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The Barents Sea is a shelf sea in the European Arctic and is influenced by the North Atlantic Current and the Arctic Ocean. The North Atlantic Current is composed of warmer, saline waters compared to the cold, fresher waters from the Arctic Ocean. Over the last century, the Barents Sea mean temperature has increased by 1.5°C above a depth of 60m and by 0.5-0.8°C below 60m. This warming is disproportionately high compared to other areas of the globe. A consequent loss of sea ice and changes in the salinity of the Arctic Ocean has been observed, especially since the 1980s. It is important to monitor distribution changes in marine invertebrates because they can reflect fluctuations of their environment and can potentially have strong impacts on the ecosystem. Marine invertebrates occupy many ecological niches and go through diverse life stages, including external fertilisation and often a planktonic larval stage. They are therefore dependent on environmental conditions both in the benthos (in case of seafloor dwellers) and the water column. For example, those with a planktonic life stage are distributed by marine currents and each species has a temperature and salinity optimum for their development. It is therefore expected that changes in the environment, such as increased ocean temperatures due to climate change, will lead to changes in the distribution of marine organisms. However, changes of invertebrates’ distributions in the Barents Sea over the entire past century have not been studied yet and this is the objective of this study. I used data mediated by the Global Biodiversity Information Facility (GBIF) to characterize species distributions and their potential change over more than a hundred years. The periods were separated into five blocks starting before 1900 and go until 2010. I took the acceleration of the warming past 1980 into account by shortening the later time ranges. The study area was divided into warm (south-western Barents Sea), mixed (central Barents Sea) and cold (North-eastern Barents Sea) zones based on bottom water temperature. I found that, over time, distribution of invertebrates has changed in different ways. First, using a correspondence analysis, I visualized similarity patterns in taxon occurrences among time period-region groups. Depending on the geographical zone in the Barents Sea some invertebrate distributions were similar in their temporal spatial pattern up until a turning point. In the cold and mixed zones, for instance, that turning point was 1980, where afterward I could observe a drastic change in species composition patterns. In the southern area in contrast, I found a consistent, more gradual change through time. Then we used a log-linear model to analyse changes in number of occurrences, accounting for changes in observation pressure. Overall, out of 364 species investigated across 11 phyla with a Log-linear model 71% of them and all but one phylum presented a change of taxon occurrences in at least one period. Changes of the distribution of marine invertebrates are discussed based on changes in their spatial occurrence patterns and difference of the number of occurrences compared to before 1900. For instance, two arctic species, Stegophiura nodosa and Ophiocten sericeum, significantly decreased in number of occurrences in 1900-1950 and 1980-2000, respectively. Other notable species are boreal mollusc, Cuspidaria lamellosa and Euspira montagui, who increased in occurences consistently after 1950, but also show a movement north in their distribution on a map of the Barents Sea. In conclusion, the species composition of invertebrates in the Barents Sea has shifted in its recent history, most likely due to the warming waters caused by climate changes.

Whether gill area constrains fish metabolism through oxygen limitation is a debated topic. Here, the authors provide insights into this question by analysing mass-specific metabolic rates across 44 teleost fishes extracted from FishBase. They explore whether species deviations from metabolic rates predicted by body mass can be explained by species gill area. They show that the gill area explains c. 26%–28% of species-level deviations from mass-specific metabolic rates. Their findings suggest that gill area might indeed be one of the factors limiting metabolic rate in fishes.

Small copepods (≤ 2 mm) are highly abundant mesozooplankton in Arctic and sub-Arctic marine ecosystems. However, small copepods are generally underestimated in zooplankton studies because of the use of large-meshed nets (≥ 180-µm mesh size) and are relatively poorly studied compared to large copepods. This thesis investigates the egg hatching rates, population dynamics and production rates of three abundant small copepod species in high-latitude ecosystems: Oithona similis, Microsetella norvegica, and Microcalanus pusillus. The egg hatching rates were investigated at between 1.3°C and 13.2°C by incubating ovigerous females. Field work was conducted in three sub-Arctic fjords and around Svalbard to obtain the biomass, age structure, and production of the populations sampled with a 64-µm mesh net. The egg hatching rates had various species-specific temperature response, which suggested different thermal plasticity among the three similar-sized species. The population dynamics of the three studied species also showed different patterns. M. norvegica has a time-limited reproduction resulting in a short but intense reproductive season and overwintering as adults. In contrast, O. similis and M. pusillus have continuous reproduction, where winter is used as recruitment period for a second generation. The most important findings of this thesis is the high biomass and production revealed by the quantitative sampling of small copepods. The biomass and production of small copepods could be equal to or even higher than that of large copepods, regardless of the season but with large spatial and interannual variations. In Balsfjord, the total production of the three small copepods could equal 0.3 to 27% of the primary production, meaning that small copepods have a significant role in the food web and carbon cycling of high-latitude ecosystems.

The release of red king crab (Paralithodes camtschaticus) into the Barents Sea by Russian scientists in the 1960s had the goal of establishing a self-sustaining stock for the purpose of commercial harvest. The red king crab has been successfully established as a stock in Norwegian waters where it has become an important commercial resource in the eastern regions of Finnmark. The Norwegian red king crab fishery is separated into two different regimes to meet two management goals, with a free fishery west of North Cape (26ºE) and a quota regulated fishery east of North Cape. This study examines the general size composition of the red king crab, investigates the female reproductive strategy, factors affecting reproductive investment and fecundity over time from when the research fishery started in 1994 till present. The findings presented in this study are general decreasing trends in size composition for both males and ovigerous (roe carrying) females during the sampling period. Clear trends toward a reduction the size where 50% of females are ovigerous in all fjords and over years. Temporal and spatial variability in measured individual egg weight has been demonstrated, and a standardized ovigerous female crab with a 125 mm carapace length (CL) is overall less fecund, compared to previous studies. These changes in population size composition, size at maturity (OL50), reproductive investment and fecundity have been suggested to be result of heavy fishing pressure on large crabs of both sexes, as a result sperm limitation may occur due to low numbers of large males. The decrease in reproductive investment and fecundity could also be related to a reduction in availability of high-quality food items for the crabs.

Gelatinous zooplankton, here referring to the phylum Cnidaria and Ctenophora and the class Appendicularia are important components of marine ecosystems due to their very diverse diets, life cycles and seasonal dynamics. In the Barents Sea, several studies reviewed the gelatinous zooplankton diversity, yet few studies documented the diversity and distribution patterns between the advected Atlantic Waters (AW) and Polar waters (PW) and no study reviewed the gelatinous zooplankton seasonality over a full year. The aim of this master thesis is to (1) investigate diversity and distribution patterns between the different water masses and regions of the northern Barents Sea and (2) investigate the gelatinous zooplankton seasonality over a whole year in the northern Barents Sea. For this purpose, during the Nansen Legacy project four seasonal surveys covered the full seasonal cycle of the Barents Sea marginal ice zone and collected both biological and physical data, including gelatinous zooplankton. Overall, 31 gelatinous zooplankton taxa were identified. Fritillaria borealis, Oikopleura sp. and Aeginopsis laurentii were the most abundant taxa and AW and PW influenced areas had different seasonal dynamics and composition, which were significantly influenced by the primary production, the zooplankton biomass, the inflow of AW and the latitudes. Keywords: Gelatinous zooplankton, Barents Sea, Seasonality, Distribution, Atlantic Waters, Polar Waters, Nansen Legacy

Micro- and nanoplastic particles, produced from the breakdown of larger plastic debris, represent a whole new field of hazardous pollution with the potential to cause great damage on a range of marine species. This escalating concern of plastic pollution in marine environments and its consequential impacts on marine species, has emphasized the urgency for an in-depth understanding of the uptake and resultant impacts of plastic particles at both cellular and tissue levels, in aquaculture species such as Atlantic salmon (Salmo salar L.). To date, there is limited information regarding the uptake and subsequent effects of nanoplastics in salmon species, especially at cellular level. This study aimed to provide insight on the uptake of polystyrene nanoplastics (PS-NPs) and the potential responses that are activated in salmonids cells by the ingestion of these external stressors. The immortalized cell lines Atlantic Salmon Kidney cells and Salmon Head Kidney cells (ASK-1 and SHK-1), as well as Chinook Salmon (Oncorhynchus tshawytscha) Embryonic cells (CHSE-214) were exposed to nanoplastic (NP) particles embedded with fluorescent dye. The presence of the NPs within the cells was confirmed through fluorescent microscopy and DeltaVision™ deconvolution imaging, as well as a fluorescence-based flow cytometry, demonstrating their internalization by the cells. A comprehensive proteomic analysis utilizing liquid chromatography-mass spectrometry (LC-MS), revealed a diverse set of differentially expressed proteins. These proteins play crucial roles in essential cellular biological functions, including oxidative stress, apoptosis, endocytosis and immune responses, suggesting a perturbation of these biological processes by NPs in the salmonid cells. The findings from this study provided new information on the uptake and potential impact of polystyrene nanoplastic exposure on the proteomes of ASK-1, SHK-1 and CHSE-214 cells. These results will contribute to our understanding of the effects of nanoplastic exposure on fish health, and broadly provide valuable insights into the implications of nanoplastic pollution in the aquatic ecosystems.

