Sea ice plays a critical role in regulating the global climate and serves as a unique habitat for diverse microbial communities. Still, our understanding of viruses in these communities remains limited. To further uncover the diversity and functional potential of viruses in polar and subarctic sea ice, we explored the viral component of Arctic, Baltic Sea, and Antarctic sea ice metagenomes. Altogether, 550 viral operational taxonomic units (vOTUs) were recovered, most of which were putatively classified within the class Caudoviricetes, which comprises bacterial and archaeal tailed double-stranded DNA viruses. Hosts were predicted for 187 vOTUs, with Gammaproteobacteria and Bacteroidia being the most prevalent viral host groups. Potential functions were assigned for 56% of predicted viral gene products, including putative auxiliary metabolic genes (AMGs) involved in oxidative metabolism, photosynthesis, and metabolism regulation under stress conditions. Related viral genomes carrying similar AMGs were detected in other Arctic and more geographically distant freshwater, marine, and ice environments. Genus- and/or family-level links between the studied vOTUs were detected across samples. Our results suggest diverse and complex virus-host interactions in sea ice and highlight the essential roles viruses may play in sea ice ecosystem dynamics across polar and subpolar environments.

Kongsfjorden, a glaciated fjord in the Arctic, is shaped by Atlantic water inflow and seasonal glacier melt. This study compared prokaryotic community dynamics during spring and summer in two contrasting years, 2019 and 2020. In spring 2019, warmer Atlantic water increased temperatures and nutrients, supporting a Phaeocystis pouchetii-dominated bloom. In contrast, spring 2020 was colder, with extensive sea ice and a bloom succession from diatoms to P. pouchetii. Summer surface waters showed strong glacier runoff influence and low-biomass, flagellate-dominated phytoplankton. Greater prokaryotic diversity was observed in 2019, particularly at the surface, which was supported by network analysis revealing fewer interactions among prokaryotes in surface waters during 2020 compared to 2019. Prokaryotic community composition clearly differed between the 2 years, showing vertically homogeneous communities and higher cyanobacterial abundance in 2019, while in 2020 communities were more vertically heterogeneous and potentially locally adapted. These patterns reflect greater Atlantic water influence in 2019 versus more stratified Arctic conditions in 2020. Surface and bottom communities showed contrasting trends, shaped by depth-specific environmental conditions. Our findings highlight the sensitivity of Arctic microbial communities to shifts in hydrography and bloom dynamics and provide important perspectives on the ecological stability and dynamics of Arctic microbial communities under changing environmental conditions.

Unicellular marine eukaryotes (protists) are important components of the biological carbon pump and form the base of marine food webs, ultimately supporting commercially important fish stocks. However, seasonally resolved data on protist composition, abundance and biomass are rare, especially in high latitude ice-covered oceans. This study presents unique seasonal data on protist community composition and abundance from the northwestern Barents Sea, spanning a latitudinal gradient from open Atlantic waters south of the Polar Front to the seasonally ice-covered Barents Sea shelf and across the shelf break into the Arctic Nansen Basin. Observations were conducted during seven cruises (2018 to 2022) covering periods from early winter to late summer. Protist standing stocks and community composition exhibited pronounced seasonality. Dinoflagellates, particularly Gymnodinium species, dominated the low protist stocks in winter whereas summer communities were more diverse and characterized by dinoflagellates, ciliates and flagellates. Mixo- and heterotrophic feeding modes enable these taxa to persist through the polar night and thrive during the stratified and nutrient-poor conditions in summer. Seasonal differences between dinoflagellates and ciliates likely reflect contrasting feeding and survival strategies. The onset of the phytoplankton spring bloom, dominated by rapidly growing centric diatoms of the genera Thalassiosira and Chaetoceros, occurred in a deeply mixed water column in both open and ice-covered waters. This may have been facilitated by diatoms’ effective defense against protozoan grazers and temporal mismatch between the phytoplankton spring bloom and copepod grazers in Atlantic influenced waters. Our seasonally resolved observations highlight the strong seasonality in both the quantity and quality of protist plankton in the Barents Sea, dynamics which will likely shift with the ongoing decline in sea-ice cover.

