Characteristics of Patchy Wetlands in a Polar Desert Environment, Arctic Canada

CHARACTERISTICS OF PATCHY WETLANDS IN A POLAR DESERT ENVIRONMENT, ARCTIC CANADA

Ming-ko Woo1, Kathy L. Young2

1. School of Geography and Geology, McMaster University, Hamilton, Ontario, Canada L8S 4K1 e-mail: [email protected]

2. Department of Geography, York University, Toronto, Ontario, Canada M3J1P3 e-mail: [email protected]

Abstract

The occurrence of patchy wetlands in the polar desert is predicated upon focused water supply and a shallow frost table to inhibit deep percolation. Several such wetlands near Resolute, Cornwallis Island, Canada, are compared. Meltwater from a late-lying snowbank or suprapermafrost groundwater seepage creates high water tables in these wetlands. The wetlands have thin thawed zones compared with their adjacent non-wetland locations 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.

Introduction lithosol (coarse gravelly and bouldery materials), polar desert (a pebbly loam) and wetland soils (with an organic layer overlying the original lithosol or polar Patches of wetland covering 101-103 m2 are common desert soil). in the otherwise arid polar deserts. They provide hydrological and ecological conditions important to the The field study was carried out in 1996 which had a plants, insects and rodents, and sometimes they are an cold, wet summer. Mean daily air temperature did not impediment to the preparation of road beds and buil- rise above 0¡C until June 26 and winter arrived early, ding sites. In the Canadian High Arctic, hydrological with temperatures hovering around 0¡ to 2¡C between studies of wetlands have been made by Buttle and July 18 and August 6. Snowfall on August 8 and there- Fraser (1992), Glenn and Woo (1997), RydŽn (1977), and after did not undergo significant melting until the sub- Woo and Xia (1995), but there is no comprehensive dis- sequent spring. Many late-lying snowbanks remained cussion on the interactions of moisture and the frozen throughout the summer. Four patchy wetlands and a soil in a wetland. The objective of this paper is to relate site with strongly humified peat were studied (Figure the occurrence of patchy wetlands in the polar desert to 1). Site 1 is a wetland below a late-lying snowbank; Site their frost and hydrological characteristics. The results 2 occupies a slight topographic depression; Site 3 is should enhance our understanding of the preservation located below a slope concavity and is the same wet- or deterioration of patchy wetlands in the High Arctic. land reported in Woo and Xia (1995); and Site 4 is a wet- land along a rill that underwent drainage modification Study sites and methods about three decades ago, leaving the higher zones of this wetland in a partially dry state. A comparison of the vegetation cover on the study wetlands and their The area around Resolute, Cornwallis Island, adjacent non-wetland zones shows that the latter areas Northwest Territories, Canada (74¡43'N, 93¡45'W) is have greater percentages of gravelly surfaces (20-50%) typical of the polar desert. The mean daily temperature than the wetlands (3-13%). The gravels may have an between June and September is above 0¡C but tempera- algal cover but that is mainly of the Nostoc sp. There is tures often fall below -30¡C in February and March. usually no surface water on the non-wetlands except at Annual precipitation, when corrected for snow gauge Site 1 below a melting snowbank. The wetlands have underestimation, is about 200 mm (Woo et al., 1983). larger coverage of mosses (25-46%) and vascular plants The undulating terrain is underlain by Silurian lime- (23-43%) than the non-wetlands. stone of the Read Bay Formation (Thorsteinsson, 1958). Cruickshank (1971) mapped the surficial deposits into

Ming-ko Woo, Kathy L. Young 1141 Figure 1. Distribution of patchy wetlands and the study sites near Resolute, Northwest Territories, Canada. At each wetland and its adjacent non-wetland zone, Hydrology frost table depth was measured by pounding a steel rod into the ground until the frost was encountered. Water Wetlands are found where the local water supply table depth was monitored in perforated plastic pipes exceeds the losses during the thawed season so as to which fully penetrated the active layer. Soil samples maintain a large water storage to enable ground satura- were dug up from the frozen ground for the determina- tion. In non-tidal areas of the Arctic, such a situation is tion of ice content. A meteorological station was set up found where: (1) water supply is sustained during the to record air temperature, radiation and summer pre- summer by a late-lying snow cover; (2) groundwater is cipitation. Snow surveys were carried out on and discharged to the ground surface; (3) frequent inunda- around the wetlands in late May, using a method tion occurs along the edges of lakes, streambanks or described by Woo et al. (1983).

