Oceanic Biological Pump Research Articles

Dynamics of ecosystem services along ecological network seascapes

Abstract Human societies depend on services provided by ecosystems, from local needs such as clean water and pest control to global services like the ozone layer and the ocean biological pump. Ecosystem services are linked to the states of the ecosystem, which are, in turn, governed by a web of ecological interactions. These interactions, along with the services they support, are under threat from environmental changes driven by human activities. Therefore, safeguarding these vital services requires an understanding of how the structure and dynamics of ecological interactions are affected by environmental change. A critical step towards this goal is the development of a theoretical framework that can elucidate how ecosystem services are sustained or impaired by interactions within ecosystems in fluctuating environments. Recent years have seen progress in characterizing the organization and dynamics of ecological networks. However, linking temporally varying network structure in fluctuating environments, the seascapes of ecological networks, and their impact on services remains a challenge. We propose an approach based on merging ecological network analysis with Boolean functions and modeling of fluctuating environments to address how services are affected by environmental change. We review aspects of Boolean Network models and illustrate the approach using biologically inspired Boolean rules that involve predator-prey cycles, trophic cascades, and mutualisms formed by plants and their frugivores. This approach aims to contribute to the study of how the organization of ecological interactions affects the persistence of ecosystem services. Specifically, we discuss how this approach can provide new insights into how environmental change affects the relationship between ecological networks and ecosystem services. The combination of information on the natural history of species interactions and ecosystem services, Boolean networks, and models for fluctuating environments may contribute to conservation strategies for preserving biodiversity and ecosystem services in the face of ongoing environmental change.

Satellite retrieval of oceanic particulate organic carbon: Towards an accurate and seamless dataset for the global ocean

Particulate organic carbon (POC) plays crucial roles in the global ocean carbon cycle and the oceanic biological pump. Satellite remote sensing has been demonstrated to be an effective technique for the retrieval of surface oceanic POC concentration. However, the complex spatiotemporal variations of the relationships between POC and oceanic optical properties across different waters posed challenges for accurate retrieval of POC concentration from satellite observations. Additionally, interference factors, such as cloud cover and sun glint, resulted in severe data missing problems and impeding daily coverage of the global ocean. With an attempt to generate accurate, seamless and readily available POC products for the global ocean, this study aimed to develop accurate satellite POC retrieval models for the Moderate Resolution Imaging Spectroradiometer (MODIS) data from both Terra and Aqua satellites, and to explore the possibility of using the empirical orthogonal function interpolation technique (DINEOF) to reconstruct satellite-retrieved POC data to generate gap-free global oceanic POC products. Results showed that the eXtreme Gradient Boosting (XGBoost) method could accurately retrieve POC with R2 approximately 0.80 and RMSE about 0.20 in log10 scale, obviously outperforming the operational blue-to-green band ratio algorithm and the hybrid polynomial algorithm based on two multi-band indices; and the DINEOF method, which could reconstruct approximately 88 % missing pixels for the global ocean, contributed to better revealing the global oceanic POC variations at a daily scale than the satellite-retrieved POC products. Based on the developed models, a suit of long time-series accurate and seamless POC products of the global surface ocean were generated, which is readily available for other applications and should be helpful to investigate the spatiotemporal variations of POC concentrations over global ocean and its roles in the global carbon cycle. The generated seamless products are openly accessible via the DOIs listed in the data availability section.

Decoding drivers of carbon flux attenuation in the oceanic biological pump

The biological pump supplies carbon to the oceans’ interior, driving long-term carbon sequestration and providing energy for deep-sea ecosystems1,2. Its efficiency is set by transformations of newly formed particles in the euphotic zone, followed by vertical flux attenuation via mesopelagic processes3. Depth attenuation of the particulate organic carbon (POC) flux is modulated by multiple processes involving zooplankton and/or microbes4,5. Nevertheless, it continues to be mainly parameterized using an empirically derived relationship, the ‘Martin curve’6. The derived power-law exponent is the standard metric used to compare flux attenuation patterns across oceanic provinces7,8. Here we present in situ experimental findings from C-RESPIRE9, a dual particle interceptor and incubator deployed at multiple mesopelagic depths, measuring microbially mediated POC flux attenuation. We find that across six contrasting oceanic regimes, representing a 30-fold range in POC flux, degradation by particle-attached microbes comprised 7–29 per cent of flux attenuation, implying a more influential role for zooplankton in flux attenuation. Microbial remineralization, normalized to POC flux, ranged by 20-fold across sites and depths, with the lowest rates at high POC fluxes. Vertical trends, of up to threefold changes, were linked to strong temperature gradients at low-latitude sites. In contrast, temperature played a lesser role at mid- and high-latitude sites, where vertical trends may be set jointly by particle biochemistry, fragmentation and microbial ecophysiology. This deconstruction of the Martin curve reveals the underpinning mechanisms that drive microbially mediated POC flux attenuation across oceanic provinces.