Ecological systems, such as marine ecosystems, are complex adaptive systems in which large scale system properties (e.g., trophic structure, energy flux patterns, etc.) emerge from interactions between ecosystem components or species. This makes them difficult to understand, predict, and model. The Norwegian and Barents Seas support multiple commercial fisheries, including those for herring, cod, and mackerel. Fisheries extract around 2.61 million tonnes of fish annually and marine mammals consume 11.7 million tonnes of fish and zooplankton annually in the region. The gradual change from conventional fisheries management towards ecosystem-based fisheries management (EBFM) requires that interactions and trade-offs between exploitation and conservation of fish and marine mammals be considered. Such trade-offs occurs when there is direct or food-web mediated competition for resources between fisheries and marine mammals. Recent diet studies suggest that there is limited direct competition between marine mammals and fisheries. Food-web mediated interactions may occur when mammals consume the prey of commercial fish species, or when fisheries target the prey of marine mammals, but evidence for such interactions is still lacking. Using Chance and Necessity modelling (CaN) we reconstruct possible dynamics of 12 trophospecies in the Norwegian and Barents Sea ecosystems during the period 1988-2021. The reconstructed dynamics are consistent with multiple observations of biomass, diet, consumption, and life-history characteristics of the species groups. We use these reconstructions to establish the level of empirical support for food-web mediated interactions between marine mammals and fisheries. The results of the model analysis indicate that there is limited evidence to support direct competitive interactions between marine mammals and fisheries in the Norwegian Sea, and mixed evidence for such interactions in the Barents Sea. The results showed that most direct interactions between the two groups were bottom-up driven, and that only demersal fish, aside from cod, demonstrated a direct competitive interaction. As for food-web mediated interactions, the model provided evidence in support of a competitive interaction between marine mammals and capelin, between marine mammals and Barents Sea fish, and between marine mammals and all fish included in the model domain. However, the analysis also revealed the presence of a bottom-up trophic control in the food-web mediated interactions, particularly involving capelin and juvenile herring as prey. Thus, our model results show the presence of both opportunistic feeding and food-web mediated competitive interactions in the Norwegian and Barents Sea ecosystem.

The Arctic is warming rapidly. Atmospheric temperatures in the Barents Sea region are increasing 5 – 7 times faster than the global average. As a result, Arctic sea ice extent is declining by nearly 12% per decade during the summer months. The study site in Kongsfjorden, Svalbard, has been mostly ice free since 2006. These changes in the physical environment likely affect the ecology of coastal Arctic ecosystems. Increased underwater irradiance seems to improve the growth of kelp forests on rock shores. This thesis is a continuation of a time series to estimate historic developments in abundance, biomass, and taxonomic composition of seaweed-associated fauna, comparing data from 2021 with data from 2012/13 and 1996/98. At each time point, samples were collected along the same gradient from 2.5 m to 15 m water depth. There is limited evidence for a difference in taxonomic composition between 2021 and 2012/13, while taxonomic composition in 1996/98 was significantly different from both other time points. Moreover, both faunal abundance and biomass increased about two-fold between 2012/13 and 2021. These results might suggest that the seaweed-associated fauna at Hansneset reached a new stable state after initial shifts in the taxonomic composition between 1996/98 and 2012/13. Differences in taxonomic composition, based on biomass data, were largely a result of an increase in cirripedian and decrease in bryozoan biomass. These taxonomic groups possibly show different responses to some biotic and abiotic factors, such as macroalgae cover or sedimentation.

The fishery for Northern Atlantic cod (Gadus morhua) off Newfoundland and Labrador, Eastern Canada, presents the most spectacular case of an exploited stock crashed in a few decades by an industrial bottom trawl fishery under a seemingly sophisticated management regime after half a millennium of sustainable fishing. The fishery, which had generated annual catches of 100000 to 200000 tonnes from the beginning of the 16th century to the 1950s, peaked in 1968 at 810000 tonnes, followed by a devastating collapse and closure 24 years later. Since then, stock recovery may have been hindered by premature openings, with vessels targeting the remains of the cod population. Previous research paid little attention towards using multicentury time series to inform sustainable catches and recovery plans. Here, we show that a simple stock assessment model can be used to model the cod population trajectory for the entire period from 1508 to 2019 for which catch estimates are available. The model suggests that if fishing effort and mortality had been stabilized in the 1980s, precautionary annual yields of about 200000 tonnes could have been sustained. Our analysis demonstrates the value of incorporating prior knowledge to counteract shifting baseline effects on reference points and contemporary perceptions of historical stock status.

Planktonic early life stages of marine benthic invertebrates play an important role in dispersal and at times represent a substantial proportion of the zooplankton. Information about the larval communities present on the Arctic inflow shelves of the Barents and Chukchi Seas, however, is incomplete and limited because of infrequent sampling and low resolution of identification. The aims of the work described in this thesis were to characterize the meroplanktonic communities of the Barents and Chukchi Sea shelves in greater taxonomic (using DNA barcoding), spatial and temporal resolution than previously achieved, and to investigate the possible origins of the identified larvae. Key findings from this thesis include 1) much higher diversity of invertebrate larvae was found on the Arctic inflow shelves than previously, 2) larvae are present on the shelves year-round but their abundances peak in late summer and fall, 3) the larval community is composed mainly of taxa known to the region but there is evidence of some species drifting in from further south. Findings from this thesis highlight the immense potential gained from species-level identification and shine a light on critical yet poorly-known life stages of the rich Arctic shelf benthic communities in a time of rapid climate change.

Competition is assumed to shape niche widths, affecting species survival and coexistence. Expectedly, high interspecific competition will reduce population niche widths, whereas high intraspecific competition will do the opposite. Here we test in situ how intra- and interspecific competition affects trophic resource use and the individual and population niche widths of two lacustrine fish species, Arctic charr and brown trout, covering a 40 year study period with highly contrasting competitive impacts prior to and following a large-scale fish culling experiment. Initially, an overcrowded Arctic charr population dominated the study system, with brown trout being nearly absent. The culling experiment reduced the littoral Arctic charr density by 80%, whereupon brown trout gradually increased its density in the system. Thus, over the study period, the Arctic charr population went from high to low intraspecific competition, followed by increasing interspecific competition with brown trout. As hypothesized, the relaxed intraspecific competition following the experimental culling reduced individual diet specialization and compressed population niche width of Arctic charr. During the initial increase of the brown trout population, there was a large dietary overlap between the two species. Over the subsequent intensified interspecific competition from the population build-up of brown trout, their trophic niche overlap chiefly declined due to a dietary shift of Arctic charr towards enhanced zooplankton consumption. Contrary to theoretical expectations, the individual and population niche widths of Arctic charr increased with intensified interspecific competition. In contrast, the diet and niche width of brown trout remained stable over time, confirming its competitive superiority. The large-scale culling experiment and associated long-term research revealed pronounced temporal dynamics in trophic niche and resource use of the inferior competitor, substantiating that intra- and interspecific competition have large and contrasting impacts on individual and population niches.

Summary In the last decade, the ocean has absorbed a quarter of the Earth’s greenhouse gas emissions through the carbon (C) cycle, a naturally occurring process. Aspects of the ocean C cycle are now being incorporated into climate change mitigation and adaptation plans. Currently, too little is known about marine vertebrate C functions for their inclusion in policies. Fortunately, marine vertebrate biology, behavior, and ecology through the lens of C and nutrient cycling and flux is an emerging area of research that is rich in existing data. This review uses literature and trusted data sources to describe marine vertebrate C interactions, provides quantification where possible, and highlights knowledge gaps. Implications of better understanding the integral functions of marine vertebrates in the ocean C cycle include the need for consideration of these functions both in policies on nature-based climate change mitigation and adaptation, and in management of marine vertebrate populations.

The role of small prey (Clupea harengus) as a case study, as it is a key species with high commercial and ecological importance. In autumn and winter, the potential PZP and MZP prey was dominated by cells Pronoctiluca pelagica,Tripos spp. and Strombidium spp.), while copepod nauplii and copepodites where more abundant in autumn than in winter. Based on their trophic enrichment (∆15 N), larvae preferentially grazed on small MZP 15N but different δ13C). Growth rates (based on RNA/DNA estimates) were similar in autumn and winter, suggesting that growth was not affected by station-specific differences in the composition of the prey field. Our results not only underscore the important role of MZP on larval herring diets both in autumn and wintertime, but also emphasize the limitations of bulk stable isotope analysis. Given the current low recruitment in North Sea herring, these results provide significant information for future monitoring approaches relevant to stock assessment of this species.

Gene flow shapes spatial genetic structure and the potential for local adaptation. Among marine animals with nonmigratory adults, the presence or absence of a pelagic larval stage is thought to be a key determinant in shaping gene flow and the genetic structure of populations. In addition, the spatial distribution of suitable habitats is expected to influence the distribution of biological populations and their connectivity patterns. We used whole genome sequencing to study demographic history and reduced representation (double-digest restriction associated DNA) sequencing data to analyse spatial genetic structure in broadnosed pipefish (Syngnathus typhle). Its main habitat is eelgrass beds, which are patchily distributed along the study area in southern Norway. Demographic connectivity among populations was inferred from long-term (~30-year) population counts that uncovered a rapid decline in spatial correlations in abundance with distance as short as ~2 km. These findings were contrasted with data for two other fish species that have a pelagic larval stage (corkwing wrasse, Symphodus melops; black goby, Gobius niger). For these latter species, we found wider spatial scales of connectivity and weaker genetic isolation-by-distance patterns, except where both species experienced a strong barrier to gene flow, seemingly due to lack of suitable habitat. Our findings verify expectations that a fragmented habitat and absence of a pelagic larval stage promote genetic structure, while presence of a pelagic larvae stage increases demographic connectivity and gene flow, except perhaps over extensive habitat gaps.

