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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.

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.

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.

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.

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 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.

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.

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.

Marine fungi comprise a group of organisms that have been overlooked for a long time. Research interest has increased with the realization of the important ecological role and rich chemistry of marine fungi. Marine fungi have yielded thousands of new natural products the last decade, but many taxa remain unstudied. Marine fungi from the Arctic have not been reported in literature in regard to bioprospecting campaigns and represent a novel source of natural products. The aim of this thesis is to assess the potential of Arctic marine fungi to produce bioactive secondary metabolites by fermentation and genome analysis. This was achieved in three steps. First, fungi were isolated from the Svalbard archipelago. The 20 isolates obtained were characterized based on molecular markers and their antibacterial activity was tested using an agar diffusion assay (Paper 1). Secondly, three distinct marine fungi were whole genome sequenced and characterized. One of the fungi represented a putatively novel species which was circumscribed based on morphology and phylogenetic inference (Paper 2). Finally, a metabolite from one fungus among the 20 obtained around Svalbard was isolated and the bioactivities characterized (Paper 3). In Paper 1, half of the fungal isolates showed activity against pathogenic bacteria and every third isolate represents potentially new species of fungi. Five of these isolates are strictly marine fungi belonging to the order of Lulworthiales. The study showed that the Arctic can yield novel marine fungal diversity that can be utilized in bioprospecting. For Paper 2, three marine fungi were whole genome sequenced and their biosynthetic gene clusters were characterized. Mapping of the biosynthetic gene clusters (BGCs) within the Emericellopsis genome confirmed the detection of the secondary metabolite helvolic acid produced during fermentation. The study revealed numerous unknown biosynthetic gene clusters and a range of carbohydrate active enzymes. Each of the three genomes provides the first genome of their respective taxa and can contribute to understanding their evolutionary adaption to the marine environment. In Paper 3, a novel compound from the fermentation broth of Mytilinidion sp. was isolated and its bioactivity was characterized using seven different bioactivity assays. The compound turned out to be a modified medium component with IC50 of 43 µM in an ACE-inhibitory assay. The compound was novel and this is the first report of its bioactivity. Molecular networking could perhaps have provided early indications that the compound was a modified medium component. Marin sopp er en gruppe av organismer som har vært oversett i lang tid. Interessen for marin sopp har steget i takt med økt forståelse for dens økologi og rike kjemi. Mange tusen nye naturprodukter har blitt beskrevet fra marin sopp de siste tiårene, men det er fremdeles mange grupper som ikke er godt studert. Marin sopp fra Arktis er ikke godt undersøkt i forbindelse med bioprospektering og representerer en ny kilde til naturprodukter. Målet med avhandlingen var å vurdere potensialet til Arktisk marin sopp for å produsere bioaktive metabolitter ved å bruke fermentering og genomanalyse. Dette ble gjennomført i tre steg. Først ble 20 marine sopp isolert i løpet av ett forskningstokt rundt Svalbard. Disse soppene ble karakterisert ved hjelp av molekylære markører og evnen til å produsere antibakterielle forbindelser ble undersøkt ved å bruke agar-diffusjons analyse (Artikkel 1). Det neste som ble gjort var at tre forskjellige marine sopp ble helgenomsekvensert og genomene ble karakterisert. En av soppene er antageligvis ny for vitenskapen og ble beskrevet basert på morfologisk og fylogenetisk analyser (Artikkel 2). Til sist ble ett stoff fra en av soppene fra Artikkel 1 isolert og bioaktiviteten til stoffet ble beskrevet (Artikkel 3). I den første artikkelen viste halvparten av de isolerte soppene antibakteriell aktivitet mot sykdomsfremkallende bakterier. Hvert tredje isolat representerer muligens nye arter basert på tilgjengelige referansesekvenser. Fem av de potensielt nye artene tilhører ordenen Lulworthiales som kun finnes i havet. Studien har vist at det er stort potensiale for å avdekke nye arter og at mange av disse viser evne til å produsere antibakterielle forbindelser. Dette viser at de kan utnyttes i bioprospektering. I Artikkel 2 ble tre ulike marine sopper helgenomsekvenseert og genomet ble karakterisert med tanke på hvilke biosyntetiske genklynger og karbohydrat-aktive enzymer som ble detektert. Karakteriseringen av genklyngene i Emericellopsis bekreftet deteksjonen av metabolitten helvolsyre som soppen produserte under fermentering. Studien avslørte mange ukjente genklynger og en rekke karbohydrat-aktive enzymer. De tre genomene er de første helsekvenserte genomene i deres respektive slekter og kan bidra til å gi ny kunnskap om de evolusjonære tilpasningene de har til det marine miljø. I den siste artikkelen ble ett ukjent stoff fra fermenteringen av Mytilinidion isolert. Bioaktiviteten til stoffet ble karakterisert ved hjelp av syv forskjellige bioaktivitetstester. Stoffet viste seg å være en delvis nedbrutt og modifisert mediekomponent som hadde en IC50 verdi på 42.3 µM i en ACE-inhiberingstest. Analyse ved hjelp av molekylært nettverk kunne kanskje avdekket at stoffet var relatert til dyrkningsmediumet.

The transmission of salmon lice (Lepeophtheirus salmonis Krøyer) from farm salmon to wild salmonids and the impact on wild populations are sources of intense debate in all salmon producing countries, and it is currently an issue of high political and economic relevance in Norway. Solid science-based evidence is therefore needed by decision makers to apply sound management strategies. Given the complexity of the systems, coupled with methodological challenges, the causal relation between the in-farm production of lice and increased lice abundances on wild salmonids have been difficult to demonstrate. In this thesis, mean salmon lice abundances on sea trout (Salmo trutta L.) were shown to be directly correlated with the estimated number of lice emanating from nearby fish farms. Although temperature was found to also have an impact on lice epidemiology on both farmed and wild fish, temperature alone could not explain the high numbers of lice parasitizing sea trout in areas were farmed and wild fish coexist in close vicinity. This corroborates the hypothesis that salmon farms act as a main source of lice for sea trout in aquaculture-intensive areas in Norway, even after the effect of temperature has been accounted for. These conclusions were drawn based on the statistical analysis of a large and comprehensive data set combining information on standing stock and mean lice abundances on farmed fish from Norwegian salmon farms, together with observations of lice infestations on wild sea trout collected all along the Norwegian coast between 2010 and 2011. In combination, these two large and comprehensive data sets provided a unique opportunity to investigate the role of salmon farms as a source of lice for local sea trout populations. Moreover, results from this thesis suggest that the Norwegian National Salmon Fjords, i.e. protected areas subjected to strict limitations on aquaculture activities, can effectively protect wild salmonids against infestations with lice of farm origin, but only if the size and configuration of the protected area is adequate. Lice abundances on sea trout caught inside large National Salmon Fjords were found to be consistently low over time and are assumed to represent natural levels. Lice infestations on sea trout caught inside smaller National Salmon Fjords, on the contrary, showed large variation between years, oscillating in accordance with the variations in stocked biomass in the surrounding farms. Lice prevalence and intensities in these areas occasionally matched and even exceed those previously reported in areas of intensive farming in Norway. These results suggest that the production and transport of planktonic lice larvae from farms situated outside the limits of the protected areas might still be an important source of lice for local populations of sea trout. Although individual studies for each National Salmon Fjord may be necessary to evaluate the degree of protection, one can expect that the capacities of the smallest fjords for preventing the transmission of lice from farm salmon to wild fish are probably limited, and it is reasonable to assume that the protection of large areas contributes best to the protection of wild salmonids. The optimal design will in any case depend on the target species: while sea trout may benefit from the establishment of a local protected area covering their marine home range, the protection of Atlantic salmon may require the protection of entire fjords or the establishment of “clean corridors” along their migration routes. Last, this thesis delivers direct evidence of a causal relationship between salmon lice and marine mortality of sea trout in nature. Based on results from exposure field studies, combined with acoustic telemetry, this thesis shows a mortality hazard ratio of 2.7 (95% CI = 1.04−7.13) for lice-infested trout post-smolts compared to non-infested control individuals. In other words, the probability of a trout post-smolt to survive the first summer at sea was reduced by almost two-thirds when infested with a heavy lice load, although there is a big uncertainty around this estimate. These results apply to fish carrying a relative intensity of 2.4 lice g−1, on average. Although such lice intensities are well beyond the estimated mortality-inducing threshold of 0.3 lice g−1 (Taranger et al. 2015), they are still representative of lice infestations observed on sea trout in some farm-intensive areas in Norway. Furthermore, this thesis documents an altered migration behavior of lice-infested individuals, including the abrupt interruption of the feeding migration after just a few weeks at sea and a preference for feeding areas closer to the river, in the inner part of the fjord. While control fish spent on average 100 days at sea before return, lice-infested sea trout returned to freshwater after only 18 days at sea, meaning that almost 80% of the feeding time at sea was lost because of salmon lice. This can be expected to significantly reduce marine growth and further have an impact at the population level. Studies that accurately quantify the impact of salmon lice on wild sea trout and their populations are still needed, and this thesis makes an important contribution in this direction.

Environmental change in the Arctic is occurring at an unprecedented rate with a loss of sea ice and warmer sea temperatures, simultaneously as increased human activity in the Arctic poses a risk of petroleum pollution. The potential future of a key Arctic forage fish, polar cod (Boreogadus saida), was investigated using laboratory simulations of oil spills during sensitive life-history stages: maturation, post-spawning, and early life stages. I hypothesized that exposure to crude oil would result in adverse effects on individual fitness during these sensitive life-history stages. Changes in growth, metabolism, reproduction, and survival were followed to provide an integrated response to determine the risk to individual fitness to deliver more robust predictions for effects at the population and ecological levels. Findings reveal the physiological robustness of mature stages of polar cod chronically exposed to low levels (post-spill concentrations) or acutely to high (present-spill) crude oil concentrations. Effects observed on sperm motility under dietary exposure and gonadal development in females exposed to burned oil residues, an oil spill response action, require follow-up examination, especially in light of the importance of gamete quality to individual fitness. Reduced energy reserves and condition in the post-spawning stage suggests increased physiological sensitivity of this life-history stage. The high sensitivity of eggs and larvae to low levels of crude oil was further amplified by a 2.3°C increase in water temperature. The interactive effects of warmer water and low levels of petroleum pollution demonstrate the vulnerability of polar cod early life stages. Determining how the sentinel species, polar cod, will respond to these environmental and ecological stressors and what influence this will have on the resilience of the Arctic marine ecosystem is the future aim of this research.