Although sea-ice ridges are prominent features of the Arctic Ocean, very little is known about their role as habitats and in biogeochemical cycles. Here, we show that ridges provide complex sea-ice habitats which host unique and diverse biological communities. Seasonally, ridges appear to transition from a biological repository in winter to biological hotspots in summer, surpassing algal biomass in level ice and surface waters by up to eight-fold. In summer, ridges can contain up to 80% of the total area integrated sea-ice algal biomass, emphasizing their importance in the Arctic sea-ice ecosystem. However, environmental shifts, such as meltwater infiltration and freezing inside the ridge in late summer, alter microbial communities from being predominantly autotrophic to heterotrophic. Our work provides evidence of contrasting roles of sea-ice ridges for Arctic carbon cycling in summer and shows that the habitats in the ridge interior harbor unique microbial communities, adding complexity to Arctic biodiversity.

The Arctic Ocean is undergoing drastic changes in its sea ice cover, but is also optically complex. Observations from summer 2022 across the western Eurasian Basin, show increased colored dissolved organic matter (CDOM) absorption from terrestrial-derived runoff within the Transpolar Drift (TPD) in the Amundsen Basin, extending down to a depth of 100 m. This is in stark contrast to the low-CDOM waters of the Atlantic-influenced Nansen Basin. The higher CDOM absorption increases the diffuse attenuation coefficient by around 35% relative to Atlantic-influenced waters, and changes the spectral quality of light at depth. An irradiance model demonstrates that the decreased light availability can delay or inhibit under-ice phytoplankton blooms, affect zooplankton behavior, and cause enhanced solar heating in the upper 10 m. These findings show that simplified representations of light attenuation in present Earth System Models can hinder accurate assessments of light limitations on Arctic marine ecosystems.

Background Fjords in Svalbard are undergoing significant changes due to climate warming.Those along the west coast of Spitsbergen are particularly affected by the increasing influence of "warm"Atlantic Water (AW), a process known as Atlantification. We compared Kongsfjorden, a relatively "warm"fjord on the west coast, with Rijpfjorden, a typical cold Arctic fjord on the north coast of Nordaustlandet, combining physical and biogeochemical data with 16S rRNA gene amplicon and shotgun metagenomic sequencing. We hypothesize that differences in fjords'water masses and prokaryotic communities provide insight into the effects of Atlantification as it expands eastwards along the shelf north of Svalbard. Results We found that warm AW dominated in Kongsfjorden, whereas Rijpfjorden was dominated by cold Arctic Water and Winter Cooled Water. Our results suggest that the Atlantic-influenced Kongsfjorden is a nutrient sink, whereas Rijpfjorden showed similar behavior only in 2016, a particularly warm year, otherwise no clear sink/source role could be identified. Analysis of 16S rRNA gene sequences revealed that Proteobacteria had higher relative abundances in Kongsfjorden while Bacteroidota dominated in Rijpfjorden. Ammonium and nitrite-oxidizing prokaryotes were most prevalent in deeper water masses of both fjords. The archaeal taxa of the ammonia-oxidizing community, mainly Nitrosopumilus and Nitrosopelagicus, were consistently more dominant than ammonium and nitrite-oxidizing bacteria. Denitrification and nitrogen fixation genes differed between the fjords, with Kongsfjorden having a higher coverage of diazotroph genes. Conclusions Kongsfjorden and Rijpfjorden displayed distinct hydrographic conditions, with Kongsfjorden being under a stronger influence of Atlantification. Our results suggest that warmer water masses are linked to higher nutrient uptake.The clear association between microbial communities and water masses offers insight into changes driven by Atlantification.

Abstract There is no light for photosynthesis by phytoplankton and sea-ice algae during the polar night, but microbial grazers remain active through the dark winter months in the Arctic Ocean. Where the energy to sustain these organisms comes from is unknown. Here we observed active tintinnid ciliates during the polar night, heterotrophic protists known to feed on phytoplankton and smaller heterotrophic protists. Our calculations indicate that the pelagic microbial loop transferring energy from bacterial production through microbial grazers was not sufficient to sustain the observed tintinnid biomass. However, the sea ice contained frozen-in particulate organic carbon produced during the previous growth seasons. We show that enough food particles can be released by mechanical break-up during sea-ice ridging, which together with bacterial production, sustained the observed tintinnid biomass in the water column. This is an important but overlooked mechanism for winter survival of plankton in the Arctic Ocean.

