Description: High primary productivity in the equatorial Atlantic and Pacific oceans is one of the key features of tropical ocean biogeochemistry and fuels a substantial flux of particulate matter towards the abyssal ocean. How biological processes and equatorial current dynamics shape the particle size distribution and flux, however, is poorly understood. Here we use high-resolution size-resolved particle imaging and Acoustic Doppler Current Profiler data to assess these influences in equatorial oceans. We find an increase in particle abundance and flux at depths of 300 to 600 m at the Atlantic and Pacific equator, a depth range to which zooplankton and nekton migrate vertically in a daily cycle. We attribute this particle maximum to faecal pellet production by these organisms. At depths of 1,000 to 4,000 m, we find that the particulate organic carbon flux is up to three times greater in the equatorial belt (1° S–1° N) than in off-equatorial regions. At 3,000 m, the flux is dominated by small particles less than 0.53 mm in diameter. The dominance of small particles seems to be caused by enhanced active and passive particle export in this region, as well as by the focusing of particles by deep eastward jets found at 2° N and 2° S. We thus suggest that zooplankton movements and ocean currents modulate the transfer of particulate carbon from the surface to the deep ocean.

Description: In ocean waters, anaerobic microbial respiration should be confined to the anoxic waters found in coastal regions and tropical oxygen minimum zones, where it is energetically favourable. However, recent molecular and geochemical evidence has pointed to a much broader distribution of denitrifying and sulfate-reducing microbes. Anaerobic metabolisms are thought to thrive in microenvironments that develop inside sinking organic aggregates, but the global distribution and geochemical significance of these microenvironments is poorly understood. Here, we develop a new size-resolved particle model to predict anaerobic respiration from aggregate properties and seawater chemistry. Constrained by observations of the size spectrum of sinking particles, the model predicts that denitrification and sulfate reduction can be sustained throughout vast, hypoxic expanses of the ocean, and could explain the trace metal enrichment observed in particles due to sulfide precipitation. Globally, the expansion of the anaerobic niche due to particle microenvironments doubles the rate of water column denitrification compared with estimates based on anoxic zones alone, and changes the sensitivity of the marine nitrogen cycle to deoxygenation in a warming climate.

Description: Planktonic organisms play crucial roles in oceanic food webs and global biogeochemical cycles(1,2). Most of our knowledge about the ecological impact of large zooplankton stems from research on abundant and robust crustaceans, and in particular copepods(3,4). A number of the other organisms that comprise planktonic communities are fragile, and therefore hard to sample and quantify, meaning that their abundances and effects on oceanic ecosystems are poorly understood. Here, using data from a worldwide in situ imaging survey of plankton larger than 600 mu m, we show that a substantial part of the biomass of this size fraction consists of giant protists belonging to the Rhizaria, a super-group of mostly fragile unicellular marine organisms that includes the taxa Phaeodaria and Radiolaria ( for example, orders Collodaria and Acantharia). Globally, we estimate that rhizarians in the top 200 m of world oceans represent a standing stock of 0.089 Pg carbon, equivalent to 5.2% of the total oceanic biota carbon reservoir(5). In the vast oligotrophic intertropical open oceans, rhizarian biomass is estimated to be equivalent to that of all other mesozooplankton ( plankton in the size range 0.2-20 mm). The photosymbiotic association of many rhizarians with microalgae may be an important factor in explaining their distribution. The previously overlooked importance of these giant protists across the widest ecosystem on the planet(6) changes our understanding of marine planktonic ecosystems.

Description: Distribution patterns of fragile gelatinous fauna in the open ocean remain scarcely documented. Using epi-and mesopelagic video transects in the eastern tropical North Atlantic, which features a mild but intensifying midwater oxygen minimum zone (OMZ), we established one of the first regional observations of diversity and abundance of large gelatinous zooplankton. We quantified the day and night vertical distribution of 46 taxa in relation to environmental conditions. While distribution may be driven by multiple factors, abundance peaks of individual taxa were observed in the OMZ core, both above and below the OMZ, only above, or only below the OMZ whereas some taxa did not have an obvious distribution pattern. In the eastern eropical North Atlantic, OMZ expansion in the course of global climate change may detrimentally impact taxa that avoid low oxygen concentrations (Beroe, doliolids), but favour taxa that occur in the OMZ (Lilyopsis, phaeodarians, Cydippida, Colobonema, Haliscera conica and Halitrephes) as their habitat volume might increase. While future efforts need to focus on physiology and taxonomy of pelagic fauna in the study region, our study presents biodiversity and distribution data for the regional epi- and mesopelagic zones of Cape Verde providing a regional baseline to monitor how climate change may impact the largest habitat on the planet, the deep pelagic realm.

