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The ocean biological carbon pump (BCP) transports organic matter from the surface to the deep ocean. Accurately quantifying the efficiency of the BCP is essential for understanding potential climate feedbacks and entails measuring the flux of organic material in and out of the mesopelagic layer (approximately 100–1,000 m). Observational estimates are often restricted to measuring the BCP efficiency over short timescales. Here we use an ocean biogeochemical model to diagnose where, and on what timescales, the mesopelagic is sufficiently in steady state that balancing the carbon budget may be possible. For the majority of the ocean the sources and sinks of organic carbon in the mesopelagic do not balance on timescales shorter than 1 year. Assuming steady state risks falsely inferring the existence of missing processes or the magnitudes of known ones to close the budget and will lead to incorrect estimates of the strength of the BCP. Plain Language Summary Up to 90% of the sinking organic carbon created in the surface ocean is broken down back into its dissolved state within the mesopelagic (approximately 100–1,000 m), therefore any changes in the governing processes have a significant impact on how much carbon is locked away in the deep ocean. Balancing mesopelagic carbon budgets, which involves assessing the relative contributions of the inputs and outputs of carbon, helps us understand the flow of organic carbon and hence the amount stored in the deep ocean. We use a simplified but perfectly understood model system to analyze the seasonal budget of the different processes acting on organic carbon in the mesopelagic. While the sources and sinks balance over 1 year, they rarely balance on shorter timescales. This is due to the complex interplay between the seasonal variability in the amount of organic carbon sinking from the surface, vertical mixing, and horizontal movement, and may be why balancing the biological carbon pump has proved difficult using observations. Understanding these interacting effects will help reduce uncertainty in the efficiency with which carbon is transported from the surface ocean and stored in the deep ocean. Key Points Assessing the different sources and sinks of mesopelagic (100–1,000 m) carbon helps us understand the flow of carbon from the surface to depth Sources and sinks rarely balance over the period for which oceanic measurements are taken, due to seasonal variations in fluxes Assuming steady state when calculating mesopelagic carbon budgets overlooks significant seasonal factors and is ill‐advised

A common dynamical paradigm is that turbulence in the upper ocean is dominated by three classes of motion: mesoscale geostrophic eddies, internal waves and microscale three‐dimensional turbulence. Close to the ocean surface, however, a fourth class of turbulent motion is important: submesoscale frontal dynamics. These have a horizontal scale of O(1–10) km, a vertical scale of O(100) m, and a time scale of O(1) day. Here we review the physical‐chemical‐biological dynamics of submesoscale features, and discuss strategies for sampling them. Submesoscale fronts arise dynamically through nonlinear instabilities of the mesoscale currents. They are ephemeral, lasting only a few days after they are formed. Strong submesoscale vertical velocities can drive episodic nutrient pulses to the euphotic zone, and subduct organic carbon into the ocean's interior. The reduction of vertical mixing at submesoscale fronts can locally increase the mean time that photosynthetic organisms spend in the well‐lit euphotic layer and promote primary production. Horizontal stirring can create intense patchiness in planktonic species. Submesoscale dynamics therefore can change not only primary and export production, but also the structure and the functioning of the planktonic ecosystem. Because of their short time and space scales, sampling of submesoscale features requires new technologies and approaches. This paper presents a critical overview of current knowledge to focus attention and hopefully interest on the pressing scientific questions concerning these dynamics. Key Points Submesoscale physics control ecology locally, but also feedback to basin scales Strong gradients in community structure are created at the submesoscale Despite recent innovations, sampling the submesoscale remains a major challenge

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 observations Seasonality in flux attenuation can reconcile contrasting TE maps reported in the literature Resolving and understanding seasonality are key for an accurate evaluation of the biological carbon pump under climate change

Oligotrophic subtropical gyres are the largest oceanic ecosystems, covering >40% of the Earth's surface. Unicellular cyanobacteria and the smallest algae (plastidic protists) dominate CO2 fixation in these ecosystems, competing for dissolved inorganic nutrients. Here we present direct evidence from the surface mixed layer of the subtropical gyres and adjacent equatorial and temperate regions of the Atlantic Ocean, collected on three Atlantic Meridional Transect cruises on consecutive years, that bacterioplankton are fed on by plastidic and aplastidic protists at comparable rates. Rates of bacterivory were similar in the light and dark. Furthermore, because of their higher abundance, it is the plastidic protists, rather than the aplastidic forms, that control bacterivory in these waters. These findings change our basic understanding of food web function in the open ocean, because plastidic protists should now be considered as the main bacterivores as well as the main CO2 fixers in the oligotrophic gyres.

