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Changes in vertical and spatial distributions of zooplankton and small pelagic fish impact the biological carbon pump and the distribution of larger piscivorous fish and marine mammal species. However, their distribution and abundance remain poorly documented at high latitudes because of the difficulties inherent to sampling relatively fast-moving organisms in ice-covered waters. This study documents the under-ice distribution of epipelagic and mesopelagic organisms at the Arctic-Atlantic gateway in spring, during the midnight sun period, using ice-tethered and ship-based echosounders. An epipelagic surface scattering layer composed of copepods consistently occupied the top 60 m and was associated with cold polar surface water (mean temperature of −1.5 °C). A mesopelagic deep scattering layer (DSL), partly composed of fish, persisted between 280 m and 600 m and was associated with modified Atlantic water. Backscattering strength within the DSL was higher than previously reported in the Arctic and north Atlantic, and increased by two orders of magnitude over the continental slope where one of the Atlantic water pathways enters the Arctic Ocean. Mesopelagic organisms did not perform diel vertical migrations. The consistent segregation between copepods at the surface and their predators at mesopelagic depths suggests limited predator–prey interactions during the midnight sun period, even under the ice cover. Predation on copepods by mesopelagic organisms, including fish, could thus be limited to very pulsed events during the seasonal vertical migration of copepods to and from overwintering depths. This suggests that the arctic mesopelagic food web may be decoupled from secondary production in the epipelagic layer throughout most of the year. This is an open access article distributed under the terms of the Creative Commons CC-BY license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

The Koster Trench is the deepest part of the Kosterfjord (Skagerrak, North Sea), which stretches southward along the west coast of Sweden. Since 2009 Kosterfjord has been included in the Marine Protected Area Kosterhavet National Park. To effectively manage national parks, long-term time series of ecological data are needed and those can be derived from local sediment archives. In this study we present multiproxy geochemical (bulk TOC, C/N and heavy metals) and micropaleontological (dinoflagellate cysts, selected palynomorphs and benthic foraminifera) data from a sediment core taken in the southern part of the Koster Trench. Radiometric dating by 210Pb and 137Cs shows that the core archived the environmental changes that took place between 1988 and 2012. The TOC, heavy metals and foraminiferal indices indicate mainly high to good Ecological Quality Status, with moderate ecological quality for arsenic concentrations. Dinoflagellate cysts suggest a major change occurring in the upper water column around 2002. The cysts of Pentapharsodinium dalei peak around 1992 and show overall slightly higher relative abundances between 1988 and 2002. Increased abundance of Biecheleria baltica cysts, heterotrophic species, oligotrichids, pollen and spores characterise the upper core part deposited from 2002 to 2012. Also, there is a clear increase of Alexandrium cysts in the top of the core (~2008–2012). Benthic foraminifera show a major faunal change reflected in a significant increase of agglutinated species (mainly Textularia earlandi) from 2007 towards present day, while the lower part of the core (1988–2007) is dominated by calcareous species. Overall, the changes demonstrated by the dataset suggest an increased freshwater input or a higher river/land runoff, as supported by climatic and hydrographical data showing increased precipitation over the study area, decreasing salinity, and an increasing trend in particulate organic carbon in the surface waters. These changes, in combination with trawling activities, which have been moved to the deepest part of the trench since 2009, seem to favour dinoflagellate and benthic foraminiferal species with mixotrophic and omnivorous feeding strategies. Similar to the dinocysts, calculated foram-AMBI and NQIforam indices show a shift around 2002 suggesting that environmental changes occurring in the study area are likely linked to darkening of coastal waters.

The Koster Trench is the deepest part of the Kosterfjord (Skagerrak, North Sea), which stretches southward along the west coast of Sweden. Since 2009 Kosterfjord has been included in the Marine Protected Area Kosterhavet National Park due to the presence of cold-water coral reefs. In this study we present multiproxy data from the sediment core KSK12-01D taken in the southern part of the Koster Trench. The core has been dated by 137Cs and was subject to bulk geochemistry (TOC, C/N and heavy metals: Ni, Cu, Zn, As, Cd and Pb) and micropaleontological (benthic foraminifera, dinocysts and selected palynomorphs) analyses. Results show that the core is an archive for the environmental changes that took place between ~1988 and 2012. Both TOC and heavy metals indicate high/good to moderate ecological quality status, as defined by the Swedish Environmental Protection Agency. Dinocysts suggest a major change occurring in the upper water column around 2002. The cysts of Pentapharsodinium dalei peak around 1992 and shows overall slightly higher relative abundances between 1988 and 2002. Increased abundance of cysts produced by Protoceratium reticulatum (i.e. Operculodinium centrocarpum sensu Wall and Dale 1966), cysts produced by Biecheleria baltica and heterotrophic dinoflagellates, oligotrichids, pollen and spores characterise the upper core part deposited from 2002 to 2012. Furthermore, there is a clear increase of calcareous Alexandrium cysts in the top of the core (~2008-2012). Benthic foraminifera show a major faunal change reflected in a drastic increase of agglutinated species (mainly Textularia earlandi) from 2007 towards present day, while the lower part of the core (1988-2007) is dominated by calcareous species (Stainforthia fusiformis, Epistominella exigua, Cassidulina laevigata, Bulimina marginata andHyalinea balthica). Overall, the changes in the dataset suggest an increased freshwater input or a higher river/land runoff, as supported by a local climate and hydrography data showing increasing precipitation and particulate organic carbon in the surface waters over the time period covered by our record.

