Under-ice plankton abundance and lipid dynamics in a subarctic lake.

Abstract

Shortening winters and changing ice and snow cover conditions are examples of the many shifts expected in subarctic lakes due to climate change. Well-documented changes in ice and snow cover conditions highlight the need to understand under-ice ecosystems and the consequences a changing climate brings to the freshwater biota. This study investigated the impact of under-ice environmental conditions and seasonal progression on the plankton community and lipid dynamics in Takvatn, a well-studied subarctic oligotrophic lake in Northern Norway. In particular, this study focused on two actively overwintering copepods, Cyclops scutifer, and Eudiaptomus graciloides. The two species differed in their life strategies and fatty acid composition; C. scutifer overwintered in the C-IV to C-V stage, while E. graciloides reproduced under ice-cover. A rapid decrease in lipid content throughout the ice-covered period was observed for both E. graciloides (from 60% to 38% DW-1) and C. scutifer (from 73% to 33% DW-1). The results indicate the importance of lipid accumulation before ice-cover for the actively overwintering copepods to survive the ice-cover period. In addition, under-ice primary production, which was apparent during the last months of ice-cover, provided a food source for E. graciloides. Compared to the previous winter, this winter zooplankton had a higher proportion of lipids to body mass. This suggests the current winter to have favourable conditions and might allow zooplankton to survive harsher/longer periods of ice-cover. Further, E. graciloides had large amounts of poly unsaturated fatty acids (in particular Stearidonic acid, 18:4n-3) which decreased rapidly and might be linked to under-ice reproduction in this species. Future changes in ice-cover duration and snow and ice conditions due to climate change effects may play a significant role in the quantity and quality of lipids of an organism. This study contributes to the understanding of plankton and lipid dynamics during the understudied winter period under ice-cover and provides a reference point for future under-ice studies.