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Phytoplankton, the foundational organisms in ocean food webs, have been little studied in the Indonesian Throughflow region of the eastern Indian Ocean, the spawning area of Southern Bluefin Tuna. Here, we assess phytoplankton abundance, biomass, size structure, pigment composition, taxonomic diversity and percent functional mixotrophs of that region based on complementary approaches of flow cytometry, microscopy, taxonspecific pigments and rRNA gene sequencing. During summer (January–February) 2022, 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. EZ mean CHLa was low in the upper layer (85 ng L−1) and 3.8 times higher (320 ng L−1) at the pronounced deep CHLa maximum. EZ-integrated phytoplankton carbon averaged 1229 mg C m−2. Prochlorococcus dominated throughout the EZ, but eukaryotic carbon biomass was ~4- times greater in the lower than upper EZ, along with a distinct community. In the upper EZ, haptophytes, dinoflagellates and prasinophycean taxa without prasinoxanthin contributed most to monovinyl chlorophyll a (MV-CHLa). In the more diverse lower EZ, haptophytes, dinoflagellates, prasinophycean taxa with prasinoxanthin, pelagophytes, and cryptophytes were the main contributors to MV-CHLa. Diatoms were a minor part of the community. A higher percentage of the upper EZ community showed mixotrophy (35–84%) relative to the lower EZ (30–51%). Nitrogen-fixing organisms (as symbionts of diatoms and free-living cyanobacteria taxa) were ubiquitous, but low in abundance. Overall, community characteristics were similar to those at the Hawaii Ocean Time-series site and the central Gulf of Mexico. 

Prey removal incubations were conducted in the Argo Basin (eastern Indian Ocean) to investigate the trophic ecology of the zooplankton community supporting Southern Bluefin Tuna larvae. Copepod grazing and selectivity were evaluated considering prey trophic mode and size as food quality descriptors and compared with microzooplankton grazing. Copepods ingested between 3.4 and 138 ng carbon (C) ind−1 d−1. Diet was dominated by mixotrophic (5–89 %) and heterotrophic (0–84 %) prey, with autotrophs contributing 2–17 %. Nanoplankton provided the highest C intake to copepods (62–99 %) versus picoplankton (0.8–38 %), despite more picoplankton cells ingested. No measurable reduction in chlorophyll a (Chla) concentration occurred from copepod grazing through food removal, suggesting an indirect trophic pathway, although gut content revealed ingestion of 0.8 μg Chla ind−1 d−1. Calculations from disappearance incubations imply copepod selection on picoplankton (E = 0.3) over nanoplankton, but picoplankton were likely ingested as aggregates or depressed due to a trophic cascade of copepods ingesting nano-microzooplankton. Copepods ingested protistan consumers and/or metazoans fed on 15N-cyanobacteria with ~5-fold higher N uptake from 1 to 2 μm 15N-Synechococcus than from <1 μm 15NProchlorococcus. Microzooplankton grazing on eukaryotes (0.07–2.5 d−1) and prokaryotes (0.3–2.1 d−1) greatly exceeded copepod grazing. Microzooplankton diet consisted mostly of heterotrophs (25–59 %) and mixotrophs (13–41 %) followed by autotrophs (12–33 %), with more nano- (95–98 %) than picophytoplankton (2–5 %). Overall, microzooplankton removed most daily production (111 %) in contrast to 7 % for copepods. Our findings indicate that mixotrophy, intraguild grazing and nutrient channeling support the food web in this oligotrophic region. 

Oligotrophic ocean regions are characterized by strong nutrient limitation, low standing phytoplankton biomass, and highly efficient nutrient recycling. During the BLOOFINZ-IO expedition (February 2022), we quantified nutrient inventories, primary productivity and N2 fixation in the Argo Basin of the eastern Indian Ocean, the sole known spawning ground for Southern Bluefin Tuna. Low concentrations of surface nitrate (<0.02 μmol L−1) and persistent residual phosphate indicated N as the limiting macronutrient, with photophysiological indices consistent with iron colimitation. Depth-integrated net primary production (NPP), from 14C-based in-situ incubations during 4 Lagrangian experiments, averaged ~460 mg C m−2d−1, generally agreeing with mean satellite-based NPP estimates (459 mg C m−2d−1) but with spatial discrepancies. Nitrogen fixation provided a consistent new nitrogen source, contributing ~16 % to local NPP in the upper euphotic zone. Gross primary production (GPP), from fast-repetition-rate-fluorometry-based estimates of electron transport, revealed significant autotrophic respiration losses, with mean GPP:NPP ratios of ~1.8 consistent with metabolic costs under nutrient limitation. Net community production (NCP) from O2/Ar ratios averaged ~20 % of NPP in the upper 30 m. This result, in combination with N2 fixation measurement, indicates that N2 fixation supports most of the export production in the region. Together, the multimethod approach revealed a recycling-dominated ecosystem affected by episodic mixing events, where primary productivity is maintained primarily through efficient nitrogen recycling and physiological photoacclimation. These results provide a comprehensive baseline of bottom-up support of ecosystem productivity for the Argo Basin for assessing future climate-driven changes in stratification, nutrient cycling, and food-web dynamics. 

