. Young2

1. School of Geography and Geology, McMaster University, Hamilton, Ontario, Canada L8S 4K1e-mail: woo@mcmaster.ca

2. Department of Geography, York University, Toronto, Ontario, Canada M3J1P3e-mail: klyoung@yorku.ca

At each wetland and its adjacent non-wetland zone,frost table depth was measured by pounding a steel rodinto the ground until the frost was encountered. Watertable depth was monitored in perforated plastic pipeswhich fully penetrated the active layer. Soil sampleswere dug up from the frozen ground for the determina-tion of ice content. A meteorological station was set upto record air temperature, radiation and summer pre-cipitation. Snow surveys were carried out on andaround the wetlands in late May, using a methoddescribed by Woo et al. (1983).

Hydrology

Wetlands are found where the local water supplyexceeds the losses during the thawed season so as tomaintain a large water storage to enable ground satura-tion. In non-tidal areas of the Arctic, such a situation isfound where: (1) water supply is sustained during thesummer by a late-lying snow cover; (2) groundwater isdischarged to the ground surface; (3) frequent inunda-tion occurs along the edges of lakes, streambanks or

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Figure 1. Distribution of patchy wetlands and the study sites near Resolute, Northwest Territories, Canada.

Ming-ko Woo, Kathy L. Young 1143

Figure 2. Topography of Site 2 (top) and Site 3 (left).

within parts of some tundra ponds. This paper exa-mines wetlands associated with the first two groups.

In June 1996, the snow cover on the wetlands aver-aged 156 mm and the summer precipitation (June 29-August 8) was 41 mm. During the main snowmelt peri-od, all wetlands received meltwater input from theiroverlying snow cover and from runoff down their adja-cent slopes. After that, Site 1 continued to receive sur-face inflow from a late-lying snowbank upslope. AtSite 2, inflow ceased on the northern flank not longafter the snow disappeared from the steep slope butsubsurface flow was sustained by seepage from thegravels on its eastern fringe. Groundwater inflow at itssoutheastern corner was derived mainly from infiltra-tion of water from an extensive wetland that lies at aslightly higher elevation. Exfiltration of supraper-mafrost groundwater at Site 3, together with some melt-water from the snowbank that lingered at the slope con-cavity, allowed this wetland to be saturated for most ofthe summer. Similarly, exfiltration at Site 4, fed bygroundwater from upslope, sustained a saturated zonein the wetland which then discharged to the lowerslope. Thus, lateral inflow is one major source of watersupply to the wetlands.

Frequent saturation produces a distinguishing featurethat the water table is often close to the ground surface.Figure 3 summarizes the probability distributions of thewater table elevations relative to the ground for thestudy wetlands and their adjacent non-wetland zones,based on observations during the thaw season of 1996.Clearly, the wetland water table was higher than itsnon-wetland counterpart and it rose frequently abovethe ground. Among the four wetlands, Site 1 fed by la-teral inflow from snowmelt usually had a higher watertable, followed by Site 2. Site 3, maintained by ground-water input from ground ice melt, showed a lowerwater table. Site 4 had a wet zone along the rill and dryground with organic soils further away from the rill.The former zone retained a high water table, but the lat-ter had been affected by artificial drainage and lowerwater table positions were observed.

Effects on frozen soils

The presence of a wetland modifies the thermal pro-perties and the energy balance of the soil, both of whichaffect the freeze-thaw conditions. Several effects areobserved at the study sites.

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Figure 3. Probability distribution of water table positions in the study wetlands and their adjacent non-wetland zones during the thawed season of 1996. Drymeadow at Site 4 refers to a segment of the original wetland affected by artificial drainage.

(1) Rich plant growth and slow decomposition favourthe formation of a peat layer in the wetlands, attaininga thickness from <0.1 m at parts of Site 1 to >0.25 m.The high porosity of this layer, when filled with wateror air in the summer, greatly reduces the thermal con-ductivity and therefore insulates the substrate againstsummer heating.

(2) A substantial amount of water is held in the wet-lands at the time of freeze-up, allowing much groundice to be formed in the active layer. The ice contents inthe surface layer (0-0.03 m in peat and living plants) ofall the wetlands average about 0.8, compared with 0.4-0.7 in the mineral soil below. For the profile as a whole,wetland soils have larger ice contents than the polardesert soil (which averages 0.3-0.35), therefore requiringmuch latent heat to thaw the active layer in the wet-lands (Woo and Xia, 1996).

