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In the open ocean, elevated carbon flux (ECF) events increase the delivery of particulate carbon from surface waters to the seafloor by severalfold compared to other times of year. Since microbes play central roles in primary production and sinking particle formation, they contribute greatly to carbon export to the deep sea. Few studies, however, have quantitatively linked ECF events with the specific microbial assemblages that drive them. Here, we identify key microbial taxa and functional traits on deep-sea sinking particles that correlate positively with ECF events. Microbes enriched on sinking particles in summer ECF events included symbiotic and free-living diazotrophic cyanobacteria, rhizosolenid diatoms, phototrophic and heterotrophic protists, and photoheterotrophic and copiotrophic bacteria. Particle-attached bacteria reaching the abyss during summer ECF events encoded metabolic pathways reflecting their surface water origins, including oxygenic and aerobic anoxygenic photosynthesis, nitrogen fixation, and proteorhodopsin-based photoheterotrophy. The abundances of some deep-sea bacteria also correlated positively with summer ECF events, suggesting rapid bathypelagic responses to elevated organic matter inputs. Biota enriched on sinking particles during a spring ECF event were distinct from those found in summer, and included rhizaria, copepods, fungi, and different bacterial taxa. At other times over our 3-y study, mid- and deep-water particle colonization, predation, degradation, and repackaging (by deep-sea bacteria, protists, and animals) appeared to shape the biotic composition of particles reaching the abyss. Our analyses reveal key microbial players and biological processes involved in particle formation, rapid export, and consumption, that may influence the ocean's biological pump and help sustain deep-sea ecosystems., Competing Interests: The authors declare no competing interest., (Copyright © 2021 the Author(s). Published by PNAS.)

Siliceous diatoms are one of the most prominent actors in the oceans, and they account for approximately 40% of the primary production and particulate organic carbon export flux. It is believed that changes in carbon flux caused by variations in diatom distribution can lead to significant climate shifts. Although the fundamental pathways of diatom-driven carbon sequestration have long been established, there are no reports of CaCO3 precipitation induced by marine diatom species. This manuscript introduces novel details regarding the enhancement of aragonite precipitation during photosynthesis in Skeletonema costatum in both artificial and natural seawater. Through direct measurements of cell surfaces via a pH microelectrode and zeta potential analyzer, it was determined that the diatom-mediated promotion of CaCO3 precipitation is achieved through the creation of specific microenvironments with concentrated [CO32−] and [Ca2+] and/or the dehydrating effect of adsorbed Ca2+. Based on this mechanism, it is highly plausible that diatom-mediated calcification could occur in the oceans, an assertion that was supported by the significant deviation of total alkalinity (TA) from the conservative TA-salinity mixing line during a Skeletonema costatum bloom in the East China Sea and other similar occurrences. The newly discovered calcification pathway establishes a link between particulate inorganic and organic carbon flux and thus helps in the reassessment of marine carbon export fluxes and CO2 sequestration efficiency. This discovery may have important ramifications for assessing marine carbon cycling and predicting the potential effects of future ocean acidification. [ABSTRACT FROM AUTHOR]

Marine mesozooplankton play an important role for marine ecosystem functioning and global biogeochemical cycles. Their size structure, varying spatially and temporally, heavily impacts biogeochemical processes and ecosystem services. Mesozooplankton exhibit size changes throughout their life cycle, affecting metabolic rates and functional traits. Despite this variability, many models oversimplify mesozooplankton as a single, unchanging size class, potentially biasing carbon flux estimates. Here, we include mesozooplankton ontogenetic growth and reproduction into a 3‐dimensional global ocean biogeochemical model, PISCES‐MOG, and investigate the subsequent effects on simulated mesozooplankton phenology, plankton distribution, and organic carbon export. Utilizing an ensemble of statistical predictive models calibrated with a global set of observations, we generated monthly climatologies of mesozooplankton biomass to evaluate the simulations of PISCES‐MOG. Our analyses reveal that the model and observation‐based biomass distributions are consistent (rpearson ${\mathrm{r}}_{\mathit{pearson}}$ = 0.40, total epipelagic biomass: 137 TgC from observations vs. 232 TgC in the model), with similar seasonality (later bloom as latitude increases poleward). Including ontogenetic growth in the model induced cohort dynamics and variable seasonal dynamics across mesozooplankton size classes and altered the relative contribution of carbon cycling pathways. Younger and smaller mesozooplankton transitioned to microzooplankton in PISCES‐MOG, resulting in a change in particle size distribution, characterized by a decrease in large particulate organic carbon (POC) and an increase in small POC generation. Consequently, carbon export from the surface was reduced by 10%. This study underscores the importance of accounting for ontogenetic growth and reproduction in models, highlighting the interconnectedness between mesozooplankton size, phenology, and their effects on marine carbon cycling. Key Points: Incorporating mesozooplankton growth and reproduction alters carbon cycling pathways, reducing carbon export at 100 m by 10%Cohort dynamics lead to significant variations in seasonal dynamics across mesozooplankton size classes without affecting export seasonalityStatistical predictive models demonstrate consistency between modeled and observed mesozooplankton dynamics globally [ABSTRACT FROM AUTHOR]

The ocean is responsible for taking up approximately 25% of anthropogenic CO2 emissions and stores >50 times more carbon than the atmosphere. Biological processes in the ocean play a key role, maintaining atmospheric CO2 levels approximately 200 ppm lower than they would otherwise be. The ocean's ability to take up and store CO2 is sensitive to climate change, however the key biological processes that contribute to ocean carbon storage are uncertain, as are how those processes will respond to, and feedback on, climate change. As a result, biogeochemical models vary widely in their representation of relevant processes, driving large uncertainties in the projections of future ocean carbon storage. This review identifies key biological processes that affect how ocean carbon storage may change in the future in three thematic areas: biological contributions to alkalinity, net primary production, and interior respiration. We undertook a review of the existing literature to identify processes with high importance in influencing the future biologically‐mediated storage of carbon in the ocean, and prioritized processes on the basis of both an expert assessment and a community survey. Highly ranked processes in both the expert assessment and survey were: for alkalinity—high level understanding of calcium carbonate production; for primary production—resource limitation of growth, zooplankton processes and phytoplankton loss processes; for respiration—microbial solubilization, particle characteristics and particle type. The analysis presented here is designed to support future field or laboratory experiments targeting new process understanding, and modeling efforts aimed at undertaking biogeochemical model development. Plain Language Summary: The storage of carbon in the ocean forms an essential component of the Earth's carbon cycle. The contribution of ocean biology to carbon storage is not well constrained by observations and as a result has large uncertainties in the future model projections. There are a multitude of processes involved in the uptake, remineralization and storage of carbon, many of which have high uncertainty. Here we assess significant processes in determining net primary production, the biological contribution to alkalinity, and interior respiration. Using an extensive literature review, expert assessment and community survey, we rank processes as having high, moderate or low importance to the future biologically‐mediated storage of carbon in the ocean. This analysis is intended to support future observational studies and biogeochemical model development. Key Points: Key processes needed to improve projections of the response of ocean carbon storage to climate change identifiedThree themes are addressed: net primary production, interior respiration, and biological contributions to alkalinityAn expert assessment and community survey used to rank processes according to importance and uncertainty levels [ABSTRACT FROM AUTHOR]

The elemental ratios of carbon, nitrogen, and phosphorus (C:N:P) within organic matter play a key role in coupling biogeochemical cycles in the global ocean. At the cellular level, these ratios are controlled by physiological responses to the environment. But linking these cellular‐level processes to global biogeochemical cycles remains challenging. We present a novel model framework that combines knowledge of phytoplankton cellular functioning with global scale hydrographic data, to assess the role of variable carbon‐to‐phosphorus ratios (RC:P) on the distribution of export production. We implement a trait‐based mechanistic model of phytoplankton growth into a global biogeochemical inverse model to predict global patterns of phytoplankton physiology and stoichiometry that are consistent with both biological growth mechanisms and hydrographic carbon and nutrient observations. We compare this model to empirical parameterizations relating RC:P to temperature or phosphate concentration. We find that the way the model represents variable stoichiometry affects the magnitude and spatial pattern of carbon export, with globally integrated fluxes varying by up to 10% (1.3 Pg C yr−1) across models. Despite these differences, all models exhibit strong consistency with observed dissolved inorganic carbon and phosphate concentrations (R2 > 0.9), underscoring the challenge of selecting the most accurate model structure. We also find that the choice of parameterization impacts the capacity of changing RC:P to buffer predicted export declines. Our novel framework offers a pathway by which additional biological information might be used to reduce the structural uncertainty in model representations of phytoplankton stoichiometry, potentially improving our capacity to project future changes. Plain Language Summary: Phytoplankton play a vital role in Earth's carbon cycle. The ratios of carbon, nitrogen, and phosphorus in these tiny marine plants influence how much carbon they absorb at the surface and export to the deep ocean. Yet, many models overlook the global variability of these ratios. Our study introduces an innovative model that combines microscopic knowledge of phytoplankton growth with global ocean data. We use this model to predict how the elemental ratios in phytoplankton vary on a global scale. By comparing this detailed model to simpler models based on individual environmental controls, we reveal that the way we represent these ratios significantly impacts carbon export patterns. Different representations of these ratios lead to estimates of carbon export from the ocean's surface that differ by over 1 billion tons annually. We also find that the flexibility of elemental ratios can counteract future declines in ocean productivity to varying degrees, depending on the assumed environmental controls of these ratios. Our findings highlight the critical role of understanding the connection between phytoplankton elemental composition and their environment and underscore the need for improved models to better anticipate future changes in the ocean's carbon cycle. Key Points: New framework embeds a mechanistic cellular growth model for phytoplankton stoichiometry in a global biogeochemical inverse modelFour parameterizations of phytoplankton C:P ratios are optimized globally using an inverse model and multiple biogeochemical tracersModeled C:P patterns impact global carbon export flux by 10%, with larger regional differences, and affect sensitivity to future change [ABSTRACT FROM AUTHOR]