Description: Underwater images are used to explore and monitor ocean habitats, generating huge datasets with unusual data characteristics that preclude traditional data management strategies. Due to the lack of universally adopted data standards, image data collected from the marine environment are increasing in heterogeneity, preventing objective comparison. The extraction of actionable information thus remains challenging, particularly for researchers not directly involved with the image data collection. Standardized formats and procedures are needed to enable sustainable image analysis and processing tools, as are solutions for image publication in long-term repositories to ascertain reuse of data. The FAIR principles (Findable, Accessible, Interoperable, Reusable) provide a framework for such data management goals. We propose the use of image FAIR Digital Objects (iFDOs) and present an infrastructure environment to create and exploit such FAIR digital objects. We show how these iFDOs can be created, validated, managed and stored, and which data associated with imagery should be curated. The goal is to reduce image management overheads while simultaneously creating visibility for image acquisition and publication efforts.

Description: Anaerobic oxidation of ammonium (anammox) in oxygen minimum zones (OMZs) is a major pathway of oceanic nitrogen loss. Ammonium released from sinking particles has been suggested to fuel this process. During cruises to the Peruvian OMZ in April–June 2017 we found that anammox rates are strongly correlated with the volume of small particles (128–512 µm), even though anammox bacteria were not directly associated with particles. This suggests that the relationship between anammox rates and particles is related to the ammonium released from particles by remineralization. To investigate this, ammonium release from particles was modelled and theoretical encounters of free-living anammox bacteria with ammonium in the particle boundary layer were calculated. These results indicated that small sinking particles could be responsible for ~75% of ammonium release in anoxic waters and that free-living anammox bacteria frequently encounter ammonium in the vicinity of smaller particles. This indicates a so far underestimated role of abundant, slow-sinking small particles in controlling oceanic nutrient budgets, and furthermore implies that observations of the volume of small particles could be used to estimate N-loss across large areas.

Description: In the ocean, downward flux of particles produced in sunlit surface waters is the major component of the biological carbon pump, which sequesters atmospheric carbon dioxide and fuels deep-sea ecosystems. The efficiency of downward carbon transfer is expected to be particularly high in tropical upwelling systems where hypoxia occurring beneath the productive surface waters is thought to hamper particle consumption. However, observations of both particle feeders and carbon export in low-oxygen waters are scarce. Here, we provide evidence that hypoxia-tolerant zooplankton feed on sinking particles in the extensive Oxygen Minimum Zone (OMZ) off Peru. Using several arrays of drifting sediment traps and in situ imaging, we show geochemical and morphological transformations of sinking particles and substantial control of carbon export by zooplankton. Our findings challenge the assumption of a consistently efficient biological carbon pump in OMZs and further demonstrate the need to consider mesopelagic organisms when studying oceanic carbon sequestration.

Description: Thriving in both epipelagic and mesopelagic layers, Rhizaria are biomineralizing protists, mixotrophs or flux-feeders, often reaching gigantic sizes. In situ imaging showed their contribution to oceanic carbon stock, but left their contribution to element cycling unquantified. Here, we compile a global dataset of 167,551 Underwater Vision Profiler 5 Rhizaria images, and apply machine learning models to predict their organic carbon and biogenic silica biomasses in the uppermost 1000 m. We estimate that Rhizaria represent up to 1.7% of mesozooplankton carbon biomass in the top 500 m. Rhizaria biomass, dominated by Phaeodaria, is more than twice as high in the mesopelagic than in the epipelagic layer. Globally, the carbon demand of mesopelagic, flux-feeding Phaeodaria reaches 0.46 Pg C y −1 , representing 3.8 to 9.2% of gravitational carbon export. Furthermore, we show that Rhizaria are a unique source of biogenic silica production in the mesopelagic layer, where no other silicifiers are present. Our global census further highlights the importance of Rhizaria for ocean biogeochemistry.

