Inferring large-scale patterns of nutrient regeneration in the ocean using a global biogeochemical inverse model

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Sustained biological production in the world ocean is heavily dependent on the recycling of the decomposition products of pre-existing organisms. In particular for the microscopic algae (phytoplankton) at the base of the marine food chain, the decomposition products (for example, carbon, nitrogen, phosphorous, and silicon) of pre-existing organisms become the fertilizer (or nutrient materials) that nourishes later generations. But dead particles that fall out of the sunlit surface ocean where photosynthesis takes place do not necessarily have the same proportions of nutrient elements as that required for making a living algal cell. To understand how this system manages to keep going as well as how it can vary during global climatic change, the research team on this project will use a set of sophisticated mathematical models that account for biological production, regeneration of nutrients, and ocean circulation patterns to create a global model relating production in the surface ocean to the elemental composition of regenerated nutrients. A postdoctoral researcher will be trained in advanced modeling techniques during this study. The stoichiometric ratios of biological export production are critical to our understanding of the major biogeochemical cycles of the ocean. Limited measurements of the elemental composition in phytoplankton and in sinking particulate organic matter suggest that the ratios can vary significantly in space and time. However the number of direct measurements is inadequate to reliably determine coherent large-scale patterns. As a result most global ecosystem and biogeochemical models assume a fixed stoichiometry - an assumption that may limit their ability to correctly simulate changes in ecosystem dynamics in response to climate change. In this study, a small research team will conduct an inverse modeling study using a global ocean circulation model and a global database of hydrographic measurements of dissolved inorganic carbon, nitrate, phosphate, silicic acid and oxygen concentrations together with abiotic tracers including temperature, salinity, chlorofluorocarbons, and radiocarbon to jointly estimate the global ocean circulation and spatially varying elemental ratios (carbon : nitrogen : phosphorous : silicon : oxygen) of the organic matter exported from the euphotic zone. The proposed work has a high intellectual merit because of the innovative computational and inverse modeling strategies that will make it possible to jointly assimilate biotic and abiotic tracer information to estimate the ocean circulation and the major marine biogeochemical cycles. A global database with maps of export production stoichiometry and three-dimensional gridded fields of regeneration ratios will be made available to the community, which will help guide the development of global marine ecosystem and biogeochemical modules used in Earth System Models.