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 (δ 13 C) 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 δ 13 C 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 δ 13 C of respired DIC varied spatially, high δ 13 C 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 δ 13 C of respired DIC was related to the variation in the δ 13 C 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 CO 2 emissions from lakes.