Publisher's version (útgefin grein), Climatic impacts are especially pronounced in the Arctic, which as a region is warming twice as fast as the rest of the globe. Here, we investigate how mean climatic conditions and rates of climatic change impact parasitoid insect communities in 16 localities across the Arctic. We focus on parasitoids in a widespread habitat, Dryas heathlands, and describe parasitoid community composition in terms of larval host use (i.e., parasitoid use of herbivorous Lepidoptera vs. pollinating Diptera) and functional groups differing in their closeness of host associations (koinobionts vs. idiobionts). Of the latter, we expect idiobionts—as being less fine‐tuned to host development—to be generally less tolerant to cold temperatures, since they are confined to attacking hosts pupating and overwintering in relatively exposed locations. To further test our findings, we assess whether similar climatic variables are associated with host abundances in a 22 year time series from Northeast Greenland. We find sites which have experienced a temperature rise in summer while retaining cold winters to be dominated by parasitoids of Lepidoptera, with the reverse being true for the parasitoids of Diptera. The rate of summer temperature rise is further associated with higher levels of herbivory, suggesting higher availability of lepidopteran hosts and changes in ecosystem functioning. We also detect a matching signal over time, as higher summer temperatures, coupled with cold early winter soils, are related to high herbivory by lepidopteran larvae, and to declines in the abundance of dipteran pollinators. Collectively, our results suggest that in parts of the warming Arctic, Dryas is being simultaneously exposed to increased herbivory and reduced pollination. Our findings point to potential drastic and rapid consequences of climate change on multitrophic‐level community structure and on ecosystem functioning and highlight the value of collaborative, systematic sampling effort., We are grateful for data from the Greenland Ecosystem Monitoring Programme provided by the Department of Bioscience, Aarhus University, Denmark, in collaboration with Greenland Institute of Natural Resources, Nuuk, Greenland, and the Department of Biology, University of Copenhagen, Denmark, and data from the Greenland Ecosystem Monitoring Programme provided by Asiaq—Greenland Survey, Nuuk, Greenland. We acknowledge CSC–IT Center for Science Ltd., Espoo, Finland, for the allocation of computational resources. Rósa Erlendsdóttir kindly provided access to the sampling site in Votilækur, Iceland. We thank Marie‐Andree Giroux and Kristen Peck for their assistance in Igloolik and Marjo Kilpinen, Eija Takala and Emma Hakanen for their hard work in the laboratory. We gratefully acknowledge funding from the Academy of Finland (grants 276909, 285803 to T.R., and 276671 to M.V.K.), Nessling Foundation (grants 201700420, 201600034, 201500090 to T.K.), Jane and Aatos Erkko Foundation, Societas pro Fauna et Flora Fennica, The Danish Environmental Protection Agency (N.M.S.), French Polar Institute‐IPEV (program “Interactions 1036” to O.G. and B.S.), INTERACT, Russian Foundation for Basic Research (grant 18‐05‐60261 to A.S. and N.S.), Research Council of Norway (grant 249902/F20), the Natural Sciences and Engineering Research Council of Canada (PGS‐D award to S.K.M., CGS‐D award to A.M.S., Discovery Grant to J.B.), Churchill Northern Studies Centre (Northern Research Fund Award to A.M.S. and M.P.), Canadian Polar Commission (Northern Scientific Training Program Award to D.L.B., K.H.I.D., S.K.M., A.M.S., and M.P.), ArcticNet, Polar Continental Shelf Project, Parks Canada, Fonds Québécois de Recherche ‐ Nature et Technologies, Polar Knowledge Canada, Entomological Society of Canada, University of Guelph. The Icelandic Research Fund (Rannsóknasjóður, grant nr 152468‐051 to I.C.B.). Thanks to PhyloPic contributors Gareth Monger and Melissa Broussard.

Different sea ice models apply unique approaches in the computation of nutrient diffusion between the ocean and the ice bottom, which are generally decoupled from the calculation of turbulent heat flux. A simple molecular diffusion formulation is often used. We argue that nutrient transfer from the ocean to sea ice should be as consistent as possible with heat transfer, since all of these fluxes respond to varying forcing in a similar fashion. We hypothesize that biogeochemical models that do not consider such turbulent nutrient exchanges between the ocean and the sea ice, despite considering brine drainage and bulk exchanges through ice freezing and melting, may underestimate bottom-ice algal production. The Los Alamos Sea Ice Model (CICE + Icepack) was used to test this hypothesis by comparing simulations without and with diffusion of nutrients across the sea ice bottom that are dependent on velocity shear, implemented in a way that is consistent with turbulent heat exchanges. Simulation results support the hypothesis, showing a significant enhancement of ice algal production and biomass when nutrient limitation was relieved by bottom-ice turbulent exchange. Our results emphasize the potentially critical role of turbulent exchanges to sea ice algal blooms and thus the importance of properly representing them in biogeochemical models. The relevance of this becomes even more apparent considering ongoing trends in the Arctic Ocean, with a predictable shift from light-limited to nutrient-limited growth of ice algae earlier in the spring, as the sea ice becomes more fractured and thinner with a larger fraction of young ice with thin snow cover.

Deep-sea sharks are little resilient to targeted harvesting and bycatch fisheries due to their life history strategy characterized by slow growth, late sexual maturity, low fecundity, and few offspring. The Northeast Atlantic component of Etmopterus spinax is an example of a species where substantial population changes in terms of abundances have occurred due to intensive fisheries induced mortality. This has led to the IUCN categorization of E. spinax as Vulnerable with a negative population trend. To reverse the trend, management and conservation criteria are needed. To give management advice, knowledge about the connectivity between potential populations need to be improved, together with an understanding of their habitat and life history. In this study, the potential populations and differences between the locations sampled in the NE Atlantic, and particularly Norway, are studied by analyzing the mitochondrial NADH2 gene. Overall, at all locations the haplotype diversity is moderate to high, and the nucleotide diversity low. This can indicate relatively recent population expansion. The results also show significant population structure between the Norwegian locations and the rest of NE Atlantic. On a smaller scale, there was weak population structure within Norway and none for the rest. Against expectations, no genetic difference was found between a fjord and coastal population off Trondheim, NO. The lack of identifying genetic differences and population structure can be due to migration, or that NADH2 is not a suitable marker to identify this on a relatively small scale.

Climate warming in the Arctic has resulted in widespread changes in the physical oceanographic characteristics of the Arctic Ocean. One particularly visible change is the loss of sea ice both within the Arctic Ocean and in the surrounding shelf seas. Compared to the other shelf seas, the Barents Sea has experienced the largest declines in sea ice extent, with much of this loss occurring during winter. In response to these changes, a range of biological responses have already begun–from population declines to changes in community compositions. This thesis explored sea ice dependencies of various Arctic marine organisms. Paper 1 explored if a pelagic phase could be party of the life cycle of a so-far presumed fully sea ice-associated amphipod. Paper 2 investigated seasonal (rarely studied polar night versus commonly studied polar day) diet changes in three zooplankton and two sea ice-associated amphipod species. Paper 3 quantified the contributions of sea ice-associated and pelagic (open water) carbon sources to diets of two different Arctic seal species: the harp and ringed seals. The main findings of this thesis demonstrate that in this time of transition–where Arctic sea ice is still present but receding–the studied Arctic marine organisms exhibit a wide range of sea ice dependence. While the seal species displayed strong seasonal associations to the sea ice habitat (in terms of energetic sources), zooplankton and sea ice amphipod species had varying degrees of trophic plasticity (for example, polar night feeding activity and being supported by pelagic carbon). This thesis research increases the knowledge of potential biological response to the current state of the Arctic.

Feeding studies give an indication about the role and position of species within ecosystems and provide crucial knowledge for management. Traditional methods are based on morphological identification of prey. DNA metabarcoding is a promising tool that allow for identification of specific prey items, also when highly digested. A total of 63 stomachs from three skate species caught in the Barents Sea region were investigated by DNA metabarcoding. Differences were found between species and size classes, reflecting the distributions and respective lengths of the skates. No sexual differences were found. The smaller size classes preferred small and soft-bodied prey species. Arctic skate have been observed to perform short-term depth changes, being confirmed by a higher portion of pelagic diet compared to the other two. Spinytail skate, reaching the largest maximum size, was more piscivorous. The smaller thorny skate predominantly preyed upon soft-bodied invertebrates. A comparison with traditional methods was performed, revealing that DNA metabarcoding identified up to twenty times more species. This includes rapidly digestible prey such as Annelida, Cnidaria and Chaetognatha, although secondary predation cannot be ruled out. DNA metabarcoding is a powerful tool for mapping the range of prey diversity and is an important complementation to traditional methods.

