Abstract. The size of phytoplankton is a key metric that influences organic carbon production in the ocean and its transfer efficiency through marine food webs. There is evidence that the size structure of phytoplankton in Antarctic waters differs markedly from that in tropical oceans. However, logistical challenges have left many Antarctic regions poorly sampled. Here, we explore a dataset collected during an austral summer research cruise to the rarely sampled northwestern Weddell Sea continental shelf and open waters near the eastern Antarctic Peninsula. The dataset includes discrete measurements of size-fractionated chlorophyll-a concentration (pico- <2 μm, nano- 2–20 μm, and microphytoplankton >20 μm) obtained using in vitro fluorescence and sequential size-fractionated filtration (SFF), phytoplankton pigments measured by high-performance liquid chromatography (HPLC), and in situ radiometric (ocean-colour) observations. We find broad agreement between HPLC-based (using diagnostic pigment analysis) and SFF-based estimates of total and size-fractionated chlorophyll-a, but with systematic differences for some size classes depending on the technique used. Ocean-colour chlorophyll-a algorithms developed in other regions of the Southern Ocean perform well in the northwestern Weddell Sea and outperform standard global algorithms, which systematically underestimate chlorophyll-a. SFF measurements indicate that medium-sized phytoplankton (nanophytoplankton) dominate both shelf and open waters of the northwestern Weddell Sea. We fitted a simple three-component model to the SFF data describing the partitioning of chlorophyll-a among the three size classes as a function of total chlorophyll-a. Model parameters were similar to those derived from an independent dataset from the western Antarctic Peninsula, supporting the broader dominance of nanophytoplankton around the entire Antarctic Peninsula, but differed markedly from parameters derived from lower-latitude tropical and subtropical (50° N to 50° S) Atlantic waters, where picophytoplankton dominate. Finally, we combined satellite-derived estimates of total chlorophyll-a during the cruise period with the tuned model parameters to map the spatial distribution of phytoplankton size classes across the broader northwestern Weddell Sea region, which confirmed the widespread dominance of nanophytoplankton. These datasets provide a valuable baseline for quantifying the rate and fate of organic carbon production in this rapidly changing region.

Antarctic krill, Euphausia superba, is an important species in the Southern Ocean and is vulnerable to climate change. We studied the carbon sources for krill-specifically pelagic and ice-associated (sympagic) particulate organic matter (POM), representing phytoplankton and sea-ice algae, respectively-during March 2019 across three distinct areas of the Kong H & aring;kon VII Sea (KHS) in the eastern Weddell Gyre. Analyses of delta 13C and delta 15N composition revealed large geographic differences in pelagic POM that were reflected in krill across life stages. At Astrid Ridge (the eastern study area), elevated pelagic POM delta 13C suggested that sea-ice algae seeding influenced phytoplankton development, in line with the more extensive sea-ice cover in this area and with the dominance of typically ice-associated pennate diatoms. This was also reflected in the elevated krill delta 13C compositions at Astrid Ridge as compared to the other two areas. In contrast, in the western areas (Maud Rise and 6 degrees East), lower delta 13C values in both pelagic POM and krill suggested limited influence of sea-ice algae on phytoplankton communities and krill feeding. Moreover, no significant difference in the delta 13C and delta 15N compositions was found among krill genders and life stages throughout the study area. These geographical, rather than ontogenetic, differences suggest that krill of all life stages feed opportunistically on the most available algal sources in early autumn in the KHS. Considering the ongoing reduction in the seasonal sea-ice cover, continued research on the dietary plasticity of krill is essential to understand the resilience and health of krill stocks in a changing Southern Ocean.

Abstract Human-induced global climate change and other anthropogenic stressors are fundamentally altering our oceans. Understanding the ecological and societal implications of these changes is critical for developing mitigation strategies and conservation measures. However, major components of the marine pelagic ecosystem remain poorly understood. This is true for euphausiids (“krill”), which are a crucial part of marine food webs and play an important role in elemental cycling, including in the biological carbon pump, but for which we know surprisingly little. In this review, we first provide an overview of the ecological and socio-economic value of krill, highlighting their function in marine food webs and biogeochemical cycling. Next, we describe what is currently known regarding the response of krill to climate change and other anthropogenic stressors, focusing on changes in their biogeography, physiology, life history, as well as the impacts of krill fishing and their association with pathogens and parasites. We identify five key gaps in our current knowledge of krill: (1) the effects of krill on food web dynamics and stability, (2) the effects of changing predator and/or prey communities on krill populations, (3) the identification of important krill habitats, (4) the understanding of vertical and horizontal range shifts, and (5) the combined effects of multiple climate change and other anthropogenic stressors on krill. We also highlight the krill species, regions, and habitats that are understudied. Finally, we propose strategies to improve our understanding of this ecologically important taxonomic group, including the sustained funding for time series; implementation of novel research technologies; expanding research on understudied species and regions; and creating a global community of krill researchers.