1142 The 7th International Permafrost Conference Figure 2. Topography of Site 2 (top) and Site 3 (left).

Ming-ko Woo, Kathy L. Young 1143 Figure 3. Probability distribution of water table positions in the study wetlands and their adjacent non-wetland zones during the thawed season of 1996. Dry meadow at Site 4 refers to a segment of the original wetland affected by artificial drainage. within parts of some tundra ponds. This paper exa- Frequent saturation produces a distinguishing feature mines wetlands associated with the first two groups. that the water table is often close to the ground surface. Figure 3 summarizes the probability distributions of the In June 1996, the snow cover on the wetlands aver- water table elevations relative to the ground for the aged 156 mm and the summer precipitation (June 29- study wetlands and their adjacent non-wetland zones, August 8) was 41 mm. During the main snowmelt peri- based on observations during the thaw season of 1996. od, all wetlands received meltwater input from their Clearly, the wetland water table was higher than its overlying snow cover and from runoff down their adja- non-wetland counterpart and it rose frequently above cent slopes. After that, Site 1 continued to receive sur- the ground. Among the four wetlands, Site 1 fed by la- face inflow from a late-lying snowbank upslope. At teral inflow from snowmelt usually had a higher water Site 2, inflow ceased on the northern flank not long table, followed by Site 2. Site 3, maintained by ground- after the snow disappeared from the steep slope but water input from ground ice melt, showed a lower subsurface flow was sustained by seepage from the water table. Site 4 had a wet zone along the rill and dry gravels on its eastern fringe. Groundwater inflow at its ground with organic soils further away from the rill. southeastern corner was derived mainly from infiltra- The former zone retained a high water table, but the lat- tion of water from an extensive wetland that lies at a ter had been affected by artificial drainage and lower slightly higher elevation. Exfiltration of supraper- water table positions were observed. mafrost groundwater at Site 3, together with some melt- water from the snowbank that lingered at the slope con- Effects on frozen soils cavity, allowed this wetland to be saturated for most of the summer. Similarly, exfiltration at Site 4, fed by The presence of a wetland modifies the thermal pro- groundwater from upslope, sustained a saturated zone perties and the energy balance of the soil, both of which in the wetland which then discharged to the lower affect the freeze-thaw conditions. Several effects are slope. Thus, lateral inflow is one major source of water observed at the study sites. supply to the wetlands.

1144 The 7th International Permafrost Conference (1) Rich plant growth and slow decomposition favour Over a period of time, parts of the wetlands develop the formation of a peat layer in the wetlands, attaining palsas or ice-cored mounds which rise slightly above a thickness from <0.1 m at parts of Site 1 to >0.25 m. their surroundings. These zones are higher than the The high porosity of this layer, when filled with water general water table and become less prone to satura- or air in the summer, greatly reduces the thermal con- tion, thus allowing aerobic conditions to occur fre- ductivity and therefore insulates the substrate against quently in the peat. Examples include a peat mound at summer heating. the middle of Site 2 wetland (Figure 2) and patches of non-saturated peat overlying an ice-rich substrate on (2) A substantial amount of water is held in the wet- the slope of Site 1. lands at the time of freeze-up, allowing much ground ice to be formed in the active layer. The ice contents in (2) Stream capture and wetland desiccation: A strip of the surface layer (0-0.03 m in peat and living plants) of shallow topographic depression ("desiccated peat" site all the wetlands average about 0.8, compared with 0.4- in Figure 1) was occupied by a fen before its water sup- 0.7 in the mineral soil below. For the profile as a whole, ply was cut off by a headward retreating channel from wetland soils have larger ice contents than the polar the west. Thus separated from its water source, the desert soil (which averages 0.3-0.35), therefore requiring peat underwent decomposition under aerobic condi- much latent heat to thaw the active layer in the wet- tions that followed. Desiccation causes the pores to be lands (Woo and Xia, 1996). air-filled and cracks of various dimensions to develop, ranging from mm to several cm in width. The highly (3) Wetlands have a thinner active layer than their sur- humified peat then becomes an excellent insulator in roundings. At Site 1, the established wetland zone with the summer. The active layer at this location is thinner a peat cover of about 0.04-0.15 m has a thinner thaw (about 0.2 m) than at the nearby wetland (0.3 m) or the depth than the incipient wetland zone which has only a non-wetland polar desert site (0.55 m). poorly developed organic layer. Thaw depth is also less beneath the upslope segment which is usually exposed (3) Artificial drainage: Artificial drainage at Site 4 from the late-lying snow cover one week or later than altered the flow paths and the saturation status of the the lower slope zone. The peat cover in the wetland of wetland. Several decades after drainage modification, Site 2 insulates the ground and its active layer thickness the zone alongside the ditch is recovering its hydrologi- is 0.1-0.2 m thinner than the adjacent gravelly areas. cal status and vegetation growth. The drier peripheral Thaw depth is particularly thin (<0.3 m) at the ice-cored zone still has a lower water table, lying at a depth of 60- mound where downward penetration of the thawing 100 mm below the ground surface, compared with a front is retarded by the ample ice content which con- water table depth of <40 mm beneath the rejuvenated sumes much of the ground heat by phase change. wet zones (Figure 3).

