Under ice plankton and lipid dynamics in a subarctic lake

Abstract

Climate warming causes shorter winters and changes in ice and snow cover in subarctic lakes, highlighting the need to better understand underice ecosystem functioning. The plankton community in a subarctic, oligotrophic lake was studied throughout the ice-covered season, focusing on lipid dynamics and life history traits in two actively overwintering copepods, Cyclops scutifer and Eudiaptomus graciloides. Whereas C. scutifer was overwintering in C-IV to C-V stage, E. graciloides reproduced under ice cover. Both species had accumulated lipids prior to ice-on and showed a substantial decrease in total lipid content throughout the ice-covered period: E. graciloides (60%–38% dw) and C. scutifer (73%–33% dw). Polyunsaturated fatty acids of algal origin were highest in E. graciloides and declined strongly in both species. Stearidonic acid (18:4n-3) content in E. graciloideswas particularly high and decreased rapidly during the study period by 50%, probably due to reproduction. The copepods differed infeeding behavior, withthe omnivoreC. scutifer continuingto accumulate lipids until January, whereasthe herbivorousE. graciloides accumulated lipids from under-ice primary production during the last months of ice-cover. Our findings emphasize the importance of lipid accumulation and utilization for actively overwintering copepods irrespective of the timing of their reproduction.