LAUSR

Abstract Krill, as forage species, play a pivotal role in marine food webs by linking lower to upper trophic levels. Despite their ecological relevance, processes affecting their physiological condition and their feeding selectivity remain not well known. Here, we hypothesize that significant spatial fluctuations of environmental conditions in a Subarctic region affect the energy reserve content and feeding strategies of krill. To test this hypothesis, we assessed the fatty acid content and composition of two dominant northern krill species Meganyctiphanes norvegica and Thysanoessa inermis on a seasonal basis over more than 1000 km in the Estuary and Gulf of St. Lawrence (eastern Canada) and for the first time during the ice covered winter. M. norvegica exhibited large spatial variation of its energy reserve lipids, which were similar to seasonal variations found in a previous study in the same environment. In contrast, lipid content of T. inermis was much less affected by the spatial variations of environmental conditions but showed high seasonal variability. Feeding preferences assessed using fatty acid trophic markers highlight that the zooplankton standing stock could be more critical than phytoplankton to fulfill the metabolic requirements of these krill species and to build lipid energy reserves. In addition, diet comparison between both species highlights that trophic niche partitioning is present in the Gulf of St. Lawrence, however, modulated by spatial differences in food supply and oceanographic conditions.