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The depth at which sinking particulate organic carbon (POC) is remineralized in the ocean is tightly linked to atmospheric levels of carbon dioxide. Rapid attenuation of downward POC flux typically occurs in the upper mesopelagic (top few hundred meters of the water column), with much slower loss rates deeper in the ocean. Currently we lack understanding of the processes that drive POC attenuation, resulting in large uncertainties in the mesopelagic carbon budget. Attempts to balance POC supply to the mesopelagic in discrete depth layers with respiration by zooplankton and microbes in those layers rarely succeed, with respiration sometimes being 50% lower than apparent carbon loss in the upper mesopelagic. One term that is often poorly quantified in such budgets is particle associated bacterial respiration, which we hypothesize could serve as the''missing sink'' for carbon in the upper mesopelagic. Here we test this hypothesis by measuring particle associated microbial respiration, through direct measurements on individual marine snow aggregates collected in situ. We find very low rates of both absolute and carbon specific particle associated microbial respiration (< 3% d-1), suggesting that this term cannot close the upper mesopelagic carbon budget. The relative importance of particle associated microbial respiration increases with depth, accounting for up to 34% of POC loss in the lower mesopelagic (128-500 m). We suggest that POC attenuation in the upper mesopelagic is driven by the transformation of large, fast sinking particles to smaller, slowly sinking and suspended particles via processes such as zooplankton fragmentation, and that this shift to non-sinking POC may help to explain imbalances in the mesopelagic carbon budget. [ABSTRACT FROM AUTHOR]

A synthesis is provided of dissolved organic nitrogen (DON) and phosphorus (DOP) distributions over the Atlantic Ocean based upon field data from eight recent transects, six meridional between 50°N and 50°S and two zonal at 24° and 36°N. Over the entire tropical and subtropical Atlantic, DON and DOP provide the dominant contributions to total nitrogen and phosphorus pools for surface waters above the thermocline. Elevated DON and DOP concentrations (>5 and >0.2 μmol L-1, respectively) occur in surface waters on the eastern side of the North Atlantic subtropical gyre and equatorial sides of both the North and South Atlantic subtropical gyres, while particularly low concentrations of DOP (<0.05 μmol L-1) occur over the northern flank of the North Atlantic subtropical gyre along 36°N. This distribution is consistent with organic nutrients formed at the gyre margins supporting carbon export as they are redistributed via the gyre circulation. The effect of DON and DOP transport and cycling on export production is examined in an eddy-permitting, coupled physical and nutrient model integrated for 40 years: organic nutrients are produced in the upwelling zones off North Africa and transferred laterally into the gyre interior, facilitated in part by the mesoscale eddy circulation, as well as fluxed northward from the tropics as part of the overturning circulation. Inputs of semilabile DON and DOP to the tropical and subtropical Atlantic Ocean play an important role in sustaining up to typically 40 and 70% of the modeled particulate N and P export, particularly on the eastern and equatorward sides of the subtropical gyres. [ABSTRACT FROM AUTHOR]

Organic carbon fluxes to the deep ocean may be enhanced by association with ballast mineral material such as calcite and opal. We made simultaneous measurements of the upper ocean production of calcite (calcification), opal (silicification) and organic carbon (photosynthesis) at 14 stations between 42°S and 49°N in the Atlantic Ocean. These measurements confirm the light-dependency of calcification and photosynthesis, and the substrate dependency of silicification. We estimate that mineralizing phytoplankton represent ~5 -20% of organic carbon fixation, with similar contributions from both coccolithophores and diatoms. Estimates of average turnover times for calcite and phytoplankton carbon are ~3 days, indicative of their relatively labile nature. By comparison, average turnover times for opal and particulate organic carbon are ~10 days. Rapid turnover of calcite suggests an important role for the plankton community in removing calcite from the upper ocean. Comparison of our surface production rates to sediment trap data confirms that ~70% of calcite is dissolved in the upper 2-3 km, and only a small proportion of total organic carbon (<2%) reaches the deep ocean. [ABSTRACT FROM AUTHOR]