Each year, the biological carbon pump (BCP) transports large quantities of carbon from the ocean surface to the interior. The efficiency of this transfer varies geographically, and is a key determinant of the atmosphere‐ocean carbon dioxide balance. Traditionally, the attention has been focused on explaining perceived geographical variations in this transfer efficiency (TE) in an attempt to understand it, an approach that has led to conflicting results. Here we combine observations and modeling to show that the spatial variability in TE can instead be explained by the seasonal variability in carbon flux attenuation. We also show that seasonality can explain the contrast between known global estimates of TE, due to differences in the date and duration of sampling. Our results suggest caution in the mechanistic interpretation of annual‐mean patterns in TE and demonstrates that seasonally and spatially resolved data sets and models might be required to generate accurate evaluations of the BCP. Plain Language Summary: Each year, marine phytoplankton convert carbon dioxide into millions of tonnes of organic carbon with a fraction of it reaching the deep ocean, where it can remain for hundreds of years. The efficiency of this surface‐to‐depth carbon transfer is therefore a key determinant of the atmosphere‐ocean carbon dioxide balance. However, the variability of this transfer efficiency (TE) and its underlying causes are poorly understood, to the extent that different studies report contradicting results. We show that the existence of seasonal variability in the attenuation of sinking carbon particles can explain the observed spatial variability in annual TE and reconcile with the literature. Our findings suggest caution in interpreting results from sparse but time‐varying data sets, highlighting that seasonal variability should be considered when studying the oceanic carbon cycle. Key Points: Spatial variability in carbon transfer efficiency (TE) can be generated solely by the seasonality of flux attenuation informed by observationsSeasonality in flux attenuation can reconcile contrasting TE maps reported in the literatureResolving and understanding seasonality are key for an accurate evaluation of the biological carbon pump under climate change [ABSTRACT FROM AUTHOR]

The marine biological carbon pump (BCP) stores carbon in the ocean interior, isolating it from exchange with the atmosphere and thereby coregulating atmospheric carbon dioxide (CO2). As the BCP commonly is equated with the flux of organic material to the ocean interior, termed "export flux," a change in export flux is perceived to directly impact atmospheric CO2, and thus climate. Here, we recap how this perception contrasts with current understanding of the BCP, emphasizing the lack of a direct relationship between global export flux and atmospheric CO2. We argue for the use of the storage of carbon of biological origin in the ocean interior as a diagnostic that directly relates to atmospheric CO2, as a way forward to quantify the changes in the BCP in a changing climate. The diagnostic is conveniently applicable to both climate model data and increasingly available observational data. It can explain a seemingly paradoxical response under anthropogenic climate change: Despite a decrease in export flux, the BCP intensifies due to a longer reemergence time of biogenically stored carbon back to the ocean surface and thereby provides a negative feedback to increasing atmospheric CO2. This feedback is notably small compared with anthropogenic CO2 emissions and other carbon‐climate feedbacks. In this Opinion paper, we advocate for a comprehensive view of the BCP's impact on atmospheric CO2, providing a prerequisite for assessing the effectiveness of marine CO2 removal approaches that target marine biology. [ABSTRACT FROM AUTHOR]

Gelatinous zooplankton are increasingly recognized to play a key role in the ocean's biological carbon pump. Appendicularians, a class of pelagic tunicates, are among the most abundant gelatinous plankton in the ocean, but it is an open question how their contribution to carbon export might change in the future. Here, we conducted an experiment with large volume in situ mesocosms (~55–60 m3 and 21 m depth) to investigate how ocean acidification (OA) extreme events affect food web structure and carbon export in a natural plankton community, particularly focusing on the keystone species Oikopleura dioica, a globally abundant appendicularian. We found a profound influence of O. dioica on vertical carbon fluxes, particularly during a short but intense bloom period in the high CO2 treatment, during which carbon export was 42%–64% higher than under ambient conditions. This elevated flux was mostly driven by an almost twofold increase in O. dioica biomass under high CO2. This rapid population increase was linked to enhanced fecundity (+20%) that likely resulted from physiological benefits of low pH conditions. The resulting competitive advantage of O. dioica resulted in enhanced grazing on phytoplankton and transfer of this consumed biomass into sinking particles. Using a simple carbon flux model for O. dioica, we estimate that high CO2 doubled the carbon flux of discarded mucous houses and fecal pellets, accounting for up to 39% of total carbon export from the ecosystem during the bloom. Considering the wide geographic distribution of O. dioica, our findings suggest that appendicularians may become an increasingly important vector of carbon export with ongoing OA. [ABSTRACT FROM AUTHOR]

Diatom sinking plays a crucial role in the global carbon cycle, accounting for approximately 40% of marine particulate organic carbon export. While oceanic models typically represent diatoms as microphytoplankton (> 20 mm), it is important to recognize that many diatoms fall into the categories of nanophytoplankton (2-20 mm) and picophytoplankton (< 2 mm). These smaller diatoms have also been found to significantly contribute to carbon export. However, our understanding of their sinking behavior and buoyancy regulation mechanisms remains limited. In this study, we investigate the sinking behavior of a nanoplanktonic diatom, Phaeodactylum tricornutum (P. tricornutum), which exhibits rapid changes in sinking behavior in response to varying nutrient concentrations. Our results demonstrate that a higher sinking rate is observed under phosphate limitation and depletion. Notably, in phosphate depletion, the sinking rate of P. tricornutum was 0.79 ± 0.03md-1, nearly three times that of the previously reported sinking rates for Skeletonema costatum, Ditylum brightwellii, and Chaetoceros gracile. Furthermore, during the first 6 h of phosphate spike, the sinking rate of P. tricornutum remained consistently high. After 12 h of phosphate spike, the sinking rate decreased to match that of the phosphate repletion phase, only to increase again over the next 12 hours due to phosphate depletion. This rapid sinking behavior contributes to carbon export and potentially allows diatoms to exploit nutrient-rich patches when encountering increased nutrient concentrations. We also observed a significant positive correlation (P< 0.001) between sinking rate and lipid content (R = 0.91) during the phosphate depletion and spike experiment. It appears that P. tricornutum regulates its sinking rate by increasing intracellular lipid content, particularly digalactosyldiacylglycerol, hexosyl ceramide, monogalactosyldiacylglycerol, and triglycerides. Additionally, P. tricornutum replaces phospholipids with more dense membrane sulfolipids, such as sulfoquinovosyldiacylglycerol under phosphate shortage. These findings shed light on the intricate relationship between nutrient availability, sinking behavior, and lipid composition in diatoms, providing insights into their adaptive strategies for carbon export and nutrient utilization. [ABSTRACT FROM AUTHOR]

The gravitational sinking of particles in the mesopelagic layer (∼200–1,000 m) transfers to the deep ocean a part of atmospheric carbon fixed by phytoplankton. This process, called the gravitational pump, exerts an important control on atmospheric CO2 levels but remains poorly characterized given the limited spatio‐temporal coverage of ship‐based flux measurements. Here, we examined the gravitational pump with BioGeoChemical‐Argo floats in the Southern Ocean, a critically under‐sampled area. Using time‐series of bio‐optical measurements, we characterized the concentration of particles in the productive zone, their export and transfer efficiency in the underlying mesopelagic zone, and the magnitude of sinking flux at 1,000 m. We separated float observations into six environments delineated by latitudinal fronts, sea‐ice coverage, and natural iron fertilization. Results show a significant increase in the sinking‐particle flux at 1,000 m with increasing latitude, despite comparable particle concentrations in the productive layer. The variability in deep flux was driven by changes in the transfer efficiency of the flux, related to the composition of the phytoplanktonic community and the size of particles, with intense flux associated with the predominance of micro‐phytoplankton and large particles at the surface. We quantified the relationships between the nature of surface particles and the flux at 1,000 m and used these results to upscale our flux survey across the whole Southern Ocean using surface observations by floats and satellites. We then estimated the basin‐wide Spring‐Summer flux of sinking particles at 1,000 m over the Southern Ocean (0.054 ± 0.021 Pg C). Plain Language Summary: Phytoplankton are tiny organisms that convert CO2 to organic carbon in the sunlit layer of the ocean. A part of this carbon sinks in the form of particles and can be stored for decades to millennia in the deep ocean before returning to the atmosphere. This long‐term carbon storage is important for our global climate because it moves CO2 out of the atmosphere and into the ocean. Traditional sinking carbon measurements rely on physical collection of sinking particles using ships and moorings. These measurements can only be made sporadically and/or at very few locations. This means that it is very difficult to measure ocean carbon storage for entire oceans, leading to big uncertainty. In this study, we used autonomous underwater robots to greatly expand measurements of sinking carbon storage in the Southern Ocean. These measurements allowed us to generate a new estimate of total sinking carbon storage for the entire Southern Ocean. These measurements also uncovered clear patterns in carbon storage, which changed with season and latitude. We found that these patterns appear to be driven by particle diameter. In the springtime and closer to Antarctica, both phytoplankton and other living and dead particles grow larger, corresponding to a greater amount of carbon storage. Key Points: We developed a method for detecting the flux of large particles in the mesopelagic layer from bio‐optical measurements on profiling floatsObservations revealed strong seasonal and latitudinal variability in deep particle flux (∼1,000 m) driven by changes in transfer efficiencyBy linking deep flux to the nature of surface particles observed by satellites, we upscaled flux observations to the entire Southern Ocean [ABSTRACT FROM AUTHOR]

