Eddy driven recirculation of Atlantic Water (AW) in the Fram Strait modifies the amount of heat that reaches the Arctic Ocean, but is difficult to constrain in ocean models due to very small Rossby radius there. In this study, we explore the effect of resolved eddies on the AW circulation in a locally eddy‐resolving simulation of the global Finite‐Element‐Sea ice‐Ocean Model (FESOM) integrated for the years 2000–2009, by focusing on the seasonal cycle. An eddy‐permitting simulation serves as a control run. Our results suggest that resolving local eddy dynamics is critical to realistically simulate ocean dynamics in the Fram Strait. Strong eddy activity simulated by the eddy‐resolving model, with peak in winter and lower values in summer, is comparable in magnitude and seasonal cycle to observations from a long‐term mooring array, whereas the eddy‐permitting simulation underestimates the observed magnitude. Furthermore, a strong cold bias in the central Fram Strait present in the eddy‐permitting simulation is reduced due to resolved eddy dynamics, and AW transport into the Arctic Ocean is increased with possible implications for the Arctic Ocean heat budget. Given the good agreement between the eddy‐resolving model and measurements, it can help filling gaps that point‐wise observations inevitably leave. For example, the path of the West Spitsbergen Current offshore branch, measured in the winter months by the mooring array, is shown to continue cyclonically around the Molloy Deep in the model, representing the major AW recirculation branch in this season.

Submesoscale flows are energetic motions on scales of several kilometers that may lead to substantial vertical motions. Here we present satellite and ship radar as well as underway conductivity-temperature-depth and Acoustic Doppler Current Profiler observations of a cyclonic submesoscale filament in the marginal ice zone of Fram Strait. The filament created a 500-m thin and 50-km long sea ice streak and extends to >250-m depth with a negative/positive density anomaly within/below the halocline. The frontal jets of 0.5 m/s are in turbulent thermal wind balance while the ageostrophic secondary circulation in places appears to subduct Atlantic Water at >50 m/day. Our study reveals the submesoscale dynamics related to sea ice shapes that can be sensed remotely and shows how submesoscale dynamics contribute to shaping the marginal ice zone. It also demonstrates the co-occurrence and mixing of water masses over short horizontal scales, which has implications for ocean and sea ice models and understanding of patch formation of planktonic organisms.Plain Language Summary A sea ice streak in the marginal ice zone was observed with radar measurements. Below this streak in situ shipboard measurements of the temperature, salinity, and velocity field revealed a cyclonic submesoscale filament. This is a line of denser water of a few kilometers width bounded by strong counteracting velocities. This denser water is also associated with a different water mass and thus a change in biological properties and communities. This provides in situ confirmation for previous theoretical conclusions of how oceanic flows on kilometer scales structure the sea ice and biology in the marginal ice zone. The understanding of such small-scale processes helps improve computer models of the ocean and sea ice dynamics. It also makes it possible to interpret oceanic flows from remote sensing of sea ice. Furthermore, it gives indication over which horizontal scales biological processes vary in the ocean.

Climate models project that the Arctic Ocean may experience ice-free summers by the second half of this century. This may have severe repercussions on phytoplankton bloom dynamics and the associated cycling of carbon in surface waters. We currently lack baseline knowledge of the seasonal dynamics of Arctic microbial communities, which is needed in order to better estimate the effects of such changes on ecosystem functioning. Here we present a comparative study of polar summer microbial communities in the ice-free (eastern) and ice-covered (western) hydrographic regimes at the LTER HAUSGARTEN in Fram Strait, the main gateway between the Arctic and North Atlantic Oceans. Based on measured and modeled biogeochemical parameters, we tentatively identified two different ecosystem states (i.e., different phytoplankton bloom stages) in the distinct regions. Using Illumina tag-sequencing, we determined the community composition of both free-living and particle-associated bacteria as well as microbial eukaryotes in the photic layer. Despite substantial horizontal mixing by eddies in Fram Strait, pelagic microbial communities showed distinct differences between the two regimes, with a proposed early spring (pre-bloom) community in the ice-covered western regime (with higher representation of SAR11, SAR202, SAR406 and eukaryotic MALVs) and a community indicative of late summer conditions (post-bloom) in the icefree eastern regime (with higher representation of Flavobacteria, Gammaproteobacteria and eukaryotic heterotrophs). Co-occurrence networks revealed specific taxon-taxon associations between bacterial and eukaryotic taxa in the two regions. Our results suggest that the predicted changes in sea ice cover and phytoplankton bloom dynamics will have a strong impact on bacterial community dynamics and potentially on biogeochemical cycles in this region.