The overall aim of this study has been to provide information on how environmental conditions in Norway might affect the life history and distribution of the edible crab (Cancer pagurus). The results show that the size at which female crabs reach physiological maturity vary little, and just randomly, across northern Europe, and among latitudinally separated areas within Norway. Analysis of geographical differences in the frequency of molting show that female crabs at more northern latitudes have a slower growth, and consequently have a higher age at maturity. The results also show that peak periods of molting occur later at higher latitudes, indicating that the reproductive cycle of crabs in the north is delayed compared to further south. It is believed that these spatial differences in frequency and timing for onset of molting are due to geographical differences in temperature. Evidence that crabs in the north live under sub optimal thermal conditions is found in laboratory experiments, showing that individuals from northern and southern latitudes have the same lower thermal tolerance, and a common and rather warm thermal preference (~13 °C). Video transects conducted in Sognesjøen, on the west coast of Norway, revealed a high abundance of crabs at depths of ~400 meters, and show that the topography of Norway, with its deep fjords, force some crabs in some areas to move to depths greater than depth limitations given for this species in popular fact sheets.

Identifying drivers of population trends in migratory species is difficult, as they can face many stressors while moving through different areas and environments during the annual cycle. Their population response to environmental change may in addition be affected by consistent differences in individual behaviour, which are widespread in free-living populations. An understanding of the structure of migration in space and time across species, populations and individuals is necessary to identify potential plasticity and constraints for migratory species in a rapidly transforming physical and biological environment. This thesis uses two congeneric long-distance migrants of the genus Uria, the common (Uria aalge) and Brünnich’s guillemot (Uria lomvia) to address these questions. To this end, I utilized a large light-level geolocator tracking dataset of 887 individual guillemots breeding at 16 colonies across the Northeast Atlantic, tracked over 10 years resulting in 1740 annual tracks. Through the development of a novel method to estimate locations from twilight timings, I was able to correct biased estimates for part of my dataset, which made the overall dataset comparable. Further, with the inclusion of information about the species’ biology as well as several spatial masks, the method was able to estimate locations also during times of equinox. This in turn made it possible to investigate migratory connectivity (i.e. the connection between breeding and no-breeding regions) and individual migration strategy fidelity (i.e. consistency of individual migratory behaviour) throughout the non-breeding period. Both guillemot species are comprised of space use specialists selecting for specific sites rather than habitats. They breed in colonies displaying strong migratory connectivity, within and between species. This was apparent through a combination of colony-specific seasonal space use and occupied environmental niches, grouping Northeast Atlantic Brünnich’s guillemot populations into two and common guillemot populations into five previously undescribed spatiotemporal movement clusters. Remarkably, common guillemot populations clustered in accordance with the variable population trends exhibited by the species, while Brünnich’s guillemot populations are all declining where their trends are known. Colony-specificity was also visible in the exhibited temporal variations of individual migratory movements due to the species breeding biology. Birds were flightless during their autumn moult constricting their movement. Likewise, individuals were constricted to quasi central place foraging during spring prior to egg-laying after arrival back at their colony. These two periods were visible as constricted space and environmental use and often lack of individual specific behaviour. Arrival dates back at the colony were highly variable between species and colonies and could be best explained by colony size. Unlike timing of egg-laying, arrival date advanced considerably in recent years across the study area in both species, demonstrating that different events in seabird phenology can show different temporal trends. Migratory behaviour is likely shaped by a combination of the physical properties of the occupied environment, energetic constraints faced due to the animal’s physiology and foraging adaptations, inter- and intra-specific competition for food resources as well as nest sites, and conservative migratory behaviour. These traits might leave migrants vulnerable to large-scale perturbations of their environments, which occur at an ever increasing rate, while the compartmentalised annual distribution allows for the potential extinction of an entire population by regional threats, anthropogenic or otherwise.

The papers of this thesis are not available in Munin. Paper I: Michelsen, H. K., Svensen, C., Reigstad, M., Nilssen, E. M., Pedersen, T.: "Seasonal dynamics of meroplankton in a high-latitude fjord". Available in Journal of marine systems 2017, 168:17-30. Paper II: Michelsen, H. K., Nilssen, E. M., Pedersen, T., Reigstad, M., Svensen, C.: "Spatial patterns of spring meroplankton along environmental gradients in a sub-Arctic fjord". (Manuscript). Paper III: Michelsen, H. K., Nilssen, E. M., Pedersen, T., Svensen, C. "Temporal and spatial dynamics of the invasive Red King Crab zoea and co-existing native decapods in Norwegian waters". (Manuscript). Many benthic marine organisms produce pelagic larvae, meroplankton, which spend from hours to months in the pelagic. During this time, they form an important part of the pelagic community. Few studies have focused on the year round dynamics of meroplankton, thus many aspects of meroplankton dynamics remains unknown at high-latitudes. In the 1960s the red king crab was intentionally introduced into Russian waters and now serves as an important resource for Norway and Russia while at the same time posing a threat to the ecosystem. Currently there is no knowledge on when and where larvae are released in Norwegian waters. The objectives of this thesis were to increase our understanding on the seasonal and spatial dynamics of meroplankton at high-latitudes, identify environmental variables responsible for spawning and the potential role of meroplankton in the pelagic. The temporal and spatial dynamics of red king crab larvae was also investigated. Sampling monthly to bi-monthly over a 1.5-years in Porsangerfjord, Norway, showed that meroplankton abundance and assemblage separate into different seasonal communities. The highest abundance and greatest number of taxa occurs in spring and summer, correlating with increased chlorophyll a and temperature. Numerically, meroplankton contributed highly to the mesozooplankton community these seasons, suggesting that they are an important component of the pelagic food web. In spring, different meroplankton assemblages were found along the fjord and correlated with temperature and chlorophyll a. This provides evidence that larval release by benthic invertebrates is coupled with variations in these environmental variables. In a warming ocean this coupling could cause a shift in reproductive phenology and a range expansion of benthic organisms. Larvae of the red king crab were continually released over a period of six months and the greatest densities of larvae was found at protected stations. Having a prolonged larval release is advantageous for this invasive species as it ensures some larvae coincides with favorable conditions. These findings coupled with a wide tolerance to environments as larvae and adults means that they have the potential to continue their expansion in Norway and north into Svalbard waters.

The papers I, II and III of this thesis are not available in Munin. Paper I: Bailey, A., Thor, P., Browman, H. I., Fields, D. M., Runge, J., Vermont, A., Bjelland, R., Thompson, C., Shema, S., Durif, C. M. F., Hop, H.: “Early life stages of the Arctic copepod Calanus glacialis are unaffected by increased seawater pCO2”. Available in ICES Journal of Marine Science 2016. Paper II: Bailey, A., De Wit, P., Thor, P., Browman, H. I., Bjelland, R., Shema, S., Fields, D. M., Runge, J. A., Thompson, C., Hop, H.: “Regulation of gene expression underlies tolerance of the Arctic copepod Calanus glacialis to CO2-acidified seawater”. (Manuscript). Paper IV: Thor, P., Bailey, A., Dupont, S., Calosi, P., Søreide, J., De Wit, P., Guscelli, E., Loubet-Sartrou, L., Deichmann, I., Candee, M., Svensen, C., King, A. L., Bellerby, R. G.J.: “Potential for rescue from future ocean acidification by extant physiological differences among distinct Arctic copepod populations”. (Manuscript). Uptake of anthropogenic carbon dioxide (CO2) into the world’s oceans is increasing seawater acidity in a process termed ocean acidification. This is predicted to have harmful effects on a variety of marine organisms. With the greatest and fastest changes in pH expected to occur the Arctic seas over the next 100 years, the need for understanding the effects of low pH on Arctic marine organisms is pressing. This thesis examines the effects of projected levels of ocean acidification on the physiology of a key component of the Arctic marine ecosystem, the copepod Calanus glacialis, using molecular, organismal, and evolutionary methodologies to investigate the effects of low pH throughout its lifespan, in combination with other stressors (food availability) and across geographically distant sub-populations (to predict their capacity for adaptation or acclimation in the future). Young stages of the copepod, potentially the most sensitive, were tolerant to realistic pH levels for future ocean acidification: the nauplii developed successfully from egg to naupliar stage N6 at all four pH treatments investigated (pH 7.5, 7,7, 7.9 and 8.05). Gene expression N6 nauplii supported the organismal-level tolerance observed, indicating only mild gene expression and no stress response to low pH. Detrimental effects were found, however, in the young copepodite stages (C2-C4), which indicated an energetic cost at low pH. However, differences in this response between geographically distant, and potentially genetically isolated sub-populations showed that those that lived in a low pH environment tolerated it better, indicating that C. glacialis may be able to alleviate these detrimental effects over time. C. glacialis populations face a myriad of environmental changes driven by global warming and ocean acidification in the Arctic, and while they may experience declines due to the interactions of these multiple stressors, they likely will not experience declines primarily due to ocean acidification.

The papers II and III of this thesis are not available in Munin. Paper II: Nguyen, T. T., Robertsen, E. M., Landfald, B.: “Viral assemblage variation in an Arctic shelf seafloor”. Available in Aquat Microb Ecol 78:135-145. Paper III: Nguyen, T. T., Cochrane, S. K.J., Landfald, B.: “Perturbation of seafloor bacterial community structure by drilling waste discharge”. (Manuscript). Studying the spatial patterns of microbial diversity is crucial in order to assess the relationship between community structure and ecosystem function. However, due to methodological limitations in the past, relatively little has been known about the biogeography of microbial communities. The introduction of next generation sequencing technologies from 2005 was a game changer, as DNA sequencing suddenly became relatively fast and cost-effective. The application of next generation sequencing in microbial ecology has revealed non-random patterns of microbial diversity in a variety of habitats. The main objective of this thesis was to assess how microbial communities were distributed in an Arctic shelf seafloor across geographic separation and anthropogenic impact by using next generation sequencing approaches. To assess spatial variations of bacterial, archaeal, and viral communities, and possible coincidences in their biogeographical patterns, surface sediment samples were collected along a 640 km Barents Sea transect. Previous findings were confirmed and extended in the explored marine sediments, as Deltaproteobacteria and Gammaproteobacteria were dominant bacterial classes, while Thaumarchaeota were predominant among the archaeal groups. The viral assemblages appeared dominated by single-stranded DNA (ssDNA) viruses, mostly of the Eukaryotic Circular Rep-encoding ssDNA (CRESS-DNA) group, which include the families Circo-, Nano-, and Germiniviridae. However, the high representation of ssDNA viruses was likely a consequence of amplification bias caused by the use of multiple displacement DNA amplification of the viral preparations. Although stability in higher taxa composition across geographical distances, were expected due to moderate environmental variations in the sampling area, significant distance-decay of both bacterial and archaeal communities in the Barents Sea seafloor was observed. Beta-diversity analyses of prokaryotes and viruses showed some degree of community structuring in accordance with the south-north spatial separation. While archaeal communities appeared largely influenced by environmental factors, bacterial communities seemed structured by a relatively equal contribution from environmental and spatial factors. The viral grouping into a southern and a northern region was principally associated with changes in the relative abundance and composition of eukaryotic ssDNA viruses. To determine whether offshore drilling waste, i.e discharge of rock cuttings and drilling muds, could cause changes in the bacterial communities, sediment corer samples were collected at variable distances from a newly drilled well. The results showed that the bacterial community compositions in the uppermost sediment layer close to the drilling site was significantly different from those of unaffected areas. The bacterial groups most conspicuously associated with the community change were representatives of the orders Clostridia and Desulfuromonadales and the class Mollicutes. These are candidates as microbial bioindicators of the spatial extent and persistence of drilling waste discharge. The effects of drilling waste discharges on bacterial composition were observable in 100 m radius around the well. The drilling waste caused oxygen depletion in the upper sediment layer in close proximity to offshore drilling site.

