Atlantic cod (Gadus morhua)feeding over deep water in the high Arctic

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Ocean acidification state variability of the Atlantic Arctic Ocean around northern Svalbard

The Svalbard shelf and Atlantic Arctic Ocean are a transition zone between northward flowing Atlantic Water and ice-covered waters of the Arctic. Effects of regional ocean warming, sea ice loss and greater influence of Atlantic Water or “Atlantification” on the state of ocean acidification, i.e. calcium carbonate (CaCO3) saturation (Ω) are yet to be fully understood. Anomalies in surface layer Ω for the climatically-vulnerable CaCO3 mineral aragonite (ΔΩ) were determined by considering the variability in Ωaragonite during late summer each year from 2014 to 2017 relative to the four-year average. Greatest sea ice extent and more Arctic-like conditions in 2014 resulted in ΔΩ anomalies of −0.05 to −0.01 (up to 45% of total ΔΩ) as a result of lower primary production. Conversely, greater Atlantic Water influence in 2015 supplied the ice-free surface layer with nitrate, which prolonged primary production to drive ΔΩ anomalies of 0.01 to 0.06 (up to 45% of total ΔΩ) in more Atlantic-like conditions. Additionally, dissolution of CaCO3 increased carbonate ion concentrations giving ΔΩ anomalies up to 0.06 (up to 52% of total ΔΩ). These processes enhanced surface water Ω, which ranged between 2.01 and 2.65 across the region. Recent sea ice retreat in 2016 and 2017 (rate of decrease in ice cover of ∼4% in 30 days) created transitional Atlantic-Arctic conditions, where surface water Ω varied between 1.87 and 2.29 driven by ΔΩ anomalies of −0.10 to 0.01 due to meltwater inputs and influence of Arctic waters. Anomalies as low as −0.12 from reduced CaCO3 dissolution in 2016 further supressed Ω. Wind-driven mixing in 2017 entrained Atlantic Water with low Ω into the surface layer to drive large ΔΩ anomalies of −0.15 (up to 58% of ΔΩ). Sea-ice meltwater provided a minor source of carbonate ions, slightly counteracting dilution effects. Ice-free surface waters were substantial sinks for atmospheric CO2, where uptake of 20.5 mmol m−2 day−1 lowered surface water Ω. “Atlantification” could exacerbate or alleviate acidification of the Arctic Ocean, being highly dependent on the numerous factors examined here that are intricately linked to the sea ice-ocean system variability.publishedVersio

Numerical models and long term monitoring - How can numerical models be used to support in situ sampling and survey design for long term hydrographic monitoring in standard sections?

As a part of regular monitoring of the marine environment, IMR conducts 10 fixed transects on a multiannual basis during which hydrographical, chemical and plankton data are collected at the same positions several times a year. The transect data sets, which in some cases span up to seven decades, have been vital to the understanding of long-term variability and trends in environmental and climate conditions. As an alternative approach to assemble physical data, numerical circulation models are widely used. There is a large variety of model data archives available, both internally at the IMR and from publicly open data portals, but it is difficult to consider the precision of the different models as they have different properties, resolution, coverage area etc. This report assesses how well existing model products developed and/or intensively used by the Oceanography and Climate Research Group can be utilised to assess and support the shipboard monitoring on the transects. Main focus is on TOPAZ, which is the only fully operational model with a model domain covering all the transects considered here. The results show that TOPAZ reproduces interannual variability and multiannual trends well. However, temperature, salinity and current velocity values, as well as seasonal variability and extreme conditions are less well represented. The operational (internal) Norkyst model show the best skill reproducing current velocities, but do not cover all transects. Using TOPAZ to assess how well the present sampling strategy captures the spatial variability in hydrographic variables suggests that the current sampling strategy is well designed in terms of the horizontal spacing of the fixed transects, although the sections in the northern Norwegian Sea and southern Barents Sea show a significant co-variability of Atlantic Water towards the Arctic Ocean. Assessing the impact of sampling frequency on long-term monitoring efforts in one of the transects, suggests a minimum sampling frequency of 3-4 transects per year to prevent loss of information relating to interannual variability and trends. We note, however, that a full assessment of the impact of sampling frequency on the transects must include also chemical and plankton observations and models, as well as the need of capturing short-term variations like the seasonal cycle.Numerical models and long term monitoring - How can numerical models be used to support in situ sampling and survey design for long term hydrographic monitoring in standard sections?publishedVersio

Climate change dynamics and mercury temporal trends in Northeast Arctic cod (Gadus morhua) from the Barents Sea ecosystem