The Argo Basin of the eastern Indian Ocean in austral summer (February 2022) was characterized by warm (28.5–30.6 ◦C), oligotrophic surface waters (nitrate and phosphate ≤0.1 μM), with relatively shallow mixed layers and deep chlorophyll biomass maxima. From euphotic zone depth-resolved samples analyzed by for DNA and acid vacuole staining (Hoechst and LysoTracker Green) by ship-board flow cytometry, we found that autotrophic populations were dominated by Prochlorococcus, followed by mixotrophs (58 and 28 % of autotrophic community biomass, respectively), with only 14 % obligate phototrophic phytoplankton (i.e., plastidic cells without acid vacuole fluorescence). Acid vacuole-containing microbes (mixotrophs and heterotrophs) were 34 % of the microbial community, and 80 % of the eukaryotic biomass. In shallow waters, the eukaryotic chlorophyllcontaining community was comprised of pico-sized obligate phototrophs and mixotrophs (233–325 cells mL−1), nano-sized obligate phototrophs and mixotrophs (72 and 374 cells mL−1, respectively), with all groups increasing several-fold in the deep chlorophyll maxima. Mixotrophs were a higher proportion of the chlorophyll-containing community in the shallow nutrient-poor mixed layer, consistent with a nutrient-acquisition argument for their prevalence. Heterotrophic eukaryotes averaged 524 ± 36 cells mL−1 in the euphotic zone, changing little with depth and showing a significant positive relationship with Prochlorococcus, but not any other group. In contrast, mixotrophs were positively correlated with heterotrophic bacteria, but not with Prochlorococcus. Overall, the high proportion of mixotrophs in the microbial community may channel more productivity to higher trophic levels than expected given the region’s nutrient-poor status. 

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. 

Abstract Gelatinous filter feeders (e.g., salps, doliolids, and pyrosomes) have high filtration rates and can feed at predator:prey size ratios exceeding 10,000:1, yet are seldom included in ecosystem or climate models. We investigated foodweb and trophic dynamics in the presence and absence of salp blooms using traditional productivity and grazing measurements combined with compound-specific isotopic analysis of amino acids estimation of trophic position during Lagrangian framework experiments in the Southern Ocean. Trophic positions of salps ranging 10–132 mm in size were 2.2 ± 0.3 (mean ± std) compared to 2.6 ± 0.4 for smaller (mostly crustacean) mesozooplankton. The mostly herbivorous salp trophic position was maintained despite biomass dominance of ~10-µm-sized primary producers. We show that potential energy flux to >10-cm organisms increases by approximately an order of magnitude when salps are abundant, even without substantial alteration to primary production. Comparison to a wider dataset from other marine regions shows that alterations to herbivore communities are a better predictor of ecosystem transfer efficiency than primary-producer dynamics. These results suggest that diverse consumer communities and intraguild predation complicate climate change predictions (e.g., trophic amplification) based on linear food chains. These compensatory foodweb dynamics should be included in models that forecast marine ecosystem responses to warming and reduced nutrient supply. 

Abstract The 2014–2015 warm anomaly (aka “the Blob”), the largest of periodic and intensifying marine heat wave (MHW) perturbations in the northeast Pacific, may provide some insight about the future warmer ocean. Here, we use mixed‐layer carbon estimates for total phytoplankton, major size classes and functional groups from 45 CalCOFI cruises to: (1) compare 2014–2015 MHW impacts in the southern California Current System to baseline estimates from 2004 to 2013 and (2) to test a space‐for‐time exchange hypothesis that links biomass structure to variability of nitracline depth (NCD). Seasonal and inshore‐offshore analyses from nine stations revealed almost uniform 2°C MHW warming extending 700 km seaward, fourfold to sixfold declines in nitrate concentration and 18‐m deeper NCDs. Phytoplankton C decreased 16–21% compared to 45–65% for Chla, with the threefold difference due to altered C : Chla. Among size classes, percent composition of nanoplankton decreased and picophytoplankton increased, driven by higherProchlorococcusbiomass, whileSynechococcusand picoeukaryotes generally declined. Diatom and dinoflagellate C decreased in both onshore and offshore waters. Seasonally, the MHW delayed the normal winter refresh of surface nitrate, resulting in depressed stocks of total phytoplankton and nanoplankton,Synechococcusand picoeukaryotes during winter. Consistent with the space‐for‐time hypothesis, biomass variations for baseline and MHW cruises followed similar (not significantly different) slope relationships to NCD. All biomass components, exceptProchlorococcus, were negatively related to NCD, and community biomass structure realigned according to regression slopes differences with NCD variability. Empirically derived biomass‐NCD relationships could be useful for calibrating models that explore future food‐web impacts in this coastal upwelling system. 

Not AvailabSouthern Bluefin Tuna (SBT, Thunnus maccoyii) range broadly in rich feeding grounds of the Southern Hemisphere but spawn only in a small tropical region off northwestern Australia directly downstream of the Indonesian Throughflow. Here, we describe goals, physical context, design and major findings of an end-to-end process study conducted during the peak SBT spawning season (January–March 2022) to understand nutrient sources, productivity, pelagic food web structure and their relationships to larval SBT feeding, growth and survival. Mesoscale variability was investigated by continuous underway measurements of surface waters and station sampling along the cruise track. Biogeochemical and community relationships, process rates, and trophic interactions were determined in four multi-day Lagrangian experiments in the southern Argo Basin. The study revealed strong system balances among nitrogen fluxes, phytoplankton production, grazing processes, and export. Highly selective feeding on appendicularians allows efficient trophic transfer from picophytoplankton-dominated production to SBT larvae. Plankton productivity, phytoplankton carbon and zooplankton biomass were proportionately elevated compared to similar measurements from the Atlantic bluefin larval habitat in the Gulf of Mexico, but with less advective input from the coastal margins. Individual-based otolith and stable isotope analyses identify larvae of lower trophic position, narrow diet, and narrow maternal diet as the fastest growers most likely to contribute to stock recruitment. Our study highlights the importance of system-level studies to document and understand the subtleties of how food webs of oligotrophic regions respond to climate change, which may not be predictable from the acquired knowledge of historical studies.