Fish and other metazoans play a major role in long-term sequestration of carbon in the oceans through the biological carbon pump1. Recent studies estimate that fish can release about 1,200 to 1,500 million metric tons of carbon per year (MtC year-1) in the oceans through feces production, respiration, and deadfalls, with mesopelagic fish playing a major role1,2. This carbon remains sequestered (stored) in the ocean for a period that largely depends on the depth at which it is released. Cephalopods (squid, octopus, and cuttlefish) have the potential to sequester carbon more effectively than fish because they grow on average five times faster than fish3,4 and they die after reproducing at an early age4,5 (usually 1-2 years), after which their carcasses sink rapidly to the sea floor6. Deadfall of carcasses is particularly important for long-term sequestration because it rapidly transports carbon to depths where residence times are longest1,6. We estimate that cephalopod carcasses transfer 11-22 MtC to the seafloor globally. While cephalopods represent less than 5% of global fisheries catch7, fishing extirpates about 0.36 MtC year-1 of cephalopod carbon that could otherwise have sunk to the seafloor, about half as much as that of fishing large fish8.

Quantifying the sequestration potential of biologically driven carbon fluxes in the ocean depends critically on residence times – how long carbon remains stored in reservoirs before being re-exposed to the atmosphere. Simple mass balance provides estimates for many of the major ocean biogenic carbon reservoirs. For vegetated coastal ecosystems (mangroves, sea grass meadows, salt marshes) that globally store 20 to 40 PgC, this is 200 to 500 years, while for the biological carbon pump, a reservoir of about 2000 PgC, it is between 200 to 800 years. Over these time scales respective reservoirs reach equilibrium if left undisturbed. Importantly, near equilibrium of ocean reservoirs during the Holocene can be inferred from the near steady atmospheric concentrations during this period. The degradation of habitats and the over-exploitation of living marine resources particularly in the last 75 years have tipped these natural processes out of balance, to the extent where many are now net emitters of legacy carbon back to the atmosphere. The analysis exposes a conflict between how sequestration is reported in oceanographic literature and how it is understood with regards durable carbon capture and storage. Nature-based solutions can be sought to address parts of the climate crisis, by improving ecosystem health and biodiversity, but are unlikely to provide solutions to carbon management on a scale commensurate with anthropogenic emissions. The best we can do is to limit net emissions by restoring what we can, and to ensure that future practices do not further tip ocean carbon reservoirs out of balance. Significance Statement Marine animals and plants maintain large pools of carbon in the ocean and coastal areas that have been laid down by generations past. This legacy carbon is continuously being recycled on time scales of 100s of years. Left undisturbed, as they were for most of the last 10000 years, these carbon pools tend to equilibrium; flux in equals flux out. Human activities such as over fishing and coastal construction, particularly in the past 75 years, have tipped these natural cycles out of balance to the extent where many pools are now net emitters of carbon. Conservation and restoration of marine habitats can bring these cycles back into balance but cannot be counted as offsetting fossil fuel emissions. ### Competing Interest Statement The authors have declared no competing interest.

Abstract. A mechanistic approach linking the population dynamics of plankton communities to the export of detrital material to the oceans interior, remains a largely unresolved component of global bio-geochemical models. We propose that the self-similarity of aggregation provides a tractable modelling framework for simulating the dynamics and sinking speed of natural marine particle aggregates. It provides a means to track both size and excess density of aggregates as they are formed and transformed by aggregation, degradation and fragmentation processes. A self-similarity parameter a in the range 1.8 to 2.1 is well supported by direct observations drawn from an extensive database of aggregate size and sinking speed. We provide a simple model, SISSOMA, that uses a 2 dimensional state-space representation of aggregate dynamics for which we conduct sensitivity analyses for the self-similarity parameter, stickiness, turbulent dissipation rate and the production rate of primary particles. The model provides size and density resolved estimates of the export flux of detrital material generated by a diverse community of primary producers. While open to improvement in several aspects, the model compares well with observations of aggregate size spectra covering the global ocean.

