Fifty-five paleolimnological records from lakes in the circumpolar Arctic reveal widespread species changes and ecological reorganizations in algae and invertebrate communities since approximately anno Domini 1850. The remoteness of these sites, coupled with the ecological characteristics of taxa involved, indicate that changes are primarily driven by climate warming through lengthening of the summer growing season and related limnological changes. The widespread distribution and similar character of these changes indicate that the opportunity to study arctic ecosystems unaffected by human influences may have disappeared.climate change ͉ paleolimnology ͉ Anthropocene ͉ warming ͉ indicators P olar amplification of anthropogenic warming is consistently predicted by general circulation models, largely because of positive feedback mechanisms involving cryospheric processes (1). This heightened climatic sensitivity is supported by recent accelerations of glacier retreat (2), sea-ice thinning (3), and permafrost degradation (4). Although the instrumental record of temperature across the Arctic is incomplete and generally of short duration, warming appears to be concentrated in the decades between approximately anno Domini 1915-1940 and approximately anno Domini 1965. However, proxy data indicate that much of the Arctic began to warm considerably earlier, in the mid-19th century (6). Such generalized trends, however, are neither spatially nor temporally uniform (7), because of regional differences in continentality, ocean heat transport, glacier and sea ice distribution, topography, and vegetation. For example, whereas much of Beringia and central Siberia have warmed Ͼ0.5°C in the last 50 years, there has been little change or even cooling (7) in parts of the North Atlantic sector. Nonetheless, all subregions of the Arctic are considered highly ecologically sensitive, implying that anthropogenic warming will test ecosystem resilience and potentially induce dramatic shifts in community composition.In the absence of long-term climatic and environmental monitoring data, proxy data from the sediments of lakes and ponds, which are ubiquitous features of most arctic landscapes, can be used to provide a long-term perspective of environmental change (8, 9). Siliceous algal remains, specifically the valves of diatoms (Bacillariophyceae) and the stomatocysts and scales of chrysophytes (Chrysophyceae and Synurophyceae), as well as chitinous invertebrate remains (Chironomidae, Diptera and Cladocera, Crustacea), are the primary paleoindicators in lake sediments that provide reliable records of changes in water quality, habitat, and catchment processes (10). Here, we synthesize a large number of paleolimnological records from arctic lakes and ponds, providing a circumpolar assessment of recent ecological changes. These data show that striking and often unprecedented ecological changes have occurred within the last Ϸ150 years, following several millennia of relatively stable communities.High-latitude lakes are extremely responsive t...

A compilation of paleoclimate records from lake sediments, trees, glaciers, and marine sediments provides a view of circum-Arctic environmental variability over the last 400 years. From 1840 to the mid-20th century, the Arctic warmed to the highest temperatures in four centuries. This warming ended the Little Ice Age in the Arctic and has caused retreats of glaciers, melting of permafrost and sea ice, and alteration of terrestrial and lake ecosystems. Although warming, particularly after 1920, was likely caused by increases in atmospheric trace gases, the initiation of the warming in the mid-19th century suggests that increased solar irradiance, decreased volcanic activity, and feedbacks internal to the climate system played roles.

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...

A characteristic feature of most Arctic regions is the many shallow ponds that dot the landscape. These surface waters are often hotspots of biodiversity and production for microorganisms, plants, and animals in this otherwise extreme terrestrial environment. However, shallow ponds are also especially susceptible to the effects of climatic changes because of their relatively low water volumes and high surface area to depth ratios. Here, we describe our findings that some high Arctic ponds, which paleolimnological data indicate have been permanent water bodies for millennia, are now completely drying during the polar summer. By comparing recent pond water specific conductance values to similar measurements made in the 1980s, we link the disappearance of the ponds to increased evaporation/precipitation ratios, probably associated with climatic warming. The final ecological threshold for these aquatic ecosystems has now been crossed: complete desiccation.climatic change ͉ desiccation ͉ environmental change ͉ limnology T he effects of climatic change are expected to be greatly amplified in polar regions because of various positive feedback mechanisms (1). However, because long-term monitoring data are generally lacking, little is known about the ecological repercussions of recent warming. In an attempt to determine the nature and magnitude of environmental changes in these high-latitude, climatically sensitive regions, we have been systematically monitoring a suite of study ponds on Cape Herschel (78°37ЈN, 74°42ЈW; Ellesmere Island, Nunavut, Canada; Fig. 1). We used identical techniques (2, 3) to collect detailed limnological data from the ponds in the summers of 1983, 1984, 1986, 1987, 1995, 1998, 2001, 2004, and 2006. Additional observations were available from other scientists working at Cape Herschel since the 1970s (3). Collectively, these data represent the longest record of systematic limnological monitoring from the high Arctic.In this paper, we summarize over two decades of limnological observations and dovetail these records with our paleolimnological studies. Using these data, we show that some high Arctic pond ecosystems, which represent the most common aquatic habitat in many polar regions, have desiccated as a consequence of climate change, i.e., increasing evaporation/precipitation (E/P) ratios. Results and DiscussionOver the 24-year monitoring window covered by our detailed surveys, we recorded evidence of recent lower water levels and changes in water chemistry data (e.g., increased specific conductance) consistent with increased E/P and warmer temperatures. Until recently, our study sites were permanent features of the landscape, as they contained water until freeze-up in September. However, a major threshold was crossed, associated with the most recent warming trends in the Arctic, because, by early July 2006, several of our main study ponds had dried up, whereas others had very much reduced water levels.The Cape Herschel study sites, many of which have existed for millennia (4, 5), are typical o...