Quantifying the strength and efficiency of the Southern Ocean biological carbon pump (BCP) and its response to predicted changes in the Earth's climate is fundamental to our ability to predict long-term changes in the global carbon cycle and, by extension, the impact of continued anthropogenic perturbation of atmospheric CO2. There is little agreement, however, in climate model projections of the sensitivity of the Southern Ocean BCP to climate change, with a lack of consensus in even the direction of predicted change, highlighting a gap in our understanding of a major planetary carbon flux. In this review, we summarize relevant research that highlights the important role of fine-scale dynamics (both temporal and spatial) that link physical forcing mechanisms to biogeochemical responses that impact the characteristics of the seasonal cycle of phytoplankton and by extension the BCP. This approach highlights the potential for integrating autonomous and remote sensing observations of fine scale dynamics to derive regionally optimized biogeochemical parameterizations for Southern Ocean models. Ongoing development in both the observational and modelling fields will generate new insights into Southern Ocean ecosystem function for improved predictions of the sensitivity of the Southern Ocean BCP to climate change. This article is part of a discussion meeting issue 'Heat and carbon uptake in the Southern Ocean: the state of the art and future priorities'. [ABSTRACT FROM AUTHOR]

The great whales (baleen and sperm whales), through their massive size and wide distribution, influence ecosystem and carbon dynamics. Whales directly store carbon in their biomass and contribute to carbon export through sinking carcasses. Whale excreta may stimulate phytoplankton growth and capture atmospheric CO 2 ; such indirect pathways represent the greatest potential for whale-carbon sequestration but are poorly understood. We quantify the carbon values of whales while recognizing the numerous ecosystem, cultural, and moral motivations to protect them. We also propose a framework to quantify the economic value of whale carbon as populations change over time. Finally, we suggest research to address key unknowns (e.g., bioavailability of whale-derived nutrients to phytoplankton, species- and region-specific variability in whale carbon contributions). As climate change accelerates, there is increasing interest in the ability of whales to trap carbon (i.e., whale carbon), yet it is currently undetermined if and how whale carbon should be used in climate-change mitigation strategies. Restoring whale populations will enhance carbon storage in whale biomass and sequestration in the deep sea via whale falls, though the global impact will be relatively small. Whale-stimulated primary productivity via nutrient provisioning may sequester substantially more carbon, though there is uncertainty regarding the carbon fate in these food webs. Recovery of whale populations via reduction of anthropogenic impacts can aid in carbon dioxide removal but its inclusion in climate policy needs to be grounded in the best available science and considered in tandem with other strategies known to directly reduce greenhouse gas emissions. [ABSTRACT FROM AUTHOR]

The current narrative of artificial upwelling (AU) is to translocate nutrient rich deep water to the ocean surface, thereby stimulating the biological carbon pump (BCP). Our refined narrative takes the response of the solubility pump and the CO2 emission scenario into account. Using global ocean‐atmosphere model experiments we show that the effectiveness of a hypothetical maximum AU deployment in all ocean areas where AU is predicted to lower surface pCO2, the draw down of CO2 from the atmosphere during years 2020–2100 depends strongly on the CO2 emission scenario and ranges from 1.01 Pg C/year (3.70 Pg CO2/year) under RCP 8.5 to 0.32 Pg C/year (1.17 Pg CO2/year) under RCP 2.6. The solubility pump becomes equally effective compared to the BCP under the highest emission scenario (RCP 8.5), but responds with CO2 outgassing under low CO2 emission scenarios. Plain Language Summary: Artificial upwelling (AU) is a proposed marine carbon dioxide removal (CDR) method, which suggests deploying pipes in the ocean to pump deep water to the ocean's surface. This process theoretically has several different impacts on the surface layer including an increase in the nutrient concentration, as well as a decrease in surface water temperature. Changes in the carbon cycle and associated with biological components are covered by the biological carbon pump (BCP), while changes via physical‐chemical processes are covered by the solubility pump. Using numerical ocean modeling and simulating almost globally applied AU between the years 2020 and 2100 under several different atmospheric CO2 emission scenarios, we show that AU leads under every simulated emission scenario to an additional CO2‐uptake of the ocean, but the potential increases under higher emission scenarios (up to 1.01 Pg C/year (3.70 Pg CO2/year) under the high CO2‐emission scenario RCP 8.5). The individual contribution via the BCP is under every emission scenario positive, while the processes associated with the solubility pump can lead to CO2‐uptake under higher emission scenarios and CO2 outgassing under lower emission scenarios. Key Points: Artificial upwelling (AU) effectiveness to draw down CO2 from the atmosphere is strongly dependent on the future CO2 emission scenarioThe solubility pump becomes as effective as the biological carbon pump under high emission scenariosOrganic matter transfer efficiency decreases under AU, likely due to higher water temperatures below the ocean's surface [ABSTRACT FROM AUTHOR]

Through the constant rain of sinking marine particles in the ocean, carbon (C) trapped within is exported into the water column and sequestered when reaching depths below the mesopelagic zone. Atmospheric CO2 levels are thereby strongly related to the magnitude of carbon export fluxes in the mesopelagic zone. Sinking particles represent the main source of carbon and energy for mesopelagic organisms, attenuating the C export flux along the water column. Attempts to quantify the amount of C exported versus consumed by heterotrophic organisms have increased in recent decades. Yet, most of the conducted estimations have led to estimated C demands several times higher than the measured C export fluxes. The choice of parameters such as growth efficiencies or various conversion factors is known to greatly impact the resulting C budget. In parallel, field or experimental data are sorely lacking to obtain accurate values of these crucial overlooked parameters. In this study, we identify the most influential of these parameters and perform inversion of a mechanistic model. Further, we determine the optimal parameter values as the ones that best explain the observed prokaryotic respiration, the prokaryotic production, and the zooplankton respirations. The consistency of the resulting C-budget suggests that such budgets can be adequately balanced when using appropriate parameters. [ABSTRACT FROM AUTHOR]

The interactions between marine bacteria and particulate matter play a pivotal role in the biogeochemical cycles of carbon and associated inorganic elements in the oceans. Eutrophic plumes typically form around nutrient-releasing particles and host intense bacterial activities. However, the potential of bacteria to reshape the nutrient plumes remains largely unexplored. We present a high-resolution numerical analysis for the impacts of nutrient uptake by free-living bacteria on the pattern of dissolution around slow-moving particles. At the single-particle level, the nutrient field is parameterized by the Péclet and Damköhler numbers (0 < Pe < 1000, 0 < Da < 10) that quantify the relative contribution of advection, diffusion and uptake to nutrient transport. In spite of reducing the extent of the nutrient plume in the wake of the particle, bacterial uptake enhances the rates of particle dissolution and nutrient depletion. These effects are amplified when the uptake timescale is shorter than the plume lifetime (Pe/Da < 100, Da > 0.0001), while otherwise they are suppressed by advection or diffusion. Our analysis suggests that the quenching of eutrophic plumes is significant for individual phytoplankton cells, as well as marine aggregates with sizes ranging from 0.1 mm to 10 mm and sinking velocities up to 40 m per day. This microscale process has a large potential impact on microbial growth dynamics and nutrient cycling in marine ecosystems. [ABSTRACT FROM AUTHOR]

Enhanced vertical carbon transport (gravitational sinking and subduction) at mesoscale ocean fronts may explain the demonstrated imbalance of new production and sinking particle export in coastal upwelling ecosystems. Based on flux assessments from 238U:234Th disequilibrium and sediment traps, we found 2 to 3 times higher rates of gravitational particle export near a deep-water front (305 mg C⋅m-2⋅d-1) compared with adjacent water or to mean (nonfrontal) regional conditions. Elevated particle flux at the front was mechanistically linked to Fe-stressed diatoms and high mesozooplankton fecal pellet production. Using a data assimilative regional ocean model fit to measured conditions, we estimate that an additional ∼225 mg C⋅m-2⋅d-1 was exported as subduction of particle-rich water at the front, highlighting a transport mechanism that is not captured by sediment traps and is poorly quantified by most models and in situ measurements. Mesoscale fronts may be responsible for over a quarter of total organic carbon sequestration in the California Current and other coastal upwelling ecosystems.