Abstract The carbon sequestration potential of open-ocean pelagic ecosystems is vastly under-reported compared to coastal vegetation ‘blue carbon’ systems. Here we show that just a single pelagic harvested species, Antarctic krill, sequesters a similar amount of carbon through its sinking faecal pellets as marshes, mangroves and seagrass. Due to their massive population biomass, fast-sinking faecal pellets and the modest depths that pellets need to reach to achieve sequestration (mean is 381 m), Antarctic krill faecal pellets sequester 20 MtC per productive season (spring to early Autumn). This is equates USD$ 4 − 46 billion depending on the price of carbon, with krill pellet carbon stored for at least 100 years and with some reaching as far as the North Pacific. Antarctic krill are being impacted by rapid polar climate change and an expanding fishery, thus krill populations and their habitat warrant protection to preserve this valuable carbon sink.

A mouthful of water while swimming in a lake is unpleasant but nothing compared to the same situation during a swim in the ocean. A sudden mouthful seawater leaves you gasping for a glass of water to wash the salty taste from your mouth. But have you ever stopped to consider why the sea is salty? In this article, we will dive into the realm of ocean salinity (salt concentration) and show that there is more to it than you may have thought. Where does the ocean’s salt come from? What is it made of and how is salinity measured? Finally, why should the saltiness of the ocean interest us at all?

The daily vertical migrations of fish and other metazoans actively transport organic carbon from the ocean surface to depth, contributing to the biological carbon pump. We use an oxygen-constrained, game-theoretic food-web model to simulate diel vertical migrations and estimate global carbon fluxes and sequestration by fish and zooplankton due to respiration, fecal pellets, and deadfalls. Our model provides estimates of the carbon export and sequestration potential for a range of pelagic functional groups, despite uncertain biomass estimates of some functional groups. While the export production of metazoans and fish is modest (~20% of global total), we estimate that their contribution to carbon sequestered by the biological pump (~ 800 PgC) is conservatively more than 50% of the estimated global total (~1300 PgC) and have a significantly longer sequestration time scale (~250 years) than previously reported for other components of the biological pump. Fish and multicellular zooplankton contribute about equally to this sequestered carbon pool. This essential ecosystem service could be at risk from both unregulated fishing on the high seas and ocean deoxygenation due to climate change.

Here we review, synthesize, and analyse the size-based approach to model unicellular plankton cells and communities. We first review how cell size influences processes of the individual the cell: uptake of dissolved nutrients and dissolved organic carbon, phototrophy, phagotrophy, and metabolism. We parameterise processes primarily from first principles, using a synthesis of existing data only when needed, and show how these processes determine minimum and maximum cell size and limiting resource concentrations. The cell level processes scale directly up to the structure and function of the entire unicellular plankton ecosystem, from heterotrophic bacteria to zooplankton. The structure is described by the Sheldon size spectrum and by the emergent trophic strategies. We develop an analytical approximate solution of the biomass size spectrum and show how the trophic strategies of osmotrophy, light- and nutrient-limited phototrophy, mixotrophy, phagotrophy depend on the resource environment. We further develop expressions to quantify the functions of the plankton community: production, respiration and losses, and carbon available to production of higher trophic levels, and show how the plankton community responds to changes in temperature and grazing from higher trophic levels. We finally discuss strengths and limitations of size-based representations and models of plankton communities and which additional trait axes will improve the representation of plankton functional diversity.