Abstract Global warming is amplified in the Arctic, accelerating glacier melt and freshwater runoff. At tidewater glaciers, runoff typically enters fjords at depth and generates buoyancy‐driven circulation that enhances glacier‐ocean exchanges of energy and matter, influencing macronutrient delivery and marine primary production. However, most studies lack the temporal resolution to capture low‐frequency, high‐magnitude events, leaving their impacts poorly understood. Here, we combine glacier observations with high‐frequency fjord and glacier‐lake sampling to examine the 2021 glacier lake outburst flood (GLOF) from Lake Setevatnet into Kongsfjorden (Svalbard). We show how evolving subglacial conditions before and during the GLOF shaped macronutrient supply to the inner fjord through both direct runoff and entrainment of bottom waters. Early in summer, nutrient delivery was dominated by direct runoff, supplying nitrate (NO 3 − ) and silicate via an inefficient drainage system. As the melt increased, an efficient system formed, generating a subglacial plume and initiating buoyancy‐driven circulation that entrained nutrient‐rich deep water. Despite high NO 3 − lake concentrations, the flood barely affected fjord NO 3 − levels. Instead, it produced a seasonal maximum in nitrite (NO 2 − ). Comparisons with conservative mixing estimates and nitrogen budget analyses reveal a non‐conservative nutrient signal. Although sedimentary sources cannot be excluded, the timing and spatial pattern of the NO 2 − anomaly suggest subglacial modification during floodwater transit. These findings indicate that Kongsfjorden functions as a summer nitrogen sink, partly shaped by subglacial transformations. Overall, nutrient delivery from tidewater glaciers depends not only on runoff volume but also on the subglacial drainage system characteristics, which evolve during high‐magnitude events such as GLOFs.

The Barents Sea is a hotspot for ongoing Arctic climate change, manifested in a rapid warming of the ocean and the atmosphere and a strong decline of the winter sea-ice cover. These changes in the physical environment have large consequences for marine ecosystems, including commercial fish populations. In a warmer future climate, both physical and ecological changes are expected to intensify. Here, we provide a first comprehensive overview of future climate change projections for the Barents Sea, and the associated physical, biogeochemical, and ecological consequences based on climate models and end-to-end ecosystem models. We also discuss potential future changes in human activities and their impacts, including changes in shipping activity and contaminants. We analyze results for two time horizons—the near-future (2040–2050) and the far-future (2090–2100)—and for two different emission scenarios: one with moderate future greenhouse gas emissions (SSP2-4.5) and one high-emission scenario (SSP5-8.5). The projections show that the future Barents Sea will be warmer, less ice-covered, more acidic, and more productive, with fish populations and spawning sites moving northward. There are small differences in multi-model mean physical and biogeochemical projections between the two emission scenarios by 2050, while large scenario differences emerge toward the end of the century. The implications of these results are far-reaching, including identifying the sensitivity of ecosystem change to future emissions, informing regional management strategies, and potentially identifying needs for adaptation to changes already likely to occur.

The projected transition of the central Arctic Ocean (CAO) into a warmer, seasonally ice-free ocean requires more knowledge of this environment to predict changes in the structure and dynamics of its ecosystems. We aimed to compare the state and underlying processes of Nansen Basin and Amundsen Basin ecosystems observed in August–September 2021 and assess impacts of Atlantic Water inflow and fresher Transpolar Drift waters, respectively, on these ecosystems. The basins differed in features of sea ice, hydrography, and chemical and biological compositions. The near-slope open water in western Nansen Basin showed a clear fingerprint of warm, saline Atlantic Water, with larger vertical turbulent fluxes facilitating nutrient transport across the pycnocline and supporting larger standing stocks of bacteria, protists, and zooplankton. Pelagic primary production and microbial and faunal stocks decreased northward and into Amundsen Basin, likely due to lower nutrient concentrations, stronger stratification, and reduced light through the more continuous and thicker ice and snow cover in Amundsen Basin, possibly also impacted by seasonally declining light levels. Transpolar Drift signals included lower salinity, stronger stratification, and higher silicate concentrations in Amundsen Basin surface waters. Similarities to earlier observations included the increase in small-sized algae from Nansen Basin into Amundsen Basin and overall low faunal abundances in the CAO, suggesting that overarching patterns remained unchanged over past decades. Examples of species range extensions and notable taxon absences relative to earlier studies, however, could be due to borealization and changes in sea-ice conditions, respectively. Higher density ecosystem sampling and consistent time series are recommended to confirm such conclusions. The distinct basin differences call for a regional approach to future management of the CAO. We especially caution against using the area of strong Atlantic Water inflow in southern Nansen Basin as representative of the entire basin, let alone Amundsen Basin or the CAO.

