Collaborative Research: Dissolved organic phosphorus controls on marine nitrogen fixation and export production

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Phytoplankton play an important role in the Earth's elemental cycles of carbon and nitrogen. In addition to sunlight, phytoplankton living in the surface waters of the oceans require the elements nitrogen and phosphorus for growth. Much of these nutrients are supplied in their inorganic forms from mixing of deep waters towards the surface during the winter months when vertical stability of the water column breaks down. However much of the low latitude oceans, 45degS45degN, suffer from limited nutrient input to sunlit surface waters due to strong thermal stratification (vertical stability) of the upper water column. As a consequence, tropical and subtropical phytoplankton have devised alternative ways of acquiring nitrogen and phosphorus. Marine nitrogen fixation is a process by which specialized microbes utilize the abundant nitrogen gas from the atmosphere to convert elemental nitrogen into the bioavailable form ammonia. These nitrogen fixing phytoplankton and many others also use organic forms of phosphorus in the low latitude ocean where inorganic nutrients are often scarce. This project will significantly increase the number of dissolved organic nitrogen and dissolved organic phosphorus concentration measurements, especially from the currently under-sampled Pacific and Indian Oceans. Changes in the concentration of organic nutrients across the surface ocean will be used to infer rates of organic nutrient use by phytoplankton in numerical models. Specifically, the role for the biological uptake of dissolved organic phosphorus to stimulate the processes of marine nitrogen fixation and photosynthesis in the low latitude ocean will be quantified from the combined data and model output. The project will train one graduate student and several undergraduate students in both laboratory chemical analysis techniques and numerical simulation of ocean biological and chemical processes. New scientific knowledge will be shared with the public via a social media campaign and will inform the development of the next generation of global climate models. The marine biogeochemical modeling community has identified the lack of dissolved organic nitrogen (DON) and especially dissolved organic phosphorus (DOP) concentration measurements from the upper 300 m of the global ocean as crucial gaps in our ability to accurately model export production and N2 fixation rates in the subtropics. The proposed work will significantly increase global data coverage of marine DON and DOP concentration measurements, in particular from under-sampled ocean regions in the Indian Ocean, western, central, and eastern tropical South Pacific, Gulf of Alaska, eastern subtropical and subpolar South Pacific, Southern Ocean, subtropical North Atlantic, and tropical South Atlantic Ocean basins. These new measurements will be assimilated in state-of-the-art biogeochemical models to constrain the relative cycling rates of DOP and DON and to quantify the role of preferential DOP consumption as a P source supporting export production and N2 fixation in the low latitude ocean. Model output will solve for the regionally-resolved fraction of new production that accumulates as DON and DOP, autotrophic DOP uptake rates, as well as the remineralization rates for DON and DOP. The model output will also include the first regionally variable rate estimates of euphotic zone DOP consumption sustaining export production and N2 fixation to be constrained by observations from the Pacific and Indian Oceans. Thus, the new concentration measurements and diagnostic modeling will allow us to evaluate the quantitative role for regional variability in DOP consumption and recycling that supports export production and N2 fixation in the low latitude ocean.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.