The introduction of pink salmon in the White Sea area during the second half of the 20th century has resulted in the establishment of a self-sustaining population of pink salmon in rivers draining to the Barents Sea and North Atlantic Ocean. The size and abundance of pink salmon is dependent on survival during the marine phase, where they acquire more than 95% of their body weight. Investigating marine growth of pink salmon caught in Norwegian rivers is important to understand population dynamics, and their geographical expansion and variation in abundance and size over time. The aim of this study was to investigate how early marine growth affect adult size of pink salmon caught in Norwegian rivers prior to spawning. Scales from pink salmon caught in the river Skallelva in Finnmark county in 2019 and 2021, and scales from pink salmon caught in Central Norway in 2021, were used in scale analyses to compare annual, seasonal and regional growth characteristics. Scale analyses of pink salmon in this study showed that fish entering Norwegian rivers in 2021 seemed to have had better growth conditions from sea entry to the winter, compared to fish entering rivers in 2019, despite large individual variation. The growth pattern showed a short period of reduced growth during summer and early autumn that was characteristic in fish caught in both Skallelva in 2019, Skallelva in 2021 and Central Norway in 2021. This seems a typical trait of growth in pink salmon when they are believed to stay in coastal areas and has been described for pink salmon native to the Pacific Ocean. During autumn the growth increased and stabilized before a pronounced period of reduced growth during winter. The autumn and winter growth had more effect on fish size at river entry than the first period of marine growth. This study is the first to provide detailed information about scale growth in pink salmon between years and regions. The results from this work is important in understanding early growth in pink salmon and what may affect their size as adults entering Norwegian rivers to spawn. Pink salmon - scale analysis - growth patter - winter zone - Atlantic Ocean

There has been a considerable number of reports on Hg concentrations in Arctic mammals since the last Arctic Monitoring and Assessment Programme (AMAP) effort to review biological effects of the exposure to mercury (Hg) in Arctic biota in 2010 and 2018. Here, we provide an update on the state of the knowledge of health risk associated with Hg concentrations in Arctic marine and terrestrial mammal species. Using available population-specific data post-2000, our ultimate goal is to provide an updated evidence-based estimate of the risk for adverse health effects from Hg exposure in Arctic mammal species at the individual and population level. Tissue residues of Hg in 13 species across the Arctic were classified into five risk categories (from No risk to Severe risk) based on critical tissue concentrations derived from experimental studies on harp seals and mink. Exposure to Hg lead to low or no risk for health effects in most populations of marine and terrestrial mammals, however, subpopulations of polar bears, pilot whales, narwhals, beluga and hooded seals are highly exposed in geographic hotspots raising concern for Hg-induced toxicological effects. About 6% of a total of 3500 individuals, across different marine mammal species, age groups and regions, are at high or severe risk of health effects from Hg exposure. The corresponding figure for the 12 terrestrial species, regions and age groups was as low as 0.3% of a total of 731 individuals analyzed for their Hg loads. Temporal analyses indicated that the proportion of polar bears at low or moderate risk has increased in East/West Greenland and Western Hudson Bay, respectively. However, there remain numerous knowledge gaps to improve risk assessments of Hg exposure in Arctic mammalian species, including the establishment of improved concentration thresholds and upscaling to the assessment of population-level effects.

We examined spatial variation in total mercury (THg) concentrations in 100 hair samples collected between 2008 and 2016 from 87 polar bears (Ursus maritimus) from the Norwegian (Svalbard Archipelago, western Barents Sea) and Russian Arctic (Kara Sea, Laptev Sea, and Chukchi Sea). We used latitude and longitude of home range centroid for the Norwegian bears and capture position for the Russian bears to account for the locality. We additionally examined hair stable isotope values of carbon (δ13C) and nitrogen (δ15N) to investigate feeding habits and their possible effect on THg concentrations. Median THg levels in polar bears from the Norwegian Arctic (1.99 μg g−1 dry weight) and the three Russian Arctic regions (1.33–1.75 μg g−1 dry weight) constituted about 25–50% of levels typically reported for the Greenlandic or North American populations. Total Hg concentrations in the Norwegian bears increased with intake of marine and higher trophic prey, while δ13C and δ15N did not explain variation in THg concentrations in the Russian bears. Total Hg levels were higher in northwest compared to southeast Svalbard. δ13C and δ15N values did not show any spatial pattern in the Norwegian Arctic. Total Hg concentrations adjusted for feeding ecology showed similar spatial trends as the measured concentrations. In contrast, within the Russian Arctic, THg levels were rather uniformly distributed, whereas δ13C values increased towards the east and south. The results indicate that Hg exposure in Norwegian and Russian polar bears is at the lower end of the pan-Arctic spectrum, and its spatial variation in the Norwegian and Russian Arctic is not driven by the feeding ecology of polar bears.

Trait-based approaches connect the traits of species to ecosystem functions to estimate the functional diversity of communities and how they may respond to environmental change. For the first time, we compiled a traits matrix across 11 traits for 28 species of Arctic ice meiofauna, including Copepoda (Subclass), Nematoda (Phylum), Acoela (Order), Rotifera (Phylum), and Cnidaria (Phylum). Over 50 years of pan-Arctic literature were manually reviewed, and trait categories were assigned to enable future trait–function connections within the threatened ice-associated ecosystem. Approximately two-thirds of the traits data were found at the genus or species level, ranging from 44% for Nematoda to 100% for Cnidaria. Ice meiofauna were shown to possess advantageous adaptations to the brine channel network within sea ice, including a majority with small body widths

Mitochondrial DNA (mtDNA) has been extensively used to explore phylogenetics since the its use became available. This science is considered useful in discerning relationships and evolutionary histories that are not possible with morphological studies alone. The sleeper sharks (genus: Somniosus) are a group of poorly understood, long-lived, iconic species whose taxonomic status within the genus have been argued about for many years. The aim of this study was to sequence and characterize mitogenomes of all Somniosus species to gain a better phylogenetic understanding of the genus and to discriminate the delimitations between species. Using next-generation sequencing, complete to nearly-complete mitogenomes were assembled of all species within Somniosus. Clear divergence was shown between the subgenera S. Somniosus and S. Rhinoscymnus, estimated at 7.8 million years ago, with no discernable differentiation between Somniosus pacificus and Somniosus antarcticus, nor between Somniosus rostratus and Somniosus longus. The results suggest a panmictic population of Somniosus pacificus and Somniosus antarcticus, as well as between Somniosus rostratus and Somniosus longus. Combined, the results of this study show that a thorough revision is needed of the Somniosus genus to improve taxonomic resolution and allow more informed conservation related decisions to be made.

This work summarizes ice algal studies, presented as biomass and species temporal and spatial distribution, during 11 cruises conducted between 1986 and 2012. The majority of the biomass was found as loosely attached sub-ice algal layers, and sampling required diving. A maximum of 40 mg chlorophyll m−2 and 15.4 × 109 cells m−2 was measured in May. The species diversity was separated in zones based on ice thickness, with the highest biodiversity in the medium-thick ice of 30–80 cm. Nitzschia frigida was the most common species. There was a significant positive relationship between the dominance of this species and ice thickness, and it dominated completely in thick ice. Other common species, such as N. promare and Fossulaphycus arcticus reacted oppositely, by becoming less dominant in thick ice, but the positive correlation between total cell numbers and number of these three species indicated that they would most likely dominate in most populations. Melosira arctica was found several times below medium-thick annual ice. Algae occurred from top to bottom in the ice floes and in infiltration layers, but in very low numbers inside the ice. The bipolar dinoflagellates Polarella glacialis inhabited the ice, both as vegetative cells and cysts. The algal layers detached from the ice and sank in late spring when melting started. The cells in the sediments form an important food source for benthic animals throughout the year. Fjord populations survive the winter on the bottom and probably form next year’s ice algal inoculum. A few ‘over-summer’ populations found in sheltered locations might provide supplementary food for ice amphipods in late summer. The future faith of the ice flora is discussed in view of a warmer climate, with increased melting of the Arctic ice cover.

Understanding Arctic deep-sea ecosystem structure and functioning is an urgent task because ongoing sea-ice reduction modifies and opens up these regions for resource exploitation. This thesis aims to increase knowledge on epibenthos in the Chukchi Borderland (deep Arctic Ocean). Paper I documents low epifaunal densities and biomass, a total of 152 taxa, and dominance of Echinodermata, Arthropoda and Cnidaria for taxon richness and/or abundance/biomass. Most taxa had Atlantic biogeographic affinity. Ridge and plateau community structure did not differ but western and eastern communities did, linked to food input and drop-stone availability. Paper II findings generally support the current view of common trait modalities in deep-sea epibenthos. Higher functional diversity at mid-depth than deep stations indicated more variable resource use, while higher functional redundancy in basin communities suggested adaptation to more homogeneous environments by fewer, shared traits. Paper III suggests tighter pelagic-benthic coupling in 2005 (high sea-ice cover) than 2016 (low sea-ice cover), reflected in higher isotopic-niche overlap and shorter isotopic-distances between pelagic and benthic systems in 2005. Isotope values suggest higher ice algal contribution in 2005 and more refractory food consumed by benthos in 2016. This study adds much-needed biological and ecological knowledge with implications for vulnerability and resilience and highlights the need for deep-sea monitoring.

The net productivity of sea ice is determined by the physical and geochemical characteristics of the ice–ocean system and the activity of organisms inhabiting the ice. Differences in habitat suitability between first-year and multi-year sea ice can affect the ice algal community composition and acclimation state, introducing considerable variability to primary production within each ice type. In this study, we characterized the biogeochemical variability between adjacent first-year and multi-year sea ice floes in the Lincoln Sea of the Canadian High Arctic, during the May 2018 Multidisciplinary Arctic Program—Last Ice sampling campaign. Combining measurements of transmitted irradiance from a remotely operated underwater vehicle with laboratory-based oxygen optode incubations, this work shows widespread heterotrophy (net oxygen uptake) in the bottom 10 cm of both ice types, particularly in thick multi-year ice (>2.4 m) and early morning of the 24-h day. Algal acclimation state and species composition varied between ice types despite similar net community production due to widespread light and nutrient limitation. The first-year ice algal community was increasingly dominated over spring by the potentially toxin-producing genus Pseudonitzschia that was acclimated to high and variable light conditions characteristic of a thinner ice habitat with mobile snow cover. In comparison, the multi-year ice harbored more shade-acclimated algae of mixed composition. This work highlights the potential for heterotrophy in sea ice habitats of the High Arctic, including first measurements of such O2-uptake in multi-year ice floes. Observed differences in photophysiology between algae of these sea ice types suggests that a shift toward higher light availability and a younger sea ice cover with climate change does not necessarily result in a more productive system. Instead, it may favor future sea ice algal communities of different species composition, with lower photosynthetic potential but greater resilience to stronger and more variable light conditions.

