COASTAL HOTSPOTS OF BIOLOGICAL NITROGEN FIXATION IN UNDERSAMPLED REGIONS OF THE INDIAN OCEAN

Biological nitrogen fixation (BNF), the process by which dinitrogen (N₂) is converted into a bioavailable form for the uptake by organisms, is crucial for determining ecological fertility and carbon sequestration in oceans and terrestrial landscapes. BNF in the oligotrophic Indonesian Throughflow (ITF) is especially under-researched, and is an important spawning site for Southern Bluefin Tuna, an economically important species in the area. In 2022, the BLOOFINZ cruise conducted a range of experiments in this region, including the first deployment of the Flow-Through Incubation Acetylene Reduction Assays by Cavity Ring Down Laser Absorption Spectroscopy (FARACAS) method in the ITF region. This method is one of the first to conduct continuous measurements of surface BNF, allowing the spatial heterogeneity in BNF to be observed. Overall rates ranged from below detectable levels to over 20 nmol N/L/day, indicating that BNF may have been a minor source of nitrogen overall. A significant hotspot occurred near the coast of Australia, in the Timor Sea, of over 49 nmol N/L/day. These hotspots support previous claims of higher BNF rates in coastal regions, going against conceptions of BNF being prevalent exclusively in open-ocean oligotrophic regions. Our findings suggest an oligotrophic region of low BNF activity, with the exception of local Australian coastal hotspots. Understanding the nutrient constraints and BNF hotspots in the region is essential for being able to predict mechanisms supporting the growth of larval tuna, as well as the system’s vulnerabilities to climate change.

Presentation Preference: Either

Primary Presenter: Ariana de Souza, Duke University (ariana.desouza@duke.edu)

Authors:

Ariana de Souza, Duke University  (ariana.desouza@duke.edu)

Shuai Gu, Texas A&M University - Corpus Christi  (shuai.gu@tamucc.edu)

Sven Kranz, Rice University  (sk235@rice.edu)

Jared Rose, Florida State University  (jr191@mailbox.sc.edu)

Robert Lampe, University of California - San Diego  (rlampe@ucsd.edu)

Hongbin Liu, The Hong Kong University of Science and Technology  (liuhb@ust.hk)

Michael Landry, University of California - San Diego  (mlandry@ucsd.edu)

Nicolas Cassar, Duke University  (nicolas.cassar@duke.edu)