North-south discrepancy in the contributors to CB153 accumulation in the deep water of the Sea of Japan

The deep-water environment and its ecosystem are becoming the ultimate sinks for Polychlorinated Biphenyls (PCBs). A three-dimensional hydrodynamic-ecosystem-PCB coupled model was applied to the Sea of Japan (SoJ), where deep water is isolated from the surrounding oceans, to elucidate the accumulation processes of CB153 and assess the contributions of physical and biological processes to the accumulation. We suggest that the dissolved CB153 concentration formed a three-layer vertical structure in the SoJ: the highest concentration is in the intermediate layer (100–600 m), followed by those in the deep (600 m to the bottom) and surface layers (0–100 m). Different accumulation mechanisms in the northern and southern SoJ were discovered. The oceanic biological pump enhances the accumulation in the northern SoJ by taking CB153 out of the thermocline in summer and contributes 70 % to the accumulation in the intermediate layer; while the vertical advection contributes 70 % to the accumulation in the intermediate and deep layer in the southern SoJ.

Southern Ocean Biological Pump Role in Driving Holocene Atmospheric CO2: Reappraisal

AbstractChanges in Southern Ocean biological pump efficiency are frequently invoked as a source of glacial/interglacial CO2 variability. It was recently suggested that Southern Ocean biological pump weakening also contributed to Holocene CO2 increase. Here we test the causes and downstream effects of biological pump inefficiency during the Holocene. We first provide evidence that a southward shift of the South Westerly Winds, was likely the cause of increasing upwelling in the Southern Ocean leading to local biological pump weakening as reflected in fossil‐bound δ15N. We then introduce a series of ultra‐high resolution (6 m/ky) productivity records from the Chilean Margin to determine the fate of Southern Ocean nutrients. A compilation of records from Pacific sites supports export and complete consumption of preformed Southern Ocean nutrients, which might have reduced the contributions of Southern Ocean marine source to the Holocene increase in atmospheric CO2.

A Circum‐Antarctic Plankton Isoscape: Carbon Export Potential Across the Summertime Southern Ocean

AbstractThe Southern Ocean accounts for ∼30% of the ocean's CO2 sink, partly due to its biological pump that transfers surface‐produced organic carbon to deeper waters. To estimate large‐scale Southern Ocean carbon export potential and characterize its drivers, we measured the carbon and nitrogen isotope ratios of surface suspended particulate matter (δ13CSPM, δ15NSPM) for samples collected in summer 2016/2017 during the Antarctic Circumnavigation Expedition (364 stations). Concurrent measurements of phytoplankton community composition revealed the dominance of large diatoms in the Antarctic and nano‐phytoplankton (mainly haptophytes) in open Subantarctic waters. As expected, δ13CSPM was strongly dependent on pCO2, with local deviations in this relationship explained by phytoplankton community dynamics. δ15NSPM reflected the nitrogen sources consumed by phytoplankton, with higher inferred nitrate (versus recycled ammonium) dependence generally coinciding with higher micro‐phytoplankton abundances. Using δ15NSPM and a two‐endmember isotope mixing model, we quantified the extent of nitrate‐ versus ammonium‐supported growth, which yields a measure of carbon export potential. We estimate that across the Southern Ocean, 41 ± 29% of the surface‐produced organic carbon was potentially exported below the seasonal mixed layer during the growth season, with maximum export potential (50%–99%) occurring in the Antarctic Circumpolar Current's southern Boundary Zone and near the (Sub)Antarctic islands, reaching a minimum in the Subtropical Zone (<33%). Alongside iron, phytoplankton community composition emerged as an important driver of the Southern Ocean's biological pump, with large diatoms dominating regions characterized by high nitrate dependence and elevated carbon export potential and smaller, mainly non‐diatom taxa proliferating in waters where recycled ammonium supported most productivity.

