Description: Most studies on the potential impacts of deep-sea mining in the Clarion Clipperton Zone (CCZ) have largely focused on benthic ecosystems but ignore the pelagic environment. To model full-scale impacts, it is important to understand how sediment discharge might affect the pelagic zone as well. This study combines in situ optics, hydrography, and remote sensing to describe particle abundance and size distribution through the entire water column in the CCZ (German sector). CCZ surface waters were characterized as productive over the year. During the winter, we observed the formation of a sharp transition zone in Chla concentration, identifying the area as a productive transitional zone toward a more depleted ocean gyre. In the German sector, median particle size was small (± 77 μm), and large particles (〉300 μm) were rare. By assessing particle flux attenuation, we could show that the presence of a thick oxygen minimum zone (OMZ) plays an essential role in export and transformation of settling aggregates, with strong diel variations. We suggest that the combination of small aggregate size, bottom currents and slow seafloor consolidation may explain the extremely low sedimentation rate in the CCZ. We conclude that sediment incorporations and ballasting effect on settling particulate matter represent the most significant hazard on midwater and benthic ecosystems.

Description: Our understanding of the small-scale processes that drive global biogeochemical cycles and the Earth’s climate is dependent on accurate estimations of interfacial diffusive fluxes to and from biologically-active substrates in aquatic environments. In this study, we present a novel model approach for accurate calculations of diffusive fluxes of dissolved gases, nutrients, and solutes from concentration profiles measured across the substrate-water interfaces using microsensors. The model offers a robust computational scheme for automatized determination of the interface position and enables precise calculations of the interfacial diffusive fluxes simultaneously. In contrast to other methods, the new approach is not restricted to any particular substrate geometry, does not require a priori determination of the interface position for the flux calculation, and, thus, reduces the uncertainties in calculated fluxes arising from partly subjective identification of the interface position. In addition, it is robust when applied to measured profiles containing scattered data points and insensitive to reasonable decreases of the spatial resolution of the data points. The latter feature allows for significantly reducing measurement time which is a crucial factor for in situ experiments.

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: The collection of zooplankton swimmers and sinkers in time-series sediment traps provides unique insight into year-round and interannual trends in zooplankton population dynamics. These samples are particularly valuable in remote and difficult to access areas such as the Arctic Ocean, where samples from the ice-covered season are rare. In the present study, we investigated zooplankton composition based on swimmers and sinkers collected by sediment traps at water depths of 180–280, 800–1320, and 2320–2550 m, over a period of 16 yr (2000–2016) at the Long-Term Ecological Research observatory HAUSGARTEN located in the eastern Fram Strait (79°N, 4°E). The time-series data showed seasonal and interannual trends within the dominant zooplankton groups including copepoda, foraminifera, ostracoda, amphipoda, pteropoda, and chaetognatha. Amphipoda and copepoda dominated the abundance of swimmers while pteropoda and foraminifera were the most important sinkers. Although the seasonal occurrence of these groups was relatively consistent between years, there were notable interannual variations in abundance, suggesting the influence of various environmental conditions such as sea-ice dynamic and lateral advection of water masses, for example, meltwater and Atlantic water. Statistical analyses revealed a correlation between the Arctic dipole climatic index and sea-ice dynamics (i.e., ice coverage and concentration), as well as the importance of the distance from the ice edge on swimmer composition patterns and carbon export.

Description: Phytoplankton forms the base of aquatic food webs and element cycling in diverse aquatic systems. The fate of phytoplankton-derived organic matter, however, often remains unresolved as it is controlled by complex, interlinked remineralization and sedimentation processes. We here investigate a rarely considered control mechanism on sinking organic matter fluxes: fungal parasites infecting phytoplankton. We demonstrate that bacterial colonization is promoted 3.5-fold on fungal-infected phytoplankton cells in comparison to non-infected cells in a cultured model pathosystem (diatom Synedra, fungal microparasite Zygophlyctis, and co-growing bacteria), and even ≥17-fold in field-sampled populations (Planktothrix, Synedra, and Fragilaria). Additional data obtained using the Synedra–Zygophlyctis model system reveals that fungal infections reduce the formation of aggregates. Moreover, carbon respiration is 2-fold higher and settling velocities are 11–48% lower for similar-sized fungal-infected vs. non-infected aggregates. Our data imply that parasites can effectively control the fate of phytoplankton-derived organic matter on a single-cell to single-aggregate scale, potentially enhancing remineralization and reducing sedimentation in freshwater and coastal systems.

Description: Large amounts of atmospheric carbon can be exported and retained in the deep sea on millennial time scales, buffering global warming. However, while the Barents Sea is one of the most biologically productive areas of the Arctic Ocean, carbon retention times were thought to be short. Here we present observations, complemented by numerical model simulations, that revealed a deep and widespread lateral injection of approximately 2.33 kt C d−1 from the Barents Sea shelf to some 1,200 m of the Nansen Basin, driven by Barents Sea Bottom Water transport. With increasing distance from the outflow region, the plume expanded and penetrated into even deeper waters and the sediment. The seasonally fluctuating but continuous injection increases the carbon sequestration of the Barents Sea by 1/3 and feeds the deep sea community of the Nansen Basin. Our findings combined with those from other outflow regions of carbon-rich polar dense waters highlight the importance of lateral injection as a global carbon sink. Resolving uncertainties around negative feedbacks of global warming due to sea ice decline will necessitate observation of changes in bottom water formation and biological productivity at a resolution high enough to quantify future deep carbon injection.

