Award: OCE-1829916

The major plant nutrients nitrogen and phosphorus are most often in very low abundance and supply within sunlit surface waters across vast stretches of the ocean surface centered on the tropics and subtropics. Marine phytoplankton (algae) have evolved adaptations to circumvent this limited supply of the necessary nutrients, for instance by fixing their own bioavailable nitrogen from the inert nitrogen gas found in the atmosphere, but these adaptations usually have a cost to the organism such as an additional expenditure of energy or the requirement of a supply of other environmental factors (nutrients, vitamins), resulting in reduced growth rates. This project investigated the role for the alternative phosphorus source, dissolved organic phosphorus (DOP), to serve has an important additional plant nutrient sustaining phytoplankton growth and production in ocean surface waters. Greater than seven hundred seawater samples collected from around the globe were analyzed for DOP concentration focused in the Pacific basin, augmenting the extant marine DOP dataset by ~20%, which was previously concentrated within the Atlantic Ocean. Analysis of the resulting dataset uncovered a previously undocumented role for iron supply, an important micronutrient to phytoplankton, in helping control when and where in the ocean DOP accumulates versus when it is consumed as an alternative phosphorus source to phytoplankton. It was found that DOP use as a phosphorus source to marine phytoplankton is greatest in regions where the plankton are (inorganic) phosphorus stressed but alleviated from iron stress. This finding further unravels the mechanisms by which the geochemical cycling of iron, phosphorus, nitrogen, and carbon are intimately linked via their interactions with marine biology, ultimately advancing our ability to predict the sensitivities of marine ecosystems to changes in ocean physics and chemistry into the future as well as to resolve what has happened previously in Earth?s history. The new marine DOP dataset generated from this project was used to validate a numerical model of the marine phosphorus cycle to quantitatively diagnose the importance of DOP supply to marine phytoplankton primary productivity and subsequent export of the resulting organic matter to the ocean interior, helping sustain the ocean?s role as a sink of atmospheric carbon dioxide. It was found that approximately one-quarter of the organic phosphorus matter marine phytoplankton produce each year accumulates as DOP in the surface ocean (the remainder comprises marine biomass and sinking particulate organic phosphorus) with a mean surface ocean lifetime of 0.5-2 years. This lifetime allows DOP to be transported away from regions of accumulation, typically in regions that aren?t experiencing phosphorus stress to the plankton community, to be delivered to phosphorus impoverished regions where it is utilized as an alternative phosphorus source sustaining phytoplankton production. Globally, this process supplies 14% of the needed phosphorus with regional contributions as large as 80% in the subtropical North Atlantic and North Pacific. Broader impacts of this work include furthering the scientific understanding of how marine nutrient cycling affects the patterns and rates at which the ocean exports and stores carbon within the ocean interior, thereby altering atmospheric carbon dioxide levels on annual to centennial timescales. The training of one person for the scientific workforce was achieved, earning a M.S. and upcoming Ph.D. in Oceanography from Florida State University. Dissemination of the project findings to a general public audience was achieved via university press releases and the social media campaign, #SaveOur70, curated by the University of New Hampshire Principal Investigator. Last Modified: 01/11/2023 Submitted by: Robert T Letscher