Temporal variability in lipid dynamics of three coexisting dominant krill species in the St. Lawrence Estuary

The St. Lawrence Estuary (Canada) is a subarctic ecosystem characterised by a strong stratification with a cold intermediate layer providing different thermal habitats occupied by different krill species (Meganyctiphanes norvegica, Thysanoessa raschii, Thysanoessa inermis), ranging from temperate to arctic. Krill is an important consumer of phyto/zooplankton allowing energy transfer (mostly lipids) towards upper trophic levels. However, lipid dynamics and trophic interactions between these krill species are poorly understood in the St. Lawrence Estuary. Here, we describe the seasonal variability in their lipids classes and fatty acids composition in relation to biotic and abiotic factors. Lipid content of the temperate species M. norvegica showed high levels of triacylglycerol (TAG) comparatively to the arctic species T. inermis. In this latter species,the wax ester (WE) predominated, as in many other arctic species. In contrast to many other regions of its geographical distribution, we detected a significant quantity of WE in M. norvegica that could represent an adaptation of this species to subarctic environments in order to survive during the wintertime. M. norvegica had however less lipid content per individual than T. raschii and T, inermis, whereas M. norvegica is the bigger species. In addition, lipids content of M. norvegica was related to copepod density in the St. Lawrence Estuary suggesting dominance of “carnivorous feeding”. This feeding dominance was confirmed by fatty acids composition of tissues. In opposite, Thysanoessa sp. seems to be more generalist (omnivorous) although with a preference for phytoplankton. Our results are discussed in the context of climate change predicted.