Award: OCE-1829797

The goal of the project was to evaluate the role of dissolved organic phosphorus ("DOP") in supporting the fertility of surface oceans when inorganic forms of phosphorus ("phosphate") are scarce. Previously, there were not enough high-quality surface ocean DOP concentration measurements from the global ocean to evaluate this question, even though lab-based experiments and modeling work suggested that DOP should be important in supporting the fertility of low-latitude surface oceans. This project confirmed that DOP is an important source of phosphorus supporting fertility, or photosynthesis, of global surface oceans. Estimates from a modeling paper resulting from this work (Letscher et al., 2022, Global Biogeochemical Cycles) indicate that DOP supports 14% of marine photosynthesis annually, but that its role varies spatially, and that DOP represents up to 80% of the phosphorus supply in regions such as the Sargasso Sea. These estimates were made possible by the creation of new, publicly available, marine DOP concentration database (Liang et al., 2022, Scientific Data), which also enabled the first evaluation of the controls on surface ocean DOP concentrations. In Liang et al., 2022 (Nature Geosciences) we showed that surface ocean phosphate concentrations were the primary control on surface ocean DOP concentrations, such that when phosphate concentrations are low there is increased consumption of DOP as a source of phosphorus for plant growth in the surface ocean. However, the same study also showed a novel relationship between surface ocean DOP concentration and iron availability. Since iron availability influences the fertility of the ocean, iron availability is a secondary control on surface ocean DOP concentration, determining whether DOP can be accessed by organisms in the surface ocean. Additional work supported by this project has taken advantage of the new DOP concentration database to evaluate ocean basin-scale trends of DOP concentration that delineate where DOP is produced and consumed. These novel observations confirm hypotheses that DOP cycles differently between the Atlantic and Pacific oceans. We found increasing DOP consumption from east to west across the Pacific Ocean, and from south to north across the Atlantic Ocean. These trends are related to other large-scale biogeochemical processes occurring in each basin, including denitrification in the oxygen deficient zones of the eastern Pacific, and atmospheric dust deposition to the north Atlantic. Other work supported by this project (Bonnet et al., 2023, Science and Forrer et al., 2023, Frontiers in Marine Science) showed that the southwest Pacific Ocean is unique because hydrothermal vents fertilize the surface ocean with iron, supporting exceptionally high rates of photosynthesis and nitrogen fixation, processes that support the base of the food web. Finally, additional manuscripts are in preparation, including one that quantifies the role of DOP in supporting nitrogen fixation in the surface ocean, which is also expected to be regionally variable. Broader Impacts supported by this project included the training of undergraduate and Ph.D. students at Florida State University, outreach to the broader public via "Ask a Scientist" in person and online events, giving a lecture to a class at Stockholm University, recorded and shared on YouTube, and sharing the results of this work at national and international conferences and the sharing of all data and model code via free, open-access platforms. Last Modified: 12/12/2023 Submitted by: AngelaNKnapp