The papers I, III and IV of this thesis are not available in Munin. Paper I: Grote U., Pasternak A., Arashkevich E., Halvorsen E., Nikishina A.: "Thermal response of ingestion and egestion rates in the Arctic copepod Calanus glacialis and possible metabolic consequences in a warming ocean". Available in Polar Biology 2015, 38:1025-1033. Paper III: Pasternak, A.F., Arashkevich, E.G., Grote, U., Nikishina, A.B., Solovyev, K.A.: Different effects of increased water temperature on egg production of Calanus finmarchicus and C. glacialis. Available in Oceanology 2013, 53(5):547-553. Paper IV: Grote U., Eisenhauer L.: "Modelling population level responses of Calanus glacialis and C. finmarchicus under different climate warming regimes in the Barents Sea area". (Manuscript). Climate change is particularly pronounced in the Arctic and water temperatures are predicted to increase substantially. This has important implications for Arctic marine organism, especially ectotherms, such as copepods. The calanoid copepods Calanus glacialis and C. finmarchicus are key species of different pelagic food webs at high latitudes and changes in temperature will likely affect them differently, further affecting higher trophic level organisms feeding on Calanus spp. One objective of this thesis was therefore to investigate physiological processes affecting growth and reproduction, such as feeding, egestion, respiration, and egg production rates of especially C. glacialis in response to temperature (0 10 °C) and to discuss the potential metabolic balance. The other objective of this thesis was to investigate the effect of increasing temperature on C. glacialis and C. finmarchicus populations using the SINMOD model. C. glacialis ingestion and egestion rates increased linearly with temperature in one experiment, while ingestion and respiration rates reached a thermal optimum at 2.5 °C and 6 °C, respectively, in another experiment, indicating plasticity in feeding responses, likely due to local adaptation. Comparing the Q10 values of ingestion, egestion and respiration rates of C. glacialis indicates that the physiological processes connected to metabolic losses (egestion, respiration) have a higher thermal sensitivity than the one connected to metabolic gains (feeding), resulting in relatively less energy being available for growth and reproduction at warmer temperatures. A direct comparison of carbon specific ingestion and respiration rates of C. glacialis indicates a metabolic mismatch at temperatures above 5-6 °C, potentially resulting in negative effects on growth and survival. At temperatures above 5 °C, egg production rates of C. glacialis decreased, indicating that metabolic balance was negatively affected by increasing temperature, while egg production rates of C. finmarchicus increased with temperature. The simulated response of Calanus spp. populations with increasing temperature suggests that C. finmarchicus biomass and net production will increase in the southern Barents Sea. C. glacialis biomass and net production will likely increase in the northern Barents Sea up to summer seawater temperatures of 5-6 °C. Further warming will reduce C. glacialis biomass and net production due to the observed metabolic mismatch negatively affecting growth. Changes in food availability due to nutrient limitation can have pronounced effects on population biomass and productivity as well, suppressing the potentially positive effects of initially increasing temperatures.

The papers of this thesis are not available in Munin. Paper I: Hamilton, C. D., Lydersen, C., Ims, R. A., Kovacs, K. M.: “Predictions replaced by facts: a keystone species’ behavioural responses to declining arctic sea-ice”. Available in Biol. Lett. 2015, 11:20150803. Paper II: Hamilton, C. D., Lydersen, C., Ims, R. A., Kovacs, K. M.: “Coastal habitat use by ringed seals Pusa hispida following a regional sea-ice collapse: importance of glacial refugia in a changing Arctic”. Available in Mar. Ecol. Prog. Ser. 2016, 545:261-277. Paper III: Hamilton, C. D., Kovacs, K. M., Ims, R. A., Aars, J., Lydersen, C.: "An Arctic predator-prey system in flux: climate change impacts on coastal space use by polar bears and ringed seals." (Manuscript). Published version available in J. Anim. Ecol. 2017, 86(5):1054–1064. Paper IV: Hamilton, C. D., Kovacs, K. M., Ims, R. A., Aars, J., Strøm, H., Lydersen, C.: "An Arctic predator, prey and scavenger system in a changing climate". (Manuscript). Published version with title "Spatial overlap among an Arctic predator, prey and scavenger in the marginal ice zone" available in Mar. Ecol. Prog. Ser. 2017, 573:45-59. The Arctic is currently in a rapid state of change, with temperature increases in this region being three times the rate of the global average and sea-ice extent declining rapidly. In 2006, a sudden shift in the sea-ice regime in Svalbard, Norway, occurred with the altered sea-ice conditions persisting to the present day. In order to study the impacts of the environmental changes occurring in the Svalbard region, the movement patterns, behaviour and degree of spatial overlap for ringed seals (Pusa hisapida, n=60), polar bears (Ursus maritimus, n=160) and ivory gulls (Pagophila eburnea, n=40) were investigated using data from biotelemetry devices equipped on these species before (2002-2004; ringed seals and polar bears) and after (2010-2013; all three species) the shift in the sea-ice regime occurred. These three species are trophically linked; ringed seals are the primary prey of polar bears and ivory gulls scavenge on polar bear kills. Coastal ringed seals and ringed seals that took offshore foraging-migration trips had an increase in foraging effort and thus energetic costs and also a change in foraging behaviour following the sea-ice collapse, indicating that alterations in their prey base have occurred throughout the Svalbard region. Coastal polar bears spent less time in front of tidal glacier fronts in the summer (Jun-Aug) after the sea-ice collapse, leading to a significant decrease in spatial overlap between coastal polar bears and ringed seals. Following the sea-ice collapse, coastal polar bears moved greater distances per day in the summer and spent more time close to ground-nesting bird colonies, indicating increased predation on terrestrially-based prey sources. Offshore polar bears, ringed seals and ivory gulls travel to the Marginal Ice Zone (MIZ) of the Barents Sea in the summer and autumn to forage. All three species preferred areas with sea-ice concentrations between 40-80%. The main spatial overlap areas for this species assemblage occurred slightly north of the 50% sea-ice contour. The predicted seasonal disappearance of the MIZ and the continued retreat of tidal glacier fronts will have consequences for the growth, distribution and abundance of ringed seals, polar bears and ivory gulls in Svalbard. The effects on each of these ice-obligate, top trophic Arctic animals and the resultant changes in biological interactions will have ramifications for the wider Arctic marine and terrestrial ecosystems.

The papers I and II of this thesis are not available in Munin. Paper I: Grenvald, J. C., Callesen, T. A., Daase, M., Hobbs, L., Darnis, G., Renaud, P., Cottier, F., Nielsen, T. G., Berge, J.: “Plankton community composition and vertical migration during polar night in Kongsfjorden”. Available in Polar Biology 2016, 39(10): 1879–1895. Paper II: Grenvald, J. C., Vader, A., Reinardy, H., Berge, J., Renaud, P., Gabrielsen, T. M.: “Does the cryptochrome 2 gene (cry2) act as a circadian clock gene in krill (Thysanoessa spp.) during polar night in Kongsfjorden (Svalbard)? (Manuscript). Recent Arctic studies contradict a long-held paradigm of winter quiescence by documenting activity at many trophic levels in the marine food web even during the darkest months of winter. Previous studies have failed to identify which species are performing patterns of vertical migrations during polar night in Kongsfjorden (Svalbard). In the present study, acoustic data supplemented by zooplankton net sampling demonstrated that polar night migration involved several migration patterns. Synchronized diel vertical migration (DVM) was present during the first and final parts of the polar night period, when day-night cycles were more distinct. During the darkest months of mid-winter, synchronized DVM was absent. Mid-winter migration patterns were restricted to surface waters, of unsynchronized character, and did not follow a clear diurnal pattern. Periodically, mid-winter migrations became more synchronized possibly due to enhanced light sources other than solar, such as lunar light. Copepods, chaetognaths and krill (Thysanoessa spp.) were abundant species but krill were responsible for the acoustic migration patterns. Spectral sensitivity tests showed that krill (T. inermis) were able to perceive solar background illumination during mid-winter. A clock gene investigated in krill required light to show circadian rhythmicity. The molecular and electrophysiological experiments suggested that polar night migration was exogenous governed by ambient light levels rather than endogenously controlled, although, light intensity had to be at a certain threshold to show diurnal rhythms. Light levels were probably too low during mid-winter, which explains unsynchronized migration patterns and lack of clock gene rhythms. Predation pressure was not evaluated in the present study but it may be that the ultimate driver for synchronized migration patterns was an anti-predatory response. This study presents a very first glimpse into a biosphere in which the dominant light source appears to trigger a response in zooplankton species, which does not always follow a diurnal pattern.

The papers II and III of this thesis are not available in Munin. Paper II: Vader, A., Marquardt, M., Meshram, A. R., Gabrielsen, T. M.: “Key Arctic phototrophs are widespread in the polar night”. Available in Polar Biology 2015, 38(1):13–21. Paper III: Marquardt, M., Skogseth, R., Wiedmann, I., Vader, A., Reigstad, M., Cottier, F., Gabrielsen, T. M.: “Fate of marine pelagic protists in an ice-free high arctic fjord (Adventfjorden, West Spitsbergen)”. (Manuscript). There is an increasing awareness of the importance and diversity of the microbial community in Arctic regions, but their role in the ecosystem is still largely unknown. Sampling was conducted in Svalbard waters with focus on the Adventfjorden-Isfjorden time series station (IsA, West Spitsbergen) that was sampled frequently from December 2011 to December 2012. The community composition of microbial eukaryotes (size: 0.45-10 µm and > 10µm) and their contribution to the vertical particle flux were determined using PCR screening and 454 sequencing of the 18S V4 region amplified from both DNA and RNA. The autotrophs Micromonas pusilla and Phaeocystis pouchetii were widely distributed and active at almost all locations and depths around Svalbard during the polar night and played important roles during the spring bloom at IsA. Small Gyrodinium species were predominant throughout the year. The winter and early-spring communities, in which heterotrophs such as Marine Stramenopiles, Picozoa and the parasitoid Marine Alveolates dominated, were more diverse than the spring and summer/autumn communities. An advective event in March was potentially responsible for a change in the community composition of small eukaryotes. Small cells (< 10 µm) contributed significantly to the vertical flux during autumn and winter, possibly due to increased flocculation and ballasting processes. In contrast, larger and more typical spring bloom taxa (e.g. diatoms) dominated both the water column and sediment traps during spring. Molecular tools not only revealed novel taxa contributing to the vertical export, but also suggested new mechanisms for vertical export exemplified by parasite-host induced transport. These results emphasize the extreme seasonality of Arctic microbial eukaryotic communities driven by the environment (e.g. light and nutrient availability) but point to the necessity of a thorough knowledge of hydrography to fully understand their succession and fate.

