Linking phytoplankton physiology to the biogeochemistry of silicon and carbon in the ocean

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Marine phytoplankton are key components of the Earth system. They contribute to half of the world's carbon fixed by photosynthesis annually, play a fundamental role in nutrient cycling, and are at the base of marine ecosystems. Of all phytoplankton groups, diatoms account for ~20% of Earth's photosynthesis and are the main biological drivers of processes involved in the cycling of silicon, carbon and other nutrients. Production of silica cell walls by diatoms in surface waters, dissolution of this biogenic silica through the water column, and export of biogenic silica and associated carbon from surface to deep waters are some of the most important fluxes in the oceanic silicon and carbon cycles. Despite silicon being essential for diatom growth and the second most abundant element on Earth, our knowledge of these processes is still limited compared to those of other nutrient cycles. This proposal describes an integrated field and laboratory program that will advance our understanding of these critical, but poorly known, processes of the silicon cycle and how they couple with those of other elements, such as carbon, in high-latitude oceans. Climate variability is disproportionally affecting the oceans surrounding Canada, which are experiencing changes in sea-ice cover, water mass circulation, nutrient inputs, temperature, salinity, and ocean acidity. It is therefore critical to experimentally evaluate phytoplankton responses to change and their impact on silicon and carbon biogeochemistry in sub-Arctic and Arctic Oceans. During the next 5 years we will study: (A) the linkage between diatom silicon use and natural silicon isotopic signals from the ocean surface to sediments, which is crucial to quantify for resolving broad spatial and temporal variations in silicon and carbon cycles, and diatom production from the past to the present ocean, and (B) the impact of diatom assemblage composition (size and taxa) on silica production and carbon export, the response of diatom assemblage silicification and structure to ocean acidification, and the relationships between silicification, particle aggregation and carbon export potential to the deep ocean. These short-term objectives will contribute to the long-term goals of my research program, which are to better understand the links between phytoplankton eco-physiology and biogeochemical cycling in the ocean. This research will employ leading-edge methodologies, and provide outstanding training and collaborative opportunities for students who will be exposed to multi-disciplinary science. Our data will critically improve the predictive capacity of computer ocean models to assess the effects of climate variability on the atmospheric-oceanic carbon equilibrium and sustainability of marine ecosystems. Ultimately, policy makers can use this knowledge to create plans and policies that mitigate the effects of climate on marine resources and provide services of benefit to Canadians.