We study the ecology and biogeochemical role of phytoplankton, the unicellular photosynthetic organisms that live in the surface waters of all aquatic ecosystems. In particular we focus on phytoplankton diversity and community structure, and key metabolic processes such as primary production, nitrogen fixation, calcification, and respiration. Our approach is based on combining field observations, experiments with phytoplankton populations and microbial plankton communities, data meta-analysis, and modelling. The ultimate goal of this research is to obtain a mechanistic understanding of the linkage between environmental variability, community structure, and biogeochemical fluxes in the ecosystem. Current topics under investigation include: Effect of environmental fluctuations on phytoplankton community structure and metabolism Role of temperature and resources in controlling metabolic rates Size-scaling of phytoplankton abundance, growth and metabolism