The Northeast Arctic cod (Gadus morhua) is the world's northernmost stock of Atlantic cod and is of considerable ecological and economic importance. Northeast Arctic cod are widely distributed in the Barents Sea, an environment that supports a high degree of ecosystem resiliency and food web complexity. Here using 121 years of ocean temperature data (1900–2020), 41 years of sea ice extent information (1979–2020) and 27 years of total mercury (Hg) fillet concentration data (1994–2021, n = 1999, ≥71% Methyl Hg, n = 20) from the Barents Sea ecosystem, we evaluate the effects of climate change dynamics on Hg temporal trends in Northeast Arctic cod. We observed low and consistently stable, Hg concentrations (yearly, least-square means range = 0.022–0.037 mg/kg wet wt.) in length-normalized fish, with a slight decline in the most recent sampling periods despite a significant increase in Barents Sea temperature, and a sharp decline in regional sea ice extent. Overall, our data suggest that recent Arctic amplification of ocean temperature, “Atlantification,” and other perturbations of the Barents Sea ecosystem, along with rapidly declining sea ice extent over the last ∼30 years did not translate into major increases or decreases in Hg bioaccumulation in Northeast Arctic cod. Our findings are consistent with similar long-term, temporal assessments of Atlantic cod inhabiting Oslofjord, Norway, and with recent investigations and empirical data for other marine apex predators. This demonstrates that Hg bioaccumulation is highly context specific, and some species may not be as sensitive to current climate change-contaminant interactions as currently thought. Fish Hg bioaccumulation-climate change relationships are highly complex and not uniform, and our data suggest that Hg temporal trends in marine apex predators can vary considerably within and among species, and geographically. Hg bioaccumulation regimes in biota are highly nuanced and likely driven by a suite of other factors such as local diets, sources of Hg, bioenergetics, toxicokinetic processing, and growth and metabolic rates of individuals and taxa, and inputs from anthropogenic activities at varying spatiotemporal scales. Collectively, these findings have important policy implications for global food security, the Minamata Convention on Mercury, and several relevant UN Sustainable Development Goals.publishedVersio

Atlantic Water Pathways Along the North-Western Svalbard Shelf Mapped Using Vessel-Mounted Current Profilers

A large amount of warm Atlantic water (AW) enters the Arctic as a boundary current through Fram Strait (West Spitsbergen Current [WSC]) and is the major oceanic heat source to the Arctic Ocean. Along the north‐western Svalbard shelf, the WSC splits into the shallow Svalbard Branch, the Yermak Branch that follows the slope of the Yermak Plateau, and the Yermak Pass Branch flowing across the plateau. The WSC has previously been studied using moorings, dedicated oceanographic transects, and models. In this study, we mapped the circulation patterns and AW flow around Svalbard using Vessel‐Mounted Acoustic Doppler Current Profiler data from multiple surveys during four consecutive summers (2014–2017). Despite the scattered nature of this compiled data set, persistent circulation patterns could be discerned. Spatial interpolation showed a meandering boundary current west of Svalbard and a more homogeneous AW flow, centered around the 1,000‐m isobath north of Svalbard. In all summers, we observed a northward jet between 79 and 80°N and the 1,000‐ and 500‐m isobaths, before the WSC divided into the three branches. North of Svalbard, the shallow Svalbard Branch reunited with the Yermak Pass Branch between 10 and 15°E and a part of the AW circulated within Hinlopen Trench. The calculated volume transport of 2 Sv in the upper 500 m compares well with model results and previous observations. Our results further show that the Yermak Pass Branch can be as important as the Svalbard Branch in transporting AW across the Yermak Plateau during summer.publishedVersio

Origin of marine invertebrate larvae on an Arctic inflow shelf

Many benthic invertebrate taxa possess planktonic early life stages which drift with water currents and contribute to dispersal of the species, sometimes reaching areas beyond the current ranges of the adults. Until recently, it had been difficult to identify planktonic larvae to species level due to lack of distinguishing features, preventing detection of expatriate species. Here, we used DNA metabarcoding of the COI gene to obtain species-level identification of early life stages of benthic invertebrates in zooplankton samples from the Barents Sea and around Svalbard, where, regionally, large volumes of warm Atlantic Water enter the Arctic from the south. We compared the larval community in the water column to the adult community on the seafloor to identify mismatches. In addition, we implemented particle tracking analysis to identify the possible areas of origin of larvae. Our results show that 30-45% of larval taxa—largely polychaetes and nudibranchs—were not local to the sampling area, though most were found nearby in the Barents Sea. In the particle tracking analysis, some larvae originating along the Norwegian coast were capable of reaching the northwest coast of Svalbard within 3 mo, but larvae found east of Svalbard had a more constrained possible area of origin which did not extend to the Norwegian coast. This study highlights largely regional-scale larval connectivity in the Barents Sea but demonstrates the potential for some long-lived larval taxa to travel to Svalbard and the Barents Sea from further south.publishedVersio

