MARINE PRODUCTIVITY AND N2 FIXATION IN THE OLIGOTROPHIC INDONESIAN THROUGHFLOW: BOTTOM-UP SUPPORT FOR SOUTHERN BLUEFIN TUNA LARVAE

Economically important Southern Bluefin Tuna (SBT) spawn exclusively in oligotrophic waters downstream of the Indonesian Throughflow (ITF). Understanding phytoplankton community composition, primary productivity and nitrogen sources in this poorly studied region is critical for evaluating the relative importance of various drivers and mechanisms that support feeding and growth of SBT larvae under low-nutrient conditions. During the 2022 BLOOFINZ research cruise, we conducted multi-day Lagrangian experiments to quantify primary productivity and N₂ fixation in ITF waters by multiple methods. Despite very low nitrate but significant phosphorus in surface waters, N₂ fixation contributed only 10-15% to marine productivity during the peak spawning season, likely constraining overall productivity. The phytoplankton community, primarily Prochlorochoccus in the upper euphotic zone and picoeukaryotes in the deep chlorophyl maximum, relied primarily on recycled N. Fast Repetition Rate fluorometry and Advanced Laser Fluorescence analysis further identified areas with limited potential for primary production, suggesting deficiency of an essential factor, such as iron. Our data suggest a contained system, with nitrate, and possibly iron constraining productivity potential of the phytoplankton assemblages. Feeding and growth success of SBT larvae appears to depend more on selecting prey that can streamline the transfer inefficiencies of a picophytoplankton-dominated system rather than the magnitude of primary production itself.

Presentation Preference: Oral

Primary Presenter: Sven Kranz, Rice Univeristy (skranz@rice.edu)

Authors:

Jared Rose, University of South Carolina  (JR191@mailbox.sc.edu)

Natalie Yingling, Florida State University  (nyingling@fsu.edu)

Joaquim Goes, Columbia University  (jig@ldeo.columbia.edu)

Karen Selph, University of Hawaii at Manoa  (selph@hawaii.edu)

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

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

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

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

Michael Stukel, Florida State University  (mstukel@fsu.edu)