We assessed responses of Arctic Isochrysis sp. grown under a matrix of temperature (2°C vs. 6°C), light intensity (55 vs. 160 μ mol photons m −2 s −1 ) and pCO 2 (400 vs. 1000 μ atm CO 2 ). Next to acclimation parameters (growth rates, particulate and dissolved organic C and N, chlorophyll a content), we measured physiological processes in vivo (electron transport rates and net photosynthesis) using fast‐repetition rate fluorometry and membrane‐inlet mass spectrometry. Within the applied driver ranges, elevated temperature had the most pronounced impacts, significantly increasing growth (~ 40%) and particulate organic carbon production (up to ~ 140%). Light stimulations manifested prominently under high temperature (~ 30–35%), underlining its role as a “master‐variable.” pCO 2 was the least effective driver, exerting mostly insignificant effects. The obtained data were used for a simplistic upscaling simulation to investigate potential changes in Isochrysis ' bloom dynamics in the Fram Strait with increasing temperatures over the 21 st century. Results suggest that global warming will accelerate bloom dynamics, with earlier onsets of blooms and higher peak biomasses. Despite remaining uncertainties about the magnitude of these effects, data strongly suggest that increasing temperatures over the coming century will affect the phenology of Isochrysis and other Arctic phytoplankton with likely important implications for higher trophic levels.

Abstract. Fluxes of nutrients at the ice-ocean interface are affected by the smooth or turbulent nature of the flow under the ice. The nature of the flow depends on the friction velocity, which determines the thickness of the laminar sublayer, and the roughness of the ice surface. Based on in situ boundary layer studies, the range of variability of the thickness of the laminar sublayer and that of surface roughness suggest that the flow under sea ice may easily shift from smooth to turbulent. This transition enhances nutrient exchanges at the ice-ocean interface. Despite the importance of such turbulent nutrient exchanges for sea ice algae, no current biogeochemical model accounts for the dependence of fluxes on the nature of the flow, while different approaches were previously implemented to compensate for the perceived overestimation of the nutrient limitation of ice algae growth. In the present study, we implement and test a Reynolds number-based parameterization that accounts for shifts between smooth and turbulent flow, weighing the contributions of viscosity and turbulence, in two sea-ice biogeochemical models. The results of three different case studies show that with increasing roughness, the turbulent nature of the flow contributes to larger fluxes of nutrients from the ocean to the ice. Nutrients accumulate during the winter, up to concentrations comparable to surface waters of the ocean. However, when light levels are sufficient to initiate algal growth, enhanced fluxes can support higher total production over a longer period, resulting in biomass accumulation more than twice that achieved under smooth flow conditions. In nutrient-rich waters, turbulence can supply sufficient nutrients to bring model outputs closer to observations. However, other processes, such as brine drainage in the vertically resolved model, appear to limit agreement between the two models. Our parameterization provides a more realistic representation of nutrient exchange at the sea ice–ocean interface, avoiding the need to “overtune” other model processes to reproduce observations.

Abstract Over the satellite period, Arctic and Antarctic sea ice extent seemed to follow opposite pathways. Arctic sea ice showed a strong and prolonged decrease until 2007 and then stalled in its rate of decline. Antarctic sea ice extent, on the other hand, displayed a small but significant increase until 2015. After 2015, Antarctic sea ice extent experienced a sharp decline, and has since been characterized by a lower mean state with enhanced variability, suggested as a new sea-ice regime. Arctic sea ice has continued transitioning towards a regime of substantially lower summertime coverage with reduced multiyear ice, increasingly resembling Antarctic sea ice in conditions and seasonality. We aim to provoke discussion by suggesting that the parallel changes in Arctic and Antarctic sea-ice behavior share a common mechanism: the breakdown of ocean stratification sustaining sea-ice decline, whilst episodic atmospheric forcing increasingly influences high-frequency sea-ice variability. In both the Antarctic and the Eurasian Basin of the Arctic, positive surface layer salinity anomalies have caused a weakening of the ocean halocline. The associated erosion of stratification makes sea ice more vulnerable to subsurface heat, and appears to underpin recent sea-ice loss and elevated variability. The convergence of the polar regions toward a seasonally dominated sea-ice regime leads us to call for continued and enhanced knowledge transfer and coordination between polar communities to accelerate understanding of recent sea-ice changes. A better understanding of the mechanisms driving changes in the coupled atmosphere–ocean–sea-ice system is needed to improve projections of future sea-ice loss and assess the cascading risks it poses to global climate and regional ecosystems.