Life strategies, ecophysiological performances and diel vertical migration (DVM) of zooplankton key species affect the efficiency and strength of the biological carbon pump (BCP). However, it is unclear to what extent different functional groups affect the BCP. Depth-stratified day and night samples (0-800 m) from the subtropical South Atlantic were analyzed focusing on the calanoid copepod community. Calanoid abundance, biomass distribution and species-specific impact on the passive (fecal pellets) and active (via DVM) vertical flux of carbon were determined. Species were assigned to different migrant groups where, their contributions were estimated by using the proportion of the migratory community instead of simple day-night differences in biomass. This novel approach leads to more robust flux estimates, particularly for small sample sizes. According to migration ranges and day/night residence depth, functional groups were characterized, i.e. small- and large-scale epipelagic and mesopelagic migrants. Epipelagic small-scale migrants transported respiratory (1.5 mg C m-2 d-1) and fecal pellet (1.1 mg C m-2 d-1) carbon from the upper to the lower epipelagic zone, where the latter can fuel the microbial loop, and thus deep chlorophyll maxima, or be ingested by other zooplankton. Large-scale migrants actively transported up to 10.5 mg C m-2 d-1 of respiratory carbon from the epipelagic layer into the twilight zone. The majority was transported by Pleuromamma borealis (5.7 mg C m-2 d-1) into the upper mesopelagic. In addition, up to 8.0 mg C m-2 d-1 was potentially egested as fecal material by large-scale zone shifters. Mesopelagic migrants transported respiratory (0.2 mg C m-2 d-1) and fecal pellet carbon (0.1 mg C m-2 d-1) even deeper into the ocean. Community consumption of migrants in the epipelagic layer during the night was 98 mg C m-2 d-1, while non-migrants consumed 98-208 mg C m-2 d-1 in the epipelagic zone, with a potential subsequent egestion of 29-62 mg C m-2 d-1. This carbon may fuel omnivorous-detritivorous feeding, the microbial loop and/or may sink as fecal pellets. This case study shows how calanoid functional groups mediate carbon fluxes in the subtropical South Atlantic Ocean and demonstrates how detailed community analyses can elucidate the complexity of pelagic carbon budgets.

Taiwan has pledged to achieve net-zero carbon emissions by 2050, but the current extent of carbon sinks in Taiwan remains unclear. Therefore, this study aims to first review the existing nature-based carbon sinks on land and in the oceans around Taiwan. Subsequently, we suggest potential strategies to reduce CO 2 emissions and propose carbon dioxide removal methods (CDRs). The natural carbon sinks by forests, sediments, and oceans in and around Taiwan are approximately 21.5, 42.1, and 96.8 Mt-CO 2 y−1, respectively, which is significantly less than Taiwan's CO 2 emissions (280 Mt-CO 2 y−1). Taiwan must consider decarbonization strategies like using electric vehicles, renewable energy, and hydrogen energy by formulating enabling policies. Besides more precisely assessing both terrestrial and marine carbon sinks, Taiwan should develop novel CDRs such as bioenergy with carbon capture and storage, afforestation, reforestation, biochar, seaweed cultivation, and ocean alkalinity enhancement, to reach carbon neutrality by 2050. • The marine carbon sink removes ~96 Mt-CO 2 y−1 in Taiwan, ~33 % of its emissions. • Adopting natural carbon sinks, Taiwan has a ~ 162 Mt-CO 2 deficit to reach net-zero. • Switching to electric vehicles and renewable energy can cut 20–50 Mt-CO 2 by 2050. • Taiwan has to develop CO 2 capture techniques and removal ways to reach net-zero. • Carbon sinks in forests, seas and sediments in Taiwan need to be well surveyed. [ABSTRACT FROM AUTHOR]

The dissolved inorganic carbon (DIC) produced by carbonate weathering may be lost by degassing or be biologically fixed as it transports from inland water to the ocean. Knowledge of DIC stability under these two processes in surface water systems is crucial for evaluating the role of carbonate weathering in the global carbon cycle. Here, we evaluated the net ecosystem productivity of aquatic ecosystems (NEPs), water‒air CO 2 exchange fluxes (FCO 2), and Carbon budgets (Carbon budget = ( T O C o u t p u t + 0.5 D I C o u t p u t ) − ( T O C i n p u t + 0.5 D I C i n p u t )) in five simulated spring-pond systems controlled by carbonate weathering and different land uses. Using these parameters and the unique structure of our simulation site, the stability of the carbonate weathering-driven carbon sink (DIC) in surface water was discussed. The results showed that the NEP and FCO 2 of the study site presented significant negative relationships and were mainly controlled by aquatic ecosystem metabolisms than abiotic processes. Most aquatic ecosystems in our study site were autotrophic (NEP>0) due to the strong biological carbon pump effect (BCP) induced by carbonate weathering. Moreover, the DIC input enhanced the FCO 2 in the warm season. The intensive BCP enhanced both CO 2 release and atmospheric CO 2 invasion. Moreover, we found that the stability of DIC was positively related to DIC input, which can be influenced by BCP and human land use. After considering the net changes in Carbon budget through the groundwater-surface water systems, a high average carbon stabilization rate (CSR) of approximately 103.7% of carbonate weathering carbon sink was found in our study site. This indicated that BCP could retain most of the DIC that was driven by carbonate weathering. Therefore, due to the high stability of the carbonate weathering carbon sink under the BCP, we stress that it should be considered a significant carbon sink in the global carbon cycle. [Display omitted] • The degassing of DIC in karst water is dominated by the aquatic metabolism. • Atmospheric CO 2 can directly invade karst water with strong aquatic photosynthesis. • Adjustment of land use will increase the stability of DIC. • The carbonate weathering carbon sink is a vital part of the global carbon cycle. [ABSTRACT FROM AUTHOR]

The biological carbon pump (BCP) stores ~1,700 Pg C from the atmosphere in the ocean interior, but the magnitude and direction of future changes in carbon sequestration by the BCP are uncertain. We quantify global trends in export production, sinking organic carbon fluxes, and sequestered carbon in the latest Coupled Model Intercomparison Project Phase 6 (CMIP6) future projections, finding a consistent 19 to 48 Pg C increase in carbon sequestration over the 21st century for the SSP3-7.0 scenario, equivalent to 5 to 17% of the total increase of carbon in the ocean by 2100. This is in contrast to a global decrease in export production of -0.15 to -1.44 Pg C y-1. However, there is significant uncertainty in the modeled future fluxes of organic carbon to the deep ocean associated with a range of different processes resolved across models. We demonstrate that organic carbon fluxes at 1,000mare a good predictor of long-term carbon sequestration and suggest this is an important metric of the BCP that should be prioritized in future model studies. [ABSTRACT FROM AUTHOR]

The biological carbon pump is a key component of the marine carbon cycle. This surface‐to‐deep flux of carbon is usually assumed to follow a simple power law function, which imposes that the surface export flux is attenuated throughout subsurface waters at a rate dictated by the parameterization exponent. This flux attenuation exponent is widely assumed as constant. However, there is increasing evidence that the flux attenuation varies both spatially and seasonally. While the former has received some attention, the consequences of the latter have not been explored. Here we aim to fill the gap with a theoretical study of how seasonal changes in both flux attenuation and sinking speed affect nutrient distributions and carbon fluxes. Using a global ocean‐biogeochemical model that represents detritus explicitly, we look at different scenarios for how these varies seasonally, particularly the relative "phase" with respect to solar radiation and the "strength" of seasonality. We show that the sole presence of seasonality in the model‐imposed flux attenuation and sinking speed leads to a greater transfer efficiency compared to the non‐seasonal flux attenuation scenario, resulting in an increase of over 140% in some cases when the amplitude of the seasonality imposed is 60% of the non‐seasonal base value. This work highlights the importance of the feedback taking place between the seasonally varying flux attenuation, sinking speed and other processes, suggesting that the assumption of constant‐in‐time flux attenuation and sinking speed might underestimate how much carbon is sequestered by the biological carbon pump. Key Points: The inclusion of seasonal variability in the flux attenuation parameter is found to increase the particulate organic carbon transfer to depthThe increase observed is sensitive to the relative phase between the flux attenuation and the solar radiationA constant‐in‐time flux attenuation might underestimate how much carbon is sequestered by the ocean [ABSTRACT FROM AUTHOR]

Biological carbon pump (BCP) inefficiency was first observed in the South China Sea (SCS) throughout the 1997–1999 El Niño Southern Oscillation (ENSO) event, but the BCP usually recovers its efficiency when the climate conditions change from El Niño to La Niña conditions in the Pacific Ocean. Enhanced stratification and Kuroshio intrusion led to weak mixing and oligotrophic conditions in the sunlit layer of the SCS during the 1997/1998 El Niño phase, but the particulate organic carbon (POC) flux was comparable to the climatological mean due to the ballast effect of increased lithogenic material and CaCO3 flux. The deepened thermocline rendered the recovered mixing less effective in replenishing subsurface nutrients and subsequently lowered POC flux during the 1998/1999 La Niña phase. Both scenarios were characterized by decreased siliceous plankton but a stimulated calcareous plankton contribution, which reduced BCP efficiency, resulting in a unique asymmetric response to the ENSO in the SCS. Plain Language Summary: The El Niño‐Southern Oscillation (ENSO) in the Pacific Ocean is one of the most important ocean‐atmosphere coupled climatic phenomena on Earth, and the ENSO in turn influences marine biogeochemical cycles and the biological carbon pump (BCP). Usually, as climate conditions transition from the El Niño to La Niña phases, the weakened BCP rebounds to a highly productive level in the central and eastern Pacific. However, the BCP exhibits an asymmetric response to the ENSO in the South China Sea (SCS), the largest tropical marginal sea of the Pacific Ocean. We show that strengthened stratification of the upper water column and enhanced oligotrophic Kuroshio intrusion led to impoverished in bioavailable nutrients near the surface, caused fewer siliceous plankton compared to calcareous plankton, and thus resulted in an inefficient BCP in the 1997/1998 El Niño phase. In contrast, the deepened thermocline prevented the replenishment of nutrients from the subsurface, subsequently inhibiting the rebound of primary production and deep biogenic flux during the 1998/1999 La Niña phase. This study may aid in understanding the response of the BCP to ENSO events in marginal seas and improve predictions of global ocean carbon storage. Key Points: The South China Sea biological carbon pump does not rebound from the 1997/1998 El Niño to La Niña phase as expectedThe enhanced stratification and Kuroshio intrusion are responsible for the low opal:CaCO3 ratio of sinking particles in the El Niño phaseThe deepened thermocline prevents the diapycnal nutrient supply and the recovery of the biological carbon pump in the La Niña phase [ABSTRACT FROM AUTHOR]