The warming trend of the Arctic is punctuated by several record-breaking warm years with very low sea ice concentrations. The nature and reversibility of marine ecosystem responses to these multiple extreme climatic events (ECEs) are poorly understood. Here, we investigate the ecological signatures of three successive bottom temperature maxima concomitant with surface ECEs between 2004 and 2017 in the Barents Sea across spatial and organizational scales. We observed community-level redistributions of fish concurrent with ECEs at the scale of the whole Barents Sea. Three groups, characterized by different sets of traits describing their capacity to cope with short-term perturbations, reacted with different timing and intensity to each ECE. Arctic species co-occurred more frequently with large predators and incoming boreal taxa during ECEs, potentially affecting food web structures and functional diversity, accelerating the impacts of long-term climate change. On the species level, responses were highly diversified, with different ECEs impacting different species, and species responses (expansion, geographical shift) varying from one ECE to another, despite the environmental perturbations being similar. Past ECEs impacts, with potential legacy effects, lagged responses, thresholds, and interactions with the underlying warming pressure, could constantly set up new initial conditions that drive the unique ecological signature of each ECE. These results highlight the complexity of ecological reactions to multiple ECEs and give prominence to several sources of process uncertainty in the predictions of climate change impact and risk for ecosystem management. Long-term monitoring and studies to characterize the vertical extent of each ECE are necessary to statistically link demersal species and environmental spatial–temporal patterns. In the future, regular monitoring will be crucial to detect early signals of change and understand the determinism of ECEs, but we need to adapt our models and management to better integrate risk and stochasticity from the complex impacts of global change. Successive extreme climatic events lead to immediate, large-scale, and diverse responses from fish in the Arctic

Populations living close together in space are likely to experience more similar environmental fluctuations, and thus may display similar temporal changes in population parameters, in other words synchrony. Interspecific synchrony has implications for ecosystem stability, and highly synchronous populations and communities tend to have a higher risk of extinction. In this thesis, I used long-term data from six seabird species, breeding at Hornøya colony in the Barents Sea, to analyse the degree of synchrony in breeding success. I then investigated potential environmental drivers of inter-specific synchrony, and also asynchrony, in their breeding success. A generalized linear model (GLMs) was fitted to the breeding success data of each species. Pairwise correlations of the model residuals were calculated for all species combinations. Model selection was performed including climate and oceanographic covariates, which were considered to possibly influence breeding success, to identify the best model of breeding success for each species. If a covariate was included in the best-fitting model for multiple species, I tested the extent to which that shared effect led to interspecific synchrony. A higher level of synchrony in breeding success was found between auks and between gulls, than between these groups of birds. Atlantic water-inflow during winter was identified as a driver of breeding success, with potential synchronizing effect for kittiwakes (Rissa tridactyla), great black-backed gulls (Larus marinus) and herring gulls (Larus argentatus). For Atlantic puffins (Fratercula arctica) and razorbills (Alca torda), a lagged effect of Atlantic water-inflow during winter on breeding success was found. Sea surface temperature was identified as a potentially synchronizing driver of breeding success for great black-backed gulls and herring gulls. The results indicate that climate-induced changes in the Barents Sea ecosystem can have pronounced effects on seabirds breeding on Hornøya.

This thesis presents a study of the impact of climate warming on deep demersal fish communities East of Greenland, in a transition zone between Atlantic and Arctic waters. Climate change was expected to affect fish community composition and functional characterization via poleward distributional shifts and changes in abundance. To assess the climate driven effects on fish communities, I used long term survey data (1998 – 2016) covering a depth range of nearly 1500m. Data on taxonomic composition and abundance were combined with functional traits information to address temporal changes in diversity, community structure and functional characterization at different depths. The rapid warming registered during the study period was associated with a decline in both taxonomic and functional diversity, with the sharpest negative trends at depths between 300 and 1000m. Overall, the abundance of boreal species with a generalist diet, like greater argentine (Argentina silus) and Tusk (Brosme brosme) increased, whereas the abundance of Arctic fish benthivores like Lycodes spp. and northern wolffish (Anarhichas denticulatus) decreased. Functional reorganization was most pronounced at depths from 300 to 750m. Species living at great depths down to 1000m were affected by climate events detected at the surface, displaying rapid responses that were likely mediated by behaviour. These findings suggest that deep sea demersal species can respond just as fast to environmental change as species living in the shallower waters of shelf seas. The diversity loss was not consistent with the general expectations of a climate driven increase in Arctic marine biodiversity, observed in other high latitude ecosystems. Clearly, regional conditions may decide the climate driven effects on diversity, as the combination of currents and topography East of Greenland seems to inhibit colonization by boreal species. The documented taxonomic and functional reorganization of the fish communities east of Greenland are likely to affect ecosystem functioning and vulnerability. The increasing importance of generalist species, also feeding in the pelagic food web compartment, changes the main pathways of energy and material flow, and tends to connect more tightly the food web. The increased connectivity of the food web may promote the spread of perturbations within an ecosystem that is losing adaptive capacity due to a decline in diversity. Since the East Greenland Ecosystem seems to be highly vulnerable to environmental change, future monitoring and a precautionary approach to fisheries and ecosystem management is suggested.

The substantial influx of freshwater to High Arctic coastal ecosystems influences nutrient, organic matter, and sediment dynamics, stratification, and light availability throughout the melt season. These changes shape pelagic microbial community composition and functioning, though little is known about impacts on nearshore benthic bacteria. Globally, mudflats are hotspots for biogeochemical cycling, and expected climate change driven increases in terrestrial runoff to coastal areas have highlighted a need to understand the influence these inputs from land might have on Arctic estuarine tidal flat bacteria. In this study, I investigated microbial community composition and function in an estuarine tidal flat through a full melt season, using a combined approach of metabarcoding and carbon-source utilization assays under different salinity treatments. I found that bacterial communities varied through both space and time, as environmental conditions shifted due to riverine inputs and local processes, with salinity as a key structuring gradient. Arctic freshwater bacteria demonstrated higher capacities for degrading a wider range of carbon substrates than Arctic marine microbial communities, indicating higher potential for degradation of complex terrestrially derived organic material in freshwater systems. Terrestrial and riverine taxa were transported with melt water and deposited in sediments, composing up to 60% of sequences in downstream communities. However, their unique functional capabilities appear to be inhibited by the high salinities found in subtidal mudflats, and the highest potential for utilization of terrestrially derived organic matter may be limited to areas where sediments are permeated by freshwater. With anticipated increases in riverine discharge and permafrost thaw in a warming Arctic, tidal flats will likely be more frequently inundated with freshwater and the resident bacteria will have increased access to bioavailable terrestrial organic matter, extending the region where terrestrial organic matter can be readily utilized by microbial communities further towards the sea.

Despite our growing understanding of the global carbon cycle, scientific consensus on the drivers and mechanisms that control dissolved organic carbon (DOC) turnover in aquatic systems is lacking, hampered by the mismatch between research that approaches DOC reactivity from either intrinsic (inherent chemical properties) or extrinsic (environmental context) perspectives. Here we propose a conceptual view of DOC reactivity in which the combination of intrinsic and extrinsic factors controls turnover rates and determines which reactions will occur. We review three major types of reactions (biological, photochemical, and flocculation) from an intrinsic chemical perspective and further define the environmental features that modulate the expression of chemically inherent reactivity potential. Finally, we propose hypotheses of how extrinsic and intrinsic factors together shape patterns in DOC turnover across the land-to-ocean continuum, underscoring that there is no intrinsic DOC reactivity without environmental context. By acknowledging the intrinsic–extrinsic control duality, our framework intends to foster improved modeling of DOC reactivity and its impact on ecosystem services.

This is the peer reviewed version of the following article: "Circumpolar patterns of Arctic freshwater fish biodiversity: A baseline for monitoring", which has been published in final form at https://doi.org/10.1111/fwb.13405. This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Use of Self-Archived Versions Climate change, biological invasions, and anthropogenic disturbance pose a threat to the biodiversity and function of Arctic freshwater ecosystems. Understanding potential changes in fish species distribution and richness is necessary, given the great importance of fish to the function of freshwater ecosystems and as a resource to humans. However, information gaps limit large‐scale studies and our ability to determine patterns and trends in space and time. This study takes the first step in determining circumpolar patterns of fish species richness and composition, which provides a baseline to improve both monitoring and conservation of Arctic freshwater biodiversity. Information on species presence/absence was gathered from the Circumpolar Biodiversity Monitoring Program's Freshwater Database and used to examine patterns of freshwater fish γ‐, α‐, and β‐diversity across 234° of longitude in the Arctic. The metrics of diversity provided information on species richness and composition across hydrobasins, ecoregions, and Arctic zones. Circumpolar patterns of fish species biodiversity varied with latitude, isolation, and coarse ecoregion characteristics; patterns were consistent with historic and contemporary barriers to colonisation and environmental characteristics. Gamma‐diversity was lower in the high Arctic compared to lower latitude zones, but α‐diversity did not decrease with increasing latitude below 71°N, reflecting glacial history. Alpha‐diversity was reduced to a single species, Arctic charr Salvelinus alpinus, in ecoregions above 71°N, where γ‐diversity was the lowest. Beta‐diversity indicated little variation in the composition and richness of species across the High Arctic; at lower latitudes, ecoregions contained more species, although species composition turned over across large spatial extents. In an analysis of five ecoregions in the circumpolar Arctic, physical isolation, and ecoregion area and topography were identified as strong drivers of γ‐, α‐, and β‐diversity. Physical isolation reduced the γ‐ and α‐diversity, and changes in β‐diversity between adjacent locations were due mainly to losses in species richness, rather than due to differences in species composition. Heterogeneity of habitats, environmental gradients, and geographic distance probably contributed to patterns of fish dissimilarity within and across ecoregions. This study presents the first analysis of large‐scale patterns of freshwater fish biodiversity in the circumpolar Arctic. However, information gaps in space, time, and among taxonomic groups remain. Future inclusion of extensive archive and new data will allow future studies to test for changes and drivers of the observed patterns of biodiversity. This is important given the potential impacts of ongoing and accelerating climate change, land use, and biotic exchange on Arctic fish biodiversity.