Sea-ice biogeochemical models are key to understanding polar marine ecosystems. We present an intercomparison of six one-dimensional models, assessing their ability to simulate algal phenology and nutrient dynamics using physical-biogeochemical data from an Arctic drift expedition in spring 2015. While no model fully captured observed bloom dynamics with default settings, tuning improved biomass but had a limited impact on nutrients. The experiment revealed challenges in simulating short-lived, dynamic ice habitats, which are expected to become more common in a changing Arctic. Variability in tuning strategies underscores key knowledge gaps and highlights the need for coordinated future model developments to improve reliability and predictive capacity.

The need to understand how the ongoing rapid sea-ice changes affect marine biogeochemistry and ecosystems fosters the development of numerical models of sea-ice biogeochemistry. We review how models can be used to understand and upscale processes, to project them into the future and to study their impacts on other components of the Earth System. The focus is then put on the particular case of the one-dimensional modelling of sea-ice algae. A general framework for biogeochemical tracers in sea ice is introduced. The technical choices to be done are explained. In particular, we show how sea-ice physical and biogeochemical processes are typically parameterized and illustrate how this shapes the model seasonality and vertical distribution of sea-ice algae. We conclude by reviewing the current model limitations and the future research perspectives.

Seasonal vertical migration of large lipid-rich copepods is often described as a mass descent of animals when primary production ceases, with important implications for mesopelagic food webs and global carbon sequestration. This view ignores the existence of surface-resident individuals, but here we show that non-migrants can form a substantial part of the populations of polar migrant species. In the Central Arctic Ocean, the biomass-dominant Calanus hyperboreus was evenly distributed throughout the water column from November 2019 to March 2020, with ~20% of subadults and adult females remaining in the upper 200 m and ~41% migrating to 1000-2000 m. These vertical positions aligned with differences in the copepods' cholesterol content, which can enhance the tissue density at higher temperatures. Gonad development and the vertical distribution of their offspring indicate that both non-migrant and migrant females contribute to the population recruitment. We reinterpret copepod seasonal migration as a bet-hedging strategy that balances nutritional benefits near the surface with survival benefits at depth, and thereby contributes to the species' resilience under climatic change.

Arctic sea ice is highly heterogeneous and composed of a mosaic of different habitats. Our understanding of the impact of climate change on Arctic sea ice and especially on the ice-associated ecosystems is hindered by both a lack of data and a limited understanding of the processes associated with different sea-ice habitats. In particular sea-ice ridges are one of the most under-sampled and poorly understood components of the Arctic sea-ice system. During a spring campaign in the Arctic Ocean, we combined a number of sampling approaches to quantify: 1) the spatial variability of sea-ice algae at single floe and multiple floe scales; 2) the contribution of ridges to ice algal spatial variability; and 3) the role of ridges in shaping the sea ice as a habitat. For upscaling purposes, algal biomass retrieved from ice cores was compared with biomass estimates based on under-ice profiles covering a total of 36 km. Our results show that the level-ice spatial variability measured on a single ice floe can be representative of the larger scale variability. However, only when ridges are included in the analysis we are able to obtain a comprehensive picture of the large-scale ice algal biomass variability. In spring, ridges let more light pass through the ice due to their geometry and their effects on snow distribution, they thus offer a potentially favorable environment for algae to grow within, and they can act as funnels of light for pelagic organisms. On a large scale, ridges contribute more than 50% percent of the potential habitable space for ice algae for snow-covered Arctic sea ice in spring.

Pressure ridges, formed by sea ice deformation, affect momentum transfer in the Arctic Ocean and support a larger biomass than the surrounding-level ice. Although trends in Arctic sea ice thickness and concentration are well documented, changes in ridge morphology remain unclear. This study provides airborne-based evidence of a shift towards a smoother ice surface, with fewer pressure ridges and reduced surface drag, attributed to the loss of old ice. Furthermore, an increase in seasonal ice cover enhances overall deformation in the Arctic and acts as a negative feedback mechanism on pan-Arctic ridge morphology: the greater the proportion of seasonal ice, the higher the pan-Arctic mean ridge rate, dampening an overall decline in ridges with age. While thinner and less frequent ridges benefit industries such as shipping, these changes are likely to have profound impacts on the energy and mass balance and the ecosystem of the Arctic Ocean. Pressure ridges, a characteristic feature of Arctic sea ice, play an important role in the ecosystem but pose challenges to shipping. Here the authors use aircraft measurements to document a decline in both the frequency and height of these pressure ridges in recent decades.

