Understanding biogeochemical dynamics in the ocean and predicting responses of marine ecosystems to climate change and other stressors require a good description and knowledge of the structure and biological processes in pelagic food webs. In marine ecosystems, zooplankton occupy a key trophic position in plankton food webs, playing a pivotal role in marine biogeochemical cycling and fish recruitment. Knowledge of zooplankton predator-prey interactions is essential for better comprehension of the factors regulating the structure of marine food webs and therefore, for an integrated understanding of the marine ecosystems dynamics.Traditionally, models of pelagic marine food webs quantify interactions between species or functional types, but attempts to embrace the inherent complexity of marine food webs make these models very complex. An emerging alternative approach, the trait based approach, proposes to replace the thousands of species with organisms that are characterized by a few key traits (i.e. those characteristics/features that are essential to the success – fitness - of an organism) and their associated trade-offs (what are the costs and benefits of a particular trait). The present project aims to experimentally quantify the feeding tradeoffs (i.e., feeding efficiency vs. metabolic cost and mortality risk) associated with the three main feeding behaviors in zooplankton and to construct trait-based models to predict zooplankton trait distributions in the ocean. Overall, the proposed project will increase our ability to understand the factors that govern the structure and function of pelagic food webs, and predict their changes under different environmental conditions, including global climate change scenarios.