Description: The ALKOR cruise AL547 represents a concluding milestone of the Helmholtz innovation project ARCHES (Autonomous Robotic Networks to Help Modern Societies). The aim was to implement a heterogeneous robotic sensing network to simultaneously monitor changes in the water column and at the seafloor. The network has been developed by a consortium of partners from AWI, DLR, GEOMAR and the University of Kiel. The participating sensing platforms allow for real-time data transfer and the entire network shall be able to autonomously respond to environmental changes in the ocean. The network comprised seven different mobile and stationary platforms. Tests were conducted at the Mittelgrund working area in the entrance of the Eckernförde Bay (western Baltic Sea). During 47 stations the various sensing platforms were deployed and recovered for maintenance. A total of 87853 messages were sent using hydro-acoustics, of which 71734 messages contained O 2 data, 15177 were status messages, 926 messages were commands to trigger a change of the measurement behavior of a platform and 16 messages represented broadcasts about the environmental status. We synoptically recorded short-term O 2 time series on the different platforms, which were placed along a depth gradient in the working area. As the Eckernförde Bay is known for sporadic fish kills by anoxia we hope to contribute to a better understanding of the O 2 dynamics in coastal areas.

Description: The seabed plays a key role in the marine carbon cycle as a) the terminal location of aerobic oxidation of organic matter, b) the greatest anaerobic bioreactor, and c) the greatest repository for reactive organic carbon on Earth. We compiled data on the oxygen uptake of marine sediments with the objective to understand the constraints on mineralization rates of deposited organic matter and their relation to key environmental parameters. The compiled database includes nearly 4000 O 2 uptake data and is available as supplementary material. It includes also information on bottom water O 2 concentration, O 2 penetration depth, geographic position, water depth, and full information on the data sources. We present the different in situ and ex situ approaches to measure the total oxygen uptake (TOU) and the diffusive oxygen uptake (DOU) of sediments and discuss their robustness towards methodological errors and statistical uncertainty. We discuss O 2 transport through the benthic and diffusive boundary layers, the diffusion- and fauna-mediated O 2 uptake, and the coupling of aerobic respiration to anaerobic processes. Five regional examples are presented to illustrate the diversity of the seabed: Eutrophic seas, oxygen minimum zones, abyssal plains, mid-oceanic gyres, and hadal trenches. A multiple correlation analysis shows that seabed O 2 uptake is primarily controlled by ocean depth and sea surface primary productivity. The O2 penetration depth scales with the DOU according to a power law that breaks down under the abyssal ocean gyres. The developed multiple correlation model was used to draw a global map of seabed O2 uptake rates. Respiratory coefficients, differentiated for depth regions of the ocean, were used to convert the global O 2 uptake to organic carbon oxidation. The resulting global budget shows an oxidation of 212 Tmol C yr − 1 in marine sediments with a 5-95% confidence interval of 175-260 Tmol C yr − 1 . A comparison with the global flux of particulate organic carbon (POC) from photic surface waters to the deep sea, determined from multiple sediment trap studies, suggests a deficit in the sedimentation flux at 2000 m water depth of about 70% relative to the carbon turnover in the underlying seabed. At the ocean margins, the flux of organic carbon from rivers and from vegetated coastal ecosystems contributes greatly to the budget and may even exceed the phytoplankton production on the inner continental shelf.

Description: The ocean moderates the world's climate through absorption of heat and carbon, but how much carbon the ocean will continue to absorb remains unknown. The North Atlantic Ocean west (Baffin Bay/Labrador Sea) and east (Fram Strait/Greenland Sea) of Greenland features the most intense absorption of anthropogenic carbon globally; the biological carbon pump (BCP) contributes substantially. As Arctic sea-ice melts, the BCP changes, impacting global climate and other critical ocean attributes (e.g. biodiversity). Full understanding requires year-round observations across a range of ice conditions. Here we present such observations: autonomously collected Eulerian continuous 24-month time-series in Fram Strait. We show that, compared to ice-unaffected conditions, sea-ice derived meltwater stratification slows the BCP by 4 months, a shift from an export to a retention system, with measurable impacts on benthic communities. This has implications for ecosystem dynamics in the future warmer Arctic where the seasonal ice zone is expected to expand.