The international and interdisciplinary sea-ice drift expedition "The Multidisciplinary drifting Observatory for the Study of Arctic Climate" (MOSAiC) was conducted from October 2019 to September 2020. The aim of MOSAiC was to study the interconnected physical, chemical, and biological characteristics and processes from the atmosphere to the deep sea of the central Arctic system. The ecosystem team addressed current knowledge gaps and explored unknown biological properties over a complete seasonal cycle focusing on three major research areas: biodiversity, biogeochemical cycles, and linkages to the environment. In addition to the measurements of core properties along a complete seasonal cycle, dedicated projects covered specific processes and habitats, or organisms on higher taxonomic or temporal resolution in specific time windows. A wide range of sampling instruments and approaches, including sea-ice coring, lead sampling with pumps, rosette-based water sampling, plankton nets, remotely operated vehicles, and acoustic buoys, was applied to address the science objectives. Further, a broad range of process-related measurements to address, for example, productivity patterns, seasonal migrations, and diversity shifts, were made both in situ and onboard RV Polarstern. . This article provides a detailed overview of the sampling approaches used to address the three main science objectives. It highlights the core sampling program and provides examples of habitat- or process-specific sampling. The initial results presented include high biological activities in wintertime and the discovery of biological hotspots in underexplored habitats. The unique interconnectivity of the coordinated sampling efforts also revealed insights into cross-disciplinary interactions like the impact of biota on Arctic cloud formation. This overview further presents both lessons learned from conducting such a demanding field campaign and an outlook on spin-off projects to be conducted over the next years.

Polar cod (Boreogadus saida) is an endemic key species of the Arctic Ocean ecosystem. The ecology of this forage fish is well studied in Arctic shelf habitats where a large part of its population lives. However, knowledge about its ecology in the central Arctic Ocean (CAO), including its use of the sea-ice habitat, is hitherto very limited. To increase this knowledge, samples were collected at the under-ice surface during several expeditions to the CAO between 2012 and 2020, including the Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) expedition. The diet of immature B. saida and the taxonomic composition of their potential prey were analysed, showing that both sympagic and pelagic species were important prey items. Stomach contents included expected prey such as copepods and amphipods. Surprisingly, more rarely observed prey such as appendicularians, chaetognaths, and euphausiids were also found to be important. Comparisons of the fish stomach contents with prey distribution data suggests opportunistic feeding. However, relative prey density and catchability are important factors that determine which type of prey is ingested. Prey that ensures limited energy expenditure on hunting and feeding is often found in the stomach contents even though it is not the dominant species present in the environment. To investigate the importance of prey quality and quantity for the growth of B. saida in this area, we measured energy content of dominant prey species and used a bioenergetic model to quantify the effect of variations in diet on growth rate potential. The modeling results suggest that diet variability was largely explained by stomach fullness and, to a lesser degree, the energetic content of the prey. Our results suggest that under climate change, immature B. saida may be at least equally sensitive to a loss in the number of efficiently hunted prey than to a reduction in the prey's energy content. Consequences for the growth and survival of B. saida will not depend on prey presence alone, but also on prey catchability, digestibility, and energy content.

Microalgae are the main source of the omega-3 fatty acids eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), essential for the healthy development of most marine and terrestrial fauna including humans. Inverse correlations of algal EPA and DHA proportions (% of total fatty acids) with temperature have led to suggestions of a warming-induced decline in the global production of these biomolecules and an enhanced importance of high latitude organisms for their provision. The cold Arctic Ocean is a potential hotspot of EPA and DHA production, but consequences of global warming are unknown. Here, we combine a full-seasonal EPA and DHA dataset from the Central Arctic Ocean (CAO), with results from 13 previous field studies and 32 cultured algal strains to examine five potential climate change effects; ice algae loss, community shifts, increase in light, nutrients, and temperature. The algal EPA and DHA proportions were lower in the ice-covered CAO than in warmer peripheral shelf seas, which indicates that the paradigm of an inverse correlation of EPA and DHA proportions with temperature may not hold in the Arctic. We found no systematic differences in the summed EPA and DHA proportions of sea ice versus pelagic algae, and in diatoms versus non-diatoms. Overall, the algal EPA and DHA proportions varied up to four-fold seasonally and 10-fold regionally, pointing to strong light and nutrient limitations in the CAO. Where these limitations ease in a warming Arctic, EPA and DHA proportions are likely to increase alongside increasing primary production, with nutritional benefits for a non-ice-associated food web.

