Region 2 comprises arctic and subarctic North America and is underlain by continuous or discontinuous permafrost. Its freshwater systems are dominated by a low energy environment and cold region processes. Central northern areas are almost totally in¯uenced by arctic air masses while Paci®c air becomes more prominent in the west, Atlantic air in the east and southern air masses at the lower latitudes. Air mass changes will play an important role in precipitation changes associated with climate warming. The snow season in the region is prolonged resulting in long-term storage of water so that the spring¯ood is often the major hydrological event of the year, even though, annual rainfall usually exceeds annual snowfall. The unique character of ponds and lakes is a result of the long frozen period, which aects nutrient status and gas exchange during the cold season and during thaw. GCM models are in close agreement for this region and predict temperature increases as large as 48C in summer and 98C in winter for a 2 Â CO 2 scenario. Palaeoclimate indicators support the probability that substantial temperature increases have occurred previously during the Holocene. The historical record indicates a temperature increase of 418C in parts of the region during the last century. GCM predictions of precipitation change indicate an increase, but there is little agreement amongst the various models on regional disposition or magnitude. Precipitation change is as important as temperature change in determining the water balance. The water balance is critical to every aspect of hydrology and limnology in the far north. Permafrost close to the surface plays a major role in freshwater systems because it often maintains lakes and wetlands above an impermeable frost table, which limits the water storage capabilities of the subsurface. Thawing associated with climate change would, particularly in areas of massive ice, stimulate landscape changes, which can aect every aspect of the environment. The normal spring¯ooding of ice-jammed north-¯owing rivers, such as the Mackenzie, is a major event, which renews the water supply of lakes in delta regions and which determines the availability of habitat for aquatic organisms. Climate warming or river damming and diversion would probably lead to the complete drying of many delta lakes. Climate warming would also change the characteristics of ponds that presently freeze to the bottom and result in fundamental changes in their limnological characteristics. At present, the food chain is rather simple usually culminating in lake trout or arctic char. A lengthening of the growing season and warmer water temperature would aect the chemical, mineral and nutrient status of lakes and most likely have deleterious eects on the food chain. Peatlands are extensive in region 2. They would move northwards at their southern boundaries, and, with sustained drying, many would change form or become inactive. Extensive wetlands and peatlands are an important component of the global carbon budget, and warm...

Peat cores from five Sphagnum-dominated peatlands in boreal continental Canada were analyzed for plant macro fossils. Results indicate that peatland development was influenced both by local autogenic and regional climatic factors. The general direction in peatland development from rich fen to poor fen to bog can primarily be ascribed to internal processes, especially peat accumulation. Quantitative paleoenvironmental reconstructions based on fossil moss assemblages indicate that all five peatlands were initially dominated by brown mosses with inferred pHs of approximately 6.0, and a water table at 5–15 cm below the surface of the peatland. Subsequently, Sphagnum-dominated peatlands developed with pHs of 4.0–4.5 and a water table at 15–30 cm of depth. Chemical factors triggered a rapid transition from rich fen (pH > 6) to poor fen and bog (Ph < 5). The two most southerly peatlands are youngest, with basal dates of 4670 BP and 4230 BP. Sphagnum peat accumulation at these sites started at 2620 BP and 1790 BP, respectively. Two sites located at intermediate latitudes have basal dates of > 5140 BP and 5020 BP, while the development of Sphagnum-dominated ecosystems dates back to ≈ 3100 BP and 3710 BP, respectively. The most northerly site has the oldest basal date (> 7870 BP), and the oldest date for the initiation of Sphagnum peat accumulation (≈ 7000 BP). The younger age of the peat deposits in the four southern sites is due to warm and dry climatic conditions during the middle Holocene that prevented peatland development until after 6000 BP when the climate gradually became cooler and moister. Farther north the climate was cool and moist enough to allow peatland development during the early to middle Holocene. In three southern peatlands, the development into a Sphagnum-dominated ecosystem took > 2000 years, while at the more northerly sites Sphagnum became dominant after < 1500 years. Key words: Sphagnum, peatlands, boreal, Holocene, climate.

The development of the present day vegetation, water chemistry, and landform features of a peatland complex in northeastern Alberta, Canada, was studied through analyses of macrofossils, peat stratigraphy, and inferred profiles. The Mariana Lakes peatland (55°54′N, 112°04′W) contains two geographical areas and four landform features (open lake basin, closed lake basin, water tracks, and forested Sphagnum islands). The surface vegetation was classified using TWINSPAN into four vegetation groups: (i) shrubby, open forest of Salix pedicellaris and Carex paupercula, (ii) closed Picea mariana forest, (iii) Sphagnum lawns of Sphagnum fallax, and (iv) Sphagnum carpets of Sphagnum subsecundum and Menyanthes trifoliata. Within the peatland complex, surface water chemistry ranges from a pH of 3.6 to 6.3, reduced conductivity of 0 to 232 μS, calcium content of 1.1–15.6 mg/L, and magnesium content of 0.6–2.8 mg/L. Peat macrofossils analyzed quantitatively were arranged (by TWINSPAN) into six macrofossil assemblages and were correlated to peat calcium content. Inferred pH profiles derived from linear regression transformations, and inferred-moisture profiles derived from a weighted moisture index, describe natural acidification and water levels. The mean initial inferred pH was at 5.6 ± 0.2, and mean initial inferred moisture indexed at 3.1 ± 0.2. Mean present day inferred pH is 4.9 ± 0.7, and mean moisture is 2.8 ± 0.6. Organic deposition at Mariana Lakes began about 8200 years BP in the closed lake basin. Filling-in of the lake basin produced a floating mat that upon enclosure developed into a poor fen. Paludification of the upper fen began about 6000 years BP, subsequent to terrestrialization in the lake basin, and progressed in a downslope direction. By 2960 years ago the entire drainage path was paludified.

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