To better quantify the ocean's biological carbon pump, we resolved the diversity of sinking particles that transport carbon into the ocean's interior, their contribution to carbon export, and their attenuation with depth. Sinking particles collected in sediment trap gel layers from four distinct ocean ecosystems were imaged, measured, and classified. The size and identity of particles was used to model their contribution to particulate organic carbon (POC) flux. Measured POC fluxes were reasonably predicted by particle images. Nine particle types were identified, and most of the compositional variability was driven by the relative contribution of aggregates, long cylindrical fecal pellets, and salp fecal pellets. While particle composition varied across locations and seasons, the entire range of compositions was measured at a single well‐observed location in the subarctic North Pacific over one month, across 500 m of depth. The magnitude of POC flux was not consistently associated with a dominant particle class, but particle classes did influence flux attenuation. Long fecal pellets attenuated most rapidly with depth whereas certain other classes attenuated little or not at all with depth. Small particles (<100 μm) consistently contributed ∼5% to total POC flux in samples with higher magnitude fluxes. The relative importance of these small particle classes (spherical mini pellets, short oval fecal pellets, and dense detritus) increased in low flux environments (up to 46% of total POC flux). Imaging approaches that resolve large variations in particle composition across ocean basins, depth, and time will help to better parameterize biological carbon pump models. Plain Language Summary: Sinking particles transport carbon from the surface ocean to depth where carbon is sequestered from the atmosphere for long time periods. The amount of carbon transported by this process is difficult to quantify, in part because of the complex ecological interactions that generate sinking particles and alter downward transport. This study combined chemical measurements with detailed imagery of sinking particles collected in drifting sediment traps to survey how different types of particles affect carbon export across five locations. The biological characteristics of imaged particles can be used to reasonably predict carbon export and its attenuation with depth. Measured particles were classified into nine different categories, including various types of aggregates, organisms, and zooplankton fecal pellets. The amount of sinking carbon was not related to the dominance of any specific particle type, but each type transported carbon with differing efficiencies. Episodic presence of fecal pellets produced by gelatinous zooplankton had a large influence on transport of carbon to depth. Particles often considered too small to contribute to carbon export (<100 µm) were consistently important, especially in low flux environments, and often attenuated relatively little with depth. Resolving biological details of sinking particles improves our ability to quantify the ocean's carbon cycle. Key Points: Carbon flux into the deep ocean by sinking particles can be reasonably predicted from images of collected particle fluxSinking particle type and trophic processing affect attenuation of carbon flux with depthParticles smaller than 100 μm can be an important component of sinking carbon and some attenuated little with depth [ABSTRACT FROM AUTHOR]

Marine ecosystems regulate atmospheric carbon dioxide levels by transporting and storing photosynthetically fixed carbon in the ocean’s interior. In particular, the subantarctic and polar frontal zone of the Southern Ocean is a significant region for physically driven carbon uptake due to mode water formation, although it is under-studied concerning biologically mediated uptake. Regional differences in iron concentrations lead to variable carbon export from the base of the euphotic zone. Contrary to our understanding of export globally, where high productivity results in high export, naturally iron-fertilized regions exhibit low carbon export relative to their surface productivity, while HNLC (High Nutrient, Low Chlorophyll) waters emerge as a significant area for carbon export. Zooplankton, an integral part of the oceanic food web, play an important role in establishing these main carbon export regimes. In this mini review, we explore this role further by focusing on the impact of grazing and the production of fecal pellets on the carbon flux. The data coverage in the subantarctic region will be assessed by comparing two case studies - the iron-replete Kerguelen Plateau and the HNLC region south of Australia. We then discuss challenges in evaluating the contributions of zooplankton to carbon flux, namely gaps in seasonal coverage of sampling campaigns, the use of non-standardized and biased methods and under-sampling of the mesopelagic zone, an important area of carbon remineralization. More integrated approaches are necessary to improve present estimates of zooplankton-mediated carbon export in the Southern Ocean.

Keywords: biological carbon pump; blue carbon; chemolithoautotroph; climate change; climate remediation; greenhouse gases; marine carbon cycle; microbial carbon pump EN biological carbon pump blue carbon chemolithoautotroph climate change climate remediation greenhouse gases marine carbon cycle microbial carbon pump 1 6 6 06/26/20 20200623 NES 200623 Given the climate change crisis, it is urgent to reduce anthropogenic CO SB 2 sb emissions and explore climate geoengineering opportunities. Although the ocean's total primary production is very high (up to 50 Gt C/year), only a small fraction (<10%) is transported to the deep ocean I via i the BCP and even a smaller fraction (<1%) is sequestered for millennia (Henson et al., [45]; Bach et al., [7]; Fender et al., [36]; Giering et al., [39]). Research on fundamental processes and mechanisms of the BCP and MCP under varying oceanographic and climatic conditions is urgently needed, with a particular focus on integrating the major biogeochemical cycles and the ocean's biological, chemical, and physical processes for a better understanding of the marine carbon cycle and its response to climate change (Lucas et al., [64]; Hwang et al., [49]; Bif et al., [11]; Igarza et al., [50]; Quigley et al., [79]; Romera-Castillo et al., [87]). Advances in upcoming marine carbon cycling research may also help overcome the uncertainty and difficulty in developing environmental-friendly ocean geoengineering techniques for climate change mitigation, the success of which may as well require interdisciplinary collaborations, strategic planning, technique innovations, and systematic investigations including both BCP and MCP for integrated ocean carbon sequestration enhancement (Polimene et al., [78]; Emerson, [33]; Sloyan et al., [94]; Sogin et al., [95]; Zhang et al., [114]). [Extracted from the article]

[Display omitted] • δ13C, δ15N, and δ18O were analyzed to reveal carbon–nitrogen coupled absorption. • Stability regulation mechanism of karst reservoir was indicated by diel monitoring. • The stability promotes the biological carbon pump development of karst reservoir. • Biological carbon pump caused carbon burial in karst reservoir. While river damming has been extensively investigated, there remains unclear in the diel biogeochemical processes and carbon control mechanisms in karst reservoirs, under the influence of the biological carbon pump (BCP) effect. We investigated the diel variation of Hongjiadu (HJD) and Wujiangdu (WJD) reservoirs, which exhibit typical periodic thermal stratification, in the Wujiang River located in Southwest China. We found that diel variation of dissolved inorganic carbon (DIC) (HJD: 1668.6 ± 10.4 µmol/L; WJD: 1777.9 ± 31.5 µmol/L), δ13C DIC (HJD: −3.1 ± 0.1 ‰; WJD: −4.6 ± 0.4 ‰), and other parameters in the lentic zones were relatively stable. Moreover, the Revelle factor in the lentic zones (HJD: 44.3-49.0; WJD: 44.3-53.9) were substantially higher than those of the fluvial zones (HJD: 24.4-33.4; WJD: 14.2-35.1). The stable diel variations indicate that the HJD and WJD reservoirs may have distinct stability regulatory mechanisms. In the lentic zones, δ13C DIC , nitrogen–oxygen isotope (δ15N and δ18O), and the correlation of C-N indicated that the conversion of DIC and NO 3 – is primarily influenced by primary production, and there was coupled absorption of carbon and nitrogen nutrients during photosynthesis. Meanwhile, the C/N weight ratio (6.0-10.1) and the δ13C POC (−30.0 ‰ to −24.7 ‰) also revealed a gradual transition from soil organic matter (OM) in the fluvial zone to phytoplankton source in the intermediate-lentic zone. Thus, two reservoirs had a significant BCP effect and converted a significant amount of DIC to autochthonous organic carbon (AOC) burial in the lentic zones. The BCP effect in subtropical deep karst reservoirs buried lots of carbon, increased carbon sequestration and decreased CO 2 emissions in the lentic zone. Simultaneously, the stability of reservoirs promotes BCP effect and intensifies carbon sequestration, which is of great significance for the study of the global carbon sources-sinks pattern. [ABSTRACT FROM AUTHOR]

In the context of global climate change, ocean iron fertilization (OIF) has been suggested as a potential geoengineering strategy to enhance the growth of marine phytoplankton, subsequently promote the ocean carbon sink, and ultimately regulate atmospheric carbon dioxide (CO 2) and mitigate climate change. However, in past artificial OIF experiments, the elevation of both net primary production and carbon export to the deep ocean is far less than anticipated. This discrepancy can be attributed to several key factors that strongly influence the efficiency of OIF and biological carbon pump (BCP) including iron bioavailability and retention time, the sensitivity of phytoplankton to iron limitation, the influence of viruses and other microorganisms comprising the microbial carbon pump (MCP), and non-biological carbon cycles. Although previous reviews have extensively summarized the outcomes of the past OIF experiments, the mechanisms and key factors that influence climate regulation mediated by OIF have not been elucidated. In this review, we analyze these mechanisms throughout the whole process from OIF, to phytoplankton physiological responses, to carbon dioxide removal (CDR) and sequestration in the ocean, and ultimately to CDR in the atmosphere and its impact on climate change. By examining both natural and artificial OIF cases, as well as recent advancements in phytoplankton physiological ecology and carbon pump theory, possible reasons leading to the inconsistencies between OIF and carbon export are explored. These comprehensive analyses are helpful to identify the key issues in regulating climate change by providing a reference and scientific approach to guide potential OIF attempts in the future. [ABSTRACT FROM AUTHOR]