We assessed the responses of solitary cells of Arctic Phaeocystis pouchetii grown under a matrix of temperature (2 degrees C vs. 6 degrees C), light intensity (55 vs. 160 mu mol photons m(-2) s(-1)) and pCO(2) (400 vs. 1000 mu atm CO2, i.e., 40.5 vs. 101.3 Pa). Next to acclimation parameters (growth rates, particulate and dissolved organic C and N, Chlorophyll a content), we measured physiological processes in vivo (electron transport rates and net photosynthesis) using fast-repetition rate fluorometry and membrane-inlet mass spectrometry. Within the applied driver ranges, elevated temperature had the most pronounced impacts, significantly increasing growth, elemental quotas and photosynthetic performance. Light stimulations manifested more prominently under 6 degrees C, underlining temperature's role as a "master-variable". pCO(2) was the least effective driver, exerting mostly insignificant effects. The obtained data were used for a simplistic upscaling simulation to investigate potential changes in P. pouchetii's bloom dynamics in the Fram Strait with increasing temperatures over the 21(st) century. Although solitary cells might not be fully representative of colonial cells commonly observed in the field, our results suggest that global warming accelerates bloom dynamics, with earlier onsets of blooms and higher peak biomasses. Such a temperature-induced acceleration in the phenology of Phaeocystis and likely other Arctic phytoplankton might cause temporal mismatches, e.g., with the development of grazers, and therefore substantially affect the biogeochemistry and ecology of the Arctic.

As Arctic sea ice deteriorates, more light enters the ocean, causing largely unknown effects on the ecosystem. Using an autonomous biophysical observatory, we recorded zooplankton vertical distribution under Arctic sea ice from dusk to dawn of the polar night. Here we show that zooplankton ascend into the under-ice habitat during autumn twilight, following an isolume of 2.4 × 10 −4 W m −2 . We applied this trigger isolume to CMIP6 model outputs accounting for incoming radiation after sunset and before sunrise of the polar night. The models project that, in about three decades, the total time spent by zooplankton in the under-ice habitat could be reduced by up to one month, depending on geographic region. This will impact zooplankton winter survival, the Arctic foodweb, and carbon and nutrient fluxes. These findings highlight the importance of biological processes during the twilight periods for predicting change in high-latitude ecosystems.

Abstract As Arctic sea ice deteriorates, more light enters the Arctic Ocean, causing largely unknown effects on the ecosystem. A novel autonomous bio-physical observatory provided the first record of zooplankton vertical distribution under sea ice drifting across the Arctic Ocean from dusk to dawn of the polar night. Its measurements revealed that zooplankton ascend into the under-ice layer during autumn twilight, following an isolume of 5.5 10-4 W m-2. We applied this trigger isolume to IPCC models enabled to incorporate incoming radiation after sunset and before sunrise of the polar night. The models project that, in about three decades, the total time spent by zooplankton in the under-ice layer will be reduced by up to one month, depending on geographic region. This will impact zooplankton winter survival, the Arctic foodweb, carbon- and nutrient fluxes. These findings highlight the importance of processes in the twilight periods for predicting change in high-latitude ecosystems.