The manuscripts of the dissertation presented here encompass several aspects of adaptations in ice-associated amphipods, focussing mainly on their ability to cope with increased light levels, including the ultraviolet radiation (UVR), under thinner or more dispersed sea ice. A decrease in multi-year ice (MYI) in the Arctic, and a concurrent reduction in stable habitat for ice-associated species, is a substantial threat to the ecosystem in the Arctic Ocean, as these amphipods act as a key trophic link between primary production inside and under sea ice, and higher trophic levels. In a shift from thicker multi-year ice to thinner and less coherent first-year ice, the changes in under-ice light levels may be considered as the second-most significant factor, after the loss of the habitat itself. This thesis has as its overarching ambition to contribute to our understanding of how this change may affect the organisms living in association with the Arctic sea ice. While the reduction in stratospheric ozone and the resulting seasonal increases in ultraviolet radiation (UVR) have been halted and to some extent reversed, the recovery period is calculated to be protracted, and might even be affected negatively by the ongoing warming of the troposphere through the ongoing emission of greenhouse gases. So both the direct and indirect effects of UVR on sea ice-associated amphipods have been proven to be a significant factor, especially in the context of a reduction in both thickness and extent of the Arctic sea ice cover. The effects of pigment ingestion and accumulation have also been of particular interest, as well as the capacity of organisms to cope with and adapt to elevated, radiation-induced oxidative stress. Another aspect of this thesis has been to investigate hitherto and potentially overlooked aspects of the distribution and occurrence of ice-associated amphipods in the Antarctic. This study contributed to altering the previously held assumption that ice-associated amphipods in Antarctic waters were predominantly originating from the shallow benthos and were thus able to interact with sea ice only when it is over relatively shallow depths. One of the papers presented in this thesis documents several pelagic amphipod species living in close association with sea ice. Hence, an important conclusion from this is that there is a pelago-sympagic component of Antarctic amphipods in the Weddell Sea.

Large swarms of individuals at different spatiotemporal scales characterise the distributions of many animal species. In the ocean several mesozooplankton taxa aggregate in large patches or swarms driven by active behavioural responses to hydrographic structures, although intrinsic biotic characteristics of species´ life cycles not related with the environment can also affect spatial distributions. To understand the mechanisms of aggregation and disentangle environmental and pure spatial effects contributing to patch formation, we extensively sampled a large Calanus finmarchicus patch off the Lofoten islands, northern Norway, in spring 2017 by means of Laser Optical Plankton Counter (LOPC) cross shelf transects and biophysical samplings at fixed stations. We observed a clear association between the buoyant layer of the fresh, cold Norwegian Coastal Current (NCC) and high surface copepod abundances. Off shelf lateral displacements of the NCC along its path may retain copepods from the offshore basins. At deeper layers, copepod abundances increased markedly at regions with low sea level anomalies and reduced vertical flows. Our results also suggest that copepods performed short range diel vertical migration within the patch. Potential future changes in the seasonality and structure of the NCC and their impact on swarm occurrence and formation are discussed.

Cold seeps are locations on the seafloor where CH4 migrates from reservoirs below sediments towards the atmosphere, sustaining thereby a high microbial and macrofaunal biomass and a diversity contrasting from the surrounding seafloor. The oxidation of methane and sulphide are typically the main sources of primary productivity of these ecosystems and have therefore gained a particular attention in the global oceans. Yet, despite the ubiquitous presence of these seeping sites and the presence of gas hydrates in the Arctic Ocean and its adjacent shelves, the impact of methane on benthic and pelagic microbial communities in this region have remained limited. Recently, five gas hydrate bearing mounds with ongoing methane seeping activity were discovered south of Svalbard, in the northern Barents Sea. In this PhD project, I studied changes in the structure of microbial communities, including both prokaryotes and eukaryotes, and geochemical profiles at these mounds to highlight key microbial groups and to provide insights on their ecological roles. Different niches were addressed, including: deep anaerobic sediments (Paper I and II); niches at the sediment surface at gas flare locations and within bacterial mats and siboglinid fields (Paper III); and above gas flares in the shallow shelf water column (Paper IV). The microbial biodiversity and the structure of communities were successfully identified for each of the habitats listed above. Our investigations revealed a microbial composition similar to other cold seeps: a predominance of archaeal anaerobic methanotrophs (ANME) and sulphate-reducing bacteria (SRB) in CH4-rich sediments, a higher abundance of methane oxidizing bacteria associated to the Methylococcales in the surface sediments and water column; and a co-occurrence of other commonly found prokaryotic groups. Yet, uncommon biological traits were also uncovered at these methane seeping sites: the anaerobic oxidation of methane was merely only driven by ANME-1 without the co-occurrence of a specific SRB clade; an abundant methanotroph with little genetic similarity in databases was detected; and a strong niche differentiation of sulphide-oxidizing bacteria within the different bacterial mats. This project has thereby extended our knowledge on the microbial biodiversity at Arctic cold seeps and opened further future research perspective toward microbial activity and metabolism at these high latitudes.

Planktic foraminfera and shelled pteropods are important calcifying groups of zooplankton in all oceans. Their calcium carbonate shells are sensitive to changes in ocean carbonate chemistry predisposing them as an important indicator of ocean acidification. Moreover, planktic foraminfera and shelled pteropods contribute significantly to food webs and vertical flux of calcium carbonate in polar pelagic ecosystems. Here we provide, for the first time, information on the under-ice planktic foraminifera and shelled pteropod abundance, species composition and vertical distribution along a transect (82°–76°N) covering the Nansen Basin and the northern Barents Sea during the polar night in December 2019. The two groups of calcifiers were examined in different environments in the context of water masses, sea ice cover, and ocean chemistry (nutrients and carbonate system). The average abundance of planktic foraminifera under the sea-ice was low with the highest average abundance (2 ind. m–3) close to the sea-ice margin. The maximum abundances of planktic foraminifera were concentrated at 20–50 m depth (4 and 7 ind. m–3) in the Nansen Basin and at 80–100 m depth (13 ind. m–3) close to the sea-ice margin. The highest average abundance (13 ind. m–3) and the maximum abundance of pteropods (40 ind. m–3) were found in the surface Polar Water at 0–20 m depth with very low temperatures (–1.9 to –1°C), low salinity (Neogloboquadrina pachydermaand the subpolar speciesTurborotalita quinqueloba. The dominating polar pteropod speciesLimacina helicinawas represented by the juvenile and veliger stages. This winter study offers a unique contribution to our understanding of the inter-seasonal variability of planktic foraminfera and shelled pteropods abundance, distribution and population size structure in the Arctic Ocean.

Rising temperatures in the Arctic affect soil microorganisms, herbivores, and peatland vegetation, thus directly and indirectly influencing microbial CH4 production. It is not currently known how methanotrophs in Arctic peat respond to combined changes in temperature, CH4 concentration, and vegetation. We studied methanotroph responses to temperature and CH4 concentration in peat exposed to herbivory and protected by exclosures. The methanotroph activity was assessed by CH4 oxidation rate measurements using peat soil microcosms and a pure culture of Methylobacter tundripaludum SV96, qPCR, and sequencing of pmoA transcripts. Elevated CH4 concentrations led to higher CH4 oxidation rates both in grazed and exclosed peat soils, but the strongest response was observed in grazed peat soils. Furthermore, the relative transcriptional activities of different methanotroph community members were affected by the CH4 concentrations. While transcriptional responses to low CH4 concentrations were more prevalent in grazed peat soils, responses to high CH4 concentrations were more prevalent in exclosed peat soils. We observed no significant methanotroph responses to increasing temperatures. We conclude that methanotroph communities in these peat soils respond to changes in the CH4 concentration depending on their previous exposure to grazing. This “conditioning” influences which strains will thrive and, therefore, determines the function of the methanotroph community.

1.The cost of reproduction on demographic rates is often assumed to operate through changing body condition. Several studies have found that reproduction depresses body mass more if the current conditions are severe, such as high population densities or adverse weather, than under benign environmental conditions. However, few studies have investigated the association between the fitness components and body mass costs of reproduction. 2.Using 25 years of individual-based capture–recapture data from Svalbard reindeer Rangifer tarandus platyrhynchus, we built a novel Bayesian state-space model that jointly estimated interannual change in mass, annual reproductive success and survival, while accounting for incomplete observations. The model allowed us to partition the differential effects of intrinsic and extrinsic factors on both non-reproductive mass change and the body mass cost of reproduction, and to quantify their consequences on demographic rates.3. Contrary to our expectation, the body mass cost of reproduction (mean = –5.8 kg) varied little between years (CV = 0.08), whereas the between-year variation in body mass changes, that were independent of the previous year's reproductive state, varied substantially (CV = 0.4) in relation to autumn temperature and the amount of rain-on-snow in winter. This body mass loss led to a cost of reproduction on the next reproduction, which was amplified by the same environmental covariates, from a 10% reduction in reproductive success in benign years, to a 50% reduction in harsh years. The reproductive mass loss also resulted in a small reduction in survival.4. Our results show how demographic costs of reproduction, driven by interannual fluctuations in individual body condition, result from the balance between body mass costs of reproduction and body mass changes that are independent of previous reproductive state. We illustrate how a strong context-dependent fitness cost of reproduction can occur, despite a relatively fixed body mass cost of reproduction. This suggests that female reindeer display a very conservative energy allocation strategy, either aborting their reproductive attempt at an early stage or weaning at a relatively constant cost. Such a strategy might be common in species living in a highly stochastic and food limited environment.