(3) Wetlands have a thinner active layer than their sur-roundings. At Site 1, the established wetland zone witha peat cover of about 0.04-0.15 m has a thinner thawdepth than the incipient wetland zone which has only apoorly developed organic layer. Thaw depth is also lessbeneath the upslope segment which is usually exposedfrom the late-lying snow cover one week or later thanthe lower slope zone. The peat cover in the wetland ofSite 2 insulates the ground and its active layer thicknessis 0.1-0.2 m thinner than the adjacent gravelly areas.Thaw depth is particularly thin (<0.3 m) at the ice-coredmound where downward penetration of the thawingfront is retarded by the ample ice content which con-sumes much of the ground heat by phase change.

Feedback mechanisms

Several hydrological, thermal and biological feedbackmechanisms were observed which influence the deve-lopment of patchy wetlands. (1) A shallow active layerreduces the amount of water required to saturate theentire thawed zone, allowing the water table to remainclose to the ground surface throughout summer. (2)With water being readily accessible to the plants, vege-tation growth is sustained in the wetlands and this con-tributes to the peat accumulation. (3) Insulation bypeat, and ice formation due to abundant soil waterfavour local aggradation of the permafrost.

On the other hand, the saturated status is subject tochange because of several internal processes or externalforcing.

(1) Ice-cored mound or palsa formation: Unlimitedwater storage in the active layer before freeze back sup-ports the growth of segregated ice and pore ice.Summer thawing of the ground ice is hindered by thelow thermal conductivity of the thawed organic layer.

Over a period of time, parts of the wetlands developpalsas or ice-cored mounds which rise slightly abovetheir surroundings. These zones are higher than thegeneral water table and become less prone to satura-tion, thus allowing aerobic conditions to occur fre-quently in the peat. Examples include a peat mound atthe middle of Site 2 wetland (Figure 2) and patches ofnon-saturated peat overlying an ice-rich substrate onthe slope of Site 1.

(2) Stream capture and wetland desiccation: A strip ofshallow topographic depression ("desiccated peat" sitein Figure 1) was occupied by a fen before its water sup-ply was cut off by a headward retreating channel fromthe west. Thus separated from its water source, thepeat underwent decomposition under aerobic condi-tions that followed. Desiccation causes the pores to beair-filled and cracks of various dimensions to develop,ranging from mm to several cm in width. The highlyhumified peat then becomes an excellent insulator inthe summer. The active layer at this location is thinner(about 0.2 m) than at the nearby wetland (0.3 m) or thenon-wetland polar desert site (0.55 m).

(3) Artificial drainage: Artificial drainage at Site 4altered the flow paths and the saturation status of thewetland. Several decades after drainage modification,the zone alongside the ditch is recovering its hydrologi-cal status and vegetation growth. The drier peripheralzone still has a lower water table, lying at a depth of 60-100 mm below the ground surface, compared with awater table depth of <40 mm beneath the rejuvenatedwet zones (Figure 3).

Conclusion

In the polar desert of Arctic Canada, the presence ofpermafrost at shallow depths limits deep percolationand where the local supply of water is ample and rel-iable during the thawed season, wetland formation isfavoured. Vegetation growth and peat formation pro-vides an insulating surface layer which has lower ther-mal conductivity than the mineral soils. The saturatedcondition of the wetland in the freeze back periodenhances the accretion of ground ice, which in the fol-lowing summer, consumes much latent heat by phasechange at the expense of deep thawing of the activelayer. A shallow active layer ensures that the wetlandprofile is largely saturated throughout the summer.This thermal (permafrost) and moisture (wetland) feed-back mechanism perpetuates the existence of the patchywetlands. However, internal processes may heave partof the peat above the general wetland water table andexternal forces due to stream capture or artificialdrainage may diminish the water supply or alter thecourse of water flow. Nevertheless, a humified organiccover with air-filled pore space will continue to insulate

Ming-ko Woo, Kathy L. Young 1145

the substrate to maintain a thin active layer while theresumption of adequate water supply will saturate thedisturbed wetland and revive its vegetation.

Acknowledgments

This work was funded by a research grant from theNatural Sciences and Engineering Research Council ofCanada. Logistical support from the Polar ContinentalShelf Project is gratefully acknowledged. We thankJoan Pace and Dallas Stoltz for their assistance in thefield.

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