Marine life contribute to carbon stores helping lock carbon away from the atmosphere. Open-ocean pelagic ecosystems are vastly under-reported in terms of carbon sequestration conservation potential, compared to coastal vegetation ‘blue carbon’ systems. Here we show that a harvested organism, Antarctic krill, has similar carbon sequestration potential through its sinking faecal pellets as coastal blue carbon stores, namely seagrasses, mangroves and marshes. Building upon recent advances in krill abundance and faecal pellet carbon flux data, and combining these with an ocean circulation model, we show that from Austral spring to early autumn Antarctic krill sequester 20 Mt C into the deep ocean for at least 100 years. This equates to USD$ 4 - 46 billion per spring/summer season depending on the price of carbon. The footprint of remineralised krill pellet carbon has a global extent, with some reaching as far as the North Pacific. The vast area of ocean krill inhabit and their high abundance make their total carbon sequestered each year similar to that from coastal vegetated blue carbon stores. As Antarctic krill are being impacted by rapid polar climate change and they are harvested, both krill populations and their habitat warrant protection to preserve this valuable carbon sink.

Every year, large numbers of zooplankton migrate from the surface ocean to depths of 500-2000 m to hibernate. Through this migration, they actively transport organic carbon to the deep ocean, where it is used to fuel metabolic needs. This active transport of carbon is thought to be highly efficient, as carbon metabolized by copepods is directly injected deep into the ocean's interior. The significance of this process in view of global carbon cycling remains an open question. Here, we focus on five representative, diapausing copepod species (Calanus finmarchicus, Calanus hyperboreus, Calanoides acutus, Calanoides natalis, and Neocalanus tonsus) distributed in the Arctic, Atlantic, Indian, and Southern Oceans. For each species, we compute both carbon injection (how much carbon is transported below the euphotic zone during zooplankton migration and left there as dissolved inorganic carbon) and carbon sequestration (the amount of carbon stored in the ocean's interior following diapausing zooplankton-mediated injection). In total, the five species considered here contribute 0.4-0.8% of total biological carbon export, and 0.8-3.3% of total carbon sequestration mediated by the biological pump (assuming a total carbon export of similar to 10 PgC yr(-1) and sequestration of similar to 1300 PgC). Including other species in this inventory would increase the contribution of diapausing copepods to the biological carbon pump, but requires more precise estimates of copepods' distribution, abundance, and metabolic requirements.

Sea surface temperature (SST) in the Northeastern North Atlantic and Nordic Seas exhibits pronounced variability across seasonal to decadal time scales. These changes can be expected to be driven by a combination of altered local conditions, shifts in seasonality and large-scale regional oceanographic change. Separating the contribution from each of these offers insight into how the region is changing. Here, we present the result of an analysis of weekly satellite-derived SST data from 1979 to 2020. An empirical orthogonal function (EOF) analysis allows us to separate observed changes in SST into independent underlying timeseries. Each timeseries explains part of the variability in SST. EOF1 can be allocated with changes in seasonality and a long-term warming trend, with summer maxima warming with twice the rate (0.043 degrees C year(-1)) compared to winter minima (0.023 degrees C year(-1)). EOF2 is associated with the North Atlantic subpolar gyre and the North Atlantic Oscillation, affecting the Atlantic Water flow across the Greenland-Scotland Ridge, imposing a dipole cooling/warming pattern. Local sea-ice melt along the southeast Greenland shelf is represented by EOF3, and finally the influx of warmer water with the North Icelandic Irminger Current is captured by EOF4. Each of these disaggregated signals differ considerably in their contribution to driving temporal and spatial trends in SST. The isolated signals offer a high-resolution long-time series of valuable indicators of oceanographic change which will likely be reflected in biogeochemistry, plankton, fish, mammals, and seabirds in the region.

The biological carbon pump transports photosynthetically fixed carbon from surface waters to depths. It removes carbon from the atmosphere and sequesters it in the deep ocean, playing an important role in global climate regulation. As the biological carbon pump is directly related to biological processes, it is heavily influenced by the biomass and trophic interactions between populations in the ecosystem. However, behavioral responses and adaptations to predation risk change trophic interactions, potentially having larger impacts than direct effects on trophic interactions and population abundances. Thus, predation risk may play an important role in shaping the biological carbon pump's strength (how much carbon leaves the euphotic zone) and efficiency (what fraction of detritus reaches a certain depth without being degraded). Except in the case of active carbon transport by vertically migrating organisms, this role of risk is not generally recognized. Here, we synthesize the existing knowledge on the consequences of anti‐predation responses on the biological carbon pump. First, we consider a generic anti‐predation response and investigate the different direct, indirect, and cascading effects that the response can induce. Then, we focus on pelagic anti‐predation responses and detail how they can specifically alter the different components of the pump. Finally, we discuss points to consider in biological carbon pump studies and highlight directions for future research. In particular, there is a need for more quantitative research to evaluate the importance of anti‐predation responses in shaping the biological carbon pump.

