Crossing the final ecological threshold in high Arctic ponds

Abstract

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...

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