Seasonal shifts between autochthonous and allochthonous carbon contributions to zooplankton diets in a subarctic lake

Abstract

We addressed the extent to which autochthonous and allochthonous carbon sources contribute to zooplankton diet throughout the year in oligotrophic subarctic Lake Saanajärvi. Optical measures of dissolved organic matter (DOM) indicated high water discharge and associated terrestrial DOM during winter and low inputs in summer. Bulk particulate organic matter (POM) showed δ 13 C values consistent with allochthonous inputs of DOM. The most positive POM δ 13 C values (−27‰) occurred during winter, when heterotrophic bacteria and nanoflagellates peaked in abundance; the isotopically lighter autotrophic phytoplankton shifted the POM δ 13 C to −29‰ in summer. The δ 13 C values of zooplankton were up to 4‰ more negative than those of POM, most likely because detritus and other nonliving material with higher δ 13 C values comprised 45–90% of POM. The δ 13 C of the cladoceran Daphnia umbra mirrored the trend of POM δ 13 C, indicating a dietary shift from heavy dependence on autochthonous carbon in summer to allochthonous or heterotrophic food in winter. The δ 13 C of the most abundant zooplankton in Lake Saanajärvi, Eudiaptomus graciloides and Cyclops abyssorum , remained low and constant in winter, suggestive of little or no food intake during the 8 months of winter. There are significant differences in the zooplankton utilization of nonphytoplankton carbon in different seasons and among species. Although autochthonous carbon seemed to be mainly responsible for the growth of zooplankton, the ability to utilize allochthonous carbon‐fueled microorganisms explains the survival of Daphnia through winter in northern lakes.