Nitrogen does the rounds Some 16% of the original Amazon forest has been cleared for agriculture, but much of that land is no longer in use and is starting to regrow. Such 'secondary forests' are becoming increasingly important as tropical land-use change results in larger areas that have gone through agricultural phases. A new study of Amazon forest areas between 3 and 70 years into their recovery reveals nitrogen and phosphorus cycling processes consistent with large losses of nitrogen during land use change. Nitrogen availability is ephemeral, and readily disrupted by either natural or anthropogenic disturbance. Understanding how the nutrient cycling processes of secondary forest succession should contribute to the better management Amazonian ecosystems. Elsewhere in the nitrogen cycle, an analysis of virtually all extant data on open oceanic nitrification, in conjunction with a global ecosystem model, demonstrates that the generally accepted assumptions concerning its distribution are incorrect. Much of the nitrate taken up by the oceans is generated through recent nitrification near the surface and, at the global scale, nitrification accounts for about half of the nitrate consumed by growing phytoplankton. This means that many previous attempts to quantify marine carbon export may be significant overestimates. This study synthesizes data from several published datasets to provide a global estimate of the impact of nitrification on oceanic new production. The flux of organic material sinking to depth is a major control on the inventory of carbon in the ocean 1 . To first order, the oceanic system is at equilibrium such that what goes down must come up 2 . Because the export flux is difficult to measure directly, it is routinely estimated indirectly by quantifying the amount of phytoplankton growth, or primary production, fuelled by the upward flux of nitrate 3 . To do so it is necessary to take into account other sources of biologically available nitrogen. However, the generation of nitrate by nitrification in surface waters has only recently received attention. Here we perform the first synthesis of open-ocean measurements of the specific rate of surface nitrification 4 , 5 , 6 , 7 , 8 , 9 , 10 , 11 , 12 and use these to configure a global biogeochemical model 13 , 14 to quantify the global role of nitrification. We show that for much of the world ocean a substantial fraction of the nitrate taken up is generated through recent nitrification near the surface. At the global scale, nitrification accounts for about half of the nitrate consumed by growing phytoplankton. A consequence is that many previous attempts to quantify marine carbon export, particularly those based on inappropriate use of the f -ratio (a measure of the efficiency of the ‘biological pump’), are significant overestimates.

We present a model describing the population dynamics of benthic biota, feeding from a common resource that is supplied by a flux of sinking organic carbon arriving on the seafloor. By using allometric relationships for the physiological processes of growth, mortality and respiration, and for food limitation, the model represents the population dynamics of organisms ranging in size from bacteria (10−14 g wet weight C) to large metazoans (103 gwwt C). The effect of temperature on physiological rates is also included. The only forcing information required is the ambient temperature and the rate of supply of sinking organic carbon. The model can be used for, and tuned to, specific locations. However, a parameter set is provided that is generally applicable. The ability of the model to simultaneously reproduce biomass size distributions at five contrasting sites is demonstrated for this parameter set. Other examples of use are also shown, using the model to explore global patterns of benthic biomass, and responding to a change in food supply.

Scarcity of iron and manganese limits the efficiency of the biological carbon pump over large areas of the Southern Ocean. The importance of hydrothermal vents as a source of these micronutrients to the euphotic zone of the Southern Ocean is debated. Here we present full depth profiles of dissolved and total dissolvable trace metals in the remote eastern Pacific sector of the Southern Ocean (55–60° S, 89.1° W), providing evidence of enrichment of iron and manganese at depths of 2000–4000 m. These enhanced micronutrient concentrations were co-located with 3 He enrichment, an indicator of hydrothermal fluid originating from ocean ridges. Modelled water trajectories revealed the understudied South East Pacific Rise and the Pacific Antarctic Ridge as likely source regions. Additionally, the trajectories demonstrate pathways for these Southern Ocean hydrothermal ridge-derived trace metals to reach the Southern Ocean surface mixed layer within two decades, potentially supporting a regular supply of micronutrients to fuel Southern Ocean primary production.