Paper VI, VII and XI of this thesis are not available in Munin.Paper VI. Recent changes in the diet composition of common minke whales (Balaenoptera acutorostrata) in Icelandic waters. – A consequence of climate change? Víkingsson GA et al. Available in Marine Biology Research, 2014, vol. 10, No. 2, 138_152Paper VII. Body condition of fin whales during summer off Iceland. Víkingsson, GA. (1995). Available in Whales, seals, fish and man. Blix, AS, Walløe L. and Ulltang Ø (eds). (Developments in Marine Biology, vol. 4). Elsevier, 2015. ISBN: 978-0-444-82070-9Paper XI. First indications of autumn migration routes and destination of common minke whales tracked by satellite in the North Atlantic during 2001–2011. Víkingsson GA and Heide-Jørgensen MP. Available in Marine Mammal Science, 31(1):376–385 (2015) Five species of baleen whales are regularly encountered at their feeding grounds in Icelandic waters during summer. Systematic monitoring of distribution and abundance of cetaceans in Icelandic and adjacent waters was initiated in 1987 as a part of the North Atlantic Sightings Surveys (NASS). In this thesis, the objecive is to summarize the results of these surveys in relation to changes in the marine environment and the feeding ecology of four baleen whale species. The results presented here demonstrate an important role played by baleen whales in the marine ecosystem around Iceland. Icelandic waters represent summer feeding grounds for these migratory species, which are believed to fast or feed at much lower rates during winter. Deposition of energy reserves during the feeding season differed among reproductive classes and was highest in pregnant females in both fin and minke whales. Two independent methods to estimate daily feeding rates of fin whales gave similar results. The annual food consumption by the 12 species of cetaceans regularly occurring in Icelandic waters was estimated at around 6 million tons with around two thirds attributable to fin and minke whales. Satellite tracking experiments gave first indications of migration route and winter destinations of North Atlantic minke whales and suggested a later departure from the feeding grounds in autumn than generally assumed. A a two stage modelling approach indicates that per-capita prey abundance affects body condition of fin whales which in turn affects fecundity. These results are consistent with a density dependent response in pregnancy rate. Pronounced oceanographic changes in Icelandic waters since the mid-1990s, appear to have caused a northward shift in the distribution of several fish species, a decrease in krill abundance and a collapse in the sand eel population off southern and western Iceland. Concurrently, appreciable changes have occurred in distribution and abundance of several cetacean species. These include a pronounced decrease in the abundance of minke whales in the Icelandic shelf area since 2001, increase in fin whale abundance in the Irminger Sea and large increase in humpback whale abundance in recent decades. Some of these changes appear to be related to the observed oceanographic and biological changes, while others are harder to explain. The decrease in minke whale abundance in the Icelandic continental shelf area after 2001 seems to be related to the decrease in the abundance of important prey species, sand eel and capelin. Apparently, minke whales have responded to these environmental changes by 1) a shift in distribution away from Icelandic coastal waters and 2) a change in diet of the animals remaining in Icelandic waters from sand eel, euphausiids and capelin, to herring and gadoids. Concurrently with increasing sea temperature in the deep waters of the Irminger Sea, the distribution of fin whales expanded into this area, possibly due to an increase in the fin whale’s dominant prey species, euphausiids. In recent decades, humpback whale abundance has increased dramatically in Icelandic waters. It is hard to relate this increase to any particular biological changes in the marine environment. An apparent northward shift in distribution of blue whales may be related to the decrease in euphausiid abundance in the waters south and southwest of Iceland. The research presented in this thesis indicates that effects of warming on cetacean distribution and abundance are already evident in Icelandic and adjacent waters. The NASS series has revealed appreciable changes in distribution of several cetacean species and studies into feeding ecology and energetics provide possible explanations for these changes. Potential benefits of global warming to subarctic and/or migratory species through increased size of suitable habitat might be offset by increased competition from the south. Continued monitoring of the distribution and abundance of cetaceans as well as further studies into their feeding ecology are essential for better understanding of the recent and ongoing changes documented here.

Paper 2: Kortsch, S., Primicerio, R., Aschan, M., Lind, S., Dolgov, A.V., Planque, B.: "Food-web structure changes along environmental gradients in a high- latitude marine ecosystem." (Manuscript). Published version available at https://hdl.handle.net/10037/14267 Paper 3 (not avbailable in Munin): Kortsch, S., Primicerio, R., Beuchel, F., Renaud, P.E., Rodrigues, J., Lønne, O.J., Gulliksen, B.: "Climate-driven regime shifts in Arctic marine benthos." Available in Proceedings of the National Academy of Sciences 2012, 109: 14052-14057. This PhD deals with spatial and temporal community patterns in Barents Sea marine food webs and benthic communities in Svalbard. A main aim of this PhD project has been to study how community and food-web structure changes along environmental and climatic gradients, and to elucidate how species respond to climate warming in the Arctic. We used network analysis to study how the network of who eats whom in the Barents Sea changes from the boreal, warm-water regions in the southwest to the cold-water, arctic regions in the northeast. We documented differences in the food-web structure across the entire Barents Sea, separating biogeographic food-web regions. Our study provides further evidence that food-web structure is linked to broad-scale environmental gradients through environmental constraints on niche space and environmental filtering of species’ spatial distributions. We also showed that the current climate-driven poleward shift of boreal fish in the Barents Sea affects the arctic food web by increasing its connectivity and decreasing its modularity. Along the rocky shores of Svalbard, we documented a regime shift in the sea-floor communities most likely as a response to warming.

Detailed predictions of the downward particulate organic carbon (POC) flux in a future Arctic are challenging due to the poor understanding of potential drivers. Short-term sediment traps, partly modified with gel-containing jars, were deployed in the Barents Sea (BS) and in Adventfjorden, Svalbard, to determine the downward POC flux and the particle flux (≥ 0.05 mm equivalent spherical diameter ESDimage) in a parallel manner. In this way, the quality of the sinking material could be linked to the downward POC flux and this link gave new insights into potential drivers of the carbon flux. In the Barents Sea, the highest downward POC flux (260-670 mg POC m^-2 d^-1) occurred in ice-free, deeply mixed waters. Sinking particles were < 1.00 mm ESDimage and had a high POC: volume ratio. Pulsed nitrate injections from deeper layers most likely stimulated new production, and the downward flux was enhanced by vertical mixing and “biomass repackaging” into fast-sinking pellets by zooplankton. In Adventfjorden, the highest POC flux (770-1530 mg POC m^-2 d^-1) occurred during autumn, coinciding with glacial run-off. Large, fast-sinking particles (0.05-3.62 mm ESDimage) apparently enhanced the downward POC, despite their low POC: volume ratio. In addition, entrained terrestrial POC enhanced the downward POC flux. Ballasting effects by entrained lithogenic material, flocculation and aggregate formation by pteropods are likely, but need further investigation. The present study illustrates that a high downward POC flux in Arctic marine ecosystems may occur during the phytoplankton bloom, but the highest POC downward fluxes found in the BS and Adventfjorden were found in deep-mixed waters during a post bloom situation or during an autumn situation influenced by glacial run-off, respectively. It is also shown, that a high POC downward flux may not only be caused by large particles, but also by small particles with a high POC: volume ratio.

Paper I of this thesis is not available in Munin: Ramasco V, Biuw M, & Nilssen KT: Improving time budget estimates through the behavioural interpretation of dive bouts in harbour seals. Available in Animal Behaviour, 2014, vol. 94 The coastal cod stock of the Porsangerfjord has drastically declined in the 80’s and never fully recovered since. A population of harbour seals, known to be resident in the fjord all year round, has been hypothesised to act as predator-pit for cod in the area, affecting its lack of recovery. In order to understand the role of these seals in this local marine ecosystem, their foraging behaviour was investigated by assessing the presence of preference for certain prey and the behavioural response to the seasonal dynamics of prey distribution in the fjord. The movement patterns of individual harbour seals (n = 15) were followed between 2009 and 2013. The data obtained (i.e. GPS location, time-depth dive profiles, etc.) were first thoroughly analysed to provide a robust interpretation of the behavioural patterns of activities of the single individuals and identify the methodological caveats in the detection of foraging behaviour. Among the major results, two types of resting behaviour at sea were described, in the form of prolonged periods at surface but also as resting dives, a behaviour never documented before for this species. The patterns of activity suggested that harbour seals mainly forage during daylight in autumn and spring in this area. The foraging locations identified were then compared to the availability of potential prey in the fjord (i.e. herring, capelin, small and large codfish and sculpins). The availability of prey was assumed to be dependent on their biomass densities, their distance from the seals’ haulout sites and the accessibility of the areas where prey was located. The latter could be restricted for example by the presence of sea ice in winter and spring. Results on the analyses of prey selection suggested that harbour seals in Porsangerfjord had a preference for small size fish (< 25cm). Small codfish was preferred during autumn, but a response to the presence of pelagic fish was seen when the latter aggregated to overwinter in cold deep waters in the inner parts of the fjord. The formation of ice in these areas during the winter season, however, provoked a shift in preference to small codfish, due to the sudden inaccessibility of the pelagic fish. A strong reversed trend was observed in late spring when the ice melted and pelagic fish was preferred again. The results suggest the preference for small aggregated fish close to the haulout areas. The impact of harbour seals on the cod population of Porsangerfjord can be therefore hypothesized to change across seasons and to be lowered by the availability of highly aggregated pelagic fish during winter and spring.

The aims of this thesis were to increase knowledge on 1) susceptibility of bivalves to environmental perturbations and 2) usage of bivalve shells as sub-annual environmental proxy archives. These objectives were addressed using two experimental settings: 1) an ocean acidification (OA) experiment on bivalve gametes and early larval stages, and 2) year-long bivalve deployments on oceanographic moorings in two fjords in Svalbard followed by geochemical sampling of bivalve shells. The results indicate that Mytilus larvae are sensitive to ocean acidification, but global warming might have a larger impact on the larvae than acidification. Sperm activity of Mytilus galloprovincialis males was negatively affected by the high-end OA scenarios for the year 2100, possibly indicating a reduced fertilization success in low-density populations. Individual males demonstrated different tolerances of OA indicating that Mytilus populations might have the capacity to adapt to moderate reductions in ocean pH. Shell growth rate of Serripes groenlandicus and Ciliatocardium ciliatum correlated with temperature, and the length of the growth season was likely determined by food availability. Both species deposited a prominent winter growth line, which can be used as an indicator of low food availability. Despite the high potential as sub-annual environmental proxies, interpretation of elemental ratios in Serripes groenlandicus and Ciliatocardium ciliatum shells is complicated by multiple internal and external factors: Li/Ca could partly reflect calcium carbonate crystal growth rate, Mg/Ca was loosely connected with temperature, and Ba/Ca could reflect the elemental ratio in seawater. None of these ratios, however, can be used as straightforward proxies of temperature, salinity, fluorescence or shell growth rate. The insights into sub-annual growth of Arctic bivalves and the methods to align sample spots along chronologically deposited materials, as provided by this thesis, can be used in further proxy development studies.

