Winter respiration of allochthonous and autochthonous organic carbon in a subarctic clear-water lake

We studied a small subarctic lake to assess the magnitude of winter respiration and the organic carbon (OC) source for this respiration. The concentration and stable isotopic composition (d13C) of dissolved inorganic carbon (DIC) accumulating in the lake water under ice was analyzed over one winter (7 months). The DIC concentration increased and the d13C of DIC decreased over time, with the greatest changes at the lake bottom. Winter respiration was 26% of annual respiration in the lake. Keeling plot analysis demonstrated that the d13C of respired DIC varied spatially, high d13C values occurring at shallow (2.5 m, 221.7%) compared with intermediate (4 m, 225.1%) and deep (6 m, 227.8%) locations in the lake. The variation in the d13C of respired DIC was related to the variation in the d13C of the sediments between locations, suggesting that sediment OC supported much of the winter respiration and that the dominant OC source for respiration was OC from benthic algae at shallow locations and settled OC, of predominately terrestrial origin, at deep locations. The respiration of OC from benthic algae constituted 55% of the winter respiration, equaling 54% of the primary production by benthic algae the previous summer. The study indicates the importance of temporal and spatial variation in respiration for the metabolism and net DIC production in unproductive high-latitude lakes; both allochthonous and autochthonous carbon can contribute to winter DIC accumulation and, consequently, to spring CO2 emissions from lakes. Recent research has provided strong evidence that most unproductive lakes are net heterotrophic, i.e., that total community respiration exceeds gross photosynthetic carbon fixation (Duarte and Prairie 2005). Net heterotrophy is caused by the import and respiration of terrestrial (allochthonous) organic carbon (OC) (del Giorgio et al. 1999; Karlsson et al. 2007), but the metabolic imbalance may be enhanced by an accompanying negative effect of allochthonous OC on lake primary production (Carpenter et al. 1998; Houser et al. 2003). Respiration often exceeds primary production in the pelagic habitat of unproductive lakes (del Giorgio and Peters 1994). Although few comparative data are available, it is clear that respiration in the sediment could be high and also quantitatively important for total respiration and net CO2 production in shallow lakes (Kortelainen et al. 2006). However, in clearwater lakes the benthic respiration should be largely supported, and hence offset, by high carbon uptake by benthic algae, causing the benthic habitat to be close to metabolic balance or even net autotrophic in summer (Algesten et al. 2005). The net heterotrophy of clear-water lakes in summer is therefore mainly an effect of high respiration relative to the CO2 fixation in the pelagic system (Algesten et al. 2005). Mineralization in winter is poorly understood and seldom considered in studies of the metabolic balance of lakes. Although metabolic activity is relatively low in winter, this period could be important for the carbon cycling in lakes in regions subject to long winters (Welch and Bergmann 1985). Including the winter period in annual estimates increases the respiration versus carbon fixation ratio of lakes, the greatest relative importance of the winter period being found in lakes at high latitudes and altitudes. High winter CO2 accumulation and the release of CO2 during ice breakup in spring has been demonstrated in many lakes receiving a high input of terrestrial OC (Striegl et al. 2001). The magnitude of this process has been related to the concentration of dissolved OC (DOC) in the lakes and, from the stable isotopic composition (d13C) of dissolved inorganic carbon (DIC), explained by the mineralization of terrestrial OC in winter (Striegl et al. 2001). However, clear-water lakes can also exhibit considerable CO2 accumulation in winter (Kling et al. 1992; Striegl et al. 2001). The source of the OC that supports winter respiration in clear-water lakes is less obvious. Especially in the case of shallow clear-water lakes, in which benthic algae photosynthesize at a high rate in summer (Vadeboncoeur et al. 2003), it could be expected that part of the CO2 sequestering and buildup of autotrophic biomass in summer could support heterotrophic metabolism the following winter. If that is true, part of the CO2 emission Acknowledgments We thank Thomas Westin for field and laboratory assistance and Anders Olsson and Håkan Wallmark for the stable isotopic analyses. This study was financially supported by the Climate Impacts Research Centre (CIRC), Umeå University, and the Swedish Research Council. Limnol. Oceanogr., 53(3), 2008, 948–954 E 2008, by the American Society of Limnology and Oceanography, Inc.