Climate and cryospheric observations have shown that the high Arctic has experienced several decades of rapid environmental change, with warming rates well above the global average. In this study, we address the hypothesis that this climatic warming affects deep, ice-covered lakes in the region by causing abrupt, threshold-dependent shifts rather than slow, continuous responses. Synthetic aperture radar (SAR) data show that lakes (one freshwater and four permanently stratified) on Ellesmere Island at the far northern coastline of Canada have experienced significant reductions in summer ice cover over the last decade. The stratified lakes were characterized by strong biogeochemical gradients, yet temperature and salinity profiles of their upper water columns (5-20 m) indicated recent mixing, consistent with loss of their perennial ice and exposure to wind. Although subject to six decades of warming at a rate of 0.5uC decade 21 , these lakes were largely unaffected until a regime shift in air temperature in the 1980s and 1990s, when warming crossed a critical threshold forcing the loss of ice cover. This transition from perennial to annual ice cover caused another regime shift whereby previously stable upper water columns were subjected to mixing. Far northern lakes are responding discontinuously to climate-driven change via a cascade of regime shifts and have an indicator value beyond the regional scale.There is a broad consensus that the world is entering a period of accelerated climate change (Solomon et al. 2007). Anthropogenic emissions of greenhouse gases and the resultant increase in net radiative forcing are widely accepted as the cause of this rapidly changing climate (Hansen et al. 2006). We are now faced with the challenge of evaluating the extent and pace of this change and its effect on the biosphere.Computer models and direct observations (Serreze and Francis 2006) indicate that the highest amplitude and most rapid climate changes are occurring in the Arctic. Although there is considerable variability among regions, the annual average air temperature in the North American continental Arctic increased by 1.06uC decade 21 during the last two decades (Comiso 2003), well above the global average of 0.2uC decade 21 during the same period (Hansen et al. 2006). The thinning and decrease in extent of Arctic sea ice cover (Maslanik et al. 2007) will lead to even more pronounced changes through an albedo-driven positive feedback that would cause Arctic Ocean warming, a further decrease in sea ice extent, and more amplification of high latitude warming.Many reports have shown significant climate change and associated ecosystem effects in the Arctic since the beginning of the short observational period in this region (Serreze et al. 2000). Long-term trends, however, have been difficult to resolve in the Arctic, in part because of the lack of site-specific data and substantial interannual variability in climate, which is exacerbated by the Northern Annular Mode. These factors act to obscure underlying tre...

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We profiled five ice-covered lakes and two ice-covered fiords of Ellesmere Island at the northern limit of High Arctic Canada to examine their environmental characteristics, and to evaluate the long-term limnological consequences of changes in their surrounding landscape through time (landscape evolution). All of the ecosystems showed strong patterns of thermal, chemical, and biological stratification with subsurface temperature maxima from 0.75 to 12.15 °C; conductivities up to 98.1 mS cm–1 (twice that of seawater) in some bottom waters; pronounced gradients in nitrogen, phosphorus, pH, dissolved inorganic and organic carbon, manganese, iron, and oxygen; and stratified photo synthetic communities. These ecosystems form an inferred chronosequence that reflects different steps of landscape evolution including marine embayments open to the sea, inlets blocked by thick sea ice (Disraeli Fiord, Taconite Inlet), perennially ice-capped, saline lakes isolated from the sea by isostatic uplift (Lakes A, C1, C2), and isolated lakes that lose their ice cover in summer. The latter are subject to entrainment of saline water into their upper water column by wind-induced mixing (Lake Romulus; Lake A in 2000), or complete flushing of their basins by dilute snowmelt (Lake C3 and Char Lake, which lies 650 km to the south of the Ellesmere lakes region). This chronosequence illustrates how changes in geomorphology and other landscape properties may influence the limnology of coastal, high-latitude lakes, and it provides a framework to explore the potential impacts of climate change.