The oceanic biological carbon pump (BCP) regulates the Earth carbon cycle by transporting part of the photosynthetically fixed CO2 into the deep ocean. Suppressing this mechanism would result in an important increase of atmospheric CO2 level. The BCP occurs mainly in the form of (1) organic carbon (OC) particles sinking out the surface ocean, of (2) neutrally buoyant OC (dissolved or particulate) entrained by downward water masses movements and/or mixing, and of (3) active transport of OC by migrating animals such as zooplankton and fishes. These various pools of OC differ in size since their sinking, production and decomposition rates vary spatially and temporally. Moreover, the OC transported to depths via these various export pathways as well as their decomposition pathways all have different ecological origins and therefore may response differently to climate changes. Currently, most ocean biogeochemical models do not resolve these various of OC pathways explicitly; rather, they imply that OC is therein created and destroyed equally. In addition, the organic composition of these various pools is largely unknown, especially at depths below 500 m. Here, known processes of OC export from the surface ocean to the mesopelagic zone (100–1000 m) are briefly reviewed. Three OC export pathways and some of their sub-categories are considered. I refer to published studies of OC fluxes associated with the specific downward export pathways and identify gaps that need to be addressed to better understand the OC fluxes associated with the BCP.

Phaeodaria, which comprise one group of large, single‐celled eukaryotic zooplankton, have been largely ignored by past marine biological studies because Phaeodaria and their delicate skeletons are liable to collapse. As a result, collection and quantification of specimens are difficult, and seasonal changes of phaeodarian abundance have not been thoroughly studied. The transport of biogenic elements by sinking phaeodarians has been estimated for only a few representative species. Sinking particles >1 mm in size and swimmers have traditionally been excluded when estimating sinking particle fluxes. The focus of this study is the large number of phaeodarians among the >1‐mm sinking particles collected in the western North Pacific from June 2014 to July 2015. Careful sorting by microscopic examination and chemical analyses revealed that phaeodarians accounted for up to about 10% of the organic carbon in all sinking particles and accounted for a mean of 33% of the organic carbon in the >1‐mm sinking particles. The high‐standing stocks of phaeodarians at depths of 150–1,000 m in the mesopelagic twilight zone suggested that particles sinking from the euphotic zone as aggregates and fecal pellets can be efficiently exported to the deep sea by the ballasting effect of large phaeodarian particles rich in organic carbon. Key Points: Phaeodarians accounted for up to ~10% of the organic carbon in all sinking particles (both >1 and <1 mm in size)Phaeodarians accounted for a mean of 33% of the organic carbon in the >1‐mm sinking particlesPhaeodarians can contribute to the enlargement and aggregation of sinking particles in the mesopelagic carbon cycle [ABSTRACT FROM AUTHOR]

Abstract Zooplankton form a trophic link between primary producers and higher trophic levels, and exert significant influence on the vertical transport of carbon through the water column ('biological carbon pump'). Using a MultiNet we sampled and studied mesozooplankton communities (i.e. >0.2 mm) from six locations around Bermuda targeting four depth zones: ∼0–200 m, ∼200–400 m, ∼400–600 m (deep-scattering layer), and ∼600–800 m. Copepoda, our focal taxonomic group, consistently dominated samples (∼80% relative abundance). We report declines in zooplankton and copepod abundance with depth, concurrent with decreases in food availability. Taxonomic richness was lowest at depth and below the deep-scattering layer. In contrast, copepod diversity peaked at these depths, suggesting lower competitive displacement in these more food-limited waters. Finally, omnivory and carnivory, were the dominant trophic traits, each one affecting the biological carbon pump in a different way. This highlights the importance of incorporating data on zooplankton food web structure in future modelling of global ocean carbon cycling. Highlights • Zooplankton and copepod abundance decreased from 0 to 800 m in Bermuda. • Copepods dominated across all depths, followed by ostracods and chaetognaths. • Zooplankton diversity dipped at 600–800 m, while the reverse was true for copepods. • Omnivory and carnivory were the dominant trophic traits. • Zooplankton food web structure should be considered in ocean carbon cycling models. [ABSTRACT FROM AUTHOR]

Here we used data from six BGC-floats deployed in the southeast Pacific and southwest Atlantic Oceans, within the Southern Ocean's Subtropical Zone, to assess the seasonality of particulate organic carbon production from phytoplankton (POC phyto) and estimate POC transfer efficiencies at 100 m below the euphotic zone (T_100). While small particles <100 μM dominated the mixed layer, large particles >100 μM comprised a significant fraction of POC phyto below the mixed layer in both areas, possibly due to a "shade flora" composed by large diatoms. POC phyto was highly seasonal with highest biomass accumulation in the Atlantic side for both small and large particles. In the Pacific, the seasonal change in small particle production ΔPOC phyto was ∼66 mg m−2 versus ∼54 mg m−2 from large particles. In the Atlantic, ΔPOC phyto was ∼852 mg m−2 for small particles versus ΔPOC phyto ∼ 262 mg m−2 for large particles. Monthly T_100s in the Pacific ranged from 76% to 92% with maximum efficiencies during the deepening of the mixed layer depth. In the Atlantic, T_100s ranged from 43% to 76% with two periods of high T_100s: the first coinciding with the decline of large particles from the "shade flora", and the second coinciding with the deepening of the mixed layer during elevated small particle production. • Large phytoplankton particles accumulated below the mixed layer at mid-latitudes • The Atlantic Subtropical Zone produced more particulate organic carbon than the Pacific • Particle transfer efficiency in the Pacific was 85%, with a monthly amplitude of 76–92% • Particle transfer efficiency in the Atlantic was 60%, with a monthly amplitude of 43–76% [ABSTRACT FROM AUTHOR]

• Anthropogenic disturbances cause large shifts in dissolved carbon turnover. • River damming and agriculture modulate aquatic metabolism and CO 2 outgassing. • Dissolved carbon turnover follows the associated trajectories of biogeochemistry. • A part of dissolved carbon is cycled rapidly in karst aquatic ecosystems. • Biological carbon pump triggers overall dissolved organic carbon metabolism. Dissolved carbon (C) provides critical feedbacks to regional biogeochemical processes and global C cycling. Yet to date, the specific pathways of fluvial dissolved C turnover, particularly with human-induced shifts involved, are still poorly understood. Here, we examined dissolved inorganic (DIC) and organic C (DOC), as well as human disturbances i.e., river damming and land use in karst rivers. We show that anthropogenic activities caused unexpected shifts to dissolved C biogeochemistry. Specifically, we found that human disturbances accelerated aquatic metabolism, ultimately causing more river CO 2 generation than fixation. The extended hydrological retention by damming greatly stimulated biological utilization of dissolved C. River DOC was sourced largely from farmland and forest, while land-use fragmentation increased DOC diversity. Artificial dams and land uses intensified the transformations between DIC and DOC within karst environments. Based on these findings, we provided a process-based conceptual model regarding the rapid cycle of active C in karst waters, revealing the associated trajectories of DIC and DOC biogeochemistry. This study suggests that reducing anthropogenic disturbances essentially decelerates organic C metabolism, and therefore promotes riverine CO 2 sequestration in the context of global C neutrality. [Display omitted] [ABSTRACT FROM AUTHOR]

Sinking particles strongly regulate the distribution of reactive chemical substances in the ocean, including particulate organic carbon and other elements (e.g., P, Cd, Mn, Cu, Co, Fe, Al, and 232Th). Yet, the sinking fluxes of trace elements have not been well described in the global ocean. The U.S. GEOTRACES campaign in the North Atlantic (GA03) offers the first data set in which the sinking flux of carbon and trace elements can be derived using four different radionuclide pairs (238U:234Th ;210Pb:210Po; 228Ra:228Th; and 234U:230Th) at stations co‐located with sediment trap fluxes for comparison. Particulate organic carbon, particulate P, and particulate Cd fluxes all decrease sharply with depth below the euphotic zone. Particulate Mn, Cu, and Co flux profiles display mixed behavior, some cases reflecting biotic remineralization, and other cases showing increased flux with depth. The latter may be related to either lateral input of lithogenic material or increased scavenging onto particles. Lastly, particulate Fe fluxes resemble fluxes of Al and 232Th, which all have increasing flux with depth, indicating a dominance of lithogenic flux at depth by resuspended sediment transported laterally to the study site. In comparing flux estimates derived using different isotope pairs, differences result from different timescales of integration and particle size fractionation effects. The range in flux estimates produced by different methods provides a robust constraint on the true removal fluxes, taking into consideration the independent uncertainties associated with each method. These estimates will be valuable targets for biogeochemical modeling and may also offer insight into particle sinking processes. Plain Language Summary: Elements, like iron and carbon, are transported from the ocean's surface to its depths on sinking particles. Access to carbon, iron, and other elements is important for marine organisms, which need them to survive. Furthermore, when the organic carbon produced by organisms is transported to depth by sinking, carbon dioxide has been effectively removed from the atmosphere and moved to the deep ocean. This carbon sink is one way that the ocean reduces the heat‐trapping potential of the atmosphere. To track how much of a given element descends on particles through the ocean, we use radioisotopes. These are elements that decay at a predictable rate. We can use them like a clock to determine how fast an element is moving from one location to another. Radioisotopes with varying decay rates can tell us about short‐term processes, like seasonal blooms, and longer term events, like the impact of ice ages. There were few ocean‐scale radioisotope data sets before GEOTRACES expeditions began about 10 years ago. For the first time ever, we present four types of radioisotope data from the U.S. GEOTRACES expedition across the North Atlantic and discuss how it improves our understanding of elemental budgets in the global ocean. Key Points: North Atlantic flux of particulate carbon and trace elements constrained with four radionuclide methods over the entire water columnFluxes derived from radionuclide methods at BATS are in agreement with sediment trap fluxesC, P, and Cd have biogenic flux profiles; Mn, Co, and Cu have mixed behavior flux profiles; and Fe, Al, and Th‐232 have lithogenic flux profiles [ABSTRACT FROM AUTHOR]