The regulatory network of R2R3 MYB transcription factors in anthocyanin biosynthesis is not fully understood in blue-coloured berries containing delphinidin compounds. We used blue berries of bilberry (Vaccinium myrtillus) to comprehensively characterise flavonoid-regulating R2R3 MYBs, which revealed a new type of co-regulation in anthocyanin biosynthesis between members of MYBA-, MYBPA1- and MYBPA2-subgroups. VmMYBA1, VmMYBPA1.1 and VmMYBPA2.2 expression was elevated at berry ripening and by abscisic acid treatment. Additionally, VmMYBA1 and VmMYBPA1.1 expression was strongly downregulated in a white berry mutant. Complementation and transient overexpression assays confirmed VmMYBA1 and VmMYBA2 to induce anthocyanin accumulation. Promoter activation assays showed that VmMYBA1, VmMYBPA1.1 and VmMYBPA2.2 had similar activity towards dihydroflavonol 4-reductase (DFR) and anthocyanidin synthase (ANS), but differential regulation activity for UDP-glucose flavonoid 3-O-glucosyltransferase (UFGT) and flavonoid 3'5'-hydroxylase (F3'5'H) promoters. Silencing of VmMYBPA1.1 in berries led to the downregulation of key anthocyanin and delphinidin biosynthesis genes. Functional analyses of other MYBPA regulators, and a member of novel MYBPA3 subgroup, associated them with proanthocyanidin biosynthesis and F3'5'H expression. The existence of 18 flavonoid-regulating MYBs indicated gene duplication, which may have enabled functional diversification among MYBA, MYBPA1 and MYBPA2 subgroups. Our results provide new insights into the intricate regulation of the complex anthocyanin profile found in blue-coloured berries involving regulation of both cyanidin and delphinidin branches.

In colonially breeding marine predators, individual movements and colonial segregation are influenced by seascape characteristics. Tidewater glacier fronts are important features of the Arctic seascape and are often described as foraging hotspots. Albeit their documented importance for wildlife, little is known about their structuring effect on Arctic predator movements and space use. In this study, we tested the hypothesis that tidewater glacier fronts can influence marine bird foraging patterns and drive spatial segregation among adjacent colonies. We analysed movements of black-legged kittiwakes (Rissa tridactyla) in a glacial fjord by tracking breeding individuals from five colonies. Although breeding kittiwakes were observed to travel up to ca. 280 km from the colony, individuals were more likely to use glacier fronts located closer to their colony and rarely used glacier fronts located farther away than 18 km. Such variation in the use of glacier fronts created fine-scale spatial segregation among the four closest (ca. 7 km distance on average) kittiwake colonies. Overall, our results support the hypothesis that spatially predictable foraging patches like glacier fronts can have strong structuring effects on predator movements and can modulate the magnitude of intercolonial spatial segregation in central-place foragers.

The diatoms make up the largest and most important group of marine microalgae. They are responsible for a quarter of all inorganic carbon fixation in the ocean and are thus of immense importance for global CO2 sequestration. This makes them good candidates for biological carbon capture and utilization (CCU) by mass cultivation using CO2 emissions from industrial point sources. Large scale cultivation of marine diatoms will produce valuable biomass containing marine lipids and proteins with possible commercial application. Commercial production of biomass requires extensive knowledge on the species to be cultivated in terms of optimal cultivation conditions, but also on the bioactivity, cytotoxicity and how the species responds to changing cultivation conditions. Most studies on the bioactivity and biochemistry of marine diatoms have been performed on species cultivated in the laboratory. Cultivation conditions are known to affect the biochemistry of diatoms, and large-scale cultivation will inevitably lead to a change in the environmental conditions. The present study was therefore designed to investigate how Porosira glacialis cultivated in large scale responds to changes in abiotic and biotic cultivation conditions. The first study was designed to investigate whether the direct addition of factory smoke as a source of CO2 would trigger production of bioactive or cytotoxic compounds in P. glacialis. Bioactivity testing revealed activity in anticancer, antibacterial and antibiofilm assays, in addition to cytotoxicity against human lung fibroblasts and effect on the development of sea urchin larvae. Nonetheless, the addition of factory smoke did not increase the cytotoxicity of P. glacialis, nor did it change the beneficial antibacterial and antibiofilm activity. These results are favourable for further development of large-scale production of diatom biomass. P. glacialis was co-cultivated with zooplankton to investigate whether the bioactivity and metabolic expression of the diatom would change when exposed to grazing pressure. The bioactivity testing revealed that the presence of zooplankton increased the cytotoxicity of P. glacialis towards human normal lung fibroblasts. Investigation of active compounds and metabolomic analysis showed that the grazing pressure possibly influence the carotenoid concentration in P. glacialis. It was also found that the bioactivity might be traced to primary metabolites such as chlorophyll derived compounds. The study also showed that the use of OSMAC (one strain many compounds) could be a useful method for further investigation of the bioactivity of diatoms. Investigation of the antibiofilm activity of P. glacialis resulted in isolation of two compounds showing inhibition of biofilm formation by Staphylococcus epidermidis. The compounds isolated were methyl 3-hydroxyoctadecanoate and a pheophorbide-like compound. This is the first time compounds with antibiofilm activity have been isolated from P. glacialis as well as the first evidence of such activity from both the isolated compounds. Kiselalgene utgjør den største og viktigste gruppen av marine mikroalger. De er ansvarlige for en fjerdedel av all uorganisk karbonfiksering i havet og er dermed av enorm betydning for det globale CO2 opptaket. Dette gjør dem til gode kandidater for biologisk karbonfangst og -utnyttelse (CCU) i massedyrking ved bruk av CO2 fra industrielle punktutslipp. Storskala dyrking av marine kiselalger vil produsere verdifull biomasse som inneholder marine lipider og proteiner med mulig kommersiell anvendelse. Kommersiell produksjon av biomasse krever stor kunnskap om arten som skal dyrkes med tanke på best mulige dyrkingsforhold, men også om bioaktivitet, toksisitet og hvordan arten reagerer på endrede dyrkingsforhold. De fleste studier på bioaktivitet og biokjemi av marine kiselalger er utført på arter dyrket i laboratoriet. Det er kjent at dyrkingsforhold påvirker biokjemien til kiselalger, og dyrking i stor skala vil uunngåelig føre til en endring i miljøforholdene. Denne studien ble derfor designet for å undersøke hvordan Porosira glacialis dyrket i stor skala reagerer på endringer i abiotiske og biotiske dyrkingsforhold. Den første studien undersøkte om direkte tilsetning av fabrikkrøyk som kilde til CO2 ville endre produksjon av bioaktive eller toksiske forbindelser i P. glacialis. Bioaktivitetstesting avdekket aktivitet i alle tester: anticancer, antibakteriell og anti-biofilm. I tillegg viste resultatene toksisitet som hemmet utvikling av kråkebolle-larver og humane lungefibroblaster. Likevel økte ikke tilsetting av fabrikkrøyk toksisiteten til P. glacialis, og den hemmet heller ikke den gunstige antibakterielle og anti-biofilm-aktiviteten til algene. Disse resultatene er svært positive for videre utvikling av storskala produksjon av kiselalger. P. glacialis ble dyrket sammen med dyreplankton for å undersøke om bioaktiviteten og utrykket av metabolitter ville endre seg når de ble utsatt for stress i form av beitetrykk. Testing av bioaktivitet avslørte at tilstedeværelsen av zooplankton økte toksisiteten til P. glacialis mot humane normale lungefibroblaster. Undersøkelse av aktive forbindelser og metabolittprofiler viste at beitetrykket muligens påvirker konsentrasjonen av karotenoider i P. glacialis, og at bioaktiviteten muligens kan spores til primære metabolitter som klorofyllderivater. Studien viste også at bruk av OSMAC (one strain many compounds) kunne være en nyttig metode for videre undersøkelse av bioaktiviteten til kiselalger. Undersøkelse av anti-biofilm-aktiviteten til P. glacialis resulterte i isolasjon av to forbindelser som viser inhibering av biofilmdannelse av bakterien Staphylococcus epidermidis. De isolerte forbindelsene var metyl-3-hydroksyoktadekanoat og en pheophorbid-lignende struktur. Dette er første gang en forbindelse med anti-biofilm-aktivitet er blitt isolert fra P. glacialis, og det første beviset på slik aktivitet fra begge de isolerte forbindelsene.

