Award: OCE-1737240

Phytoplankton are microscopic plants at the base of the marine food web. Their growth is controlled by the availability of certain plant nutrients, like nitrogen and phosphorus. Our objective in this project was to understand the availability of different forms of these nutrients for phytoplankton communities in the northern Gulf of Mexico, where the Mississippi and Atchafalaya rivers are major sources of nutrients for offshore phytoplankton. During two oceanographic expeditions in summer 2018 and 2019, we showed how nitrogen and phosphorus availability decline near the ocean surface going away from land within the plume of the Mississippi river. We demonstrated that nitrogen availability drops more rapidly than phosphorus availability within the river plume, and also that phosphorus availability in the northern Gulf of Mexico has increased over several decades. Even though the availability of phosphorus is higher than that of nitrogen once far enough offshore, both nutrients remain important controls on phytoplankton growth and community composition. Interestingly, we also found that the phosphorus nutrients carried in by Mississippi and Atchafalaya Rivers are probably insufficient to account for the total supply in the northern Gulf of Mexico. This means that other sources of phosphorus in the northern Gulf of Mexico are contributing to the increase in phosphorus nutrients over time that we described. We dont currently know what those other sources of phosphorus are, but the trends suggest broader ecosystem changes are underway. Management of the Mississippi and Atchafalaya Rivers, and the northern Gulf of Mexico, should consider the sources and importance of both nitrogen and phosphorus nutrients. In this project we also made several methodological improvements to studying phytoplankton communities, both in water samples collected from ships, and in satellite data. The improved methods gave us unique insight into the phytoplankton of the northern Gulf of Mexico, and the approaches could also be applied to studies of phytoplankton and the marine food web in many other locations. Eight undergraduate students gained research experience through this project, and data collected during our expedition formed the basis of the PhD dissertations for several graduate students. More information about our research findings is available in the publications that have resulted from the project. In addition, the observational data resulting from the project are publicly available in open databases. Last Modified: 03/28/2024 Submitted by: AndrewRJuhl