Award: OCE-1434007

Marine phytoplankton are an extremely diverse set of single-celled organisms spanning several orders of magnitude in cell size and an enormous range of morphologies, biochemical function, elemental requirements and trophic strategy. Community structure, the assemblage of diverse phytoplankton types, regulates biogeochemical pathways with consequences for carbon cycling, and is an inherent feature to consider in relation to perturbations, such as climate change. In this grant we have worked to understand controls of community structure and biodiversity over large regions of the world?s ocean. A primary tool that we have utilized is a complex computer ecosystem model that has been developed as part of this grant. There are likely many tens, if not hundreds, of thousands of different species of phytoplankton. Instead of trying to model individual species, we have used a trait-based approach: Organisms are characterized by essential traits (such as size, biogeochemical function, trophic strategy, accessory pigments) that capture key aspects of their diversity. We have used the model together with observations and theory to explore aspects of phytoplankton diversity and links to biogeochemical cycling. We have: - taken leadership roles in forwarding the trait-based approach as a novel framework for understanding the complexity and structure of marine ecosystems; - developed and synthesized theoretical interpretation of plankton biogeography; - examined the ecological control on the marine nutrient environment; - explored the controlling drivers of marine biodiversity, and especially how different trait axes are controlled by different mechanisms; - linked regional shifts in diversity to ocean productivity and stability; - used new and disparate measurements of marine diversity from microscopy and ?omics datasets; - explored the role of calcification, mixotrophy, nitrogen fixation, symbiosis, differing stoichiometry, and pigment composition in controlling where certain types of phytoplankton can co-exist, survive and/or dominate; - contributed to a synthesis on how iron is modelled in the ocean; - contributed to synthesis papers on two major groups of phytoplankton: diatoms and coccolithophores; - evaluated the importance of including spectrally resolved light for phytoplankton biogeography; - explored how including complexity in a model alters the marine ecosystem response to climate change relative to simpler models usually used in such studies. As part of this grant we have published 11 peer reviewed articles, 2 workshop reports and one book chapter. An additional article has been submitted and three others are in preparation. Numerous conference proceeding and talks have also disseminated results from this grant. A graduate student was supported on this grant. The work supported by this grant is by nature highly interdisciplinary as is at the intersection of ecology, biogeochemistry and physical climate modelling. We are frequently asked to present to our findings at conferences and workshops in these very different fields, and believe that our work has been influential in facilitating new cross-discipline collaborations. The computer model code is freely available and we have worked with national and international scientists to help them use this model for their own studies. We have also provided output from the model simulations to several colleagues for their research. The model has provided beautiful imagery and movies that are frequently used in educational and outreach activities. Examples can be found at: http://darwinproject.mit.edu/media-library. In particular, the movies have been used for a Science on the Sphere dataset (https://sos.noaa.gov/datasets/phytoplankton-model/), for the interactive display Living Liquids at the San Francisco Exploratorium, and for a movie on monitoring the earth that is shown on the dome of the Planetarium in Paris, France. Last Modified: 01/02/2019 Submitted by: Stephanie W Dutkiewicz