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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. 

The eastern Indian Ocean is substantially under sampled with respect to the biological carbon pump – the suite of processes that transport the carbon fixed by phytoplankton into the deeper ocean. Using sediment traps and other ecosystem measurements, we quantified sinking organic matter flux and investigated the characteristics of sinking particles in waters overlying the Argo Abyssal Plain directly downstream of the Indonesian Throughflow off northwest Australia. Carbon export from the euphotic zone averaged 7.0 mmol C m􀀀 2 d􀀀 1, which equated to an average export efficiency (export/net primary production) of 0.19. Sinking particle flux within the euphotic zone (beneath the mixed layer, but above the deep chlorophyll maximum) averaged slightly higher than flux at the base of the euphotic zone, suggesting that the deep euphotic zone was a depth stratum of net particle remineralization. Carbon flux attenuation continued into the twilight zone with a transfer efficiency (export at euphotic depth + 100m/export at euphotic depth) of 0.62 and an average Martin's b-value of 1.1. Within the euphotic zone, fresh phytoplankton (chlorophyll associated with sinking particles, possibly contained within appendicularian houses) were an important component of sinking particles, but beneath the euphotic zone the fecal pellets of herbivorous zooplankton (phaeopigments) were more important. Changes in carbon and nitrogen isotopic composition with depth further reflected remineralization processes occurring as particles sank. We show similarities with biological carbon pump functioning in a similar semi-enclosed oligotrophic marginal sea, the Gulf of Mexico, including net remineralization across the deep chlorophyll maximum. 

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. 

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. 

The marine biological carbon pump is driven by sinking particulate organic matter (POM). Sinking speed and remineralization rate determine flux attenuation in the mesopelagic. Since the fate of all marine organic matter is either complete remineralization or transformation to more stable products, diagenetic modifications impact carbon dioxide sequestration time from the atmosphere. To investigate particle transformation at the molecular level, we characterize the water-extractable organic matter (WEOM) fraction of sinking particles from dominant biogeochemical environments using ultrahigh-resolution mass spectrometry. We find distinct, inverse associations in molecular-level nitrogen content and degree of transformation (i.e., “stability”) of organic matter across a productivity gradient from coastal upwelling to oligotrophic conditions. Nitrogen enrichment and low stability were observed at the coastal upwelling site and persisted to depths >400 m. Further, carbon flux is strongly correlated with the relative abundance of stable WEOM (“Island of Stability” molecular formulae) across productivity regimes and depth. This suggests emergent patterns in epi- and mesopelagic diagenesis, highlighting that the molecular composition of sinking organic matter exiting the euphotic zone varies more across regions than as a function of depth. This is attributed to highly variable sinking rates and the microbial diagenetic histories within the euphotic zone. The stability–flux relationship is considered a “diagenetic clock” relative to organic matter formation where the relative abundance of Island of Stability molecular formulae describes the degree of departure from the organic matter molecular-level composition at formation. This ubiquitous trajectory of the diagenetic clock further underpins a global ocean molecular signature of sinking POM. 

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 The eastern Indian Ocean is substantially under sampled with respect to the biological carbon pump – the suite of processes that transport the carbon fixed by phytoplankton into the deeper ocean. Using sediment traps and other ecosystem measurements, we quantified sinking organic matter flux and investigated the characteristics of sinking particles in waters overlying the Argo Abyssal Plain directly downstream of the Indonesian Throughflow off northwest Australia. Carbon export from the euphotic zone averaged 7.0 mmol C m^-2d^-1, which equated to an average export efficiency (export / net primary production) of 0.17. Sinking particle flux within the euphotic zone (beneath the mixed layer, but above the deep chlorophyll maximum) averaged slightly higher than flux at the base of the euphotic zone, suggesting that the deep euphotic zone was a depth stratum of net particle remineralization. Carbon flux attenuation continued into the twilight zone with a transfer efficiency (export at euphotic depth + 100m / export at euphotic depth) of 0.62 and an average Martin’sb-value of 1.1. Within the euphotic zone, fresh phytoplankton (chlorophyll associated with sinking particles, possibly contained within appendicularian houses) were an important component of sinking particles, but beneath the euphotic zone the fecal pellets of herbivorous zooplankton (phaeopigments) were more important. Changes in carbon and nitrogen isotopic composition with depth further reflected remineralization processes occurring as particles sank. We show similarities with biological carbon pump functioning in a similar semi-enclosed oligotrophic marginal sea, the Gulf of Mexico, including net remineralization across the deep chlorophyll maximum. Submitted to: Deep-sea Research II HighlightsDespite low productivity, export efficiency was 17% of primary productionFlux attenuation beneath the euphotic zone (EZ) was low for a tropical regionSinking particle flux from the upper to lower EZ exceeded export from lower EZThe deep EZ was a stratum of net particle remineralization (and net heterotrophy) 

Abstract Disturbance ecology is underdeveloped in marine pelagic ecosystems relative to terrestrial and aquatic benthic habitats, in part because, when measured relative to a fixed location, postdisturbance recovery involves the advection of entire communities in addition to biotic interactions. A Lagrangian frame-of-reference perspective alleviates this issue. Using results from the California Current Ecosystem, we highlight three approaches: in situ Lagrangian, synthetic Lagrangian, and simulated Lagrangian studies. Within a Lagrangian context, extratropical marine heatwaves and El Niños represent press disturbances or alterations to the disturbance regime. Individual upwelling events are more appropriately viewed as pulse disturbances. Upwelling disturbances stimulate rapid growth of pioneer species (diatoms), with herbivores (copepods) lagging these blooms by approximately 3 weeks. The climax community is an assemblage of small low-nutrient specialists with high Shannon diversity. We suggest that pelagic ecosystems can be ideal systems for investigating disturbance recovery because of the rapid response times of marine primary producers and herbivores. 

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.