Lipid transfers within the lower food web of western Arctic seas

Abstract

Polar marine ecosystems are undergoing major transformations due to climate change. Alterations of the physical and chemical growth conditions for plankton are likely to propagate through the food web, potentially having major consequences for the animals that Inuit harvest for subsistence. These alterations can impact the nutritional qualities of plankton through physiological responses or indirectly via changes in the taxonomic composition of assemblages. This study focuses on lipids, which provide a vital source of energy in cold waters, and on essential fatty acids (EFA), such as omega-3 and omega-6 FA, which are necessary for the function, growth, and reproduction of organisms. To better understand how EFA propagate and accumulate in the lower food web, we compared the fatty acid composition of phytoplankton and copepods across a large portion of the western Arctic. The bulk lipid profiles of copepod assemblages were determined largely by their taxonomic composition, but specific fatty acid groups (e.g., omega-6 and saturated fatty acids) exhibited strong correlations with water properties (e.g., pH, salinity). A comparative analysis of specific fatty acids suggests that the animals accumulate eicosapentaenoic acid (EPA) but metabolize its precursors, although these processes were not measured directly. The proportions of EPA, docosahexaenoic acid, and specific omega-7 FA (e.g., 16:1ω7) were much higher in copepods than in phytoplankton, and a fatty acid diatom marker accounted for over 45% of the fatty acid pool in copepods. The positive relationship between proportions of this marker in copepods and phytoplankton implies that the fatty acid content of copepods is influenced primarily by recent feeding. These results underscore the importance of long-lived subsurface chlorophyll maxima for trophic transfers of EFA in the strongly stratified waters of the Pacific-influenced sector of the Arctic Ocean and question the paradigm that copepods rely principally on brief surface blooms to rapidly store lipids for the following winter.