Ocean Oxygen, Preformed Nutrients, and the Cause of the Lower Carbon Dioxide Concentration in the Atmosphere of the Last Glacial Maximum

AbstractAll else equal, if the ocean's “biological [carbon] pump” strengthens, the dissolved oxygen (O2) content of the ocean interior declines. Confidence is now high that the ocean interior as a whole contained less oxygen during the ice ages. This is strong evidence that the ocean's biological pump stored more carbon in the ocean interior during the ice ages, providing the core of an explanation for the lower atmospheric carbon dioxide (CO2) concentrations of the ice ages. Vollmer et al. (2022, https://doi.org/10.1029/2021PA004339) combine proxies for the oxygen and nutrient content of bottom waters to show that the ocean nutrient reservoir was more completely harnessed by the biological pump during the Last Glacial Maximum, with an increase in the proportion of dissolved nutrients in the ocean interior that were “regenerated” (transported as sinking organic matter from the ocean surface to the interior) rather than “preformed” (transported to the interior as dissolved nutrients by ocean circulation). This points to changes in the Southern Ocean, the dominant source of preformed nutrients in the modern ocean, with an apparent additional contribution from a decline in the preformed nutrient content of North Atlantic‐formed interior water. Vollmer et al. also find a lack of LGM‐to‐Holocene difference in the preformed 13C/12C ratio of dissolved inorganic carbon. This finding may allow future studies to resolve which of the proposed Southern Ocean mechanisms was most responsible for enhanced ocean CO2 storage during the ice ages: (a) coupled changes in ocean circulation and biological productivity, or (b) physical limitations on air‐sea gas exchange.

Physical Mechanisms Driving Enhanced Carbon Sequestration by the Biological Pump Under Climate Warming

AbstractAs ocean Carbon Dioxide Removal techniques are being considered, it is critical that they be evaluated against our scientific understanding of the global biological carbon pump. In a recent paper Nowicki et al. (2022, https://doi.org/10.1029/2021GB007083) provide an innovative and comprehensive breakdown of the different mechanistic pathways of carbon sequestration through the present‐day biological pump but then speculate that “These results suggest that ocean carbon storage will weaken as the oceans stratify and the subtropical gyres expand due to anthropogenic climate change.” Essentially, the authors combine their steady state result that oligotrophic subtropical gyres have lower residence times than other areas with the climate change result of these areas increasing under climate warming and extrapolate—assuming “all else is equal”—that the overall ocean will suffer a reduction in carbon sequestration efficiency. Expressing global changes in carbon sequestered by the ocean's biological pump as the summation of local changes in the sequestered carbon, timescale of return to the surface, and biogeographical area, I discuss how all three terms are tightly coupled, and summarize decades of climate change modeling consistently indicating that the global scale physical sequestration response is an increase ‐ in opposition of what one would infer from changes in subtropical area alone.

Export of Dissolved Organic Carbon (DOC) compared to the particulate and active fluxes near South Georgia, Southern Ocean

Quantifying the relative contributions of the export of particulate organic carbon (POC), dissolved organic carbon (DOC) and active fluxes by migrating organisms is essential to understand the functioning and vulnerability of the ocean's biological pump. However, these fluxes are rarely measured at the same time. Here we provide a first simultaneous comparison of these biological pump components in the region of South Georgia. We use a combination of in-situ data and an inverse model to calculate the DOC export and the suspended POC export and compare them to the sinking POC and active export. We find that, in this region, the DOC total export contributes about 6.6% (23.0–37.5 mg C m−2 day−1) to the total export flux, the active flux has no discernible contribution, and the sinking POC flux is dominant with a mean value of 409 mg C m−2 day−1. Diapycnal fluxes of DOC obtained from the cruise data constitute only a minor fraction (0.05–1.28 mg C m−2 day−1) of the total DOC export estimated by the inverse model and are exceeded on average by the diapycnal flux of suspended POC. Our results also indicate that the total export of DOC is driven by isopycnal transport. Future fieldwork in the region of South Georgia should focus on quantifying the isopycnal flux of DOC. Future measurement campaigns should also aim to simultaneously measure the particulate, dissolved and active components of the biological pump at contrasting locations and at different times to resolve the variability of their relative contribution.

