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Whereas recruitment success for many fisheries depends on coincident timing of larvae with abundance peaks of their prey, less can be more in the tropical/subtropical spawning areas of bluefin tunas if lower but steady food resources are offset by reduced larval vulnerability to pelagic predators. To understand larval habitat characteristics for Southern Bluefin Tuna (SBT), we quantified microbial community carbon flows based on growth and grazing rates from depth profiles of dilution incubations and carbon biomass assessments from microscopy and flow cytometry (FCM) during their peak spawning off NW Australia (Indian Ocean) in February 2022. Two Chla-based estimates of phytoplankton production gave differing offsets due to cycling or mixotrophy, exceeding 14C net community production on average (677 ± 98 versus 447 ± 43 mg C m−2 d−1). Productivity was higher than in the Gulf of Mexico spawning area for Atlantic Bluefin Tuna but less than similar studies of oceanic upwelling regions. Microzooplankton grazing averaged 482 ± 63 mg C m−2 d−1 (71 ± 13 % of production). Two measurement variables for Prochlorococcus gave average production and grazing rates of 282 ± 36 and 248 ± 32 mg C m−2 d−1 (86 ± 6 % grazed). Prochlorococcus comprised almost half of production and grazing fluxes in the upper (0–25 m) euphotic zone where SBT larvae reside. Prochlorococcus declined and eukaryotic phytoplankton and heterotrophic bacteria increased in relative importance in the lower euphotic zone. These results describe relatively classic open-ocean oligotrophic conditions as the food web base for nutritional flows to SBT larvae. 

ABSTRACT Phytoplankton community composition during austral summer 2022 in the Argo Abyssal Plain (Argo Basin), a 5000-m deep area northwest of the Australian continent in the eastern Indian Ocean, is described in detail, including phytoplankton abundance, biomass, size structure, taxonomic identifications through DNA and pigment analyses, as well as the percent of functional mixotrophs. The region was characterized by warm (up to 30.5°C), stratified, oligotrophic (nitrogen-limited) waters, with integrated euphotic zone (EZ) chlorophyll a (CHLa) of 13 mg m^-2. The EZ mean CHLawas low in the upper layer (0.085 µg L^-1) and 0.32 µg L^-1at the pronounced deep CHLamaxima. EZ-integrated phytoplankton carbon averaged 1229 mg C m^-2.Prochlorococcuswas the dominant taxon throughout the EZ, but the lower EZ had ∼4-times more eukaryotic carbon biomass than the upper EZ, along with a distinct community. In the upper EZ, prymnesiophytes, dinoflagellates and prasinophyte taxa without prasinoxanthin had the highest contributions to monovinyl chlorophyll a (MV-CHLa). In the lower EZ the community was more diverse, with prymnesiophytes, dinoflagellates, prasinophyte taxa with prasinoxanthin, pelagophytes, and cryptophytes all comprising significant contributions to MV-CHLa. Diatoms were a minor part of the community. In the upper EZ, a higher percent of the community showed mixotrophy (35-84%) relative to the lower EZ (30-51%). Although a low abundance, nitrogen-fixing organisms (symbionts of diatoms and cyanobacteria taxa) were ubiquitous. Overall, the community was similar to that found at the Hawaii Ocean Time-series site and the central Gulf of Mexico.