The Controls Over Mesopelagic Interior Carbon Storage (COMICS) cruise DY086 took place aboard the RRS Discovery in the South Atlantic during November and December, 2017. Physical, chemical, biogeochemical and biological data were collected during three visits to ocean observatory station P3, off the coast of South Georgia, during an austral spring bloom. A diverse range of equipment including CTD-rosette, Acoustic Doppler Current Profiler (ADCP), net deployments, marine snow catchers (MSCs), Stand Alone Pump System (SAPS) and PELAGRA Sediment Traps were used to produce a comprehensive, high-quality dataset. The data can provide excellent insight into regional biological carbon pump (BCP) processes; it is recommended for use by observational scientists and modellers to enhance understanding of ecosystem interactions relating to mesopelagic carbon storage., Competing Interests: Competing interests: The COMICS project (NERC NE/M020835/2) funded the data collection effort and data synthesis and submission was funded by the Sustainable Management of Mesopelagic Resources (SUMMER) project under grant agreement No 817806. The simultaenous glider mission was supported by a European Research Council Consolidator grant (GOCART, agreement number 724416) to S. Henson, as well as South Africa’s Department of Science and Innovation (DST/CON0182/2017) and the National Research Foundation (SANAP: SNA170522231782) grants; as such, these data are published separately. There is no conflict of interest in submitting this paper for consideration., (© 2024. The Author(s).)

Photosynthesis in the surface ocean produces approximately 100 gigatonnes of organic carbon per year, of which 5 to 15 per cent is exported to the deep ocean. The rate at which the sinking carbon is converted into carbon dioxide by heterotrophic organisms at depth is important in controlling oceanic carbon storage. It remains uncertain, however, to what extent surface ocean carbon supply meets the demand of water-column biota; the discrepancy between known carbon sources and sinks is as much as two orders of magnitude. Here we present field measurements, respiration rate estimates and a steady-state model that allow us to balance carbon sources and sinks to within observational uncertainties at the Porcupine Abyssal Plain site in the eastern North Atlantic Ocean. We find that prokaryotes are responsible for 70 to 92 per cent of the estimated remineralization in the twilight zone (depths of 50 to 1,000 metres) despite the fact that much of the organic carbon is exported in the form of large, fast-sinking particles accessible to larger zooplankton. We suggest that this occurs because zooplankton fragment and ingest half of the fast-sinking particles, of which more than 30 per cent may be released as suspended and slowly sinking matter, stimulating the deep-ocean microbial loop. The synergy between microbes and zooplankton in the twilight zone is important to our understanding of the processes controlling the oceanic carbon sink.

Myrionecta rubra, a ubiquitous planktonic ciliate, has received much attention due to its wide distribution, occurrence as a red tide organism, and unusual cryptophyte endosymbiont. Although well studied in coastal waters, M. rubra is poorly examined in the open ocean. In the Irminger Basin, North Atlantic, the abundance of M. rubra was 0-5 cells/ml, which is low compared with that found in coastal areas. Distinct patchiness (100 km) was revealed by geostatistical analysis. Multiple regression indicated there was little relationship between M. rubra abundance and a number of environmental factors, with the exception of temperature and phytoplankton biomass, which influenced abundance in the spring. We also improve on studies that indicate distinct size classes of M. rubra; we statistically recognise four significantly distinct width classes (5-16, 12-23, 18-27, 21-33 microm), which decrease in abundance with increasing size. A multinomial logistic regression revealed the main variable correlated with this size distribution was ambient nitrate concentration. Finally, we propose a hypothesis for the distribution of sizes, involving nutrients, feeding, and dividing of the endosymbiont.

Diatoms are unicellular or chain-forming phytoplankton that use silicon (Si) in cell wall construction. Their survival during periods of apparent nutrient exhaustion enhances carbon sequestration in frontal regions of the northern North Atlantic. These regions may therefore have a more important role in the 'biological pump' than they have previously been attributed, but how this is achieved is unknown. Diatom growth depends on silicate availability, in addition to nitrate and phosphate, but northern Atlantic waters are richer in nitrate than silicate. Following the spring stratification, diatoms are the first phytoplankton to bloom. Once silicate is exhausted, diatom blooms subside in a major export event. Here we show that, with nitrate still available for new production, the diatom bloom is prolonged where there is a periodic supply of new silicate: specifically, diatoms thrive by 'mining' deep-water silicate brought to the surface by an unstable ocean front. The mechanism we present here is not limited to silicate fertilization; similar mechanisms could support nitrate-, phosphate- or iron-limited frontal regions in oceans elsewhere.