Papers number 5 and 6 of the thesis are not available in Munin due to publishers' restrictions: 5. Daase, M., Eiane, K., Aksnes, D. L. and Vogedes, D.: 'Vertical distribution of Calanus spp. and Metridia longa at four Arctic locations', Marine Biology Research, 4(2008), 193-207, available at http://dx.doi.org/10.1080/17451000801907948 6. Berge, J., Cottier, F., Last, K. S., Varpe, Ø., Leu, E., Søreide, J., Eiane, K., Falk-Petersen, S., Willis, K., Nygard, H., Vogedes, D., Griffiths, C., Johnsen, G., Lorentzen, D. and Brierley, A. S.: 'Diel vertical migration of Arctic zooplankton during the polar night', Biology Letters, 5(2009), 69-72, available at http://dx.doi.org/10.1098/rsbl.2008.0484 The calanoid copepods of the genus Calanus (C. finmarchicus, C. glacialis and C. hyperboreus) are key elements in the Arctic ecosystem. They link primary producers (ice algae, phytoplankton) and higher trophic levels through their ability to convert low energy carbohydrates and proteins to high energy wax esters. This energy is stored in a separate and conspicuous lipid sac. This ability to store large amounts of compact energy is considered an essential adaptation towards the strong seasonality in the Arctic environments. Also the calanoid copepods are known to conduct pronounced seasonal vertical migration and diapause at depth. Due to the large amount of stored energy, Calanus spp. is a preferred prey for a number of planktivorous predators such as herring, polar cod and little auks. The three Calanus spp. share a very similar morphology but have a very different life history and they are bound to different types of water masses: C. finmarchicus has a one year life cycle, indicator of Atlantic water; C. glacialis 1-3 year cycle, indicator of Arctic shelf water; C. hyperboreus 1-5 years, indicator of Arctic water. For this reason the Calanus spp. have long been considered a reliable climate indicator species. Also, they vary in size and thus in total amount of stored lipids, an important factor for those species foraging on Calanus spp. The aim of this thesis was to evaluate the ecological role of the important climate indicator Calanus spp. Two technical papers provide tools for Calanus spp. research. In paper 1 the potential misidentification of C. glacialis as C. finmarchicus is discussed. Genetic analysis revealed that at several localities a variable percentage of C. finmarchicus is wrongly identified as C. finmarchicus when traditional morphological identification methods are applied. Paper 4 provides an equation to convert copepod lipid sac area obtained by optical measurements to energy content and thus allows comparison of individual state of copepods within and across all three species. Prey selection of chick-rearing little auks depending on zooplankton availability in the adjacent waters was the focus of paper 2 and 3. Little auks displayed a bimodal foraging strategy, presumably as a response to poor feeding conditions close to the colony. Prey count and travel time recordings indicate long travels to the shelf edge for self-maintenance feeding on high energetic C. hyperboreus and C. glacialis and short trips for collection of food for the chicks. An unexplained mismatch of prey ratio in returning birds versus samples at sea remains. Close to the bird colony a high-resolution zooplankton sampling grid was applied which revealed the occurrence of a zooplankton patch of 0.2 to 2 km. Total energy content of little auk prey m-3 was 16 times higher in the patch than in the place of lowest abundance. The presence of zooplankton patches is highly relevant not only for little auks searching for their food (concentration of food – less feeding effort), but in fact also for zooplankton research itself (hitting or missing a patch during a sampling campaign will give largely different results). In paper 5 the focus is on variations in vertical distribution of Calanus spp. and Metridia longa in autumn. Older and larger copepodite stages were located deeper in the water column and the distribution reflected the variation in seasonality on a south-north gradient. Diel vertical migration (DVM) was only observed for M. longa. It was a paradigm that biological activity in the polar night more or less comes to a halt, but in paper 6 a pattern of synchronized DVM could for the first time be demonstrated for two Arctic fjords, Kongsfjorden and Rijpfjorden, during the darkest time of the year. In this paper calanoid copepods were suggested as actively migrating during winter.

The papers of the thesis are not available in Munin: 1. Lynghammar, A., Christiansen, J.S., Mecklenburg, C.W., Karamushko, O.V., Møller, P.R. & Gallucci, V.F.: 'Species richness and distribution of chondrichthyan fishes in the Arctic Ocean and adjacent seas', Biodiversity 14(1): 57-66 (2013). Available at http://dx.doi.org/10.1080/14888386.2012.706198 2. Lynghammar, A., Christiansen, J.S., Griffiths, A.M., Fevolden, S-E., Hop, H. & Bakken, T.: 'DNA barcoding of the northern Northeast Atlantic skates (Chondrichthyes, Rajidae), with remarks on the widely distributed starry ray', Zoologica Scripta Volume 43, Issue 5, published online: 23 June 2014. Available at http://dx.doi.org/10.1111/zsc.12064 3. Lynghammar, A., Præbel, K., Fevolden, S-E. & Christiansen, J.S., 'Population genetic structure of an abundant skate (Amblyraja radiata) in the North Atlantic Ocean.' Manuscript The basal prerequisite for managing species and understanding ecosystems is correct species and population identification. Without knowing the species, we cannot with accuracy detect changes in distribution or abundance due to climate change, fisheries, diseases or any other types of impact. This thesis gives the first complete overview of chondrichthyan fishes (sharks, skates, rays and chimaeras), recorded in the Arctic Ocean and adjacent seas. A total of 49 species were found to occur, and the most specious regions were the Bering Sea, the Norwegian Sea and the Barents Sea, also known as the Arctic Gateways. The Bering Strait is an effective zoogeographic barrier, separating the Pacific from the Atlantic species. In particular, skates are prone to misidentifications. Species complexes are known to occur, and the group is commonly lumped as “Skates & Rays” in fishery statistics or in scientific publications. By use of genetic methods (DNA barcoding), the skate species reported from the northern Northeast (nNE) Atlantic was investigated. A total of 12 species were found to occur in the area, and three of those were new for the region. Skates are commonly encountered in the nNE Atlantic, but ~95 % are the smaller-sized Amblyraja radiata. Larger skates may be landed and they are necessarily a mix of the remaining 11 species, some of which are considered threatened. There were strong indications of the disappearance of Leucoraja fullonica, as well as over-reporting of the critically endangered common skate complex formerly known as Dipturus batis. Amblyraja radiata was suspected to be cryptic due to large differences in size-at-maturity across its distributional range. However, the mitochondrial gene cytochrome c oxidase subunit 1 (COI) did not support this hypothesis. To further elucidate this question, a higher resolution method was applied, and 10 microsatellite markers were investigated from specimens across the North (N) Atlantic. Pairwise fixation indexes (FST) indicated that there may be three major clusters in the N Atlantic: Northwest Atlantic, Mid Atlantic (Greenland) and NE Atlantic. However, more rigorous tests revealed high physical mixing of individuals over a large geographical area. The microsatellite results did not support the hypothesis of A. radiata being cryptic. Although the population structure of A. radiata could not be fully explained, baselines for occurrence of chondrichthyan fishes are established, and a new identification guide for skates in the nNE Atlantic is provided. Hopefully, this will pave the way for a better management of the skates by improving data quality.

The papers of this thesis are not available in Munin: 1. Windsland, K., Hvingel, C., Nilssen, E. M., Sundet, J. H.: 'Evaluation of von Bertalanffy growth curves for the introduced red king crab (Paralithodes camtschaticus) in Norwegian waters', Fisheries Research (2013), vol. 145:15-21. Available at http://dx.doi.org/10.1016/j.fishres.2013.03.003 2. Windsland, K., Hvingel, C., Nilssen, E. M., Sundet, J. H.: 'Dispersal of the introduced red king crab (Paralithodes camtschaticus) in Norwegian waters: a tag-recapture study', ICES Journal of Marine Science (2014), online before print. Available at http://dx.doi.org/10.1093/icesjms/fst241 3. Windsland, K.: 'Total and natural mortality of red king crab (Paralithodes camtschaticus) in Norwegian waters: Catch curve analysis and indirect estimation methods' (manuscript). To improve local economy, the red king crab (Paralithodes camtschaticus) was introduced to the Barents Sea by Soviet scientists in the 1960s. Adapting well to the new environment, the growing population gradually invaded new areas along the coast of northern Norway. The red king crab has become a valuable marine resource but as an introduced species, it is also a potential threat to the ecosystem. To meet the management goals, more knowledge about the growth, mortality and dispersal of the species is essential. This thesis attempts to provide this knowledge. In paper I, we developed Bayesian hierarchical models to estimate growth parameters of the von Bertalanffy growth function using tag-recapture data. Allowing for individual variability led to increased precision in estimated lengths and to more realistic values for the growth parameters. The length-at-relative age of small males were overestimated, while those of small females were not. This is believed to be due to differences in growth patterns after maturity. In paper II, we explored the dispersal behaviour and extent of dispersal of red king crab in Norwegian waters. The results indicated that the range expansion of this species is a result of both the existence of long-distance dispersers and time-dependent slow migration by short distance-dispersers and that there is a substantial dispersal of crabs out of the quota-regulated area. In paper III, estimates of annual total mortality were calculated using length-converted catch curves in three time-periods that represented different levels of exploitation. Natural mortality was estimated using a linear regression of total mortality and exploitation rate and by using indirect methods based on life-history parameters. The steady increase in total mortality of males is consistent with the increase in exploitation level while the increase in female mortality is surprising. The estimates of natural mortality ranged from 0.23-0.44 year-1; the larger values are probably an overestimate due to unaccounted bycatch mortality while the lower ones are consistent with life-expectancy.