The Weddell Sea, Southern Ocean, is a highly productive location of deep-water formation and a globally important site of carbon sequestration. Here, the biological carbon pump is dominated by carbon-rich particulates which are both actively and passively transported to deep water (e.g. zooplankton faecal pellets and phytoplankton detritus). However, climate driven changes in sea ice have the potential to disrupt these processes, highlighting a need for contemporary observations. This study quantified the flux of particulate organic carbon (POC) and nitrogen (PON) across three depths (50, 100, 150 m) at five locations (including shelf, off shelf, ice covered and ice-free environments) in the western Weddell Sea using a drifting sediment trap. POC and PON fluxes were greater on shelf than off-shelf, likely reflecting increased nutrient supply and productivity on shelf. No strong patterns between sea ice and ice-free stations were present, likely because the ice pack was constantly shifting, with most sites influenced by sea ice. The POC flux remained stable or increased with depth at most stations, ranging from 42.5–364.1 mg C m−2 d−1 (mean of 123.2 mg C m−2 d−1). Krill faecal pellets represented 98 % of all pellets, which contributed an estimated 17 %–99 % (median of 48 %) of the POC flux. The faecal pellet flux peaked at 100 m across the shelf, suggesting krill defecating at depth effectively counteracted attenuation in the upper ocean. Our findings emphasise the importance of zooplankton-mediated processes in determining the particle flux and the benefits of resolving the vertical flux at a resolution which incorporates their ecology. It is unclear how changing sea ice dynamics will impact zooplankton, so a process-driven understanding of biogeochemical fluxes is integral for predicting the future of carbon cycling in the Southern Ocean.

Abstract. Antarctic krill (Euphausia superba, hereafter "krill") are an important component of Southern Ocean food webs, are efficient in sequestering carbon and support a major fishery. Knowledge of their early life cycle is key to understanding krill population dynamics and essential for fisheries management in a warming climate. Many data have been collected over the years on the distribution of krill larvae, but the data remain fragmented and hard to re-use. Here we have put these disparate data sources together into a large database of 10,762 net-sampling records with numerical abundance data on the various larval stages. This new KRILLBASE-larvae database complements two existing and circumpolar KRILLBASE open-access databases, namely KRILLBASE-abundance (numerical abundance of postlarval krill and salps) and KRILLBASE-length frequency (length, sex and maturity stage of postlarval krill). By completing the set to include larvae, we provide datasets that can underpin a more holistic appreciation of krill dynamics; for example to model the krill life cycle, population dynamics, response to climate change and to help manage the krill fishery. KRILLBASE-larvae is circumpolar, albeit with most data concentrated in the SW Atlantic sector which appears to be the major spawning ground and where the fishery operates. The data span 1926–2024 with >50 seasons of coverage spanning two epochs: 1926–1937 and 1976–2024. The database is based on net haul data on densities (numbers per m-2) of eggs, nauplii, metanauplii, calypotope- and furcilia stages, alongside key sampling information such as sampling depths, net type, net mesh size, water depth, temperature etc. This data paper provides a description of KRILLBASE-larvae, mapping data coverage in terms of space, time and sampling depth, providing pointers and caveats to its use. The KRILLBASE-larvae database is available here for reviewers: http://ramadda.data.bas.ac.uk/repository/entry/show?entryid=946546c8-b24f-422f-96e1-3bd872506c5f with user id reviewer_02221 and password VchJWSANpH1T5Wpj with instructions https://www.bas.ac.uk/data/polar-data-centre/reviewer-access/ [Pending publication and any subsequent amendment on review, the final dataset will be freely available with a doi and single click to download]. We request that this data doi and the data paper are cited when the data are used.