The biological carbon pump (BCP) effect of dissolved inorganic carbon (DIC)-enriched karst surface waters is an important carbon sink mechanism that has substantial implications for the study of global missing carbon sinks and carbon cycling. However, the operation of the BCP is not established well in wider inland waters, particularly not in the surface waters of the Chinese Loess Plateau (CLP) that are also DIC-enriched but have high turbidity. In this study, we combined analyses of stable isotopes (δ2H, δ18O, and δ13C) and C/N ratios, ultraviolet absorption spectroscopy, and excitation-emission matrix fluorescence approaches to investigate the sources and compositions of particulate organic carbon (POC) and dissolved organic carbon (DOC) in two river–reservoir ecosystems [the Wulihe (WLH)-river to WLH-reservoir and Honghe (HH)-river to HH-reservoir] on the CLP. Both DOC and POC signals from autochthonous production gradually increased from the river inflow to the reservoir areas along the river flow, particularly during the wet season with the same duration of rain and heat. The WLH-reservoir had higher δ2H, δ18O, and δ13C values, more protein-like fluorescence components, higher chlorophyll-a and DOC concentrations, and lower CO 2 efflux than the HH-reservoir. Our results showed that the BCP intensity of the WLH-reservoir was higher than that of the HH-reservoir, probably because of the longer water residence time of the WLH-reservoir. Although the amount of autochthonous dissolved organic matter (Auto-DOM) produced by the BCP in the surface water on the CLP was lower than that in the karst regions of southwest China (CKR), the proportion of Auto-DOM in the CLP was close to that in the CKR, indicating the significance of Auto-DOM in the surface water on the CLP. Furthermore, the BCP-derived Auto-DOM correlated negatively with the partial pressure of CO 2 , and the WLH-reservoir with a higher BCP effect exhibited CO 2 influx in the wet season (−1.47 ± 0.02 g m−2 d−1), indicating that the enhanced BCP process contributed to the water–air interface carbon sink. This study highlighted that enhanced BCP could generate carbon sinks in DIC-enriched but high-turbidity surface waters, providing the underlying theory for extending the BCP carbon sink model to a wider range of surface waters flowing through areas of soil carbonate. [Display omitted] • The operational mechanisms of the BCP in DIC-enriched loess basins were explored. • Multi-tracer confirmed the presence of autochthonous organic carbon. • High-turbidity water could have high biological carbon pumping (BCP) potential. • Enhanced BCP processes contribute to the water–air interface C sink. [ABSTRACT FROM AUTHOR]

On geological time-scales, the production and degree of recycling of biogenic carbon in the marine realm and ultimately its removal to sediments, exerts a dominant control on atmospheric CO 2 and hence variability in climate. This is a highly complex system involving a myriad of inter-connected biological, chemical, and physical processes. For this reason alone, linking observations, often highly abstracted in the form of proxies, to the primary processes involved and ultimately to explanatory hypotheses for specific geological events and transitions, is challenging. The past few decades have seen a progressive improvement in theoretical and process-based understanding of the various components that make up the marine carbon cycle and, hand-in-hand with this, the development of numerical model representations of the complete system. Models have also been designed and/or adapted with paleoclimate questions in mind and applied to quantitatively explore the role of the marine carbon cycle in both perturbations and long-term geologic evolutionary trends in global climate, and possible feedbacks between them. However, we must ask whether paleoclimate models incorporate sufficiently appropriate representations of the dynamics and sensitivities of the marine carbon cycle, and indeed, whether in the geological context, we really know what these dynamics are. Here we provide a comprehensive overview of how marine carbon cycling and the biological carbon pump is treated in available paleoclimate models, with the aim of critically evaluating their ability to help interpret past marine carbon cycle and climate dynamics. To this end, we first provide an overview of commonly used paleoclimate models and some of their associated paleo-applications, drawing from a wide range of global carbon cycle box models and Earth system Models of Intermediate Complexity (EMICs). Secondly, we review and evaluate the three dominant processes involved in the cycling of organic and inorganic carbon in the marine system and how they are represented in models, namely: biological productivity at the ocean surface, remineralisation/dissolution of particulate carbon within the water column, and the benthic-pelagic coupling at the seafloor. We generate and employ illustrative examples using the model GENIE to show how different parameterisations of water-column and sediment processes can lead to significantly different model projections. Our compilation reveals the prevalence of static parameterisations of marine carbon cycling among existing paleoclimate models, which are commonly empirically derived from present-day observations. Although such approaches tend to represent carbon transfer in the modern ocean well, they are potentially compromised in their ability to reflect the true degree of freedom and strength of feedbacks with respect to past climate events, particularly those characterised by environmental boundary conditions that differ fundamentally from today. Finally, we discuss the importance of using models of different complexities and how questions of model uncertainty may start to be addressed. [ABSTRACT FROM AUTHOR]

Transparent Exopolymer Particles (TEP) have received considerable attention since they were first described in the ocean more than 20 years ago. This is because of their carbon-rich composition, their high concentrations in ocean’s surface waters, and especially because of their ability to promote aggregation due to their high stickiness (i.e. biological glue). As large aggregates contribute significantly to vertical carbon flux, TEP are commonly seen as a key factor that drives the downward flux of particulate organic carbon (POC). However, the density of TEP is lower than that of seawater, which causes them to remain in surface waters and even move upwards if not ballasted by other particles, which often leads to their accumulation in the sea surface microlayer. Hence we question here the generally accepted view that TEP always increase the downward flux of POC via gravitational settling. In the present reassessment of the role of TEP, we examine how the presence of a pool of non-sinking carbon-rich particulate organic matter in surface waters influences the cycling of organic carbon in the upper ocean at daily to decadal time scales. In particular, we focus on the role of TEP in the retention of organic carbon in surface waters versus downward export, and discuss the potential consequences of climate change on this process and on the efficiency of the biological carbon pump. We show that TEP sink only when ballasted with enough high-density particles to compensate their low density, and hence that their role in vertical POC export is not solely linked to their ability to promote aggregation, but also to their contribution to the buoyancy of POC. It follows that the TEP fraction of POC determines the degree of retention and remineralization of POC in surface waters versus its downward export. A high TEP concentration may temporally decouple primary production and downward export. We identify two main parameters that affect the contribution of TEP to POC cycling; TEP stickiness, and the balance between TEP production and degradation rates. Because stickiness, production and degradation of TEP vary with environmental conditions, the role of TEP in controlling the balance between retention versus export, and hence the drawdown of atmospheric CO 2 by the biological carbon pump, can be highly variable, and is likely to be affected by climate change. [ABSTRACT FROM AUTHOR]

• Creation of a new agent-based model allows investigation of zooplankton community structure and behavior effects on fecal pellet carbon flux in the ocean. • Diel vertical migration behavior in the model resulted in model fecal pellets fluxes that were closer to that of field data, compared to scenarios with no diel vertical migration. • Validation with field data shows that trait-based relationships for physiological traits can be used to give insight into particle flux variability in the deep ocean. Fecal pellets are a significant component of total particulate organic carbon (POC) flux in the ocean and a key component of the biological pump. The effect of copepod community structure, feeding behavior, and diel vertical migration (DVM) on fecal pellet carbon flux was investigated using an agent-based model, which was applied to two contrasting open ocean regions: an oligotrophic (Hawaii Ocean Time-series site ALOHA) and a mesotrophic (Japanese time-series site K2) system. Data from the VERtical Transport in the Global Ocean (VERTIGO) project was used to parameterize the model, and modelled fecal pellet fluxes were compared to field-derived neutrally buoyant sediment trap data to examine how community structure and DVM impact fecal pellet carbon flux in the water column. The model produces copepod fecal pellet fluxes that are consistent with sediment trap data from K2 and ALOHA and with other modeling studies. The vertical distribution of copepods matching nighttime field data resulted in a greater magnitude of fecal pellet flux at all model depths compared to daytime distributions. Communities dominated by omnivores resulted in higher fecal pellet carbon flux and lower POC attenuation than those dominated by carnivores or detritivores. Incorporating DVM into the simulations produced lower fecal pellet fluxes in some cases, while also marginally improving the agreement between the modeled and observed median fecal pellet fluxes. The relatively simplified model presented here shows that an agent-based model that uses trait-based relationships for physiological rates can be used to give insight into the variability of particle flux in the deep ocean. [ABSTRACT FROM AUTHOR]