Paper 2 of this thesis is not available in Munin: 2. Voronkov A., Hop H., Gulliksen B.: 'Zoobenthic communities on hard-bottom habitats in Kongsfjorden, Svalbard' (manuscript). Subtidal hard-bottom habitats constitute substantial parts of the Arctic fjord ecosystem. There is a deficiency in complex faunistic surveys of the composition of hard-bottom fauna in the Arctic. The aim of this study was to increase the understanding of how animals are distributed on hard-bottom and their role in the marine ecosystem in the Arctic. A baseline study of hard-bottom zoobenthos in relation to environmental gradients in Kongsfjorden, a glacial fjord in Svalbard, is presented, based on collections from 1996 to 1998. The inventory resulted in a total of 403 species in 62 samples from 0 to 30 m depth along five transects. Of these, 47 species are new for Svalbard waters. The diversity generally decreased towards the tidal glaciers in the inner Kongsfjorden. It was consistently low at shallow depths, whereas the non-linear patterns of species composition of deeper samples indicated a presence of different hydrographical conditions between surface and deeper water masses at 15-20 m depth. Among eight environmental factors, depth and distance from the glacier were most important for explaining variance in the composition of the zoobenthos. Biogeographic composition was sensitive to variation of environmental characteristics over the studied depth range and revealed features of both Arctic and sub-Arctic properties of the fauna. Shifts in climate-related environmental variables are expected to influence the composition of the biota. It will likely result in shifts in ecosystem functioning. A list of recorded species with distribution and abundance characteristics is given in the Appendix. We contributed to the knowledge of Kongsfjorden ecosystem not only with the biotic data, but also with a description of physical environment at sampling sites. Seven zoobenthic community types were identified on hard substrata in Kongsfjorden. Method of adequate determination of community types included consideration of species’ potential role in community, accounting for dominant species and similarity in species composition. Specific approach to the diversity study of hydroids in Kongsfjorden allowed evaluation of the role of a particular taxon in hard-bottom habitats and estimation of importance of abiotic factors in validation of species separation for taxonomic purposes. The studied habitats and transects should be counted as useful for climate change-related monitoring of diversity on hard-bottom and research on ecosystem functioning. Future studies could reveal new species at these sites related to altered distribution ranges and invasions of species.

Papers 1, 3, 4 and 5 of this thesis are not available in Munin: 1. Halme P, Heilmann-Clausen J, Rämä T, Kosonen T & Kunttu P.: 'Monitoring fungal biodiversity – towards an integrated approach', Fungal Ecology (2012), vol. 5(5):750–758. Available at http://dx.doi.org/10.1016/j.funeco.2012.05.005 3. Rämä T, Davey ML, Nordén J, Blaalid R, Mathiassen GH, Alsos IG & Kauserud H.: 'Fungal communities in stranded North Atlantic driftwood as revealed by high throughput amplicon sequencing' (manuscript). 4. Davey ML, Rämä T, Bjørnsgaard Aas A, Mysterud A, Ohlson M, Vrålstad T & Kauserud H.: 'Comparing culture dependent and culture independent methods for answering questions in fungal ecology' (manuscript). 5. Rämä T, Mathiassen GH & Kauserud H.: 'Marine fungi new to Norway, with an outlook to the overall diversity' (manuscript). Marine fungi in arctic waters have rarely been studied. The main aim of this thesis was to explore the diversity of driftwood inhabiting fungal communities in the cold waters of North Norway. In order to gain as comprehensive view as possible, different methodological approaches were applied on the same study substrates, and the recovered fungal communities were analysed for taxonomic, phylogenetic and ecological aspects of diversity. Altogether 925 operational taxonomic units (OTUs) and species were detected using three different study methods. As a result of the morphological study, one undescribed and 16 new species to Norway were found. With a culturing method, 143 OTUs, based on clustering at 97% sequence similarity of the internal transcribed spacer region (ITS), were detected from 50 driftwood logs. 454 amplicon pyrosequencing revealed 807 OTUs from the same logs and estimated the total diversity in the area to be 1,400 OTUs (97% ITS2 clustering). Approximately 75% of the OTUs had affinity to the phylum Ascomycota, and previously overlooked taxa were detected from the marine environment. Both methods revealed that one half of the OTUs were tentatively non-marine, and that the fungal communities were structured by many geographical and environmental variables. It was shown that the study methods used target different parts of the fungal community with low overlap. The rich and diverse mycota found in this thesis contributes to the global knowledge of wood-inhabiting marine fungi, and suggests that many overlooked and undescribed species exist in this habitat. The diversity was well characterized at higher taxonomic levels, but resolution should be increased towards the terminal branches of the fungal tree of life by means of more collecting, culturing and DNA barcoding of marine fungi. The role of many tentatively non-marine species found in driftwood will be scrutinized in the future using RNA-sequencing of environmental samples.

The papers of this thesis are not available in Munin: 1. Hovinen JEH, Welcker J, Rabindranath A, Brown ZW, Hop H, Berge J, Steen H.: 'At-sea distribution of foraging little auks relative to physical factors and food supply', Marine Ecology Progress Series (2014), vol. 503:263-277. Available at http://dx.doi.org/10.3354/meps10740 2. Hovinen JEH, Wojczulanis-Jakubas K, Jakubas D, Hop H, Berge J, Kidawa D, Karnovsky NJ, Steen H.: 'Fledging success of little auks in the high Arctic: do provisioning rates and the quality of foraging grounds matter?', Polar Biology (2014), vol. 37(5):665-674. Available at http://dx.doi.org/10.1007/s00300-014-1466-1 3. Hovinen JEH, Welcker J, Descamps S, Strøm H, Jerstad K, Berge J, Steen H.: 'Regional and local variations in climate affect the survival of a high Arctic avian predator' (manuscript) The ability of individuals to acquire and store energy for life-history traits such as reproduction and survival, is finite. This demands prioritizing some traits at the expense of other traits. Which traits to prioritize, depends on the life-history strategy of a species. ‘Slow’-living species, in contrast to ‘fast’-living species, mature late, produce small broods, have low extrinsic mortality and high life expectancy. They tend to allocate resources cautiously to current reproduction, thereby enhancing their survival and potential for future reproduction. Many seabird species employ the slow-living life-history strategy. In order to assess the effects of predicted future climate change on seabird populations, it is important to understand how life-history traits, such as reproduction and survival, are influenced by climatic and environmental variability. The aim of this thesis was to assess the effects of climatic and environmental variability on foraging (i.e. energy acquisition), reproduction and survival of the little auk (Alle alle), the most abundant seabird species breeding in the Arctic. It is in the Arctic regions, where the effects of global climate change are expected to be strongest. Data on oceanographic conditions, foraging trip durations, chick provisioning rates, chick diet, chick fledging success and adult survival was collected on Bjørnøya, a small island in the western Barents Sea, and at three colonies (Isfjorden, Kongsfjorden and Magdalenefjorden) on the western coast of Spitsbergen, Svalbard during 2006-2013. The study area is influenced by both warm, Atlantic and cold, Arctic water masses. The results show that little auk adults preferred to forage in cold water masses at the shelf-sea area, but that oceanographic conditions did not influence their foraging trip durations or chick provisioning rates. On the other hand, the number of good quality prey items delivered daily to a chick correlated negatively with ocean temperature, and both chick fledging success and adult survival was higher when ocean temperature was lower. The higher fledging and survival probabilities were most likely due to higher availability of good quality prey in the little auk’s foraging grounds. Indeed, both these life-history traits seemed highly responsive to changes in environment. High sensitivity indicates that in the future, when Arctic warming is expected to continue and favoured Arctic zooplankton is gradually replaced with Atlantic zooplankton, little auk populations are likely to decrease substantially. This in turn may influence the dynamics of the entire Arctic food web, in which little auks play an important role.

The papers of this thesis are not available in Munin: 1. Aarbakke, O. N. S., Bucklin, A., Halsband, C. and Norrbin, F.: 'Discovery of Pseudocalanus moultoni Frost 1989 in Northeast Atlantic waters based on mitochondrial COI sequence variation', Journal of Plankton Research (2011), vol. 33(10):1487-1495, availble at http://dx.doi.org/10.1093/plankt/fbr057 2. Aarbakke, O. N. S., Weydmann, A. and Fevolden, S-E.: 'Pseudocalanus (Copepoda: Calanoida) species distribution and relative abundance as indicators of changing sea temperature. Submitted to Journal of Plankton Research' (manuscript) 3. Aarbakke, O., Bucklin, A., Halsband, C. and Norrbin, F.: 'Comparative phylogeography and demographic history of five sibling copepod species in the North Atlantic Ocean' (manuscript) The seven species of the genus Pseudocalanus (Copepoda: Calanoida) are difficult to identify because of very small interspecific, and comparatively large intraspecific, divergence of morphologic and morphometric traits. Thus, despite the fact that Pseudocalanus spp. are among the most abundant metazoans in the world, our knowledge of them at the species level is limited. The main objective of this thesis was to lay the groundwork for future studies of Pseudocalanus spp. in the Northeast Atlantic Ocean and Euro-Arctic by determining which species are present. Through a combined DNA barcoding and traditional identification approach, this work reveals the presence of at least four species of Pseudocalanus, one of which was previously not believed to occur in the East Atlantic Ocean. Furthermore, the thesis reveals differences in environmental preferences and distributions of Pseudocalanus elongatus, P. minutus, P. moultoni and P. acuspes in the Northeast Atlantic Ocean and Euro-Arctic, and discusses implications of these differences in relation to rising sea temperatures. Finally, this work examines the phylogeography and demographic history of Pseudocalanus minutus, P. moultoni, P. elongatus, P. acuspes and P. newmani in the North Atlantic Ocean and based on these results suggests an evolutionary hypothesis for the divergence of Pseudocalanus spp. into one oceanic and one coastal clade. Hopefully, this thesis has achieved its main goal of providing a basis from which the biology and ecology of these fascinating copepod species can be further explored.

