Microbial community diversity and heterotrophic production in a coastal Arctic ecosystem: A stamukhi lake and its source waters

Stamukhi lakes are vast but little-explored Arctic ecosystems. They occur throughout winter, spring, and early summer near large river inflows along the Arctic coastline, and are the result of freshwater retention behind the thick barrier of rubble ice (stamukhi) that forms at the outer limit of land-fast sea ice. We examined the molecular biodiversity within all three microbial domains (Bacteria, Archaea, and Eukaryota) and the heterotrophic productivity in Lake Mackenzie, a stamukhi lake in the western Canadian Arctic, and made comparative measurements in the freshwater (Mackenzie River) and marine (Beaufort Sea) source waters. Bacterial and eukaryotic communities in the stamukhi lake differed in composition and diversity from both marine and riverine environments, whereas the archaeal communities were similar in the lake and river. Bacteria 16S ribosomal RNA sequences from the lake were mostly within freshwater clusters of Betaproteobacteria and Bacteroidetes and the Archaea were within the Lake Dagow sediment and Rice cluster-V clusters of Euryarchaeota. The eukaryotes were mainly ciliates from the subclass Choreotrichia, and there was a notable lack of flagellates. Heterotrophic production rates in the lake were lower than in the river and more similar to those in the sea, despite much higher bacterial concentrations than in either. The lake samples had markedly higher ratios of 3H leucine to 3H thymidine incorporation than in the river and sea, implying some physiological stress. Lake Mackenzie is an active microbial ecosystem with distinct physical and microbiological properties. This circumpolar ecosystem type, vulnerable to the ongoing effects of climate change, likely plays a key functional role in processing riverine inputs to the Arctic Ocean. The application of molecular microbiological techniques to temperate estuaries has uncovered diverse, active, and spatially variable microbial communities (Crump et al. 1999; Kirchman et al. 2005), reflecting the complex hydrography of such environments. Arctic estuaries are also hydrodynamically complex, but they additionally experience extreme seasonality in their physical structure caused by ice formation and melting, which in turn creates diverse conditions for biological processes (Carmack and Macdonald 2002). Arctic estuarine systems include lagoons that form along the Arctic coastline each year behind the barrier of rubble ice (stamukhi) that is generated by ice convergence at the outer edge of the land-fast sea ice. These lagoons, termed ‘‘stamukhi lakes,’’ are widely distributed and are formed especially in the vicinity of large Arctic rivers that push the ice offshore and fill the inshore zone with freshwater (Reimnitz et al. 1978). The ice dams have drafts from 5 to 20 m and are partially grounded, semiporous barriers to water flow. Stamukhi lake ecosystems may persist for over 6 months each year and are common along the coastline of Siberia (Ogorodov et al. 2005) and the Canadian Arctic (Carmack and Macdonald 2002) with their large river inflows. These rivers are major sources of organic carbon entering the Arctic Ocean. Understanding the basic properties of such ecosystems, including their biodiversity and metabolic activity, will be crucial for predicting the consequences of climate change and shifts in the Arctic ice regime. Yet the microbial diversity and productivity of Arctic estuarine systems has been little studied to date, and there are no published accounts of the microbiological properties of stamukhi lakes. A large stamukhi forms every year near the outflow of the Mackenzie River in the western Canadian Arctic (Carmack and Macdonald 2002) and over the winter a vast lagoon of brackish water called ‘‘Lake Mackenzie’’ (or ‘‘Lake Herlinveaux’’; both unofficial names) forms behind this stamukhi (Fig. 1). The lake spreads over an area of 12,000 km2 and contains 70 km3 of water, placing it within the top 20 or 30 lakes of the world by area or volume, respectively (Macdonald 2000). The stamukhi lake disappears in late June–early July, well after the peak annual 1 Corresponding author ([email protected]). 2 Present address: Unitat de Limnologia Departament d’Ecologia Continental, Centre d’Estudis Avançats de Blanes CSIC, 17300 Blanes, Spain. Acknowledgments We thank the officers and crew of the CCGS Amundsen, and C. Martineau, S. Roy, R. Terrado, M.-J. Martineau, and M. Rautio for field assistance and analysis, and R. Macdonald (Fisheries and Oceans Canada) for his insightful comments on stamukhi systems. This work was conducted within the Canada Arctic Shelf Exchange Study (CASES) led by L. Fortier. We also thank the anonymous reviewers for helpful comments and suggestions. Financial support for this study was provided by the Natural Sciences and Engineering Research Council of Canada, the Canada Research Chair program, Fonds québécois de recherche sur la nature et les technologies, and Indian and Northern Affairs Canada. Limnol. Oceanogr., 53(2), 2008, 813–823 E 2008, by the American Society of Limnology and Oceanography, Inc.