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 N-2 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 mu mol 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 C-14-based in-situ incubations during 4 Lagrangian experiments, averaged similar to 460 mg C m(-2)d(-1), generally agreeing with mean satellite-based NPP estimates (459 mg C m(-2)d(-1)) but with spatial discrepancies. Nitrogen fixation provided a consistent new nitrogen source, contributing similar to 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 similar to 1.8 consistent with metabolic costs under nutrient limitation. Net community production (NCP) from O-2/Ar ratios averaged similar to 20 % of NPP in the upper 30 m. This result, in combination with N-2 fixation measurement, indicates that N-2 fixation supports most of the export production in the region. Together, the multi-method 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.

Investigating the time evolution of physical and biochemical properties of the ocean with in situ sampling can follow two approaches: Eulerian and Lagrangian. In the Eulerian approach, repeated measurements are taken at fixed locations, whereas the sampling point moves with the displacement of a water parcel in the Lagrangian approach. During the BLOOFINZ-IO cruise off northwest Australia, four Lagrangian experiments (“cycles”) were conducted with multidisciplinary sampling done at regular intervals for several days following a satellite tracked drifter with mixed-layer drogue. To test the Lagrangian nature of these experiments, we adapted the Bindoff and McDougall (1994) approach for decomposing observed changes between adjacent CTD profiles into components due to vertical movement (heaving) and those occurring along isopycnal surfaces. Profile depth variability was mainly driven by vertical displacements of isopycnals (internal waves), while temperature, chlorophyll, oxygen and salinity were relatively stable when observed on isopycnal surfaces across all casts within the same cycle, and different on average between cycles. Our analysis clearly indicated that density surface was a more appropriate vertical coordinate than physical depth for assessing real environmental variability during each cycle and confirmed the Lagrangian character of the experiments. While the BLOOFINZ cruise serves as a case study, the methodology can be readily extended to analyze other biochemical variables in different ocean regions.

The eastern Indian Ocean (IO) is a poorly characterized oligotrophic region influenced by the Indonesian Throughflow (ITF), with limited data on nutrient uptake and microbial community structure. We conducted depth-resolved measurements of nitrate and ammonium uptake, primary production and carbon-based biomasses of pico, nano and microplankton in waters overlying the Argo Abyssal Plain, downstream of the ITF. Our findings revealed picoplankton dominance of biomass, notably by Prochloroccus and heterotrophic bacteria. Primary production and phytoplankton specific growth rates declined with increasing depth (similar to 4-fold) to the deep chlorophyll maximum (DCM), suggesting that light limitation occurred at the DCM. Ammonium uptake exceeded nitrate uptake by similar to 15-fold, peaking in the upper euphotic zone, indicating strong nutrient recycling. Nitrate uptake was highest at the DCM, where phytoplankton and heterotrophic bacteria biomass increased. Nighttime nitrate uptake and a low median f-ratio (similar to 0.05) emphasizes the importance of nutrient recycling in this region. Our study enhances our understanding of nitrogen cycling, microbial community structure, and productivity in the Argo Basin, offering a valuable baseline for assessing future changes in oligotrophic ocean productivity.