All but paper 5 of this thesis are not available in Munin: 1. Aars J, Marques T A, Buckland ST, Andersen M, Belikov S, Boltunov A & Wiig Ø.: 'Estimating the Barents Sea polar bear subpopulation size', Marine Mammal Science (2009), vol. 25(1):35-52. Available at http://dx.doi.org/10.1111/j.1748-7692.2008.00228.x 2. Andersen M, Derocher AE, Wiig Ø & Aars J.: 'Movements of two Svalbard polar bears recorded using geographical positioning system satellite transmitters', Polar Biology (2008), vol. 31:501-507. Available at http://dx.doi.org/10.1007/s00300-008-0428-x 3. Freitas C, Kovacs KM, Andersen M, Aars J, Sandven S, Mauritzen M, Pavlova O & Lydersen C.: 'Importance of fast ice and glacier fronts for female polar bears and their cubs during spring in Svalbard, Norway', Marine Ecology Progress Series (2012), 447:289-304. Available at http://dx.doi.org/10.3354/meps09516 4. Andersen M, Derocher AE, Wiig Ø & Aars J.: 'Polar bear (Ursus maritimus) maternity den distribution in Svalbard, Norway', Polar Biology (2012), vol.35:499-508. Available at http://dx.doi.org/10.1007/s00300-011-1094-y 6. Andersen M and Aars J.: 'Short-term behavioural responses of polar bears to disturbance by snowmobiles', Polar Biology (2008), vol.31:501-507. Available at http://dx.doi.org/10.1007/s00300-007-0376-x 7. Derocher AE, Wiig Ø & Andersen M.: 'Diet composition of polar bears in Svalbard and the western Barents Sea', Polar Biology (2002), vol.25:448-452. Available at http://dx.doi.org/10.1007/s00300-002-0364-0 8. Andersen M, Lie E, Derocher AE, Belikov SE, Bernhoft A, Boltunov AN, Garner GW, Skaare JU & Wiig Ø.: 'Geographic variation of PCB congeners in polar bears (Ursus maritimus), from Svalbard to the Chukchi Sea', Polar Biology (2001), vol. 24:231-238. Available at http://dx.doi.org/10.1007/s003000000201 9. Andersen M, Gwynn JP, Dowdall M, Lydersen C & Kovacs KM.: 'Radiocaesium in marine mammals from Svalbard, the Barents Sea and the North Greenland Sea', Science of the Total Environment (2006), vol. 363:87-94. Available at http://dx.doi.org/10.1016/j.scitotenv.2005.06.019 Polar bears in the Barents Sea population have been protected from hunting in Russia since 1956 and following the signing of the international Polar Bear Agreement in 1973 in Norway. This thesis seeks to summarise current knowledge on key population biology issues four decades after the Norwegian protection and almost six after the Russian. Further, it discusses threats that have developed in the decades following protection against human harvesting. It concludes with a discussion of the effect of multiple stressors on the population, and some thoughts on future research, monitoring and management. Polar bears in Svalbard and the Barents Sea area have been studied during the last 40 years with the aim of gaining knowledge regarding population biology and to evaluate potential sources of impact on the population from anthropogenic activity and changes to their habitat. The initial threat to polar bears in the region was unquestionably overharvest. Polar bear numbers were reduced quite drastically and hunting was clearly not sustainable. After the harvesting was stopped, the population grew in size to an estimated 2650 (1900-3600) in 2004. We believe that population recovery led to a wider distribution of maternity denning in the Svalbard Archipelago, compared to the period just after the protection of the population in 1973. However, during recent decades, the population has faced challenges from a variety of new anthropogenic impacts. The population has been exposed to a range of pollutants and an increasing level of human presence and activity within their range. Contaminants are bioaccumulated through the trophic levels in the marine food web, culminating in this top predator that consumes primarily ringed, bearded and harp seals. Females with small cubs use the land-fast sea ice for hunting, and are vulnerable to human disturbance. Changes in sea ice conditions also affect polar bears in the region, and reduced access to denning areas on the eastern islands of Svalbard is currently a concern. A decrease in spring land-fast ice close to important denning areas could negatively affect the survival of cubs. Research and monitoring provides advice to management bodies both locally and globally. Information on the presence of toxic compounds in High Arctic systems has resulted in progress in recent decades in having better control of harmful substances and in some cases international bans on their production and use. This has resulted in declining contaminant burdens in polar bears. Unfortunately, new harmful substances are finding their way to the Arctic, while others, such as radionuclides, are stored locally (within Russian Territories) in large quantities, representing potential sources of pollution. The protection of important habitats locally with restrictions on motorized traffic may help reduce negative impacts from human activity on polar bears in the region. The fate of polar bears with regard to climate change is uncertain, but significant negative effects have been documented and these impacts are expected to increase in the coming decades. Relevant research and monitoring of polar bears is essential for future management of the species. The arctic environment should be managed in such a way that the combined effects of stressors on polar bear populations are minimized.

The papers of this thesis are not available in Munin: 1. Silje Ramsvatn and Torstein Pedersen: 'Ontogenetic niche changes in haddock Melanogrammus aeglefinus reflected by stable isotope signatures, δ13C and δ15N' Marine Ecology Progress Series (2012), vol. 451:175–185, available at http://dx.doi.org/10.3354/meps09604 2. Torstein Pedersen, Silje Ramsvatn, Einar M. Nilssen, Marianne Nilsen, Lyne Morissette, Trond Ivarjord, Geir Systad, Ina Kolsum and Håvard Fause: 'Running headline: D iversity and Mass Flows Patterns in Fjord Ecosystems Species diversity affects marine ecosystem structure and mass flows' (manuscript). 3. Silje Ramsvatn, Torstein Pedersen, Marianne Nilsen and Einar M. Nilssen: 'Can stable isotopes be used to parameterize a marine food-web model in Ecopath?' (manuscript) Marine økosystemer er svært komplekse, og derfor bruker vi modeller for å kunne forenkle og systematisere kompliserte sammenhenger. Slike økologiske modeller brukes i utstrakt grad innen forskning og forvalting. Hva de ulike artene av fisk og krepsdyr spiser er svært viktig å vite for å kunne forstå hvordan artene påvirker hverandre, og dette kan vi finne ut med en liten vevsprøve for å måle stabile isotoper. Dette prosjektet undersøkte om denne metoden gav like god informasjon som analyse av mageinnholdet.

The papers of this thesis are not available in Munin: 1. Aubert, A.B., Svensen, C., Hessen, D., Tamelander, T.: 'CNP stoichiometry of a lipid-synthesising calanoid copepod, Calanus finmarchicus, from winter to spring bloom in a sub-Arctic sound', Journal of Marine Systems (2012), Volumes 111–112, pp.19–28. Available at http://dx.doi.org/10.1016/j.jmarsys.2012.09.004 2. Aubert, A.B., Tamelander, T.: 'Stoichiometric approach to elemental limitation of pelagic producers and grazers during the Arctic summer' (manuscript). 3. Tamelander, T., Aubert, A. B., Wexels Riser, C.: 'Export stoichiometry and contribution of copepod faecal pellets to vertical flux of particulate carbon, nitrogen and phosphorus', Marine Ecology Progress Series (2012), vol.459, pp.17-28. Available at http://dx.doi.org/10.3354/meps09733 Calanoid copepods, Calanus finmarchicus, C. glacialis and C. hyperboreus, are key species in Arctic and sub-Arctic marine ecosystems. They represent important prey for a number of species such as juvenile herring and cod because of their high but seasonally variable calorific value: they synthesize and store lipids as part of their survival strategy. Limitations in food quantity and quality can have strong impact on the copepods’ lipid synthesis and development, with implications for their predators and thus for the lipid-driven Arctic marine food web. Ecological stoichiometry, i.e. the study of the relative abundance of elements (most commonly carbon, nitrogen, and phosphorus) in biological entities, is a powerful tool to study potential limitation of aquatic organisms, particularly in primary producers and in omnivorous and herbivorous zooplankton grazers. However, studies that apply a stoichiometric approach to the high-latitude marine regions are extremely rare. The main aim of this thesis is to reveal the elemental requirements and potential limitation of high-latitude calanoid copepods by investigating their C:N:P composition in combination with information on other biochemical compounds. The impact of copepods on nutrient recycling and C:N:P export through fecal pellets is also investigated. The study was conducted in a sub-Arctic north-Norwegian fjord, and in the western part of the European Arctic Ocean, focusing on the period of intense primary production during spring. Somatic stoichiometric ratios in copepods were calculated by subtracting C in the lipid sac from the total body C. Due to the strong influence of lipid storage on total body stoichiometry, high-latitude calanoid copepods, and by implication lipid-synthesizing zooplankton in general, should be treated through a two-compartment stoichiometric approach, with lipid storage and somatic tissues separated. Both season and development stage influence the somatic elemental ratios. The somatic C:P ratios of high-latitude calanoid copepods are somewhat lower than those reported in marine copepods from other areas, suggesting that they have higher growth rates than temperate and tropical copepods. Seston C:N:P stoichiometry is used as a proxy for food quality in the calculation of copepod elemental limitation. The seston C:P and N:P ratios are among the first to be reported for the Arctic Ocean and indicate a higher relative proportion of nutrients (N and P) compared to published seston values from other marine regions. Seston C:N:P ratios varied according to bloom stage, which therefore also impacts the potential C, N or P limitation of copepods. Unless they have high assimilation efficiency for C, calanoid copepods are shown to be most likely subject to C limitation. However, variable requirements during ontogeny make specific stages, such as copepodite IV, and females more sensitive to nutrient limitation (N or P). This study is the first to present data on stoichiometric ratios including P for copepod fecal pellets in the Arctic, and shows their high potential not only to vertically export P at depth but also to provide a source of P in the upper layers during the productive season if degraded. The Arctic marine regions are experiencing the greatest climate change, and the potential future stoichiometric changes in the Arctic and how climate change affects copepod-seston interaction are discussed.

The papers of this thesis are not available in Munin: 1. Hjelset, A.M., Sundet, J.H. and Nilssen, E.M.: 'Size at sexual maturity in the female red king crab (Paralithodes camtschaticus) in a newly settled population in the Barents Sea, Norway', Journal of Northwest Atlantic Fishery Science (2009), 41:173-182. Available at http://dx.doi.org/10.2960/J.v41.m633 2. Hjelset, A.M., Nilssen, E.M. and Sundet, J.H.: 'Reduced size composition and fecundity related to fishery and invasion history in the introduced red king crab (Paralithodes camtschaticus) in Norwegian waters', Fisheries Research (2012), 121–122:73-80. Available at http://dx.doi.org/10.1016/j.fishres.2012.01.010 3. Hjelset, A.M.: 'Red king crab (Paralithodes camtschaticus) recruitment potential in Norwegian fjords' (manuscript) The red king crab (Paralithodes camtschaticus) was deliberately introduced to the Barents Sea by Russian scientists during the 1960s, with the purpose to improve coastal fishery and thus improve the local economy. Since the red king crab was caught for the first time in the Varangerfjorden area, the stock has increased in abundance and expanded westward inhabiting coastal areas along the coast of Finnmark from the Russian border in east to the area around Hammerfest in west. Today the population of red king crab supports a valuable fishery in the Barents Sea, representing an ex-vessel value of 150 million NOK in 2011. Several challenges are related to management of the red king crab, since it is an introduced species. The Norwegian management regime today has two goals, one is to keep a long term harvest within a geographical limited area and the second is to limit further spread of the crab. Basic knowledge is needed to meet both objectives, particularly knowledge on life-history traits. The main study area in this thesis has been three large fjords in Finnmark in northern Norway. There is a historical westward spread of the crab along the Norwegian coast, and it has been present in Varangerfjorden, Tanafjorden and Laksefjorden for about 35, 25 and 15 years respectively. Crabs have been sampled annually during scientific autumn cruises from 1994 until 2011. Size at maturity is a key life-history parameter in fishery management and should be monitored routinely. For female red king crabs this means the recording of presence or absence of eggs. In paper I the size at maturity was estimated in female crabs from each of the three fjords which constitute the main study area in this thesis. No temporal differences were seen and the size at maturity varied from 108 to 111 mm carapace length in the three fjords and was found to be generally higher than in native areas. Knowledge about the species individual fecundity gives information on size and number of offspring produced. Fecundity is closely related to life-history traits such as size at maturity, life-span and egg size. In this study both spatial and temporal patterns in fecundity and a reduction in individual egg weight was found. A main finding in paper II was a reduction in the individual fecundity during a part of the study period. An average female produced about 30 % less eggs in 2007 compared to 2002. Fecundity is the life-history parameters that respond most rapidly to changes in environment or other factors, such as fishery. The abundance estimates of red king crab females have increased in our area during the study period, but in the last two years, a decrease was observed in Tanafjorden and Laksefjorden. The stock size composition and the individual fecundity parameters established in paper II has been used in paper III to obtain a reproductive potential of the crab stock for the period 1995 to 2011. The male-only fishery and introduction of a female quota has had influence on the size composition and fecundity, and thus on the potential egg production in the stock. The study also showed that an observed reduction in size of large males was correlated to a size reduction of large females with a one year lag.

