Award: OCE-2404504

This project investigated how ocean physics (currents, mixing, temperature and light), ocean chemistry (inorganic nutrients). and ocean biology (nutrient cycling, ocean productivity and plankton food web dynamics) interact to support the only known spawning habitat of Southern Bluefin Tuna off northwestern Australia in the eastern Indian Ocean. The award was focused around the BLOOFINZ-IO expedition (January–March 2022), which carried out four multi-day “Lagrangian” experiments that followed drifting water masses while measuring biological, chemical, and physical processes. Despite major logistical disruptions early in the award - including COVID-19 impacts - the project produced the first comprehensive, multi-disciplinary dataset for this remote and previously undersampled region.

What we found (Intellectual Merit). The Argo Basin is strongly oligotrophic (meaning it is chronically low in nitrate), yet it supports high microbial activity and surprisingly efficient transfer of energy through the food web. Measurements of depth-resolved primary production, nutrient uptake, and nitrogen fixation showed that most nitrogen supporting plant growth in surface waters comes from rapid recycling of ammonium (regenerated nitrogen), while nitrate uptake peaks deeper near the deep chlorophyll maximum. Nitrogen fixation (conversion of N2 gas into biologically usable nitrogen) was generally low in the open ocean (typically contributing <20% of the nitrogen demand), but it was measurable and, importantly, it roughly matched the amount of nitrogen leaving the surface ocean in sinking particles, consistent with a balanced nitrogen budget in the Southern Argo Basin. Isotope measurements of nitrate supported these conclusions while also pointing to localized hotspots of nitrogen fixation, including coastal regions where continuous measurements revealed elevated rates. The project also quantified who eats whom and how carbon moves from microbes to animals that tuna larvae can eat. Grazing experiments and zooplankton analyses showed that microzooplankton consumed most primary production, but enough carbon remained to sustain positive net community production and carbon export. Sediment traps measured the sinking flux of organic matter and indicated an export efficiency of about 17%. A key biological insight was that appendicularians, gelatinous zooplankton that efficiently capture very small prey, can shorten food chains and help move energy toward larger zooplankton and larval tuna prey in this region. Comparative ecosystem modeling further suggested that, despite similar nutrient-poor conditions, the Argo Basin can be more productive and more efficient at transferring energy to larval tuna prey than the Gulf of Mexico spawning grounds, highlighting the importance of nutrient recycling, nitrogen fixation, and episodic mixing events (such as storm-driven deepening of the mixed layer) in sustaining higher trophic levels.
Why it matters (Broader Impacts). Southern Bluefin Tuna are economically valuable and culturally important, and their recruitment depends on the quality of larval habitat. By identifying the processes that support prey production, especially how microbial communities, nutrient recycling, and export dynamics work together in a warming, stratifying ocean, this project provides knowledge relevant to fisheries science and international resource management. The work also improves the scientific basis for representing nitrogen fixation, export efficiency, and plankton functional types in ecosystem and Earth system models used to project climate impacts.

The award also made strong contributions to workforce development. Graduate students, postdoctoral researchers, and early-career scientists gained hands-on training in ocean fieldwork (CTD casts, incubations, grazing experiments, sediment traps), laboratory analyses (nutrients, isotopes, flow cytometry, microscopy, FRRf photophysiology), and data synthesis (statistics and ecosystem modeling). The demanding expedition and pandemic-era challenges also built resilience and collaborative problem-solving skills. Data products have been made publicly available through repositories such as BCO-DMO, and results have been disseminated through conference sessions and a dedicated BLOOFINZ special issue in Deep Sea Research Part II. To date, this award has supported seven peer-reviewed publications, with additional manuscripts under review, and the broader BLOOFINZ-IO program is expected to yield more than 25 scientific papers. Collectively, the project delivers a transferable, end-to-end framework for understanding how oligotrophic ecosystems function, and how they may respond to future climate change, while directly informing the science that underpins sustainable management of a globally important fishery

 

Last Modified: 01/29/2026
Modified by: Sven A Kranz