NITROGEN SOURCES AND EXPORT PRODUCTION IN THE OLIGOTROPHIC EASTERN INDIAN OCEAN: INSIGHTS FROM SEDIMENT TRAP AND NITROGEN UPTAKE DATA

Southern bluefin tuna only spawn in a small low-nutrient, oligotrophic region of the eastern Indian Ocean off northwestern Australia. The nitrogen (N) sources fueling this ecosystem and the processes connecting the upper 25 m where larvae reside to the full euphotic zone are poorly understood. Potential N sources include recycled ammonium, lateral advection of particulate organic matter, upwelled nitrate, and N₂ fixation. In this study, we conducted in-situ and deckboard incubations to quantify N uptake, focusing on nitrate and ammonium assimilation. We used a combination of flow cytometry and epifluorescence microscopy to assess phytoplankton community distributions and sediment traps to quantify vertical fluxes of organic matter and pigments within and out of the euphotic zone. Our results indicate that this region relies heavily on recycled N, particularly ammonium, and while lateral advection and N₂ fixation contribute to N supply, upwelled nitrate is the dominant source of new N. Analysis of the euphotic zone vertical structure suggests that the deep chlorophyll maximum layer is primarily a site of remineralization, rather than net particle production.  Comparative ecosystem analysis with the Atlantic bluefin tuna spawning site in the Gulf of Mexico explore these processes further. Understanding N dynamics and export in these oligotrophic waters is critical for improving biogeochemical models, to assess ecosystem impacts caused by climate change, and to improve our understanding of the recruitment issues for the tuna fisheries that rely on this ecosystem.

Presentation Preference: Either

Primary Presenter: Natalia Yingling, Florida State University (ny18b@fsu.edu)

Authors:

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

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

Michael Landry, Scripps Institution of Oceanography  (mlandry@ucsd.edu)

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

Margaret Johnson, Florida State University  (mej92300@gmail.com)

Christian Fender, Florida State University  (ckfender@fsu.edu)

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