Chasing iron bioavailability in the Southern Ocean: Insights from Phaeocystis antarctica and iron speciation.

Dissolved iron (dFe) availability limits the uptake of atmospheric CO2 by the Southern Ocean (SO) biological pump. Hence, any change in bioavailable dFe in this region can directly influence climate. On the basis of Fe uptake experiments with Phaeocystis antarctica, we show that the range of dFe bioavailability in natural samples is wider (<1 to ~200% compared to free inorganic Fe') than previously thought, with higher bioavailability found near glacial sources. The degree of bioavailability varied regardless of in situ dFe concentration and depth, challenging the consensus that sole dFe concentrations can be used to predict Fe uptake in modeling studies. Further, our data suggest a disproportionately major role of biologically mediated ligands and encourage revisiting the role of humic substances in influencing marine Fe biogeochemical cycling in the SO. Last, we describe a linkage between in situ dFe bioavailability and isotopic signatures that, we anticipate, will stimulate future research.

Important Contribution of Bacterial Carbon and Nitrogen to Sinking Particle Export

AbstractPhotosynthesis in the surface ocean converts atmospheric CO2 into organic particles, with the fraction sinking to depth representing a major part of the ocean's biological pump. Although sinking particles are known to be altered by attached‐bacteria during transit, most prior organic geochemical data indicated only minor replacement of plankton‐derived particles by bacterial material. We exploit bacteria‐specific biomarkers (d‐amino acids) in a multi‐year sediment trap in the Pacific Ocean (1,200 m) and suggest a different view. Major d‐amino acids were consistently measured at abundance demonstrating widespread accumulation of bacterial material in sinking particles. Bacterial detritus was estimated to account for up to 19% of particulate organic carbon and up to 36% of particulate nitrogen, much higher than cell count‐based values. The bacterial relative contribution increased with decreasing export production. Our results indicate that bacterial material constitutes an underappreciated component of the biological pump, a role expected to rise as the ocean warms.

Accelerated light carbon sequestration following late Paleocene-early Eocene carbon cycle perturbations

Carbon releases into the climate system produce global warming and ocean acidification events that can be reversed eventually by carbon sequestration. However, the underlying controls on the timescales of carbon removal, and their dependence on the amplitude of the initial perturbation, are poorly understood. Here, we assess a series of late Paleocene-early Eocene (LPEE) carbon cycle perturbations (∼56-52 Ma) of different amplitudes to constrain carbon removal timescales. Carbon isotope excursions (CIEs) and sedimentation patterns for the largest event, the Paleocene-Eocene Thermal Maximum (PETM), allow identification of a light carbon injection that appeared ∼85 kyr after the PETM onset. This CIE may have been triggered by orbital forcing of long (∼400 kyr) and short (∼100 kyr) eccentricity maxima. The various LPEE light carbon injections were followed by exponential carbon removal trends with half-life (t1/2) estimates of ∼6-26 kyr. These values are smaller than background estimates for the modern carbon cycle (t1/2>100 kyr), which reveals accelerated light carbon sequestration. We find that one estimated t1/2 period coincided temporally with ocean acidification recovery in different locations with contrasting paleo-water depths. This pattern indicates enhanced chemical weathering following LPEE CIEs; however, chemical weathering timescales are an order of magnitude longer than the observed t1/2 estimates. This reveals that several carbon processes were optimized during LPEE CIE recovery. Similar t1/2 estimates are obtained for light carbon injections of different sizes, which suggests that carbon removal was optimized to conditions induced by the initial perturbation. Temperature controls on oxygen solubility may have accelerated the oceanic biological pump in proportion to each LPEE carbon injection. This process may have caused accelerated carbon sequestration during LPEE CIE recovery and produced the short carbon removal timescales identified by t1/2 estimates of LPEE carbon cycle perturbations.