Description: Calanoid copepods and euphausiids are key components of marine zooplankton communities worldwide. Most euphausiids and several copepod species perform diel vertical migrations (DVMs) that contribute to the export of particulate and dissolved matter to midwater depths. In vast areas of the global ocean, and in particular in the eastern tropical Atlantic and Pacific, the daytime distribution depth of many migrating organisms corresponds to the core of the oxygen minimum zone (OMZ). At depth, the animals experience reduced temperature and oxygen partial pressure (pO2) and an increased carbon dioxide partial pressure (pCO2) compared to their near-surface nighttime habitat. Although it is well known that low oxygen levels can inhibit respiratory activity, the respiration response of tropical copepods and euphausiids to relevant pCO2, pO2, and temperature conditions remains poorly parameterized. Further, the regulation of ammonium excretion at OMZ conditions is generally not well understood. It was recently estimated that DVM-mediated ammonium supply could fuel bacterial anaerobic ammonium oxidation – a major loss process for fixed nitrogen in the ocean considerably. These estimates were based on the implicit assumption that hypoxia or anoxia in combination with hypercapnia (elevated pCO2) does not result in a down-regulation of ammonium excretion. We exposed calanoid copepods from the Eastern Tropical North Atlantic (ETNA; Undinula vulgaris and Pleuromamma abdominalis) and euphausiids from the Eastern Tropical South Pacific (ETSP; Euphausia mucronata) andthe ETNA (Euphausia gibboides) to different temperatures, carbon dioxide and oxygen levels to study their survival, respiration and excretion rates at these conditions. An increase in temperature by 10°C led to an approximately 2-fold increase of the respiration and excretion rates of U.vulgaris (Q10, respiration=1.4; Q10,NH4-excretion=1.6), P. abdominalis (Q10, respiration=2.0; Q10,NH4-excretion=2.4) and E. gibboides (Q10, respiration=2.0; Q10,NH4-excretion=2.4; E. mucronata not tested). Exposure to differing carbon dioxide levels had no overall significant impact on the respiration or excretion rates. Species from the ETNA were less tolerant to low oxygen levels than E. mucronata from the ETSP, which survived exposure to anoxia at 13°C. Respiration and excretion rates were reduced upon exposure to low oxygen levels, albeit at different species-specific levels. Reduction of the excretion and respiration rates in ETNA species occurred at a pO2 of 0.6 (P. abdominalis) and 2.4 kPa (U. vulgaris and E.gibboides) at OMZ temperatures. Such low oxygen levels are normally not encountered by these species in the ETNA. E. mucronata however regularly migrates into the strongly hypoxic to anoxic core of the ETSP OMZ. Exposure to low oxygen levels led to a strong reduction of respiration and ammonium excretion in E. mucronata (pcrit respiration=0.6, pcrit NH4excretion=0.73). A drastic reduction of respiratory activity was also observed by other authors for euphausiids, squat lobsters and calanoid copepods, but was not yet accounted for when calculating DVM-mediated active fluxes into the ETSP OMZ. Current estimates of DVM-mediated active export of carbon and nitrogen into the ETSP OMZ are therefore likely too high and future efforts to calculate these export rates should take the physiological responses of migratory species to OMZ conditions into account.

Description: Marine particles of different nature are found throughout the global ocean. The term "marine particles"describes detritus aggregates and fecal pellets as well as bacterioplankton, phytoplankton, zooplankton and nekton. Here, we present a global particle size distribution dataset obtained with several Underwater Vision Profiler 5 (UVP5) camera systems. Overall, within the 64 μm to about 50 mm size range covered by the UVP5, detrital particles are the most abundant component of all marine particles; thus, measurements of the particle size distribution with the UVP5 can yield important information on detrital particle dynamics. During deployment, which is possible down to 6000 m depth, the UVP5 images a volume of about 1 L at a frequency of 6 to 20 Hz. Each image is segmented in real time, and size measurements of particles are automatically stored. All UVP5 units used to generate the dataset presented here were inter-calibrated using a UVP5 high-definition unit as reference. Our consistent particle size distribution dataset contains 8805 vertical profiles collected between 19 June 2008 and 23 November 2020. All major ocean basins, as well as the Mediterranean Sea and the Baltic Sea, were sampled. A total of 19 % of all profiles had a maximum sampling depth shallower than 200 dbar, 38 % sampled at least the upper 1000 dbar depth range and 11 % went down to at least 3000 dbar depth. First analysis of the particle size distribution dataset shows that particle abundance is found to be high at high latitudes and in coastal areas where surface productivity or continental inputs are elevated. The lowest values are found in the deep ocean and in the oceanic gyres. Our dataset should be valuable for more in-depth studies that focus on the analysis of regional, temporal and global patterns of particle size distribution and flux as well as for the development and adjustment of regional and global biogeochemical models. The marine particle size distribution dataset is available at 10.1594/PANGAEA.924375.