Warm and salty Atlantic Water is transported by the Norwegian Atlantic Current through the Nordic Seas. A fraction of it enters the Arctic Ocean where it contributes significantly to its heat budget. Resolving the complex circulation structure in the Nordic Seas, in particular eddies, presents a numerical challenge in ocean models. Here, we present a hindcast experiment for the years 1969–2009 with a global configuration of the Finite Element Sea‐ice Ocean Model, employing a high‐resolution mesh in the Nordic Seas and Arctic Ocean (4.5 km). We show that substantial improvements can be achieved in the circulation structure, hydrography and eddy kinetic energy in the Nordic Seas compared with a coarse‐resolution reference run. A better represented Norwegian Atlantic Front Current (NwAFC) in the high‐resolution setup leads to a reduction of a strong negative temperature bias in the eastern Nordic Seas. The Atlantic Water inflow through the Iceland‐Faroe Ridge is found to be very sensitive to mesh resolution, and high resolution is required to adequately represent this inflow and the downstream NwAFC. With increased mesh resolution, the simulated ocean temperature is significantly improved at Barents Sea Opening (BSO), and the Atlantic Water volume transport in Fram Strait becomes much closer to observations in terms of both magnitude and variability. By using passive tracers, the origins of water masses at BSO and Fram Strait are identified. Our study also indicates that eddy‐resolving meshes are required to further improve the representation of dynamical processes in the region, in particular in Fram Strait.

Abstract. In coupled biogeochmical–ocean models, the choice of numerical schemes in the ocean circulation component can have a large influence on the distribution of the biological tracers. Biogeochemical models are traditionally coupled to ocean general circulation models (OGCMs), which are based on dynamical cores employing quasi-regular meshes, and therefore utilize limited spatial resolution in a global setting. An alternative approach is to use an unstructured-mesh ocean model, which allows variable mesh resolution. Here, we present initial results of a coupling between the Finite Element Sea Ice–Ocean Model (FESOM) and the biogeochemical model REcoM2 (Regulated Ecosystem Model 2), with special focus on the Southern Ocean. Surface fields of nutrients, chlorophyll a and net primary production (NPP) were compared to available data sets with a focus on spatial distribution and seasonal cycle. The model produces realistic spatial distributions, especially regarding NPP and chlorophyll a, whereas the iron concentration becomes too low in the Pacific Ocean. The modelled NPP is 32.5 Pg C yr−1 and the export production 6.1 Pg C yr−1, which is lower than satellite-based estimates, mainly due to excessive iron limitation in the Pacific along with too little coastal production. The model performs well in the Southern Ocean, though the assessment here is hindered by the lower availability of observations. The modelled NPP is 3.1 Pg C yr−1 in the Southern Ocean and the export production 1.1 Pg C yr−1. All in all, the combination of a circulation model on an unstructured grid with a biogeochemical–ocean model shows similar performance to other models at non-eddy-permitting resolution. It is well suited for studies of the Southern Ocean, but on the global scale deficiencies in the Pacific Ocean would have to be taken into account.

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The Arctic marine ecosystem is shaped by the seasonality of the solar cycle, spanning from 24-h light at the sea surface in summer to 24-h darkness in winter. The amount of light available for under-ice ecosystems is the result of different physical and biological processes that affect its path through atmosphere, snow, sea ice and water. In this article, we review the present state of knowledge of the abiotic (clouds, sea ice, snow, suspended matter) and biotic (sea ice algae and phytoplankton) controls on the underwater light field. We focus on how the available light affects the seasonal cycle of primary production (sympagic and pelagic) and discuss the sensitivity of ecosystems to changes in the light field based on model simulations. Lastly, we discuss predicted future changes in under-ice light as a consequence of climate change and their potential ecological implications, with the aim of providing a guide for future research.

Compared to land‐based sources, mussel aquaculture provides food products with a high‐quality protein content and a low carbon footprint. At the same time, mussel cultures store nutrients in their tissue that are removed from the system through harvesting. However, increasingly development of suspended bivalve aquaculture in the coastal zone also comes with a price as the ecological carrying capacity of the ecosystem may be exceeded. The present study aims to support future fjord‐management by estimating the nutrient budgets and ecological impacts of intensified mussel aquaculture in a shallow semi‐enclosed system, the Limfjorden, using 3D ecosystem modeling. Model results showed a net removal of nutrients by suspended mussel cultures at basin scale, whereas at farm scale the efficiency was lower due to increased sediment fluxes. An increase in mussel farming from the current 4 kt‐fresh weight to a future projection of 104 kt‐fresh weight did not exceed the ecological carrying capacity with respect to impacts on sediment chemistry but could cause local declines in benthic bivalve populations. Intense mussel farming provided ecosystem services such as better oxygen conditions and higher Secchi depth together with lower nutrient‐ and chlorophyll a concentrations on basin scale. In addition, there was a redistribution of nutrients, chlorophyll a concentrations, and Secchi depth between sub‐basins in the fjord depending on farming location and intensity. Overall, intensified mussel farming could contribute to the mitigation of eutrophication effects by removing nutrients from land sources and by reducing the local sediment loading.