The papers of this thesis are not available in Munin: 1. Eilertsen H.C., Huseby S., Degerlund, M., Eriksen, G.K., Ingebrigtsen, R.A. and Hansen, E.: 'Sample storage effects on reproducibility of direct infusion high-resolution mass spectrometry (HRMS) microalgae extract analysis', (manuscript submitted to Limnology and Oceanography Methods). 2. Huseby, S., Hansen, E., Degerlund, M., Eriksen, G. K., Ingebrigtsen, R.A and Eilertsen, H.C.: 'Chemical diversity of six northern diatoms', (manuscript submitted to Journal of Applied Phycology). 3. Degerlund, M., Huseby, S., Zingone, A., Sarno, D. and Landfald, B.: 'Functional diversity in cryptic species of Chaetoceros socialis Lauder (Bacillariophyceae)', Journal of Plankton Research (2012), vol. 34, no. 5:416-431. Available at http://dx.doi.org/10.1093/plankt/fbs004 4. Huseby S., Degerlund M., Zingone, A. and Hansen. E.: 'Metabolic fingerprinting reveals differences between northern and southern strains of the cryptic diatom Chaetoceros socialis', (manuscript in revision in European Journal of Phycology) The main aim of this thesis was to investigate if and how metabolic fingerprinting can be applied in diatom taxonomy. Even though both morphology and gene sequences have been shown to be appropriate tools in diatom taxonomy there are cases where these give contradicting results, like in the case of cryptic species. Cryptic species have similar morphology but are genetically different. Another issue with these two tools is that they do not offer much information about the function of a species, information that is interesting in light of for example ecology and management. Metabolomics investigates the metabolites synthesized by an organism. The metabolites synthesized at a certain moment in time will be a reflection of what genes are expressed at that time and will be a product of the organisms response to the environmental and biological conditions prevailing. Direct injection mass spectrometry was used to investigate the metabolic fingerprints of different, commonly occurring, northern diatom species. The method produces mass-to-charge ratios (markers) normally with a mass precision of four decimals. Reproducibility of the method was 80% with the direct injection method applied, using a decimal precision of 0.1. The results of the analysis showed that the different species shared between 26-67% of the total markers. Even species of the same genera showed a high diversity. The two species Chaetoceros furcellatus and Chaetoceros socialis only shared 30% of the total markers. For four out of six species the difference between species increased with decreasing temperature. The expected phylogeny of these six species could not be reflected by the metabolite data. Cryptic diversity has been documented within the so-called cosmopolitan species Chaetoceros socialis. We investigated this diversity between strains collected from two geographic areas; the northeast Atlantic and Arctic and from Mediterranean waters. Monoclonal cultures were cultivated at three different temperatures, and analyzed with the aid of morphology, LSU rRNA sequencing, growth rates, photosynthetic maximum quantum yield and metabolic fingerprinting. Comparison of gene sequences of the two groups showed an unequivocal difference, while only small morphological differences in spore morphology (not in the morphology of the vegetative cells) could be found between the two groups. At all three temperatures there were clear differences in growth and maximum quantum yield. The results from the metabolic fingerprinting also supported these findings. The clear genetic as well as functional differences does not support the cosmopolitan distribution of C. socialis and we therefore conclude that this species should be revised. The results, both from the comparison of metabolic fingerprinting between diatom species as well as within a pseudo cryptic diatom species, in my opinion, is in support of the use of metabolomics in diatom taxonomy. Our results underline the need of metadata, e.g. growth rates, in metabolomics studies. I also think that increased knowledge of functional traits of species, like metabolomics, could be implemented in ecological modeling, building a bridge between taxonomy and ecology. The results of this thesis are also relevant to bioprospecting. The higher chemical diversity between species found at the lower temperatures, would indicate that it could be beneficial to cultivate diatoms at low temperatures, close to zero, in search for bioactivities.

Paper 3 and 4 of this thesis are not available in Munin: 3. Seuthe L, Töpper B, Reigstad M, Thyrhaug R and Vaquer-Sunyer R: 'Microbial communities and processes in ice-covered Arctic waters of the northwestern Fram Strait (75 – 80ºN) during the vernal pre-bloom phase', Aquatic Microbial Ecology (2011) 64:253-266. Available at http://dx.doi.org/10.3354/ame01525 4. Svensen C, Seuthe L, Vasilyeva Y, Pasternak A and Hansen E: 'Zooplankton communities across Fram Strait in autumn : are small copepods and protozooplankton important? (in press in Progress in Oceanography). This thesis investigates the structure and function of planktonic food webs at two sites between Greenland and the Svalbard Archipelago, covering a coastal ecosystem influenced by Atlantic water masses (Kongsfjorden, 78º N) and a more oceanic system off the East Greenlandic shelf, influenced by the outflow of Arctic water and sea ice from the Arctic Ocean (northwest Fram Strait, 75 - 80º N). In Kongsfjorden, a seasonal study was conducted with sampling at six occasions between March and December 2006. Logistical constrains prohibited a similar extensive seasonal investigation in the ice-covered waters of northwest Fram Strait. Sampling fell into the onset (April-May 2008) and end (September 2006/2007) of the productive season. All four studies investigated the stocks of pico- to micro-sized autotrophs and heterotrophs, i.e. heterotrophic bacteria, proto- and metazooplankton. Production rates of autotrophs and heterotrophic bacteria were measured in Kongsfjorden. Ratios of heterotrophic and autotrophic biomass (H:A) and of specific bacterial and primary production (µBac:µPhyto) allowed to asses the overall structure and function of the investigated planktonic food webs and to compare them to published data from other Arctic regions. The emerging picture provides three distinct food web modes, where mode A is characterized by H:A < 1 and µBac:µPhyto > 1, new production, and large-celled phytoplankton. Mode B and C are characterized by dominance of heterotrophs (H:A > 1), more regenerated production, and small-celled phytoplankton. The difference between mode B and C is a difference in the specific production ratio, with µBac:µPhyto smaller and larger 1, respectively. According to this scheme, Arctic plankton communities appear to prevail under mode B and C most of the year. In Kongsfjorden, e.g. five of the six months sampled fell under mode B and C, with mode C being typical for light-limited winter communities. From own and literature data, it is suggested that the food web mode controlling physical factors are the amount of incident photosynthetically active radiation (PAR), ice cover, nutrient concentration, and water column stability. Due to an approximately latitudinal change in these physical settings, the relative prevalence of the three food web modes changes from the marginal seas to the central Arctic Ocean, with mode A most likely being absent from the highest latitude waters. In general, Arctic plankton communities appear to sustain on average a 3-fold higher H:A biomass ratio for a given phytoplankton stock than the world’s coastal oceans, which may mainly be due to extensive import of long-lived copepods from sub-Arctic European seas through advection. It is argued that the large stock of heterotrophs plays a crucial role in structuring Arctic plankton communities, with the potential to prohibit phytoplankton bloom formation (mode A) through extensive grazing.

The papers of this thesis are not available in Munin: 1. Degerlund M and Eilertsen HC: 'Main species characteristics of phytoplankton spring blooms in NE Atlantic and Arctic waters(68–80°N)', Estuaries and Coasts (2010) 33: 242-269. Available at http://dx.doi.org/10.1007/s12237-009-9167-7 2. Eilertsen HC and Degerlund M: 'Phytoplankton and light during the northern high-latitude winter', Journal of Plankton Research (2010) 32: 899-912. Available at http://dx.doi.org/10.1093/plankt/fbq017 3. Degerlund M, Huseby S, Zingone A, Sarno D and Landfald B: 'Functional diversity in cryptic species of Chaetoceros socialis Lauder (Bacillariophyceae)' (submitted paper to Journal of Plankton Research). 4. Huseby S, Degerlund M, Zingone A and Hansen, E: 'Metabolite fingerprinting and physiology of the cryptic diatom Chaetoceros socialis Lauder' (manuscript) The choice of species concept when delineating phytoplankton species is decisive for our knowledge of species diversity and distribution, but will also affect our understanding of the functioning of marine ecosystems. This is particularly true for so-called cryptic species, which may show high degree of genetic and physiological heterogeneity although being morphologically similar. A precise species delimitation procedure appreciating the value of genotypic as well as phenotypic traits is therefore important. In this thesis, species concepts and functional aspects in abundant cold-water diatoms were investigated using biogeographic (species presence and abundance), taxonomic (morphological and molecular data), and functional (experimental physiological and metabolomics data) tools. A compilation of multiannual phytoplankton species abundance data gathered during the spring bloom period from north-east Atlantic and Arctic waters (68-80oN) revealed high similarities in associations of the most abundant species. Spring associations were dominated by the diatom Chaetoceros socialis and the prymnesiophyte Phaeocystis pouchetii. A corresponding investigation of the winter period in this area could not identify associations of actively growing, autotrophic phytoplankton species. Change in species composition and abundance in time could not be assessed due to lack of consistency and regularity in sampling. A high-resolution time series of phytoplankton distribution is therefore needed in this area. A case study of the so-called cosmopolitan diatom C. socialis was performed using strains from north-east Atlantic/Arctic and from Mediterranean waters. By applying a phylo-phenetic species concept the two geographical populations were found to be pseudo-cryptic. They were genetically distinct, but only slight morphological differences were observed, in this case in resting spore morphology. Furthermore, when cultivated at 2.5, 8 and 13oC, the two groups of strains were functionally different in terms of growth rates (doublings day-1), photosynthetic efficiency (maximum quantum yield), and metabolic profiles. Our results do not support the assumed cosmopolitan distribution of C. socialis and therefore the present taxonomy of this species will need revision. A taxonomic update of the pseudo-cryptic diatom formerly known as Skeletonema costatum indicated that the species present in northern Norwegian and Barents Sea waters is identical to S. marinoi. Due to reports of high genetic, metabolic, and physiological diversity within this species, a functional approach would be valuable for understanding the ecology of S. marinoi in this area. Integrated approaches combining taxonomic and functional variables are recommended for future taxonomic work on phytoplankton species. Similarly, phytoplankton ecologists are urged to acknowledge the high degree of hidden taxonomic and functional diversity present in several phytoplankton species. Functional diversity studies are suggested to be a useful spot for integration and eventual modelling phenotypic, genotypic and ecological species data. Ultimately, such an approach should be beneficial also at the ecosystem level.