Award: OCE-1827948

(736 words) Biological production in the global ocean is a key driver of the global carbon cycle and atmospheric CO2, which in turn drives global climate. The strength of the marine biological production (i.e., how much carbon is cycled through marine biota) depends significantly on the phytoplankton C:N:P ratio. The ratio is the elemental stoichiometry that relates carbon to nutrients nitrogen and phosphorus in phytoplankton biomass. A stable plankton C:N:P ratio was proposed nearly a century ago and is often called the Redfield ratio. It has since become one of the most important concepts in biogeochemical oceanography. However, recent studies show that this ratio can vary substantially by phytoplankton type and over the spatial scale of ocean basin. This project is motivated by the possibility that the phytoplankton C:N:P ratio can vary substantially under climate change with important implications for atmospheric CO2 and thus climate. This project developed a new mathematical model of flexible phytoplankton stoichiometry and investigated the importance of flexible stoichiometry on the marine carbon cycle under climate change. A critical first step in developing the power law model was a comprehensive meta-analysis of laboratory experiments of phytoplankton growth reported in the literature. The meta-analysis clarified the environmental controls on the stoichiometry of various phytoplankton types. For example, a reduction in the nutrient levels in the environment leads to a higher carbon content in all phytoplankton types, but this effect is much more prominent in the larger eukaryotes than in the smaller prokaryotes. Statistically significant environmental controls from the meta-analysis were formulated into a set of mathematical equations and embedded in numerical models of global ocean circulation and biogeochemistry. In several publications that resulted from this project, these global ocean models were used to simulate changes in the global carbon cycle under future warming scenarios as well as Pleistocene glacial conditions. Generally, the results demonstrated that as climate and ocean conditions change, the global ocean C:N:P ratio of phytoplankton can significantly vary for three reasons. First, as a physiological response, each phytoplankton type will respond as the environment changes according to the meta-analysis. Second, as a taxonomic response, the C:N:P of the phytoplankton community as a whole can vary as the community composition changes. The community C:N:P will reflect most strongly the C:N:P ratio of the most dominant phytoplankton type. Third, the global ocean C:N:P ratio will vary as the balance of the regional production changes. The global C:N:P will be more heavily influenced by the C:N:P ratio of the more productive regions of the world ocean. Results from this project indicated that under a future warming scenario, prokaryotic phytoplankton becomes more carbon-rich primarily due to warming. At the same time, eukaryotic phytoplankton also becomes more carbon-rich but chiefly for a different reason: nutrients become scarcer under a warmer, more stratified ocean. These are physiological responses. Taxonomically, the global marine phytoplankton community shifts slightly in favor of prokaryotes, which are generally more carbon-rich than eukaryotes. In terms of regional biological production changes, model results indicate that the polar regions will make a greater contribution to the global production in the future, as the growing season in polar regions increase in space and time due to sea ice retreat and warming. High latitude biological production, which is typically low in C:N:P ratios, would thus tend to lower the global ocean phytoplankton C:N:P ratio. In this project, these effects (i.e., physiology, taxonomy, and regional production) combine to yield a generally higher phytoplankton C:N:P ratio in the future ocean. The higher C:N:P ratio would buffer a future decline in the biological production, even as the world ocean becomes warmer and more stratified. The project spanned the period of the Covid pandemic, which disrupted graduate studies for many. This project supported four students, two of who obtained graduate degrees. This project supported a PhD student in his final year as a graduate student. He obtained a PhD degree in 2019 and continued to work under this project as a postdoc for one additional year at the University of Minnesota. In total, as a student and later as a postdoc, this individual published ten peer-reviewed publications. The second degree that resulted from this project was a MS degree obtained in 2022 by a female Latina student. Within a year of graduation, she published her MS thesis in a peer-reviewed journal. This project also supported two other graduate students, who worked on different aspects of this project in a more limited capacity. Last Modified: 11/04/2023 Submitted by: WilliamESeyfried