Phytoplankton productivity and community structure changes in the middle Okinawa Trough since the last deglaciation

Marine phytoplankton plays an important role in the ocean biological pump, whereas our understandings of phytoplankton growth in response to global climate and enviromment changes are still debated. Due to the forcing from the strong land-ocean interaction between the East Asia and the Indo-Pacific Warm Pool, the Okinawa Trough is characterized by high phytoplankton productivity and fast sediment accumulation, therefore it provides ideal archives to study marine productivity changes on millennial-to-orbital timescales. In this study, lipid biomarkers of C37 alkenones, dinosterol and brassicasterol, were analyzed in a sediment core M063–05 retrieved from the middle Okinawa Trough to reconstruct phytoplankton productivity and community structure changes over the last 15.8 kyr. We find that temporal variations of these lipid biomarker contents are parallel with precipitation changes in the Changjiang drainage basin during the last deglaciation, with high values during increased precipitation interval (14.5–12.7 ka) and low values during decreased precipitation intervals (15.8–15.0 ka and 12.7–11.5 ka). We propose that precipitation-induced terrestrial nutrient supply controlled phytoplankton growth in the middle Okinawa Trough during the last deglaciation. Lipid biomarker ratios reveal high (low) contributions of diatoms and dinoflagellates to total phytoplankton productivity during increased (decreased) precipitation intervals. The phytoplankton community structure record is also consistent with precipitation-induced terrestrial nutrient forcing during the deglacial stage, with higher nutrient condition favored diatom and dinoflagellate growth. Precipitation in the Changjiang drainage basin was mainly controlled by the East Asian summer monsoon change and the Intertropical Convergence Zone migration, however the deglacial sea level rise also modulated the forcing of precipitation on phytoplankton growth by affecting the delivery distance of terrestrial nutrients to our site. From the last deglaciation to the Holocene, the biomarker content decreased and the contribution of coccolithophorids to total phytoplankton productivity increased at the expense of dinoflagellates and diatoms. The most likely explanation is the regime shift of major nutrient sources supporting phytoplankton growth, from terrestrial riverine nutrients during the deglaciation to the Kuroshio subsurface water nutrients during the Holocene. Our results provide new insight of past phytoplankton responses to climate and environment changes.

The Ocean's Biological Pump: In Situ Oxygen Measurements in the Subtropical Oceans

AbstractThe magnitude and distribution of the ocean's biological pump (the downward flux of organic carbon (OC) from the ocean surface) influences the pCO2 of the atmosphere and the O2 content of the deep sea, but has not been well quantified. We determine this flux in the ocean's five subtropical gyres using upper‐ocean oxygen mass balance and measurements of T, S, and pO2 by autonomous profiling floats. Our results suggest that the biological OC pump is not globally uniform among the subtropical gyres: values in the North Pacific and Atlantic indicate distinct autotrophy (1–2 mol C m−2 yr−1) while near zero values in the S. Indian Ocean suggest the possibility of net heterotrophy. There is a correlation between the surface water iron/nitrate ratio and the magnitude of the biological pump suggesting an important role for nitrogen fixation in controlling the global distribution.

Proxy‐Based Preformed Phosphate Estimates Point to Increased Biological Pump Efficiency as Primary Cause of Last Glacial Maximum CO2 Drawdown

AbstractUpwelling deep waters in the Southern Ocean release biologically sequestered carbon into the atmosphere, contributing to the relatively high atmospheric CO2 levels during interglacial climate periods. Paleoceanographic evidence suggests this “CO2 leak” was lessened during the last glacial maximum (LGM), potentially due to increased stratification, weaker and equatorward‐shifted winds, and/or enhanced biological carbon export. The collective influences of these mechanisms on the ocean's biological pump efficiency and amount of atmospheric CO2 can be quantified by determining preformed phosphate of deep waters. We quantify preformed PO4 (Ppre,AOU) and preformed () of LGM bottom waters using a compilation of published paleo‐temperature, nutrient and oxygen estimates from benthic foraminifera. Our results show that preformed phosphate of the Pacific and Indian deep oceans was reduced by about −0.53 ± 0.13 μM and suggest that much (64 ± 28 ppmv) of the Glacial‐Interglacial CO2 drawdown resulted from changes in the ocean's biological pump efficiency. Once carbonate compensation is accounted for, this can explain the entire CO2 drawdown (87 ± 40 ppmv). Preformed shows similar results. The reconstructed LGM Ppre,AOU and oxygen are qualitatively consistent with the changes produced by a suite of numerical sensitivity experiments that roughly simulate three proposed mechanisms for an increase in LGM biological pump efficiency: an increase in biological activity, a decrease in wind‐driven upwelling, and an increase in stratification in the Southern Ocean.