ABSTRACT Using linear inverse ecosystem modeling as a data assimilation tool, we compare spawning grounds of Atlantic and Southern Bluefin Tuna (ABT and SBT, respectively) based on results from field campaigns in the Gulf of Mexico (GoM) and eastern Indian Ocean off northwest Australia (Argo Basin). Both regions are warm, stratified, low-nutrient waters dominated by cyanobacteria ( Prochlorococcus ). Despite these similarities, the Argo Basin is more productive, with ∼1.5X higher net primary production and nearly 2X higher production of top trophic levels in the model (tuna larvae, planktivorous fish, and predatory gelatinous zooplankton). Higher primary production in the Argo Basin is mainly driven by higher N 2 fixation and storm mixing of new nutrients in the upper and lower euphotic zone, respectively. Increased ecosystem efficiency (secondary production of top trophic levels / primary production) results from differences in plankton food web organization. In the GoM, protistan zooplankton are the direct consumers of nearly all phytoplankton production. In contrast, higher rates of herbivory by crustaceans feeding on nanophytoplankton combines with a higher impact of appendicularians on cyanobacteria to convert plankton production into larval tuna prey more efficiently in the Argo Basin. Despite similarities in the proportions of phytoplankton production mediated by cyanobacteria and other picoplankton in both systems, food web pathways to larval tuna and other planktivorous fish are substantially shorter in the Argo Basin. Our results highlight the impact of distinct zooplankton ecological niches on ecosystem efficiency and suggest a need for better inclusion of plankton food-web structure in models simulating climate impacts on fisheries production. HIGHLIGHTS Developed food web models of tuna spawning habitat (Indian Ocean & Gulf of Mexico) Spawning habitats in the Argo Basin and Gulf of Mexico (GoM) are both oligotrophic Argo Basin had higher net primary production in part as a result of nitrogen fixation Argo Basin had higher rates of direct herbivory by metazoan zooplankton This resulted in greater ecosystem efficiency in the Argo Basin.

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.

Many of the oceans' top predatory fishes at low to mid latitudes face an uncertain future as their vulnerable larvae deal with higher environmental temperature and reduced productivity associated with climate change. However, ocean ecosystem models that predict a general decline of zooplankton prey with warming and stratification hardly ever account for complexities in pelagic food webs that might make them resilient in providing food resources for larvae. We illustrate such complexities in comparing feeding interactions of larval Southern Bluefin Tuna (SBT) between two studies conducted 35 y apart in the same spawning region off northwest Australia. In the first (1987), SBT larvae fed and grew at low rates demonstrably limited by zooplankton prey. In the second (2022), feeding and growth were significantly higher despite substantially warmer conditions. The difference reflects a realignment in larval feeding preference from copepods to appendicularians, which allows a more direct and efficient energy transfer pathway from the microbially dominated food web base to higher-level consumers. These findings reset thinking on bluefin larvae feeding preferences, demonstrate high growth on an appendicularian diet up to 30 °C, and align with predictions of appendicularians being an environmentally selected zooplankton category in warming oligotrophic waters.

While pelagic tunicates are known to exert strong influences on ecosystem trophic and biogeochemical function, it has long been assumed that their shortcutting of traditional planktonic food webs results in a dead-end with respect to higher trophic levels given their poor nutritional quality. Growing reports of salps as prey in various fish diets challenge this notion. Here we combine insights into salp feeding with a variety of other in-situ rate measurements of New Zealand's Chatham Rise to reconstruct the most likely energy flows within the ecosystems using linear inverse ecosystem modeling. We then utilize model results to assess diets, production, and ecosystem efficiency for each of the key functional groups in the system, including economically important higher trophic levels and fisheries. We find that rather than acting as a trophic dead-end, the shorter energetic pathways introduced by salps increase overall ecosystem efficiency. Through their direct consumption by fish such as myctophids and oreo, these salp-driven energetic savings are efficiently passed on to higher trophic levels, increasing NPP-normalized secondary production by 130% relative to areas with background salp abundances. This finding challenges the hypothesis that an increased dominance of gelatinous zooplankton (brought on by climate change) will negatively impact global fisheries and instead suggests that species able to directly or indirectly access enhanced gelatinous production may instead benefit from the tighter coupling to lower trophic levels.

