Award: OCE-1851381

Southern Bluefin Tuna (SBT), an important fishery stock that was overfished and still has Endangered Species status (IUCN), spawn solely in the tropical eastern Indian Ocean, a region that is characterized by mesoscale features. Bottom-up influences on larval SBT revolve around suitable habitat for their survival, including prey availability, which in turn depends upon the phytoplankton and zooplankton in the system. Thus, characterizing the phytoplankton community and the nutrient environment leading to its composition is necessary to understand SBT larval survivorship. We collected samples and conducted in situ experiments to better understand their population dynamics during a research expedition aboard the RV Roger Revelle during the peak SBT spawning period (Jan to March 2022), investigating mesoscale variability in new production and food-web structure, amongst other goals, to provide the data to better understand SBT population dynamics. The data from this project is still being analyzed for synthesis and publication, however a small part of it (funded by this grant) shows that the phytoplankton community found in the areas where SBT larvae reside is characterized by very small microorganisms (e.g., Prochlorococcus and Synechococcus), as would be found in other open-ocean nutrient-poor areas. These tiny phytoplankton set the stage for a longer, more complex, food-web, with SBT larvae feeding several trophic levels above the base, and our data will be able to tease out the details of this food web once all collaborators have analyzed and collated their results in the coming year. We will be able to report the rates of growth and mortality of these phytoplankton underpinning the entire food web, thereby estimating potential production and turnover at the base of the ecosystem system, indispensable for modeling upper trophic level dynamics. On this expedition, we also used a newer flow cytometry method to estimate the abundance and distribution of the consumers of these tiny phytoplankton, i.e., microzooplankton. In addition to phytoplankton and microzooplankton, it is now known that mixotrophs, eukaryotic cells that used a mixed mode of nutrition, are likely ubiquitous. During this project we developed a method that is promising for estimating their abundance as well, producing a data set that is more extensive that others published to date, and potentially forwarding understanding of this understudied phenomena of mixotrophy in wild, naturally occurring microorganisms. We recommend future studies where a sensitive flow cytometer is available at sea, so that experiments can be done in real time on living microorganisms to get the best data possible for understanding microbial ecosystems and to best utilize expensive ship-time by getting as much information as possible from every sample collected. Last Modified: 01/08/2024 Submitted by: KarenESelph