Studies of Arctic food-web structure and function by means of carbon isotopic composition (δ13C) of fatty acids (FAs) have a main challenge of identifying a proper baseline for pelagic particulate organic matter (PPOM). To assess variations in δ13C values of the FAs 16:1(n-7), 20:5(n-3) and 22:6(n-3) in PPOM, seawater was collected along a latitudinal sea-ice gradient across the Barents Sea in August 2019, spanning ice-free to fully ice-covered stations. δ13C values varied strongly along the sampling transect in all three FAs (16:1(n-7): -32.2 to -28.9 ‰, 20:5(n-3): -37.4 to -29.8 ‰, 22:6(n-3): -34.7 to -29.3 ‰) and, independently of the FA, were consistently higher at ice-covered (δ13C: -30.2 ± 1.1 ‰) vs. ice-free stations (δ13C: -33.8 ± 2.2 ‰). This was likely the result of the contribution of ice-associated algae to PPOM due to ice melt as ice algae often have higher δ13C values than pelagic algae. Latitudinal differences in δ13C values of 20:5(n-3) and 22:6(n-3) displayed a similar trend, which partly differed from 16:1(n-7). This was likely related to differences in FA synthesis pathways and cellular functions between the membrane-associated FAs 20:5(n-3) and 22:6(n-3) and the storage FA 16:1(n-7). The δ13C values presented here are intended to support future food-web studies applying stable isotope mixing models to quantify carbon sources in the polar marine environment.

Behavioural plasticity is likely to influence how individuals continue to access resources under rapid climate change. Plasticity will be particularly important at highly dynamic, prey-rich foraging areas such as upwelling fronts of marine-terminating glaciers in the high Arctic, where profitability varies significantly across space and time. Understanding individual variation in plasticity and its adaptive potential is crucial to understand a populations flexibility to future climate scenarios. By analysing GPS data from 186 black-legged kittiwakes Rissa tridactyla breeding in the high Arctic over six years, we quantified individual variation in behavioural plasticity in use of glacial fronts and its relationship with the number of chicks produced. Variation in the relationship between glacial use and levels of discharged meltwater was primarily explained by differences in food availability between years. Whereas there was no significant relationship between discharge rates and glacier use in years of low zooplankton biomass, the probability of glacial front use and time spent at glaciers decreased in years when food was more abundant, despite high discharge and likely good conditions at the front. Interestingly, neither glacial use nor plasticity in foraging during the breeding season correlated with the number of surviving chicks, suggesting that all individuals still obtained enough food for reproduction. Understanding the complex nature of individual variation in plasticity and when it is likely to be adaptive will be the first step in highlighting when plasticity can be used to predict how species will respond to rapidly changing environments.