One of the most important questions in the science of global change is how to balance the atmospheric CO budget. There is a large terrestrial missing carbon sink amounting to about one billion tonnes of carbon per annum. The locations, magnitudes, variations, and mechanisms responsible for this terrestrial missing carbon sink are uncertain and the focus of much continuing debate. Although the positive feedback between global change and silicate chemical weathering is used in geochemical models of atmospheric CO, this feedback is believed to operate over a long timescale and is therefore generally left out of the current discussion of human impact upon the carbon budget. Here, we show, by synthesizing recent findings in rock weathering research and studies into biological carbon pump effects in surface aquatic ecosystems, that the carbon sink produced by carbonate weathering based on the HO-carbonate-CO-aquatic phototroph interaction on land not only totals half a billion tonnes per annum, but also displays a significant increasing trend under the influence of global warming and land use change; thus, it needs to be included in the global carbon budget. [ABSTRACT FROM AUTHOR]

Complex interactions between microbial communities in the epipelagic and mesopelagic ocean drive variability in sinking carbon flux as part of the biological carbon pump. To investigate the relative contribution of dominant prokaryotic, protistan, and phytoplankton taxa to sinking particles, we used 16S/18S rRNA gene sequencing of particles collected in sediment traps and water column communities. Comparisons were done across a coastal upwelling-to-oligotrophic environmental gradient in the southern California Current Ecosystem to assess effects of natural environmental variability on taxon-specific contributions to sinking particle export. Particle-associated microbial assemblages differ from ambient mixed-layer communities. Gammaproteobacteria, dinoflagellates, and rhizarians were dominant microbes associated with sinking particles at all sampling locations, with diatoms increasing significantly at the inshore mesotrophic site. Parasitic groups, syndiniales and apicomplexans, were also major particle-associated taxa. Relative sequence abundance on exported particles from the mesotrophic inshore site more closely resembled the water-column dominants (especially diatoms) found above, while oligotrophic communities exhibited greater dissimilarity between the microbial communities on sinking particles and in the mixed layer. Protists generally showed greater similarity between mixed-layer and sinking particle communities than prokaryotes. Synechococcus was over-represented on sinking particles (relative to eukaryotic phytoplankton and other cyanobacteria) only in oligotrophic areas. Diatoms were the only phytoplankton group consistently over-represented on sinking particles across the region. Our results highlight important variability in microbial contributions to export that seem to align with differences in dominant grazing pathways. At the upwelling influenced site where export was primarily due to the fecal pellets of large copepods, diatoms were dominant contributors to export and protistan communities on sinking particles resembled those in the water column above. When doliolids were abundant, dinoflagellates contributed substantially to export, while the dominant picoeukaryote (Ostreococcus) was only a minor contributor. In oligotrophic areas, where grazing pathways were dominated by protists and sinking particles were primarily amorphous aged aggregates, there was little similarity between mixed layer and sinking particle communities. • Mixed layer and sinking communities show high similarity in the upwelling region • Mixed layer and sinking communities show high dissimilarity in oligotrophic areas • Protists were more similar between mixed layer and sinking particles than prokaryotes • Diatoms were over-represented on sinking particles [ABSTRACT FROM AUTHOR]

In the dark ocean, the balance between the heterotrophic carbon demand and the supply of sinking carbon through the biological carbon pump remains poorly constrained. In situ tracking of the dynamics of microbial degradation processes occurring on the gravitational sinking particles is still challenging. Our particle sinking simulator system (PASS) intends to mimic as closely as possible the in situ variations in pressure and temperature experienced by gravitational sinking particles. Here, we used the PASS to simultaneously track geochemical and microbial changes that occurred during the sinking through the mesopelagic zone of laboratory-grown Emiliania huxleyi aggregates amended by a natural microbial community sampled at 105 m depth in the North Atlantic Ocean. The impact of pressure on the prokaryotic degradation of POC and dissolution of E. huxleyi-derived calcite was not marked compared to atmospheric pressure. In contrast, using global O2 consumption monitored in real-time inside the high-pressure bottles using planar optodes via a sapphire window, a reduction of respiration rate was recorded in surface-originated community assemblages under increasing pressure conditions. Moreover, using a 16S rRNA metabarcoding survey, we demonstrated a drastic difference in transcriptionally active prokaryotes associated with particles, incubated either at atmospheric pressure or under linearly increasing hydrostatic pressure conditions. The increase in hydrostatic pressure reduced both the phylogenetic diversity and the species richness. The incubation at atmospheric pressure, however, promoted an opportunistic community of "fast" degraders from the surface (Saccharospirillaceae, Hyphomonadaceae, and Pseudoalteromonadaceae), known to be associated with surface phytoplankton blooms. In contrast, the incubation under increasing pressure condition incubations revealed an increase in the particle colonizer families Flavobacteriaceae and Rhodobacteraceae, and also Colwelliaceae, which are known to be adapted to high hydrostatic pressure. Altogether, our results underline the need to perform biodegradation experiments of particles in conditions that mimic pressure and temperature encountered during their sinking along the water column to be ecologically relevant. [ABSTRACT FROM AUTHOR]

• Sources of organic carbon constrained by δ13C and Δ14C in river-reservoir system. • The effects of reservoirs on the transformation of OC are quantified. • The biological carbon pump process is greatly accelerated by the reservoir. • A new conceptual model reveals the effect of the reservoir on carbon cycling. Reservoirs have been constructed as clean energy sources in recent decades with various environmental impacts. Karst rivers typically exhibit high dissolved inorganic carbon (DIC) concentrations, whether and how reservoirs affect carbon cycling, especially organic carbon (OC)-related biogeochemical processes in karst rivers, are unclear. To fill this knowledge gap, multiple tracer methods (including fluorescence excitation-emission matrix (EEM), ultraviolet (UV) absorption, and stable carbon (δ13C) and radiocarbon (Δ14C) isotopes) were utilized to track composition and property changes of both particulate OC (POC) and dissolved OC (DOC) along river-transition-reservoir transects in the Southwest China karst area. The changes in chemical properties indicated that from the river to the reservoir, terrestrial POC is largely replaced by phytoplankton-derived OC, while gradual coloured dissolved organic matter (CDOM) removal and addition of phytoplankton-derived OC to the DOC pool occurred as water flowed to the reservoir. Higher primary production in the transition area than that in the reservoir area was observed, which may be caused by nutrient released from suspended particles. Within the reservoir, the production surpassed degradation in the upper 5 m, resulting in a net DIC transformation into DOC and POC and terrestrial DOM degradation. The primary production was then gradually weakened and microbial degradation became more important down the profile. It is estimated that ~3.1–6.3 mg L −1 (~15.5–31.5 mg-C m −2 (~10–21%)) DIC was integrated into the OC pool through the biological carbon pump (BCP) process in the upper 5 m in the transition and reservoir areas. Our results emphasize the reservoir impact on riverine OC transport, and due to their characteristics, karst areas exhibit a higher BCP potential which is sensitive to human activities (more nutrient are provided) than non-karst areas. [Display omitted] [ABSTRACT FROM AUTHOR]

• CO 2 fluxes vary from small outgassing to strong sink conditions. • Enhanced CO 2 uptake on the western domain was associated with salinity variations. • Increased haptophyte concentrations boosted the CO 2 uptake in the gyre. • Intense CO 2 uptake in the eastern domain (30°S) was associated with haptophytes. The influence of phytoplankton groups on carbon dynamics was investigated during six oceanographic spring cruises (three cruises were carried out in 2009, and another three were conducted in 2011) between the western (Brazil) and eastern (Africa) South Atlantic margins. A seventh cruise crossed the South Atlantic during the early winter of 2015. Sea surface temperature and salinity, oceanic and atmospheric partial pressure of CO 2 (p CO 2), chlorophyll a and other phytoplankton pigment data were gathered. Net CO 2 fluxes were calculated for each cruise, characterizing the ability of each region to take up atmospheric CO 2. We quantified phytoplankton chemotaxonomic groups using the HPLC/CHEMTAX approach. Thus, this study aimed to improve our understanding of the distribution of phytoplankton groups and their connection with the carbon biogeochemical cycle in the South Atlantic Ocean. Our results showed significant variations in both the zonal and meridional patterns of phytoplankton groups and the associated CO 2 uptake magnitudes. Diatoms and haptophytes dominated the coastal regions of Brazil and Africa, respectively, whereas the open ocean was dominated by haptophytes and the picoplanktonic cyanobacteria Prochlorococcus and Synechococcus. The CO 2 uptake capacity increased eastward from −7.1 mmol CO 2 m−2 d–1 on the Brazilian coast to −27.6 mmol CO 2 m−2 d–1 on the African coast. There was a significant negative relationship (p < 0.05) between the phytoplankton biomass and the difference in sea-air p CO 2 (Δ p CO 2), with increasing CO 2 uptake corresponding to increases in the biomasses of diatoms and haptophytes. Therefore, according to our analysis, haptophytes and diatoms were the main phytoplankton groups related to a high uptake of CO 2 along the South Atlantic Ocean regions covered in this study. Thus, we encourage further investigations on their traits and vulnerabilities to future environmental change scenarios. [ABSTRACT FROM AUTHOR]