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The formation of many arctic wetlands is associated with the occurrence of polygonpatterned permafrost. Existing scenarios to describe and explain surface landforms in arctic wetlands (low-center and high-center polygons and polygon ponds) invoke competing hypotheses: a cyclic succession (the thaw-lake hypothesis) or a linear succession (terrestrialization). Both hypotheses infer the predictable development of polygon-patterned wetlands over millennia. However, very few studies have applied paleoecological techniques to reconstruct long-term succession in tundra wetlands and thereby test the validity of existing hypotheses. This paper uses the paleoecological record of diatoms to investigate long-term development of individual polygons in a High Arctic wetland. Two landform processes were examined: (1) the millennial-scale development of a polygon-pond, and (2) the transition from lowcenter to erosive high-center polygons. Diatom assemblages were quantified from habitats associated with contrasting landforms in the present-day landscape, and used as an analog to reconstruct past transitions between polygon types. On the basis of this evidence, the paleoecological record does not support either of the existing models describing the predictable succession of polygon landforms in an arctic wetland. Our results indicate a need for greater paleoecological understanding, in combination with in situ observations in present-day geomorphology, in order to identify patterns of polygon wetland development and elucidate the long-term drivers of these landform transitions.

The influence of natural terrestrial disturbances on the limnology of northern lakes is little known, yet important for understanding the ecology of these remote ecosystems. The Riviere George Caribou Herd (RGCH) in northern Quebec has undergone a large population increase since the 1950s, which has been accompanied by pronounced impacts (i.e., degradation of vegetation cover, soil erosion) on terrestrial environments of the Riviere George region. The goal of our study was to evaluate if the increased caribou activity and resulting terrestrial impacts have been accompanied by impacts on adjacent aquatic ecosystems. We studied the recent diatom assemblages (i.e., about the last two centuries) preserved in six sediment cores taken from lakes located in the most heavily impacted region, and in one core from a lake showing little evidence of recent caribou activity in the catchment basin. Core chronologies based on 210Pb dating techniques indicated that average mass sedimentation rates in this subarctic region were very low (0.0039 to 0.012 g cm-2 yr-1). Perhaps surprisingly, diatom assemblages from all seven cores showed very little change, indicating remarkably stable limnological conditions throughout the past -200 yr. The lack of a signal from caribou activity is most likely due to (1) short-term impacts being too transitory to be registered in the sediment record, and (2) long-term impacts not representing a significant perturbation outside the natural variability of these aquatic ecosystems. In contrast to other arctic regions, the limnological stability of our study sites suggests that recent climatic change impacts have been negligible in this region of northern Quebec, which seems to confirm climate model predictions that northern Quebec and Labrador will remain climatically stable under global warming scenarios.

Paleolimnological analyses of two lakes located near the northern treeline in the central part of the Canadian Northwest Territories document a history of abrupt postglacial climatic and limnological changes. A diatom-based transfer function, based on weighted-averaging partial least squares regression (WA-PLS) techniques, was used to give quantitative estimates of past trends in lakewater dissolved organic carbon (DOC), a limnological variable strongly aligned with catchment vegetation and soils. The diatom record from the sediment cores provides evidence for profound limnologic change during the mid-Holocene, corresponding to maximum forest-tundra development between 5000 and 3000 '4C yr BP in both Queen's and Toronto lakes, with a diatom-inferred relative change in lakewater DOC of up to 5.8 mg L-1 between the mid-Holocene and the present-day. Comparison of the diatom-inferred environmental changes with other proxy data (pollen, stable isotopes) from the same lakes provides strong evidence for an episode of climatic amelioration and lake responses to the associated vegetational changes (from tundra to forest-tundra) at sites near the central Canadian treeline. This study illustrates the usefulness of diatoms as quantitative indicators of past climate-related environmental change in northern treeline regions, and implies that aquatic ecosystems at high latitudes might respond with extreme sensitivity to climate warming.