Arctic cod (Boreogadus saida) is the most abundant forage fish in the Arctic Ocean. Here we review Arctic cod habitats, distribution, ecology, and physiology to assess how climate change and other anthropogenic stressors are affecting this key species. This review identifies vulnerabilities for different life stages across the entire distribution range of Arctic cod. We explore the impact of environmental (abiotic and biotic) and anthropogenic stressors on Arctic cod with a regional perspective in a scenario up to the year 2050 and identify knowledge gaps constraining predictions. Epipelagic eggs and larvae are more vulnerable to climate change and stressors than adults. Increased water temperatures, sea-ice decline, altered freshwater input, acidification, changing prey field, increased interspecific competition, new predators, and pollution are the principal stressors that will affect Arctic cod populations. Detrimental effects are likely to be greater in regions characterized by the advection of warmer Atlantic and Pacific waters. In contrast, Arctic cod may benefit from ocean warming in colder areas of the High Arctic. The risk from fisheries is moderate and primarily limited to bycatch. Overall, a decrease in suitable habitat and an associated decline in total Arctic cod biomass are predicted. In most Arctic seas, the relative abundance of Arctic cod within the fish community will likely fluctuate in accordance with cold and warm periods. A reduced abundance of Arctic cod will negatively affect the abundance, distribution, and physiological condition of certain predators, whereas some predators will successfully adapt to a more boreal diet. Regional management measures that recognize the critical role of Arctic cod are required to ensure that increased anthropogenic activities do not exacerbate the impacts of climate change on Arctic marine ecosystems. Ultimately, the mitigation of habitat loss for Arctic cod will only be achieved through a global reduction in carbon emissions.

Low-salinity meltwater from Arctic sea ice and its snow cover accumulates and creates under-ice meltwater layers below sea ice. These meltwater layers can result in the formation of new ice layers, or false bottoms, at the interface of this low-salinity meltwater and colder seawater. As part of the Multidisciplinary drifting Observatory for the Study of the Arctic Climate (MOSAiC), we used a combination of sea ice coring, temperature profiles from thermistor strings and underwater multibeam sonar surveys with a remotely operated vehicle (ROV) to study the areal coverage and temporal evolution of under-ice meltwater layers and false bottoms during the summer melt season from mid-June until late July. ROV surveys indicated that the areal coverage of false bottoms for a part of the MOSAiC Central Observatory (350 by 200 m2) was 21%. Presence of false bottoms reduced bottom ice melt by 7–8% due to the local decrease in the ocean heat flux, which can be described by a thermodynamic model. Under-ice meltwater layer thickness was larger below first-year ice and thinner below thicker second-year ice. We also found that thick ice and ridge keels confined the areas in which under-ice meltwater accumulated, preventing its mixing with underlying seawater. While a thermodynamic model could reproduce false bottom growth and melt, it could not describe the observed bottom melt rates of the ice above false bottoms. We also show that the evolution of under-ice meltwater-layer salinity below first-year ice is linked to brine flushing from the above sea ice and accumulating in the meltwater layer above the false bottom. The results of this study aid in estimating the contribution of under-ice meltwater layers and false bottoms to the mass balance and salt budget for Arctic summer sea ice.

The Arctic is no longer a region dominated by thick multi-year ice (MYI), but by thinner, more dynamic, first-year-ice (FYI). This shift towards a seasonal ice cover has consequences for the under-ice light field, as sea-ice and its snow cover are a major factor influencing radiative transfer and thus, biological activity within- and under the ice. This work describes in situ measurements of light transmission through different types of sea-ice (MYI and FYI) performed during two expeditions to the Chukchi sea in August 2018 and 2019, as well as a simple characterisation of the biological state of the ice microbial system. Our analysis shows that, in late summer, two different states of FYI exist in this region: 1) FYI in an enhanced state of decay, and 2) robust FYI, more likely to survive the melt season. The two FYI types have different average ice thicknesses: 0.74 ± 0.07 m (N = 9) and 0.93 ± 0.11 m (N = 9), different average values of transmittance: 0.15 ± 0.04 compared to 0.09 ± 0.02, and different ice extinction coefficients: 1.49 ± 0.28 and 1.12 ± 0.19 m−1. The measurements performed over MYI present different characteristics with a higher average ice thickness of 1.56 ± 0.12 m, lower transmittance (0.05 ± 0.01) with ice extinction coefficients of 1.24 ± 0.26 m−1 (N = 12). All ice types show consistently low salinity, chlorophyll a concentrations and nutrients, which may be linked to the timing of the measurements and the flushing of melt-water through the ice. With continued Arctic warming, the summer ice will continue to retreat, and the decayed variant of FYI, with a higher scattering of light, but a reduced thickness, leading to an overall higher light transmittance, may become a more relevant ice type. Our results suggest that in this scenario, more light would reach the ice interior and the upper-ocean.