Numerical simulations suggest that submesoscale turbulence may transform lateral buoyancy gradients into vertical stratification and thus restratify the upper ocean via vertical flow. However, the observational evidence for this restratifying process has been lacking due to the difficulty in measuring such ephemeral phenomena, particularly over periods of months to years. This study presents an annual cycle of the vertical velocity and associated restratification estimated from two nested clusters of meso- and submesoscale-resolving moorings, deployed in a typical midocean area of the northeast Atlantic. Vertical velocities inferred using the nondiffusive density equation are substantially stronger at submesoscales (horizontal scales of 1–10 km) than at mesoscales (horizontal scales of 10–100 km), with respective root-mean-square values of 38.0 ± 6.9 and 22.5 ± 3.3 m day −1 . The largest submesoscale vertical velocities and rates of restratification occur in events of a few days’ duration in winter and spring, and extend down to at least 200 m below the mixed layer base. These events commonly coincide with the enhancement of submesoscale lateral buoyancy gradients, which is itself associated with persistent mesoscale frontogenesis. This suggests that mesoscale frontogenesis is a regular precursor of the submesoscale turbulence that restratifies the upper ocean. The upper-ocean restratification induced by submesoscale motions integrated over the annual cycle is comparable in magnitude to the net destratification driven by local atmospheric cooling, indicating that submesoscale flows play a significant role in determining the climatological upper-ocean stratification in the study area.

To predict the impacts of climate change it is essential to understand how anthropogenic change alters the balance between atmosphere, ocean and terrestrial reservoirs of carbon. It has been estimated that natural atmospheric concentrations of CO2 are almost 200ppm lower than they would be without the transport of organic material produced in the surface ocean to depth, an ecosystem service driven by mechanisms collectively referred to as the biological carbon pump. Here we quantify potential reductions in carbon sequestration fluxes in the North Atlantic Ocean through the biological carbon pump over the 21st century, using two independent biogeochemical models, driven by low and high IPCC AR5 carbon emission scenarios. The carbon flux at 1000m (the depth at which it is assumed that carbon is sequestered) in the North Atlantic was estimated to decline between 27-43% by the end of the century, depending on the biogeochemical model and the emission scenario considered. In monetary terms, the value of this loss in carbon sequestration service in the North Atlantic was estimated to range between US $170-US$ 3,000 billion in abatement (mitigation) costs and US $23-US$ 401billion in social (adaptation) costs, over the 21st century. Our results challenge the frequent assumption that coastal habitats store more significant amounts of carbon and are under greater threat. We highlight the largely unrecognized economic importance of the natural, blue carbon sequestration service provided by the open ocean, which is predicted to undergo significant anthropogenic-driven change.

In the face of increasing anthropogenic pressures acting on the Earth system, urgent actions are needed to guarantee efficient resource management and sustainable development for our growing human population. Our oceans - the largest underexplored component of the Earth system - are potentially home for a large number of new resources, which can directly impact upon food security and the wellbeing of humanity. However, the extraction of these resources has repercussions for biodiversity and the oceans ability to sequester green house gases and thereby climate. In the search for “new resources” to unlock the economic potential of the global oceans, recent observations have identified a large unexploited biomass of mesopelagic fish living in the deep ocean. This biomass has recently been estimated to be 10 billion metric tonnes, 10 times larger than previous estimates however the real biomass is still in question. If we are able to exploit this community at sustainable levels without impacting upon biodiversity and compromising the oceans’ ability to sequester carbon, we can produce more food and potentially many new nutraceutical products. However, to meet the needs of present generations without compromising the needs of future generations, we need to guarantee a sustainable exploitation of these resources. To do so requires a holistic assessment of the community and an understanding of the mechanisms controlling this biomass, its role in the preservation of biodiversity and its influence on climate as well as management tools able to weigh the costs and benefits of exploitation of this community.