"Marine phytoplankton and zooplankton communities mediate a large flux of carbon from the atmosphere into the ocean. This biogeochemical process has traditionally been modelled with the focus primarily on the supply and consumption of essential nutrients. I propose to broaden this approach to resolve important processes occurring at higher trophic levels. I plan to use observations and food-web models to examine how trophic interactions influence and control large-scale ecology and biogeochemical cycling. I will address two outstanding questions that have not yet been examined from this perspective.1) Marine ecosystem models have traditionally classified phytoplankton as strictly photoautotrophic consumers of inorganic nutrients. This view is challenged by recent observations showing that the majority of photosynthetic plankton also graze upon a large fraction of the bacterial community. These ``mixotrophic'' species combine autotrophic and heterotrophic nutrition. They have a large capacity to increase the flux of carbon through the marine biota, but their ecology is not yet clearly understood. I will explore the mechanisms by which mixotrophs are able to compete in different environments, and I will examine how their success affects the cycling of elements at the ocean basin scale.2) Phytoplankton show large variability in their elemental composition. These changes are driven by adaptation and acclimation to different environments, and hence the changing balance of marine communities should affect elemental ratios across broad environmental gradients. This elemental variability is an important component of the global carbon cycle, but the underlying links between physiology, ecology and biogeochemistry are still poorly understood. I will ask how biodiversity can facilitate observed ecological and biogeochemical distributions on the global scale, and examine how physiological changes may affect the biogeochemical function of marine communities."