Ocean warming is projected to threaten fisheries, but the extent varies greatly between models due to a poor understanding of how complex food webs respond to change. Likewise, inequalities in socioeconomic dependence on fisheries and uneven distributions of global fishing effort make it unclear how the distribution of fisheries declines could translate into socioeconomic impacts. Here we developed a quantitative IPCC tripartite risk mapping approach, combining hazard (projected pelagic fish decline), exposure (present day pelagic fishing intensity), and vulnerability (national dependence on fisheries) to generate global maps of pelagic fishery climate risk. Using a direct, empirical method of projecting fish trends based on plankton size-spectra, our risk mapping approach identifies fishing grounds across Southeast Asia, western seaboards of Africa and South America and adjacent islands at highest risk. We project substantial declines ( ∼ 20%) in supportable fish biomass by the end of the century under a high emission scenario, which could be reduced (to ∼ 10%) by strong global climate mitigation measures. With climate mitigation being the only clear route to curbing declines in fisheries carrying capacity, our approach guides the spatial prioritisation of broader mitigation measures which reduce other human pressures on fish stocks, including effort control and the development of protected areas which support essential fish habitats. High-risk regions extend into Areas Beyond National Jurisdiction, highlighting the potential for the upcoming High Seas Treaty to introduce fisheries management measures which serve to offset a substantial socioeconomic risk under climate change. ### Competing Interest Statement The authors have declared no competing interest.

High‐latitude zooplankton can sequester millions of tons of carbon due to their seasonal migration from the surface ocean to depth, and their respiration and mortality during overwintering. This seasonal vertical migration pump (SVMP) efficiently removes carbon but not limiting nutrients such as iron from the surface layers. However, this process is not included in Earth System Models and whole Southern Ocean estimates are still lacking. Here, we compile large datasets of Southern Ocean zooplankton biomass and physiology to estimate that the SVMP transports 65 Mt carbon annually to sequestration‐achieving depths of > 500 m. Mesozooplankton are the main agents (80%), followed by krill (14%), and salps (6%), with respiration and mortality at depth contributing a similar share. This SVMP adds greatly to existing modeled or measured estimates of Southern Ocean carbon sequestration, equating to 38–56% of particulate organic carbon flux at 500 m and 78–103% of the flux at 1000 m. Given their large biomass but projected change under polar warming, understanding how zooplankton transport carbon and nutrients will underpin improved model projections of ocean carbon storage in a warmer world.

Melt ponds are a common feature of the Arctic sea-ice environment during summer, and they play an important role in the exchange of heat and water vapor between the ocean and the atmosphere. We report the results of a time-series study of the CO2 dynamics within melt ponds (and nearby lead) and related fluxes with the atmosphere during the summer-to-autumn transition in the central Arctic Ocean during the Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) expedition. In late summer 2020, low-salinity meltwater was distributed throughout the melt ponds, and undersaturation of pCO2 in the meltwater drove a net influx of CO2 from the atmosphere. The meltwater layer subsequently thinned due to seawater influx, and a strong gradient in salinity and low-pCO2 water was observed at the interface between meltwater and seawater at the beginning of September. Mixing between meltwater and underlying seawater drives a significant drawdown of pCO2 as a result of the non-linearities in carbonate chemistry. By the middle of September, the strong stratification within the meltwater had dissipated. Subsequent freezing then began, and cooling and wind-induced drifting of ice floes caused mixing and an influx of seawater through the bottom of the melt pond. The pCO2 in the melt pond reached 300 µatm as a result of exchanging melt pond water with the underlying seawater. However, gas exchange was impeded by the formation of impermeable freshwater ice on the surface of the melt pond, and the net flux of CO2 was nearly zero into the pond, which was no longer a sink for atmospheric CO2. Overall, the melt ponds in this Arctic sea-ice area (both melt ponds and lead water) act as moderate sinks for atmospheric CO2.

The rapidly changing Arctic has led to evolving natural aerosol emissions, which have strong feedback effects on its climate system. Sea spray aerosol (SSA) particles are a main source of aerosols; they influence cloud formation and cloud properties. Ocean microbiota potentially have an impact on SSA production and flux, but our understanding of the mechanisms is still limited. The potential impact of ocean microbiota on SSA fluxes is still a matter of active research. In this multidisciplinary study conducted in the central Arctic during The Multidisciplinary Drifting Observatory for the Study of Arctic Climate expedition, air-sea interactions were measured by means of in situ bubble-bursting experiments. For the first time, we studied the effect of zooplankton grazing on aerosol production. We found that surface water subjected to zooplankton grazing had a 2-fold increase in SSA production relative to controls without zooplankton. Our biogeochemical results suggest that fresh organic material-possibly dissolved organic carbon and dissolved organic nitrogen from humic-like substances-influences aerosol production. We find a strong chemical-selective process affecting SSA production, with humic-like substances much enriched (40%-280%) relative to protein-like material in the aerosol relative to the water. Our results point to a complex relationship between specific low-concentration organic components in the water phase and its relative capability of being aerosolized, with strong chemically selective processes affecting SSA production. We provide evidence for the role of zooplankton, implying a complex microbial loop that may be regulating the organic component of the SSA production. This study underlines the role of zooplankton in biogeochemical cycles, due to their large biomass and transformation of organic matter during feeding and digestion. Similar processes may occur in other regions with high zooplankton grazing impact, for example, in productive parts of the Southern Ocean and in upwelling regions.