Sea-ice algae account for a substantial part of annual primary production in ice-covered waters and are an important component of the Arctic marine food web. With climate-induced changes to snow and sea-ice cover and their impact on the surface ocean, such as earlier melt, thinner ice, and increased upper-ocean stratification, a shift toward earlier and more extensive nutrient limitation on ice algal growth can be expected. Therefore, increasing our understanding of the processes governing nutrient supply and uptake by sea-ice algae is essential. Here, we compiled a pan-Arctic dataset of concentrations of sea-ice and sub-ice nutrients and sea-ice chlorophyll a (chl a) to assess their regional and seasonal variability, as well as the relationship of sea-ice algae and nutrient dynamics in the Arctic Ocean. This dataset indicates that bottom sea-ice nutrient and chl a concentrations were highest in the central Canadian Arctic Archipelago (Resolute Passage) due to tidal-driven mixing at the ocean-ice interface, and lowest in the Arctic Ocean basins. At the regional scale, Pacific and Atlantic Water influence variability in sea-ice and sub-ice nutrient concentrations. Significant positive relationships of bottom sea-ice nutrient versus chl a concentrations were ubiquitous across the Arctic during the ice algal bloom, suggesting intracellular nutrient storage as an important mechanism to support ice algal growth. This relationship in turn alters nutrient ratios within the sea ice relative to sub-ice waters, decreasing NOx:PO4 ratios, while increasing NOx:Si(OH)4 ratios. In contrast, bottom sea-ice nutrient-chl a relationships were less common and sometimes negative when nutrient concentrations were low, likely reflecting nutrient limitation. In conclusion, we have demonstrated a pan-Arctic, yet regionally specific, influence of the ice algal community on bottom sea-ice nutrient concentrations.

Fjords in the Arctic are changing rapidly due to multiple factors including increasing air temperatures, the influx of Atlantic Water (Atlantification), sea-ice loss, retreat of tidewater glaciers, increased freshwater discharges, pollution and tourism. Understanding how these changes affect ecosystem processes and functions and, thus, services to society is critical. Net Ecosystem Metabolism (NEM) offers a holistic measure of ecosystem functioning and services, reflecting the balance between autotrophic and heterotrophic processes and the sink/source role of an ecosystem for nutrients and carbon. Using a 10-year dataset we quantify the main nutrient sources and sinks in Kongsfjorden (Svalbard) and estimate NEM using a method based on mixing diagrams combined with an ocean circulation model. We show that Kongsfjorden is a nutrient and carbon sink primarily supported by nutrient inputs from the adjacent shelf sea with terrestrial run-off playing a secondary role. Given the ongoing changes in the Arctic, driven by global warming and its associated effects, we recommend monitoring NEM as an integrated measure of the state of coastal ecosystems, considering the disproportionately large role of coastal regions in the global carbon budget.

The Barents Sea is a highly dynamic and productive marine ecosystem and a hotspot of global warming. Variability in sea ice extent is a common feature in the Barents Sea with substantial movements of the sea ice edge on short-term, seasonal to interannual time scales. Historically the northern Barents Sea (north of 75 degrees N) has been ice-covered in winter, but recently it has become the area with most winter ice loss in the Arctic, and year-round ice-free conditions are predicted for the second half of the 21st century. These environmental changes have significant implications for the marine ecosystem. In this study we used contrasting sea ice regimes in August 2018 and August 2019 to explore the response of phytoplankton and bacterial production, microbial abundance, and vertical carbon flux in the north-western Barents Sea (between 76 degrees N and 83 degrees N) to the variability of sea ice. While the study area was ice-free in August 2018, extensive areas north of 79 degrees N were ice-covered in 2019. When the northern parts of the transect were still ice covered, diatoms and other larger phytoplankton were dominant and highest abundances were observed following the receding ice edge. In contrast, under ice-free conditions in 2018, the pelagic ecosystem resembled a post-bloom stage of the seasonal succession with higher abundance of small phytoplankton and heterotrophic protists and low vertical flux throughout the water column. While phytoplankton biomass, bacterial production and downward vertical flux of particulate organic carbon in the upper 60 m were on average higher in 2019, primary production and carbon export below the euphotic layer were comparable between both years. However, overall highest primary production, bacterial production and abundance of both photosynthetic and heterotrophic microorganisms were observed in surface waters (upper 30 m) in 2019, connected to the retreating ice edge, where also vertical particle flux was higher and characterized by a strong attenuation curve. The results clearly demonstrate that differences in ice cover affect the phenology of pelagic primary production and associated biological processes in the Barents Sea.