Origin of marine invertebrate larvae on an Arctic inflow shelf

Many benthic invertebrate taxa possess planktonic early life stages which drift with water currents and contribute to dispersal of the species, sometimes reaching areas beyond the current ranges of the adults. Until recently, it had been difficult to identify planktonic larvae to species level due to lack of distinguishing features, preventing detection of expatriate species. Here, we used DNA metabarcoding of the COI gene to obtain species-level identification of early life stages of benthic invertebrates in zooplankton samples from the Barents Sea and around Svalbard, where, regionally, large volumes of warm Atlantic Water enter the Arctic from the south. We compared the larval community in the water column to the adult community on the seafloor to identify mismatches. In addition, we implemented particle tracking analysis to identify the possible areas of origin of larvae. Our results show that 30-45% of larval taxa—largely polychaetes and nudibranchs—were not local to the sampling area, though most were found nearby in the Barents Sea. In the particle tracking analysis, some larvae originating along the Norwegian coast were capable of reaching the northwest coast of Svalbard within 3 mo, but larvae found east of Svalbard had a more constrained possible area of origin which did not extend to the Norwegian coast. This study highlights largely regional-scale larval connectivity in the Barents Sea but demonstrates the potential for some long-lived larval taxa to travel to Svalbard and the Barents Sea from further south

Fluctuating Atlantic inflows modulate Arctic atlantification

Enhanced warm, salty subarctic inflows drive high-latitude atlantification, which weakens oceanic stratification, amplifies heat fluxes, and reduces sea ice. In this work, we show that the atmospheric Arctic Dipole (AD) associated with anticyclonic winds over North America and cyclonic winds over Eurasia modulates inflows from the North Atlantic across the Nordic Seas. The alternating AD phases create a “switchgear mechanism.” From 2007 to 2021, this switchgear mechanism weakened northward inflows and enhanced sea-ice export across Fram Strait and increased inflows throughout the Barents Sea. By favoring stronger Arctic Ocean circulation, transferring freshwater into the Amerasian Basin, boosting stratification, and lowering oceanic heat fluxes there after 2007, AD+ contributed to slowing sea-ice loss. A transition to an AD− phase may accelerate the Arctic sea-ice decline, which would further change the Arctic climate system.acceptedVersio

Possible future scenarios for two major Arctic Gateways connecting Subarctic and Arctic marine systems: I. Climate and physical-chemical oceanography

We review recent trends and projected future physical and chemical changes under climate change in transition zones between Arctic and Subarctic regions with a focus on the two major inflow gateways to the Arctic, one in the Pacific (i.e. Bering Sea, Bering Strait, and the Chukchi Sea) and the other in the Atlantic (i.e. Fram Strait and the Barents Sea). Sea-ice coverage in the gateways has been disappearing during the last few decades. Projected higher air and sea temperatures in these gateways in the future will further reduce sea ice, and cause its later formation and earlier retreat. An intensification of the hydrological cycle will result in less snow, more rain, and increased river runoff. Ocean temperatures are projected to increase, leading to higher heat fluxes through the gateways. Increased upwelling at the Arctic continental shelf is expected as sea ice retreats. The pH of the water will decline as more atmospheric CO2 is absorbed. Long-term surface nutrient levels in the gateways will likely decrease due to increased stratification and reduced vertical mixing. Some effects of these environmental changes on humans in Arctic coastal communities are also presented.publishedVersio

Diversity and seasonal development of large zooplankton along physical gradients in the Arctic Barents Sea

Due to ongoing climate change, a new Arctic Ocean ecosystem is emerging. Within the framework of the Nansen Legacy project, we investigated the community composition of the large zooplankton and its seasonal development along a latitudinal gradient in the northern Barents Sea. Total biomass was maximal in summer and early winter, and minimal in spring, with copepods contributing considerably in all seasons. Euphausiids represented a minor fraction of the biomass, whereas chaetognaths and other gelatinous zooplankton contributed substantially to the sampled zooplankton at all stations, particularly in winter. Amphipod biomass was high in early winter, but otherwise low. Temperature in the water column interior and bottom-depth had the highest explanatory power for the community composition of the large zooplankton, both revealing the same distinct Atlantic and Arctic domains along the studied section. The continental shelf of the northern Barents Sea had an Arctic signature and was in terms of biomass characterized by a dominance of cold-water species, such as Themisto libellula, and Calanus glacialis. The copepod Calanus hyperboreus was the dominant over the continental slope. Locations at the southern and northern end of the studied section were influenced by Atlantic Water (at intermediate depth at the northern stations), and contained a mixture of temperate species, deep-water species, and sympagic amphipods in northern ice-covered waters. In the northern Barents Sea, a seasonal change was observed in the biomass fractions of different zooplankton feeding guilds, with dominance of herbivores in summer and carnivores in winter. This suggests switching between bottom-up and top-down control through the year. On the continental slope, species that are typically considered omnivores seemed to increase in importance. The role of seasonally changing food preferences to bridge periods outside of the main primary production season is discussed in light of ecosystem resilience to the expected changes in the Arctic Ocean.publishedVersio