Antarctic sea ice showed a profound, step-wise reduction around 2016-2017, but the scarcity of modern time series means that we know very little about how Southern Ocean biota have responded. Given the paucity of direct field data, we combined satellite data with KRILLBASE, a large historical salp and krill database, to examine how the new, low-ice era has changed the feeding habitat of these key plankton species. Most (69%) of the Southern Ocean area has experienced an increase in mean summer chlorophyll a (chl a) values since the step-change in sea ice. We went beyond bulk chl a indices to classify the Southern Ocean into 14 optical water types based on their spectral reflectance. At this finer resolution, the 2 species showed more habitat differentiation than discernible from chl a alone, with salps being strongly correlated with those optical water types that increased in area after the 2017 reduction in sea ice. These water types have moderate phytoplankton concentrations (similar to 0.4 mg chl a m(-3)), and large expanses of the Indian-Pacific sector improved from being too oligotrophic into better feeding habitats for salps. We also show fundamental differences in habitat requirements of both species between the Atlantic and the Indian-Pacific sectors, but overall, our feeding habitat indices suggested that the modern low-ice era has become more favourable for salps. Antarctic sea ice is a crucial part of the climate system, and the recent era of extreme variability and record lows has major ramifications for food webs and biogeochemical cycles.

Plankton models form the core of marine ecosystem simulators, with uses from regional resource and ecosystem management to climate change projections. In this Perspective, we suggest that stronger alignment of models with empirical knowledge about plankton physiology, diversity and trophic roles will improve model utility and the reliability of their outputs regarding biodiversity, ecophysiology, trophic dynamics and biogeochemistry. We recommend key steps to resolve the disconnect between empirical research and simulation models accounting for well-established plankton processes with an aim to increase the utility of such models for applied uses. A central challenge is characterizing the complexity of plankton diversity and activity in ways that are amenable to model incorporation. We argue that experts in empirical science are best placed to advise the development of next-generation models to address these challenges, and we propose a series of actions to achieve that engagement, including involvement of these experts in the design and exploitation of plankton digital twins.

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.

Melt pond is a common and important feature of the Arctic in the summer season. Melt ponds provide unique microbial habitats with high light availability, which can promote photosynthesis. Therefore, melt ponds play an important role for nutrient cycling at the ice-ocean interface. However, the changes in nutrient dynamics in and under the sea ice resulting from melt pond formation are poorly understood. To elucidate melt pond nutrient (NO3 -, NO2 -, NH4 +, PO4 3-, and Si(OH)4) dynamics and their relationship with the melt pond bottom ice, which is sea ice right beneath the floor of a melt pond, in the Central Arctic Ocean during late summer, melt pond water and bottom sea-ice samples were collected during the MOSAiC Expedition (2019-2020). Comparison with the dilution line based on winter surface seawater, which is a source of sea ice, suggest that nutrients in the melt ponds are consumed by algae or other organisms, and then remineralized at the pond bottom. Nutrients then percolated downward through the porous bottom ice. Melt pond water was completely exchanged with surrounding seawater (lead or under-ice seawater) and snow derived water. If the surrounding seawater and snow are rich in nutrients, the exchange promotes photosynthesis within the melt pond water and can enhance nutrient accumulation within the pond bottom ice.

Under climate change, model ensembles suggest that declines in phytoplankton biomass amplify into greater reductions at higher trophic levels, with serious implications for fisheries and carbon storage. However, the extent and mechanisms of this trophic amplification vary greatly among models, and validation is problematic. In situ size spectra offer a novel alternative, comparing biomass of small and larger organisms to quantify the net efficiency of energy transfer through natural food webs that are already challenged with multiple climate change stressors. Our global compilation of pelagic size spectrum slopes supports trophic amplification empirically, independently from model simulations. Thus, even a modest (16%) decline in phytoplankton this century would magnify into a 38% decline in supportable biomass of fish within the intensively-fished mid-latitude ocean. We also show that this amplification stems not from thermal controls on consumers, but mainly from temperature or nutrient controls that structure the phytoplankton baseline of the food web. The lack of evidence for direct thermal effects on size structure contrasts with most current thinking, based often on more acute stress experiments or shorter-timescale responses. Our synthesis of size spectra integrates these short-term dynamics, revealing the net efficiency of food webs acclimating and adapting to climatic stressors.

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.