The magnitude and efficiency of particulate carbon export from the ocean surface depends not only on net primary production (NPP) but also on how carbon is consumed, respired, and repackaged by organisms. We contend that several of these processes can be captured by the size spectrum of the plankton community. However, most global models have relatively simple food‐web structures that are unable to generate plankton size spectra. Moreover, the life‐cycles of multicellular zooplankton are typically not resolved, restricting the ability of models to represent time‐lags that are known to impact carbon export and its efficiency (pe‐ratio). Here, we use a global mechanistic size‐spectrum model of the marine plankton community to investigate how particulate export and pe‐ratio relate to the community size spectrum, community composition, and time‐lags between predators and prey. The model generates emergent food‐webs with associated size distributions for organisms and detrital particles. To resolve time‐lags between phytoplankton and zooplankton, we implement the life‐cycle of multicellular zooplankton (here represented by copepods). We find that carbon export correlates best with copepod biomass and trophic level, whereas the pe‐ratio correlates best with the exponent of the size spectrum and sea surface temperature (SST). Community metrics performed better than NPP or SST for both deep export and pe‐ratio. Time‐lags between phytoplankton and copepods did not strongly affect export or pe‐ratio. We conclude by discussing how can we reconcile size spectrum theory with field sampling.

A collated and referenced data base of observed size and sinking speeds of marine particle aggregates including Reynolds number and estimated excess density.

3 The daily vertical migrations of fish and other metazoans actively transport organic carbon from the 4 ocean surface to depth, contributing to the biological carbon pump. An important but unanswered ques-5 tion is whether fish play a significant role in the biological carbon pump relative to other organisms, both 6 in terms of carbon export and sequestration. Here, we use a game-theoretic food-web model that simu-7 lates diel vertical migrations to estimate global carbon fluxes and sequestration by fish and zooplankton 8 due to respiration, fecal pellets, and deadfalls. Despite uncertainties due to poorly constrained biomass 9 estimates of some functional groups, a robust result of this model is that fish play a major role in the 10 biological carbon pump. Our model estimates that open-ocean metazoans inject ∼ 3.1 (range 1.5 - 4.7) 11 PgC/yr of a total of ∼ 10 PgC/yr into the ocean’s interior. Fish are further responsible for 47% (25-65%) 12 of the oceanic carbon sequestration mediated by metazoans. This essential ecosystem service provided 13 by fishes could be at risk from unregulated fishing in the high seas.

Individuals of different interacting populations often adjust to prevailing conditions by changing their behavior simultaneously, with consequences for trophic relationships throughout the system. While we now have a good theoretical understanding of how individuals adjust their behavior, the population dynamical consequences of co-adaptive behaviors are rarely described. Further, mechanistic descriptions of ecosystem functions are based on population models that seldom take behavior into account. Here, we present a model that combines the population dynamics and adaptive behavior of organisms of two populations simultaneously. We explore how the Nash equilibrium of a system - i.e. the optimal behavior of its constituent organisms - can shape population dynamics, and conversely how population dynamics impact the Nash equilibrium of the system. We illustrate this for the case of diel vertical migration (DVM), the daily movement of marine organisms between food-depleted but safe dark depths and more risky nutrition-rich surface waters. DVM represents the archetypal example of populations choosing between a foraging arena (the upper sunlit ocean) and a refuge (the dark depths). We show that population sizes at equilibrium are significantly different if organisms can adapt their behavior, and that optimal DVM behaviors within the community vary significantly if population dynamics are considered. As a consequence, ecosystem function estimates such as trophic transfer efficiency and vertical carbon export differ greatly when fitness seeking behavior is included. Ignoring the role of behavior in multi-trophic population modeling can potentially lead to inaccurate predictions of population biomasses and ecosystem functions. (C) 2021 Elsevier Ltd. All rights reserved.