Despite the lack of local anthropogenic mercury sources, methylated mercury (MeHg) concentrations in Arctic biota are higher than in biota from lower latitudes. The main entry route occurs during the bioconcentration of seawater monomethylmercury (MMHg) into phytoplankton. Despite the known seasonal changes in biological activity in the region, little is known about the seasonal cycling of total mercury (THg) and MeHg in the Arctic Ocean. Here, we report the concentrations of THg and MeHg in seawater sampled from the northwestern Barents Sea water column during late winter and spring. In the upper 500 m, the THg concentrations are significantly higher in spring (0.64 +/- 0.09 pmol L-1) compared to late winter (0.53 +/- 0.07 pmol L-1), driven by seasonal inputs to surface waters from atmospheric deposition and the dynamics of changing sea ice conditions. Contrastingly, the MeHg concentrations in spring were significantly lower (41 +/- 39 fmol L-1) compared to late winter (85 +/- 42 fmol L-1). We suggest that most MeHg is biotically demethylated by both phytoplankton and bacteria, with additional losses from photodemethylation and evasion. Our observations highlight the importance of demethylation during potential uptake of methylmercury coinciding with the Arctic spring bloom. Lastly, we use our new data together with previously published seasonal data in the region to construct a simplified seasonal mercury cycle in an Arctic marginal ice zone.

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.

Strong seasonality is a key feature of high-latitude systems like the Barents Sea. While the interannual variability and long-term changes of the Barents Sea are well-documented, the seasonal progression of the physical and biological systems is less known, mainly due to poor accessibility of the seasonally ice-covered area in winter and spring. Here, we use an extensive set of physical and biological in situ observations from four scientific expeditions covering the seasonal progression from late winter to late summer 2021 in the northwestern Barents Sea, from fully ice-covered to ice-free conditions. We found that sea ice meltwater and the timing of ice-free conditions in summer shape the environment, controlling heat accumulation, light and nutrient availability, and biological activity vertically, seasonally, and meridionally. In March and May, the ocean north of the Polar Front was ice-covered and featured a deep mixed layer. Chlorophyll-a concentrations increased strongly from March to May along with greater euphotic depth, indicating the beginning of the spring bloom despite the absence of surface layer stratification. By July and in September, sea ice meltwater created a shallow low-density surface layer that strengthened stratification. In open water, chlorophyll-a maxima were found at the base of this layer as surface nutrients were depleted, while in the presence of ice, maxima were closer to the surface. Solar heating and the thickness of the surface layer increased with the number of ice-free days. The summer data showed a prime example of an Arctic-like space-for-time seasonal variability in the key physical and biological patterns, with the summer situation progressing northwards following sea ice retreat. The amount of sea ice melt (local or imported) has a strong control on the conditions in the northwestern Barents Sea, and the conditions in late 2021 resembled pre-2010 Arctic-like conditions with high freshwater content and lower ocean heat content.

BackgroundHigh-throughput sequencing of the full-length 16S rRNA gene has improved the taxonomic classification of prokaryotes found in natural environments. However, sequencing of shorter regions from the same gene, like the V4-V5 region, can provide more cost-effective high throughput. It is unclear which approach best describes prokaryotic communities from underexplored environments. In this study, we hypothesize that high-throughput full-length 16S rRNA gene sequencing combined with adequate taxonomic databases improves the taxonomic description of prokaryotic communities from underexplored environments in comparison with high-throughput sequencing of a short region of the 16S rRNA gene.ResultsTo test our hypothesis, we compared taxonomic profiles of seawater samples from the Arctic Ocean using: full-length and V4-V5 16S rRNA gene sequencing in combination with either the Genome Taxonomy Database (GTDB) or the Silva taxonomy database. Our results show that all combinations of sequencing strategies and taxonomic databases present similar results at higher taxonomic levels. However, at lower taxonomic levels, namely family, genus, and most notably species level, the full-length approach led to higher proportions of Amplicon Sequence Variants (ASVs) assigned to formally valid taxa. Hence, the best taxonomic description was obtained by the full-length and GTDB combination, which in some cases allowed for the identification of intraspecific diversity of ASVs.ConclusionsWe conclude that coupling high-throughput full-length 16S rRNA gene sequencing with GTDB improves the description of microbiome profiling at lower taxonomic ranks. The improvements reported here provide more context for scientists to discuss microbial community dynamics within a solid taxonomic framework in environments like the Arctic Ocean with still underrepresented microbiome sequences in public databases.

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.