Description: The dynamics of the particulate organic carbon (POC) pool in the ocean are central to the marine carbon cycle. POC is the link between surface primary production, the deep ocean, and sediments. The rate at which POC is degraded in the dark ocean can impact atmospheric CO2 concentration. Therefore, a central focus of marine organic geochemistry studies is to improve our understanding of POC distribution, composition, and cycling. The last few decades have seen improvements in analytical techniques that have greatly expanded what we can measure, both in terms of organic compound structural diversity and isotopic composition, and complementary molecular omics studies. Here we provide a brief overview of the autochthonous, allochthonous, and anthropogenic components comprising POC in the ocean. In addition, we highlight key needs for future research that will enable us to more effectively connect diverse data sources and link the identity and structural diversity of POC to its sources and transformation processes.

Description: The ocean's biological carbon pump plays a central role in regulating atmospheric CO2 levels. In particular, the depth at which sinking organic carbon is broken down and respired in the mesopelagic zone is critical, with deeper remineralization resulting in greater carbon storage. Until recently, however, a balanced budget of the supply and consumption of organic carbon in the mesopelagic had not been constructed in any region of the ocean, and the processes controlling organic carbon turnover are still poorly understood. Large-scale data syntheses suggest that a wide range of factors can influence remineralization depth including upper-ocean ecological interactions, and interior dissolved oxygen concentration and temperature. However, these analyses do not provide a mechanistic understanding of remineralization, which increases the challenge of appropriately modeling the mesopelagic carbon dynamics. In light of this, the UK Natural Environment Research Council has funded a programme with this mechanistic understanding as its aim, drawing targeted fieldwork right through to implementation of a new parameterization for mesopelagic remineralization within an IPCC class global biogeochemical model. The Controls over Ocean Mesopelagic Interior Carbon Storage (COMICS) programme will deliver new insights into the processes of carbon cycling in the mesopelagic zone and how these influence ocean carbon storage. Here we outline the programme's rationale, its goals, planned fieldwork, and modeling activities, with the aim of stimulating international collaboration.

Description: Eastern boundary upwelling systems (EBUSs) are among the most productive marine environments in the world. The Canary Current upwelling system off the coast of Mauritania and Morocco is the second most productive of the four EBUS, where nutrient-rich waters fuel perennial phytoplankton blooms, evident by high chlorophyll a concentrations off Cape Blanc, Mauritania. High primary production leads to eutrophic waters in the surface layers, whereas sinking phytoplankton debris and horizontally dispersed particles form nepheloid layers (NLs) and hypoxic waters at depth. We used Catalyzed Reporter Deposition Fluorescence In Situ Hybridization (CARD-FISH) in combination with fatty acid (measured as methyl ester; FAME) profiles to investigate the bacterial and archaeal community composition along transects from neritic to pelagic waters within the “giant Cape Blanc filament” in two consecutive years (2010 and 2011), and to evaluate the usage of FAME data for microbial community studies. We also report the first fatty acid profile of Pelagibacterales strain HTCC7211 which was used as a reference profile for the SAR11 clade. Unexpectedly, the reference profile contained low concentrations of long chain fatty acids 18:1 cis11, 18:1 cis11 11methyl, and 19:0 cyclo11–12 fatty acids, the main compounds in other Alphaproteobacteria. Members of the free-living SAR11 clade were found at increased relative abundance in the hypoxic waters in both years. In contrast, the depth profiles of Gammaproteobacteria (including Alteromonas and Pseudoalteromonas), Bacteroidetes, Roseobacter, and Synechococcus showed high abundances of these groups in layers where particle abundance was high, suggesting that particle attachment or association is an important mechanisms of dispersal for these groups. Collectively, our results highlight the influence of NLs, horizontal particle transport, and low oxygen on the structure and dispersal of microbial communities in upwelling systems.

Description: Marine snow aggregates are microhabitats for diverse microbial communities with various active metabolic pathways. Rapid recycling and symbiotic transfer of nutrients within aggregates poses a significant challenge for accurately assessing aggregate‐associated turnover rates. Although single‐cell uptake measurements are well‐established for free‐living microorganisms, suitable methods for cells embedded in marine snow are currently lacking. Comparable cell‐specific measurements within sinking pelagic aggregates would have the potential to address core questions regarding aggregate‐associated fluxes. However, the capacity to perform microscale studies is limited by the difficulty of sampling and preserving the fragile aggregate structure. Furthermore, the application of nano‐scale secondary ion mass spectrometry (NanoSIMS) to aggregates is complicated by technical requirements related to vacuum and ablation resistance. Here, we present a NanoSIMS‐optimized method for fixation, embedding, and sectioning of marine snow. Stable isotope labeling of laboratory‐generated aggregates enabled visualization of label incorporation into prokaryotic and eukaryotic cells embedded in the aggregate structure. The current method is also amenable to various staining procedures, including transparent exopolymer particles, Coomassie stainable particles, nucleic acids, and eukaryotic cytoplasm. We demonstrate the potential for using structural stains to generate three‐dimensional (3D) models of marine snow and present a simplified calculation of porosity and fractal dimension. This multipurpose method enables combined investigations of 3D aggregate structure, spatial microbial distribution, and single‐cell activity within individual aggregates and provides new possibilities for future studies on microbial interactions and elemental uptake within marine snow.