Quantifying the Ocean's Biological Pump and Its Carbon Cycle Impacts on Global Scales.

The biological pump transports organic matter, created by phytoplankton productivity in the well-lit surface ocean, to the ocean's dark interior, where it is consumed by animals and heterotrophic microbes and remineralized back to inorganic forms. This downward transport of organic matter sequesters carbon dioxide from exchange with the atmosphere on timescales of months to millennia, depending on where in the water column the respiration occurs. There are three primary export pathways that link the upper ocean to the interior: the gravitational, migrant, and mixing pumps. These pathways are regulated by vastly different mechanisms, making it challenging to quantify the impacts of the biological pump on the global carbon cycle. In this review, we assess progress toward creating a global accounting of carbon export and sequestration via the biological pump and suggest a path toward achieving this goal.

The influence of black shale weathering on riverine barium isotopes

Barium and its isotopes are receiving growing interest in their capacity to record the variation of the oceanic biological pump through geological timescales. Nonetheless, only little is known about the continental sources and processes that can drive the Ba flux and isotope composition to the ocean. Whereas the role of processes such as secondary phase formation, sorption and biological uptake on Ba isotopes has been recently investigated, the potential of various rock sources in producing a range of Ba isotope compositions remains poorly known. In particular, black shales often exhibit enrichment in Ba that might potentially be associated with various Ba isotope signatures.In this study, we report the isotope composition of Ba in rivers with different relative contribution of black shale rocks in their catchments, located in the Mackenzie Basin (Northwest Canada). Both the dissolved and solid loads of the Mackenzie tributaries are enriched in heavy Ba isotopes with respect to the continental crust, which is consistent with an additional source to the weathering of igneous and siliciclastic rocks. Although the dissolved Ba abundance is partially driven by weathering processes, the river dissolved Ba isotope composition rather reflects a binary mixing between 1) a “classical” siliciclastic source (with a Ba isotope composition close to that of the upper continental crust δ138Ba ∼ 0.15 ± 0.05‰) and 2) an isotopically heavier source especially present in the Mackenzie Mountains (with a δ138Ba ∼ 0.40‰). The positive relationships between dissolved δ138Ba and radiogenic osmium isotope ratios, as well as with sulphuric acid production, indicate that the heavy Ba source is tightly linked to the weathering of black shale. Although the exact reason for such heavy Ba isotope signatures remains elusive, this study emphasises the potential of black shale weathering in shifting the Ba isotope composition of the dissolved flux to the ocean towards heavier values.

Equatorial Pacific bulk sediment δ15N supports a secular increase in Southern Ocean nitrate utilization after the mid-Pleistocene Transition

In much of the equatorial Pacific, surface nitrate is consistently available due to upwelling of sub-Antarctic Mode Water-derived nitrate from the thermocline. However, in the low-latitude western boundary current (WBC) region, surface nitrate is limited due to reduced upwelling and less nitrate-replete thermocline water masses. In order to explore South Pacific WBC paleoceanography via upper ocean nitrate dynamics, we present a new bulk sediment δ15N record from within this WBC region at International Ocean Discovery Program Site U1486 (2°22′S, 144°36′E) in the Bismarck Sea north of New Guinea. This record spans from 1420 ka to recent – surpassing nearby sediment δ15N reconstructions by over a million years and allowing for direct comparison to low-latitude Pacific sediment core δ15N records of similar length. Core-top and down-core bulk sediment δ15N, TOC/TN and δ13Corg values indicate minimal or no terrestrial influence on the organic fraction at Site U1486, which is consistent with previous studies of modern processes in the region. After comparison, differences in orbital variability and secular trends throughout the Middle and Late Pleistocene suggest that nitrate dynamics along the equator and in the WBCs were relatively disparate. We observe that δ15N at Site U1486 is consistent with patterns of eastern Pacific denitrification, while we suggest increasing δ15N after the mid-Pleistocene Transition (MPT; ∼1250–700 ka) at Sites 806 (0°19′N, 110°30′W) and 849 (0°19′N, 160°E) is linked to increasing Southern Ocean nitrate utilization. Enhanced nitrate utilization is a key indicator of the strengthened biological pump that contributed to a reduction in atmospheric pCO2 during the last glacial. Therefore, a post-MPT increase in nitrate utilization may bolster the role of the Southern Ocean biological pump in driving the deeper and longer glacial periods of the 100-kyr world.