Phytoplankton community structure is crucial to pelagic food webs and biogeochemical processes. Understanding size-based biomass distribution and carbon dynamics is essential for assessing their contributions to oceanic carbon cycling. This study quantifies plankton carbon (C) based size spectra, community composition, living to total particulate organic carbon (POC) and C:Chlorophyll a (C:Chla) ratios across biogeographical provinces in the Pacific sector of the Southern Ocean near the Subtropical Front (Chatham Rise, Aotearoa-New Zealand). We analyzed phytoplankton community composition using epifluorescence microscopy and flow cytometry, while quantifying size-fractionated Chl-a and POC to estimate normalized biomass, abundance size spectra, and C:Chla ratios. On average, subtropical-influenced waters had lower macronutrients, higher total Chla (1.1 ± 0.2 μg Chla L-1) and were dominated by nanoplankton, which accounted for 45% of the total plankton community (35.2 ± 4.6 μg C L-1). In contrast, picoplankton dominated plankton communities within the subantarctic-influenced and accounted for 35% of the total plankton community (18.5 ± 0.9 μg C L-1) in these water with higher macronutrient concentrations and lower total Chla concentrations (0.32 ± 0.06 μg Chla L-1). Subantarctic-influenced regions had steeper (more negative) slopes for the normalized biomass size spectrum (average = -1.00) compared to subtropical-influenced waters (average = -0.78) indicating greater relative dominance of small taxa. The subantarctic-influenced region had ~2-fold higher surface average C:Chla ratios compared to the subtropical-influenced region with picoplankton consistently having lower C:Chla ratios, due to low Chla values, than larger nano- or microplankton. Live plankton carbon contributed a median of 67% of total particulate organic carbon in the euphotic zone (non-living detritus comprises the remaining ~1/3), which is indicative of substantial primary production and rapid recycling by a strong microbial loop. Our study provides important insights into phytoplankton community structure, biomass distribution and their contribution to carbon sequestration in this region, highlighting the important roles of nanoplankton in subtropical productive waters and picoplankton in offshore subantarctic waters as well as a strong variation of C:Chla across different phytoplankton size classes.

Oligotrophic regions of the global ocean are characterized by strong nutrient limitation, low standing phytoplankton biomass, and highly efficient nutrient recycling. We quantified nutrient inventories, primary productivity and N 2 fixation during the BLOOFINZ-IO expedition (February 2022) in the Argo Basin located in the eastern Indian Ocean, the sole known spawning ground for Southern Bluefin Tuna. Surface nitrate concentrations were near depletion (<0.02 µmol L -1 ), with low but persistent residual phosphate (P) concentrations suggesting nitrogen as the major limiting nutrient. Depth-integrated net primary production (NPP), from 14 C-based in-situ incubations during 4 Lagrangian cycles, averaged ∼460 mg C m -2 d -1 , in good agreement with satellite-based NPP estimates. Nitrogen fixation provided a consistent new nitrogen source, contributing ∼16% to local NPP in the upper euphotic zone. Gross primary production (GPP), derived from fast-repetition-rate-fluorometry-based electron transport estimates, revealed significant autotrophic respiration losses, with GPP:NPP ratios averaging ∼1.8, consistent with metabolic costs under nutrient limitation. Net community production (NCP), estimated from O 2 /Ar ratios, remained positive across all cycles, averaging ∼20% of NPP in the upper 30 m. This result, in combination with N 2 fixation measurement indicates that N 2 fixation supports most of the export production in this region. Together, the multi-method 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 on ecosystem productivity for the Argo Basin for assessing future climate-driven changes in stratification, nutrient cycling, and food-web dynamics.

Gravitational particle sinking is the main mechanism for carbon export in the biological carbon pump. However, the export dynamics of the particle-associated protist community are not fully understood. We used 18S rRNA gene metabarcoding to characterise the exported protist community within sinking particles and bathypelagic surficial sediments in oligotrophic subtropical and high-nutrient, low-chlorophyll subantarctic waters. Sinking particles were collected with formalin-fixed and preservative-free particle interceptor traps (fixed and live traps, respectively) to identify the community involved in particle export (fixed) and protist loss from remineralisation (live). We paired this with community analysis of the upper and lower water column (mixed layer and below mixed layer to mesopelagic, respectively) to compare the relative sources of exported protists. Amplicon sequences variants (ASVs) from upper water column samples accounted for 2 to 4-fold higher proportion of reads and ASV richness compared to lower water column samples in fixed trap and sediment samples, suggesting low influence of the suspended protist community from the lower water column on export. We further traced the export patterns of upper water column protist taxa by analysing the change in taxa relative abundance across the mixed layer to mesopelagic depths. Export patterns differed between taxa, which is similarly suggested by taxa-specific loss of ASV richness between fixed and live traps, but remained the same across biogeochemically-contrasting water masses. This could imply that the drivers for protist loss during export are related to characteristics consistent across environmental conditions, such as specific microbial interactions or inherent cell properties. ### Competing Interest Statement The authors have declared no competing interest.