Understanding and managing the response of marine ecosystems to human pressures including climate change requires reliable large-scale and multi-decadal information on the state of key populations. These populations include the pelagic animals that support ecosystem services including carbon export and fisheries. The use of research vessels to collect information using scientific nets and acoustics is being replaced with technologies such as autonomous moorings, gliders, and meta-genetics. Paradoxically, these newer methods sample pelagic populations at ever-smaller spatial scales, and ecological change might go undetected in the time needed to build up large-scale, long time series. These global-scale issues are epitomised by Antarctic krill ( Euphausia superba ), which is concentrated in rapidly warming areas, exports substantial quantities of carbon and supports an expanding fishery, but opinion is divided on how resilient their stocks are to climatic change. Based on a workshop of 137 krill experts we identify the challenges of observing climate change impacts with shifting sampling methods and suggest three tractable solutions. These are to: improve overlap and calibration of new with traditional methods; improve communication to harmonise, link and scale up the capacity of new but localised sampling programs; and expand opportunities from other research platforms and data sources, including the fishing industry. Contrasting evidence for both change and stability in krill stocks illustrates how the risks of false negative and false positive diagnoses of change are related to the temporal and spatial scale of sampling. Given the uncertainty about how krill are responding to rapid warming we recommend a shift towards a fishery management approach that prioritises monitoring of stock status and can adapt to variability and change.

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.

Understanding and managing the response of marine ecosystems to human pressures including climate change requires reliable large-scale and multi-decadal information on the state of key populations. These populations include the pelagic animals that support ecosystem services including carbon export and fisheries. The use of research vessels to collect information using scientific nets and acoustics is being replaced with technologies such as autonomous moorings, gliders, and meta-genetics. Paradoxically, these newer methods sample pelagic populations at ever-smaller spatial scales, and ecological change might go undetected in the time needed to build up large-scale, long time series. These global-scale issues are epitomised by Antarctic krill (Euphausia superba), which is concentrated in rapidly warming areas, exports substantial quantities of carbon and supports an expanding fishery, but opinion is divided on how resilient their stocks are to climatic change. Based on a workshop of 137 krill experts we identify the challenges of observing climate change impacts with shifting sampling methods and suggest three tractable solutions. These are to: improve overlap and calibration of new with traditional methods; improve communication to harmonise, link and scale up the capacity of new but localised sampling programs; and expand opportunities from other research platforms and data sources, including the fishing industry. Contrasting evidence for both change and stability in krill stocks illustrates how the risks of false negative and false positive diagnoses of change are related to the temporal and spatial scale of sampling. Given the uncertainty about how krill are responding to rapid warming we recommend a shift towards a fishery management approach that prioritises monitoring of stock status and can adapt to variability and change.

The Barents Sea is a hotspot for environmental change due to global warming. These changes impact the structure and functioning of the marine ecosystem year-round, and it is therefore important to gain knowledge on trophic relationships and the energy flow from primary producers, i.e., ice algae (sympagic algae) and phytoplankton (pelagic algae) to consumers over the entire seasonal cycle. By using different lipid components as trophic markers, we provide seasonal coverage of the carbon and food-source composition of five of the most abundant and ecologically important zooplankton taxa inhabiting the Barents Sea: copepods, krill, amphipods, pteropods and chaetognaths. Based on the composition of algal-produced fatty acid (FA) markers, carbon-source composition of the zooplankton species reflected changes in the production and availability of food resources during different periods of the year. For example, relative proportions of the dinoflagellate/Phaeocystis FA marker 18:4(n-3) peaked during summer in Calanus copepods, the amphipod Themisto abyssorum and the chaetognath Pseudosagitta maxima, when the production of this FA reached maximum concentrations in phytoplankton. The composition of carnivory FAs (relative contribution of copepod-associated FAs, ratio 18:1(n-9)/18:1(n-7)) and the ratio of zoo- to phytosterols indicated that most grazers relied more on heterotrophic prey during polar night and spring while switching to a more algae-based diet during the summer. Based on source-specific highly branched isoprenoids (HBIs), sympagic carbon had generally a minor contribution to the nutrition of the zooplankton community, particularly during winter and spring when sympagic HBIs were virtually undetected in the animals. In contrast, sympagic HBI metabolites were detected in krill, amphipods and the pteropod Clione limacina during summer and autumn. The krill Meganyctiphanes norvegica was unique in terms of its HBI composition as the only species containing both sympagic and pelagic HBIs during spring. Our results indicate that the Barents Sea zooplankton community is largely based on pelagic carbon, while sympagic carbon is only supplementing species-specific diets, mostly during the second half of the year. This relatively low trophic dependency on sea-ice algae might be an indication of the resilience of this food web towards ongoing sea-ice decline that causes changes to the timing and availability of sympagic and pelagic carbon and food sources.