Amblyraja hyperborea is the cartilaginous fish with the widest distribution across the Arctic seas and shelves and yet, large parts of its biology remain unknown. In a changing climate where the ice cover in the polar regions is retreating and fisheries are expanding northward, studying poorly known species is more important than ever. To provide a more exhaustive understanding of this species, horizontal and vertical distributions and temporal trends in the North Atlantic Ocean were evaluated, length at first maturity (L50) was estimated and potential nursery grounds were searched for. Additionally, the most common patterns of ventral colouration were described, and the variation of the colouration coverage was investigated. A transboundary approach was applied with data provided by four countries. Amblyraja hyperborea was found in every area surveyed, though not being evenly distributed and clustering along the shelf breaks, and the abundance observations of was found to decrease below 65ºN. The vertical distribution did not depend on sex nor size, and the majority of the observations were made from 200 to 1000 m depth. No conclusive temporal trends could be defined. The estimated L50 for females was of 70.5 cm total length (TL) and for males of 66.8 cm TL, and some indices of potential nursery grounds were found in Iceland and the Barents Sea. Lastly, distribution was found to play a role in the ventral colouration coverage, with lighter individuals being dominant in the eastern side of the study area, and darker individuals being most present in the western side. Overall, the transboundary approach was successful in the in-depth study of A. hyperborea, even with the limitations of the data. This study can serve as a baseline for future studies regarding other poorly known transboundary species.

Subcellular compartmentation is a fundamental property of eukaryotic cells. Communication and metabolic and regulatory interconnectivity between organelles require that solutes can be transported across their surrounding membranes. Indeed, in mammals, there are hundreds of genes encoding solute carriers (SLCs) which mediate the selective transport of molecules such as nucleotides, amino acids, and sugars across biological membranes. Research over many years has identified the localization and preferred substrates of a large variety of SLCs. Of particular interest has been the SLC25 family, which includes carriers embedded in the inner membrane of mitochondria to secure the supply of these organelles with major metabolic intermediates and coenzymes. The substrate specificity of many of these carriers has been established in the past. However, the route by which animal mitochondria are supplied with NAD+ had long remained obscure. Only just recently, the existence of a human mitochondrial NAD+ carrier was firmly established. With the realization that SLC25A51 (or MCART1) represents the major mitochondrial NAD+ carrier in mammals, a long-standing mystery in NAD+ biology has been resolved. Here, we summarize the functional importance and structural features of this carrier as well as the key observations leading to its discovery.

Sampling seabirdsThe vastness of the worlds' oceans makes them difficult to monitor. Seabirds that forage and breed across oceans globally have been recognized as sentinels of ocean health. Sydemanet al.looked across seabird species of both the Northern and Southern Hemispheres and found varying patterns. Northern Hemisphere species exhibited greater signs of stress and reduced breeding success, indicative of low fish resources. Southern Hemisphere species showed less impact on reproductive output, suggesting that the fish populations there have thus far been less disturbed. The differences across hemispheres indicate different strategies for conservation, with active recovery needed in the north and enhanced protection in the south.Science, abf1772, this issue p.980

Plastic is a major component of marine litter in the world’s oceans and therefore it is critical we develop a better understanding of the fate of plastic in the marine environment, and the potential impacts on biota. Microplastics are the most abundant size class (

Small rodents are a key indicator to understand the effect of rapidly changing winter climate on Arctic tundra ecosystems. However, monitoring rodent populations through the long Arctic winter by means of conventional traps has, until now, been hampered by snow cover and harsh ambient conditions. Here, we conduct the first extensive assessment of the utility of a newly developed camera trap to study the winter dynamics of small mammals in the Low Arctic tundra of northern Norway. Forty functional cameras were motion-triggered 20 172 times between September 2014 and July 2015, mainly by grey-sided voles (Myodes rufocanus (Sundevall, 1846)), tundra voles (Microtus oeconomus (Pallas, 1776)), Norwegian lemmings (Lemmus lemmus (Linnaeus, 1758)) and shrews (Sorex spp.). These data proved to be suitable for dynamical modelling of species-specific site occupancy rates. The occupancy rates of all recorded species declined sharply and synchronously at the onset of the winter. This decline happened concurrently with changes in the ambient conditions recorded by time-lapse images of snow and water. Our study demonstrates the potential of subnivean camera traps for elucidating novel aspects of year-round dynamics of Arctic small mammal communities. Les petits rongeurs constituent un indicateur clé pour comprendre l’effet du changement rapide du climat hivernal sur les écosystèmes de la toundra arctique. Cependant, le suivi des populations de rongeurs pendant le long hiver arctique au moyen de pièges conventionnels a jusqu’à présent été entravé par la couverture neigeuse et les conditions ambiantes difficiles. Les auteurs réalisent ici la première évaluation approfondie de l’utilité d’un nouveau piège photographique pour étudier la dynamique hivernale des petits mammifères dans la toundra du bas-arctique au nord de la Norvège. Quarante appareils photo fonctionnels ont été déclenchés par le mouvement 20 172 fois entre septembre 2014 et juillet 2015, principalement par des campagnols de Sundevall (Myodes rufocanus (Sundevall, 1846)), des campagnols nordiques (Microtus oeconomus (Pallas, 1776)), des lemmings norvégiens (Lemmus lemmus (Linnaeus, 1758)) et des musaraignes (Sorex spp.). Ces données se sont avérées appropriées pour la modélisation dynamique des taux d’occupation des sites spécifiques aux espèces. Les taux d’occupation de toutes les espèces enregistrées diminuaient abruptement et de façon synchrone au début de l’hiver. Ce déclin se produisait en même temps que les changements des conditions ambiantes enregistrés par les images de neige et d’eau prises à intervalles. Cette étude démontre le potentiel des piègesphotographiques subnivaux pour élucider de nouveaux aspects de la dynamique des communautés de petits mammifères de l’Arctique tout au long de l’année.

Source at https://doi.org/10.33265/polar.v38.3345. Time series of the marine CO2 system and related parameters at the IsA Station, by Adventfjorden, Svalbard, were investigated between March 2015 and November 2017. The physical and biogeochemical processes that govern changes in total alkalinity (TA), total dissolved inorganic carbon (DIC) and the saturation state of the calcium carbonate mineral aragonite (ΩAr) were assessed on a monthly timescale. The major driver for TA and DIC was changes in salinity, caused by river runoff, mixing and advection. This accounted for 77 and 45%, respectively, of the overall variability. It contributed minimally to the variability in ΩAr (5%); instead, biological activity was responsible for 60% of the monthly variations. For DIC, the biological activity was also important, contributing 44%. The monthly effect of air–sea CO2 fluxes accounted for 11 and 15% of the total changes in DIC and ΩAr, respectively. Net community production (NCP) during the productive season ranged between 65 and 85 g C m−2, depending on the year and the presence of either Arctic water or transformed Atlantic water (TAW). The annual NCP as estimated from DIC consumption was 34 g C m−2 yr−1 in 2016, which was opposite in direction but similar in magnitude to the integrated annual air–sea CO2 flux (i.e., uptake of carbon from the atmosphere) of −29 g C m−2 yr−1 for the same year. The results showed that increased intrusions of TAW into Adventfjorden in the future could possibly lower the NCP, with the potential to reduce the CO2buffer capacity and ΩAr over the summer season.

Source at https://doi.org/10.1186/s43008-019-0006-6. © The Author(s). 2019 High-latitude environments are warming, leading to changes in biological diversity patterns of taxa. Oomycota are a group of fungal-like organisms that comprise a major clade of eukaryotic life and are parasites of fish, agricultural crops, and algae. The diversity, functionality, and distribution of these organisms are essentially unknown in the Arctic marine environment. Thus, it was our aim to conduct a first screening, using a functional gene assay and high-throughput sequencing of two gene regions within the 18S rRNA locus to examine the diversity, richness, and phylogeny of marine Oomycota within Arctic sediment, seawater, and sea ice. We detected Oomycota at every site sampled and identified regionally localized taxa, as well as taxa that existed in both Alaska and Svalbard. While the recently described diatom parasite Miracula helgolandica made up about 50% of the oomycete reads found, many lineages were observed that could not be assigned to known species, including several that clustered with another recently described diatom parasite, Olpidiopsis drebesii. Across the Arctic, Oomycota comprised a maximum of 6% of the entire eukaryotic microbial community in Barrow, Alaska May sediment and 10% in sea ice near the Svalbard archipelago. We found Arctic marine Oomycota encode numerous genes involved in parasitism and carbon cycling processes. Ultimately, these data suggest that Arctic marine Oomycota are a reservoir of uncharacterized biodiversity, the majority of which are probably parasites of diatoms, while others might cryptically cycle carbon or interface other unknown ecological processes. As the Arctic continues to warm, lower-latitude Oomycota might migrate into the Arctic Ocean and parasitize non-coevolved hosts, leading to incalculable shifts in the primary producer community.

Reductions in Arctic sea ice affect marine food webs through a multitude of direct and indirect effects. We here analysed direct, indirect, and delayed associations between winter sea ice cover and year-to-year changes in biomasses of the main zooplankton groups (copepods, krill, amphipods) and planktivorous fishes (capelin, polar cod) in the central and northern Barents Sea from 1980 to 2015. Climate effects and species interactions were estimated jointly in a Bayesian state-space model framework. We found that population biomass of capelin increased in years with low sea ice cover, while biomasses of copepods and amphipods decreased. Furthermore, results suggested strong bottom-up effects by krill on capelin and by copepods on amphipods and top-down effects by polar cod on amphipods and by capelin on krill and copepods. Sea ice effects thereby cascaded through the food web through indirect and delayed effects. The indirect and delayed effects amplified the effects of sea ice cover on capelin, copepods and amphipods. For krill, low sea ice cover most likely increased biomass in the short-term but reduced biomass in longer terms because of increased predation pressure. These findings suggest that both bottom-up and top-down processes need to be considered when projecting effects of future climate change on ecosystems.