Observations of phytoplankton abundances and community structure are critical towards understanding marine ecosystems. Common approaches to determine group-specific abundances include measuring phytoplankton pigments with high-performance liquid chromatography and DNA-based metabarcoding. Increasingly, mRNA abundances with metatranscriptomics are also employed. As phytoplankton pigments are used to develop and validate remote sensing algorithms, further comparisons between pigments and other metrics are needed to validate the extent to which these measurements agree for group-specific abundances; however, most previous comparisons have been hindered by metabarcoding and metatranscriptomics solely producing relative abundance data. By employing quantitative approaches that express both 18S rRNA genes (DNA) and total mRNA as concentrations, we show that these measurements are related for several eukaryotic phytoplankton groups. We further propose that integration of these can be used to examine ecological patterns more deeply. For example, productivity-diversity relationships of both the whole community and individual groups show a dinoflagellate-driven negative trend rather than the commonly found unimodal pattern. Pigments are also shown to relate to certain harmful algal bloom-forming taxa as well as the expression of sets of genes. Altogether, these results suggest that potential models of pigment concentrations via hyperspectral remote sensing may enable improved assessments of global phytoplankton community structure. These assessments may further support the detection of harmful algal blooms and the development of Earth system models.

Abstract A major challenge in understanding the oceanic carbon cycle is estimating the sinking flux of organic carbon exiting the sunlit surface ocean, termed carbon export. Existing algorithms derive carbon export from satellite ocean color, but neglect spatiotemporal offsets created by the temporal lag between production and export, and by horizontal advection. Here, we show that a Lagrangian “growth‐advection” (GA) satellite‐derived product, where plankton succession and export are mapped onto surface oceanic circulation following coastal upwelling, succeeds in representing in situ export off the California coast. In situ export is best represented by a combination of GA export (proportional to modeled zooplankton) and export derived from ocean color (related to local phytoplankton). Both products also correlate with a long‐term time series of abyssal carbon flux. These results provide insights on export spatiotemporal patterns and a path toward improving satellite‐derived carbon export in the California Current and beyond.

The biological carbon pump (BCP) transports carbon fixed by phytoplankton into the deep ocean via multiple pathways and leads to net carbon dioxide sequestration. The BCP is very active in highly productive eastern boundary upwelling systems (EBUSs), however, the extent to which climate change will impact this key ecosystem service remains uncertain. While this review and synthesis focuses on results from the California Current Ecosystem (the most extensively studied EBUS), similarities and differences are also noted with the Benguela, Canary, and Humboldt ecosystems. We focus on vertical carbon export mediated by sinking particles (responsible for >half of the BCP in EBUSs), as well as subduction of organic matter and vertically migrating zooplankton and fish. We suggest that a plug-flow-reactor conceptual model can be used to link BCP results with physical circulation changes within EBUSs to predict whether climate change will lead to increased or decreased biologically mediated CO2 uptake. ### Competing Interest Statement The authors have declared no competing interest. U.S. National Science Foundation, OCE-2224726

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 Chlabased estimates of phytoplankton production gave differing offsets due to cycling or mixotrophy, exceeding 14C net community production on average (677 f 98 versus 447 f 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 f 63 mg C m- 2 d- 1 (71 f 13 % of production). Two measurement variables for Prochlorococcus gave average production and grazing rates of 282 f 36 and 248 f 32 mg C m- 2 d- 1 (86 f 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.