Feedback mechanisms Conclusion

Several hydrological, thermal and biological feedback In the polar desert of Arctic Canada, the presence of mechanisms were observed which influence the deve- permafrost at shallow depths limits deep percolation lopment of patchy wetlands. (1) A shallow active layer and where the local supply of water is ample and rel- reduces the amount of water required to saturate the iable during the thawed season, wetland formation is entire thawed zone, allowing the water table to remain favoured. Vegetation growth and peat formation pro- close to the ground surface throughout summer. (2) vides an insulating surface layer which has lower ther- With water being readily accessible to the plants, vege- mal conductivity than the mineral soils. The saturated tation growth is sustained in the wetlands and this con- condition of the wetland in the freeze back period tributes to the peat accumulation. (3) Insulation by enhances the accretion of ground ice, which in the fol- peat, and ice formation due to abundant soil water lowing summer, consumes much latent heat by phase favour local aggradation of the permafrost. change at the expense of deep thawing of the active layer. A shallow active layer ensures that the wetland On the other hand, the saturated status is subject to profile is largely saturated throughout the summer. change because of several internal processes or external This thermal (permafrost) and moisture (wetland) feed- forcing. back mechanism perpetuates the existence of the patchy wetlands. However, internal processes may heave part (1) Ice-cored mound or palsa formation: Unlimited of the peat above the general wetland water table and water storage in the active layer before freeze back sup- external forces due to stream capture or artificial ports the growth of segregated ice and pore ice. drainage may diminish the water supply or alter the Summer thawing of the ground ice is hindered by the course of water flow. Nevertheless, a humified organic low thermal conductivity of the thawed organic layer. cover 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 the Acknowledgments resumption of adequate water supply will saturate the disturbed wetland and revive its vegetation. This work was funded by a research grant from the Natural Sciences and Engineering Research Council of Canada. Logistical support from the Polar Continental Shelf Project is gratefully acknowledged. We thank Joan Pace and Dallas Stoltz for their assistance in the field.

References

Buttle, J.M. and Fraser, K.E. (1992). Hydrochemical fluxes in Thorsteinsson, R. (1958). Cornwallis and Little Cornwallis a High Arctic wetland basin during spring melt. Arctic and Islands, District of Keewatin, Northwest Territories. Alpine Research, 24, 153-164. Geological Survey of Canada Memoir, 294 (134 pp.). Cruickshank, J. (1971). Soil and terrain units around Woo, M.K., Heron, R., Marsh, P. and Steer, P. (1983). Resolute, Cornwallis Island. Arctic, 24, 195-209. Comparison of weather station snowfall with winter snow accumulation in High Arctic basins. Atmosphere-Ocean, 21, Edlund, S.A. (1992). Vegetation of Cornwallis and adjacent 312-325. islands, Northwest Territories - relationships between veg- etation and surficial materials. Geological Survey of Canada Woo, M.K. and Xia, Z.J. (1995). Suprapermafrost groundwa- Paper 89-26 (24 pp.). ter seepage in gravelly terrain, Resolute, NWT, Canada. Permafrost and Periglacial Processes, 6, 57-72. Glenn, M.S. and Woo, M.K. (1997). Spring and summer hydrology of a valley-bottom wetland, Ellesmere Island, Woo, M.K. and Xia, Z.J. (1996). Effects of hydrology on the Northwest Territories, Canada. Wetlands,17, 321-329. thermal conditions of the active layer. Nordic Hydrology, 27, 129-142. Polunin, N. (1948). Botany of the Canadian Eastern Arctic, Part III, Vegetation and Ecology. Department of Mines and Resources, Bulletin No. 104, Ottawa (304 pp.). RydŽn, B.E. (1977). Hydrology of Truelove Lowland. In Bliss, L.C. (ed.), Truelove Lowland, Devon Island, Canada: A High Arctic Ecosystem. University of Alberta Press, Edmonton, Alberta, pp. 107-136.

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