Diel vertical migration of fish and other metazoans actively transports organic carbon from the ocean surface to depth, contributing to the biological carbon pump. Here, we use a global vertical migration model to estimate global carbon fluxes and sequestration by fish and metazoans due to respiration, fecal pellets, and deadfalls. We estimate that fish and metazoans contribute 5.2 PgC/yr (2.1-8.8PgC/yr) to passive export out of the euphotic zone. Together with active transport, we estimate that fish are responsible for 20% (9-29%) of global carbon export, and 32% (18-43%) of oceanic carbon sequestration, with forage and deep-dwelling mesopelagic fish contributing the most. This essential ecosystem service could be at risk from unregulated fishing on the high seas.

Diel Vertical Migration (DVM) is a key feature of pelagic and mesopelagic ecosystems, mainly driven by predator-prey interactions along a time-varying vertical gradient of light. Marine organisms including meso-zooplankton and fish typically hide from visual predators at depth during daytime and migrate up at dusk to feed in productive near-surface waters during nighttime. Specific migration patterns, however, vary tremendously, for instance in terms of residency depth during day and night. In addition to environmental parameters such as light intensity and oxygen concentration, the migration pattern of each organism is intrinsically linked to the patterns of its conspecifics, its prey, and its predators through feedbacks that are hard to understand—but important to consider. DVM not only affects trophic interactions, but also the biogeochemistry of the world’s oceans. Organisms preying at the surface and actively migrating vertically transport carbon to depth, contributing to the biological carbon pump, and directly connecting surface production with mesopelagic and demersal ecosystems. Here, we present a method based on a game-theoretic trait-based mechanistic model that enables the optimal DVM patterns for all organisms in a food-web to be computed simultaneously. The results are used to investigate the contributions of the different food-web pathways to the active component of the biological carbon pump. We apply the method to a modern pelagic food-web (comprised of meso- and macro-zooplankton, forage fish, mesopelagic fish, large pelagic fish and gelatinous organisms), shedding light on the direct effects that different trophic levels can have on the DVM behaviours of each other. The model is run on a global scale to assess the carbon export mediated by different functional groups, through fecal pellet production, carcasses sinking and respiration. Finally, the model output is coupled to an ocean inverse circulation model to assess the carbon sequestration potential of the different export pathways. Results indicate that the carbon sequestration mediated by fish is much more important than presently recognised in global assessments of the biological carbon pump. The work we present relates to contemporary ecosystems, but we also explain how it can be adapted to fit any pelagic food-web structure to assess the contribution of the active biological pump to the global carbon cycle in past ecosystems.

Nitrogen (N 2 ) fixation by heterotrophic bacteria associated with sinking particles contributes to marine N cycling, but a mechanistic understanding of its regulation and significance are not available. Here we develop a mathematical model for unicellular heterotrophic bacteria growing on sinking marine particles and can fix N 2 under suitable environmental conditions. We find that the interactive effects of polysaccharide and polypeptide concentrations, sinking speed of particles, and surrounding O 2 and NO 3 - concentrations determine the N 2 fixation rate inside particles. N 2 fixation inside sinking particles is mainly fueled by SO 4 2- respiration rather than NO 3 - respiration. Our model suggests that anaerobic processes, including heterotrophic N 2 fixation, can take place in anoxic microenvironments inside sinking particles even in fully oxygenated marine waters. The modelled rates are similar to bulk rates measured in the aphotic ocean, and our study consequently suggests that particle-associated heterotrophic N 2 fixation contributes significantly to oceanic N 2 fixation.