The effects of climate variability on Pacific salmon abundance are uncertain because historical records are short and are complicated by commercial harvesting and habitat alteration. We use lake sediment records of delta15N and biological indicators to reconstruct sockeye salmon abundance in the Bristol Bay and Kodiak Island regions of Alaska over the past 300 years. Marked shifts in populations occurred over decades during this period, and some pronounced changes appear to be related to climatic change. Variations in salmon returns due to climate or harvesting can have strong impacts on sockeye nursery lake productivity in systems where adult salmon carcasses are important nutrient sources.

Paleolimnological data from three high-arctic ponds on Cape Herschel, Ellesmere Island, Canada, show that diatom assemblages were relatively stable over the last few millennia but then experienced unparalleled changes beginning in the 19th century. The environmental factors causing these assemblage shifts may be related to recent climatic warming. Regardless of the cause, the biota of these isolated and seemingly pristine ponds have changed dramatically in the recent past and any hopes of cataloging natural assemblages may already be fruitless.

Recent and historical deposition of mercury (Hg) was examined over a broad geographic area from southwestern Northwest Territories to Labrador and from the U.S. Northeast to northern Ellesmere Island using dated sediment cores from 50 lakes (18 in midlatitudes (41-50 degrees N), 14 subarctic (51-64 degrees N) and 18 in the Arctic (65-83 degrees N)). Distinct increases of Hg overtime were observed in 76% of Arctic, 86% of subarctic and 100% of midlatitude cores. Subsurface maxima in Hg depositional fluxes (microg m(-2) y(-1)) were observed in only 28% of midlatitude lakes and 18% of arctic lakes, indicating little recent reduction of inputs. Anthropogenic Hg fluxes adjusted for sediment focusing and changes in sedimentation rates (deltaF(adj,F)) ranged from -22.9 to 61 microg m(-2) y(-1) and were negatively correlated (r = -0.57, P < 0.001) with latitude. Hg flux ratios (FRs; post-1990)/pre-1850) ranged from 0.5 to 7.7. The latitudinal trend for Hg deltaF(adj,F) values showed excellent agreement with predictions of the global mercury model, GRAHM for the geographic location of each lake (r = 0.933, P < 0.001). The results are consistent with a scenario of slow atmospheric oxidation of mercury, and slow deposition of reactive mercury emissions, declining with increasing latitude away from emission sources in the midlatitudes, and support the view that there are significant anthropogenic Hg inputs in the Arctic.

Historical catch records suggest that climatic variability has had basin-wide effects on the northern Pacific and its fish populations, such as salmon, sardines and anchovies. However, these records are too short to define the nature and frequency of patterns. We reconstructed approximately 2,200-year records of sockeye salmon abundance from sediment cores obtained from salmon nursery lakes on Kodiak island, Alaska. Large shifts in abundance, which far exceed the decadal-scale variability recorded during the past 300 years, occurred over the past two millennia. A marked, multi-centennial decline in Alaskan sockeye salmon was apparent from approximately 100 BC to AD 800, but salmon were consistently more abundant from AD 1200 to 1900. Over the past two millennia, the abundances of Pacific sardine and Northern anchovy off the California coast, and of Alaskan salmon, show several synchronous patterns of variability. But sardines and anchovies vary out of phase with Alaskan salmon over low frequency, which differs from the pattern detected in historical records. The coherent patterns observed across large regions demonstrate the strong role of climatic forcing in regulating northeastern Pacific fish stocks.