In marine ecosystems, krill is a key stone species allowing energy transfer from phytoplankton and mesozooplankton to the higher trophic levels. In the estuary and the Gulf of St. Lawrence (EGSL) the krill biomass, dominated by two genus Meganyctiphanes and Thysanoessa, is estimated at 1 billion tons. However, despite their ecological importance only few studies worldwide and no studies in the EGSL focus on the feeding behaviour and trophic relationship between these two genus and lower trophic levels. In addition, the evolution of food supply and abiotic factors on the krill’s respective physiological conditions are still unknown. To understand how these parameters effect krill population dynamics, the main objective of my PhD is to determinate the evolution of feeding behaviour and the consequential variation of the physiological condition of these dominant krill species in the EGSL. To achieve these goals we will use indirect proxies as total lipids, fatty acids, stable isotopes and RNA/DNA ratio on krill and potential food sources from “in situ” samples over spatial and temporal scales. In addition “in vivo” experiments will allow developing mechanistic functions on feeding rates.