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.

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.

Southern 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 low 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.

The current conventional paradigm of ocean food web structure inserts one full level or more of microzooplankton heterotrophic consumption, a substantial energy drop, between phytoplankton and mesozooplankton. Using a dataset with contemporaneous measurements of primary production (PP), size-fractioned mesozooplankton biomass, and micro- and mesozooplankton grazing rates from 10 tropical to temperate ocean ecosystems, we examined whether the structural inefficiencies in this paradigm allow sufficient energy transfer to support active metabolism and growth of observed zooplankton standing stocks. Zooplankton carbon requirements (ZCR) were determined from allometric equations that account for ecosystem differences in temperature and size structure. ZCRs were relatively low (similar to 30% of PP or less) for both oligotrophic systems and bloom biomass accumulation in eutrophic coastal waters. Higher relative ZCRs (>30% PP) were associated with elevated mesozooplankton grazing scenarios (bloom declines, abundant salps), advective subsidies, and open-ocean upwelling systems. Microzooplankton generally dominated as grazers of PP but were equal or secondary to direct herbivory as nutritional support for mesozooplankton in five of eight regional studies. All systems were able to satisfy ZCR within the conventional food-web interpretation, but balanced open-ocean upwelling systems required the most efficient alignments of contributions from microzooplankton grazing, direct herbivory, and carnivory to do so.

Small phytoplankton, consisting of pico and nano size fractions, are diverse in size and taxonomy. Yet, the differences in their productivity and taxonomic diversity are poorly described. Here, we measured the cell-specific carbon fixation rates of picocyanobacteria Synechococcus , picoeukaryote and nanoeukaryote populations while unveiling their taxonomic composition in oligotrophic subtropical (ST) and high-nutrient low-chlorophyll subantarctic (SA) waters. We coupled 24 h in-situ radiolabelled 14C incubations to flow cytometry sorting (FCM-sorting) and DNA metabarcoding from the same incubated samples, offering a direct account of the community associated with the carbon fixation rates measured. In both water masses, nanoeukaryotes had the highest cell-specific carbon fixation rate, followed by picoeukaryotes and Synechococcus (2.24 ± 29.99, 2.18 ± 2.08 and 0.78 ± 0.55 fgC cell-1 h-1, respectively). The cell-specific carbon fixation rates and growth rates of Synechococcus were 3-fold higher in ST compared to SA waters, while the rates of picoeukaryotes and nanoeukaryotes had no significant difference between the biogeochemically-contrasting water masses. Despite significant differences in their taxonomic composition, the FCM-sorted picoeukaryote and nanoeukaryote populations in SA waters were dominated by taxa with reported phago-mixotrophic strategies (Chrysophyceae, Dinophyceae and Prymnesiophyceae), suggesting phago-mixotrophy might alleviate nutrient stress in iron-limited conditions for discrete small photosynthetic eukaryote populations.### Competing Interest StatementThe authors have declared no competing interest.

Coastal upwelling supplies nutrients supporting primary production while also adding the toxic trace metal mercury (Hg) to the mixed layer of the ocean. This could be a concern for human and environmental health if it results in the enhanced bioaccumulation of monomethylmercury (MMHg). Here, we explore how upwelling influences Hg cycling in the California Current System (CCS) biome through particle scavenging and sea-air exchange. We collected suspended and sinking particle samples from a coastal upwelled water parcel and an offshore non-upwelled water parcel and observed higher total particulate Hg and sinking flux in the upwelling region compared to open ocean. To further investigate the full dynamics of Hg cycling, we modeled Hg inventories and fluxes in the upper ocean under upwelling and non-upwelling scenarios. The model simulations confirmed and quantified that upwelling enhances sinking fluxes of Hg by 41% through elevated primary production. Such an enhanced sinking flux of Hg is biogeochemically important to understand in upwelling regions, as it increases the delivery of Hg to the deep ocean where net conversion to MMHg may take place.