Submesoscale flows in the ocean are energetic motions, O(1–10 km), that influence stratification and the distributions of properties, such as heat and carbon. They are believed to play an important role in sea‐ice‐impacted oceans by modulating air‐sea‐ice fluxes and sea‐ice extent. The intensity of these flows and their response to wind forcing are unobserved in the sea‐ice regions of the Southern Ocean. We present the first submesoscale‐resolving observations in the Antarctic marginal ice zone (MIZ) collected by surface and underwater autonomous vehicles, for >3 months in austral summer. We observe salinity‐dominated lateral density fronts occurring at sub‐kilometer scales. Surface winds are shown to modify the magnitude of the mixed‐layer density fronts, revealing strongly coupled atmosphere‐ocean processes. We posture that these wind‐front interactions occur as a continuous interplay between front slumping and vertical mixing, which leads to the dispersion of submesoscale fronts. Such processes are expected to be ubiquitous in the Southern Ocean MIZ.

Bottom trawls suspend sediments by physical contact and the drag created by the gear when towed over the seafloor. Increased turbidity and redistribution of sediments may be of concern as some organisms are vulnerable to increased levels of sediment particles in the water column. This study investigates the distribution and fate of trawling-induced suspension of sediments in a coastal marine protected area (MPA). Bottom trawling is allowed in a deep trench of the MPA and regulated to weekdays with weekend closures. We use the closed period as controls to experimentally investigate sediment resuspension generated by the trawling fleet. We find that the turbidity is elevated at trawled depths and originates from small particles of silt–mud seafloor origin that remain suspended for days preventing background levels to be reached during closures. Peaks in the plumes behind trawlers reach levels critical to organisms but decays within hours. Currents in the trench are too weak to resuspend sediments; however, wind-induced resuspension events of elevated turbidity likely originating from nearby shallower areas may occur. Short-term closures and small-scale MPAs are concluded to be of limited use as management tools to reduce effects of resuspension by bottom trawling.

A unique dataset of atmospheric observations over the Nordic Seas has been analyzed to investigate the role of convective available potential energy (CAPE) for the energetics of polar lows. The observations were made during the flight campaign of the Norwegian International Polar Year (IPY) and The Observing System Research and Predictability Experiment (THORPEX) in February and March 2008, which specifically targeted polar lows. The data reveal virtually no conditional instability and very limited CAPE. It is suggested that the significance of CAPE values should be assessed by calculating the time scale tCAPE that is necessary for the heat fluxes from the ocean to transfer the corresponding amount of energy. Even the largest CAPE values have a tCAPE of less than 1 h. These CAPE values are associated with unconditional instability. It is concluded that the observed CAPE should be seen as a temporary stage in an energy flux rather than as an energy reservoir. Based on the findings in this investigation, it is proposed that significant reservoirs of CAPE over the marine Arctic atmosphere are impossible since CAPE production will automatically trigger convection and CAPE is consumed as it is produced.

Possible surface warming by strong wind-forcing from polar lows in the North-Atlantic has been investigated using a numerical model for vertical entrainment of waters from a subsurface warm core, and microwave satellite images of sea-surface temperature during polar low events. The hypothesis is based on the frequently observed subsurface warm core in oceans influenced by the North-Atlantic current (NAC) or by outflowing surface water from the Arctic Ocean. CTD-soundings from the Nordic Seas reveal that the waters from the NAC are located under colder and less saline surface waters in winter. For sufficiently strong wind events, turbulent entrainment of this subsurface warm core may lead to a rapid surface warming. Our main findings is that the surface warming of more than 1 °C may take place within a few hours. The result is based on model runs with initial temperature and salinity profiles from CTD-observations. Observational evidence of surface temperatures that support the hypothesis are found in microwave satellite observations from a polar lowevent. In the case presented here, increased sea-surface temperatures between 1 and 2 °C were observed. We believe that rapid surface warming of this magnitude may be a potential positive feedback mechanism for the cyclone intensity.