Plankton community composition and productivity near the Subantarctic Prince Edward Islands archipelago in autumn

The Subantarctic Ocean is a sink for atmospheric CO2, largely due to its biological pump, which is enhanced by the influence of the Subantarctic islands on the plankton ecosystem (the so‐called “island mass effect”). The influence of the Prince Edward Islands archipelago in the Indian Subantarctic on the surrounding hydrography and benthos has been well studied; however, over the last two decades, little attention has been paid to the functioning and productivity of its plankton ecosystem. Here, we present the first measurements of primary production at the archipelago in over 20 years and the first‐ever rates of secondary production, interpreted in the context of hydrographic, biogeochemical, and plankton community composition data. In autumn 2017, after the late‐summer bloom, nanophytoplankton dominated the near‐island waters and regenerated nutrients fuelled 76% of phytoplankton growth. Primary production and carbon export potential (inferred from nitrate uptake) reached a local maximum in the inter‐island region, which we attribute to water‐mass retention and stratification over the inter‐island plateau (a manifestation of the island mass effect). We observed a diverse mesozooplankton community, likely a remnant of the late‐summer bloom rather than representing the consumers feeding on the in situ (nano)phytoplankton. We estimate that even after the decay of the late‐summer bloom, roughly a quarter of the planktonic carbon was potentially exportable. This finding implies that the archipelago's upper‐ocean ecosystem sequesters atmospheric CO2 at least through autumn despite high rates of recycling and inefficient trophic transfer, thereby contributing to the island mass effect‐associated strengthening of the Subantarctic Ocean's biological pump.

A first assessment of particle flux over the South Brazil Bight continental slope

The vertical export of particulate organic carbon (POC) is a key component of the oceanic biological pump, which regulates the distribution of elements associated with primary production in the upper euphotic zone. To better understand how oceanic biogeochemical processes will respond in climate change scenarios, it is necessary to comprehend the mechanisms that regulate the magnitude and variations of POC flux among distinct oceanic domains, including the boundaries between continental margins and oceanic basins. This work is a first assessment of particle flux over the South Brazil Bight continental slope, a tropical western boundary system dominated by the Brazil Current. Total mass flux, and specific carbon, nitrogen and biogenic silica fluxes were measured periodically by two sediment traps installed at 450 m deep over the 900 m isobath, and at 750 m deep over the 2000 m isobath, from November 2016 to September 2017. Due to strong contribution of advected particles, high and extremely irregular fluxes were registered at the 900 m isobath site (mean = 1.1 g m-2 d− 1, stand. dev. ± 2.9 g m-2 d-1). The captured material suggests this sediment trap was affected by turbidity flows, which precludes the estimation of the biological pump contribution over this isobath. Over the 2000 m isobath, POC flux (3.7 mg m-2 d-1; ±2.5 mg m-2 d− 1) followed the seasonal pattern of primary production measured by satellite. POC flux was strongly correlated with biogenic silica content (r = 0.79; p < 0.01). This ballast mineral was more efficient than CaCO3 in transferring carbon from the base of the euphotic zone to 750 m. We suggest intra-seasonal variations found in the POC flux over the 2000 m isobath to be related to Brazil Current instabilities. Measurements of δ15N suggest that higher POC flux occurred during subsurface nitrate-based production regimes. A regional exponential export model is proposed, with a mean exponent (b value) of 1.4 (±0.3). Although initial, the results of this work allow us to estimate that 2.2 × 1012 g C is exported annually over the South Brazil Bight continental slope.