Kongsfjorden is a glacial fjord in the Arctic (Svalbard) that is influenced by both Atlantic and Arctic water masses and harbours a mixture of boreal and Arctic flora and fauna. Inputs from large tidal glaciers create steep environmental gradients in sedimentation and salinity along the length of this fjord. The glacial inputs cause reduced biomass and diversity in the benthic community in the inner fjord. Zooplankton suffers direct mortality from the glacial outflow and primary production is reduced because of limited light levels in the turbid, mixed inner waters. The magnitude of the glacial effects diminishes towards the outer fjord. Kongsfjorden is an important feeding ground for marine mammals and seabirds. Even though the fjord contains some boreal fauna, the prey consumed by upper trophic levels is mainly Arctic organisms. Marine mammals constitute the largest top‐predator biomass, but seabirds have the largest energy intake and also export nutrients and energy out of the marine environment. Kongsfjorden has received a lot of research attention in the recent past. The current interest in the fjord is primarily based on the fact that Kongsfjorden is particularly suitable as a site for exploring the impacts of possible climate changes, with Atlantic water influx and melting of tidal glaciers both being linked to climate variability. The pelagic ecosystem is likely to be most sensitive to the Atlantic versus Arctic influence, whereas the benthic ecosystem is more affected by long‐term changes in hydrography as well as changes in glacial runoff and sedimentation. Kongsfjorden will be an important Arctic monitoring site over the coming decades and a review of the current knowledge, and a gap analysis, are therefore warranted. Important knowledge gaps include a lack of quantitative data on production, abundance of key prey species, and the role of advection on the biological communities in the fjord.

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High-latitude environments show extreme seasonal variation in physical and biological variables. The classic paradigm of Arctic marine ecosystems holds that most biological processes slow down or cease during the polar night. One key process that is generally assumed to cease during winter is diel vertical migration (DVM) of zooplankton. DVM constitutes the largest synchronized movement of biomass on the planet, and is of paramount importance for marine ecosystem function and carbon cycling. Here we present acoustic data that demonstrate a synchronized DVM behaviour of zooplankton that continues throughout the Arctic winter, in both open and ice-covered waters. We argue that even during the polar night, DVM is regulated by diel variations in solar and lunar illumination, which are at intensities far below the threshold of human perception. We also demonstrate that winter DVM is stronger in open waters compared with ice-covered waters. This suggests that the biologically mediated vertical flux of carbon will increase if there is a continued retreat of the Arctic winter sea ice cover.

Planktonic copepods of the genus Calanus play a central role in North Atlantic/Arctic marine food webs. Here, using molecular markers, we redrew the distributional ranges of Calanus species inhabiting the North Atlantic and Arctic Oceans and revealed much wider and more broadly overlapping distributions than previously described. The Arctic shelf species, C. glacialis, dominated the zooplankton assemblage of many Norwegian fjords, where only C. finmarchicus has been reported previously. In these fjords, high occurrences of the Arctic species C. hyperboreus were also found. Molecular markers revealed that the most common method of species identification, prosome length, cannot reliably discriminate the species in Norwegian fjords. Differences in degree of genetic differentiation among fjord populations of the two species suggested that C. glacialis is a more permanent resident of the fjords than C. finmarchicus. We found no evidence of hybridization between the species. Our results indicate a critical need for the wider use of molecular markers to reliably identify and discriminate these morphologically similar copepod species, which serve as important indicators of climate responses.

Overwintering Calanus spp. were studied in four Norwegian fjords with different predator regimes and ranging in depth from 380 to 1300 m. Three fjords held both the planktivorous mesopelagic fish Maurolicus muelleri and Benthosema glaciale and invertebrate predators, whereas one lacked mesopelagic fish but had especially high abundance of several invertebrate predators. Co‐occurrence of C. finmarchicus, C. helgolandicus, and C. glacialis rendered distinction between effects of environmental conditions and inherent species properties in choice of depth difficult. The highest daily per capita mortality rate for Calanus was estimated at 0.024–0.027 d‐1 (95% CI) in a fjord with high fish abundance and with the clearest water. Predation by M. muelleri and B. glaciale alone could explain the estimated winter mortality. The fjord devoid of mesopelagic fish but particularly rich in invertebrate predators gave the lowest estimated mortality rate; 0.008–0.009 d‐1 (95% CI). Our results indicate that mesopelagic fish pose a stronger predatory threat than invertebrates to overwintering Calanus. This concurs with Calanus selection of oceanic winter habitats below depths where planktivorous fish can forage efficiently by sight.

Light is an important limiting factor for the visual foraging process in fishes, and the light regime may potentially affect the competition between visual and tactile predators. We investigated two equal‐sized fjords of quite different pelagic food web structure. Earlier studies have revealed that the jellyfish Periphylla periphylla dominates Lurefjorden, while fish predators dominate in the other fjord, Masfjorden. Furthermore, the mesozooplankton stock of Lurefjorden is larger in both total biomass and size of the individuals. Hence, earlier hypotheses linking the competitive advantage of tactile gelatinous plankton predators to smaller sized mesozooplankton communities are unable to explain the present phenomenon. To see if the difference in the pelagic biota of the two fjords could be associated with characteristics of the light regime, we measured the light absorbance in the basin water of the two fjords. We found that, due to a slightly stronger influence of coastal water in the basin water of Lurefjorden, the exponential light absorbance coefficient below 100 m is two to three times higher there than in other fjords. This results in a reduction in light flux of several orders of magnitude, effectively reducing the possibility of visual foraging. The tactile mode of predation in jellyfish, however, is not influenced, and we hypothesize that the visibility regime has a decisive role in structuring the pelagic food webs of the two fjords.

The sea butterfly Limacina helicina was col-

This thesis investigates mesozooplankton abundance, composition and distribution in Svalbard waters in relation to hydrography. Sampling was carried out in the archipelago of Svalbard mainly during summer and autumn between 2000 and 2004. From cluster analysis four species assemblages were distinguished and these reflected differences in hydrography and bottom depth. In particular the distribution of the Atlantic Calanus finmarchicus relative to that of the Arctic Calanus glacialis was associated with different hydrographic regimes. Differences in the species assemblages primarily resulted from variations in species densities rather than from taxonomical variation. For species of Atlantic and Arctic origin significant relationships with temperature and salinity were found. Regression models were used to quantify the influence of water mass characteristics on the abundance of the three different Calanus species that co-occur in the study area. About 50% of the variability in abundance of each Calanus species could be accounted for by variability in temperature and salinity. C. finmarchicus abundance was positively related to warmer and more saline waters, as expected from its distributional southern core area. Conversely, the Arctic species C. hyperboreus was more abundant in colder and fresher waters. The numbers of C. glacialis decreased with increasing temperature and salinity in shallow areas, while the opposite trend was found in deep locations. Salinity and temperature between 50-150 m depth were in most cases better predictors for Calanus spp. abundance than near-surface conditions. Variability in the vertical distribution of the three Calanus species and Metridia longa reflected life history and behavioural adaptations on diel and seasonal scale. Diel vertical migration was observed for copepodite stages of M. longa but generally not for Calanus spp. The copepodite stage composition indicated a south to north delay in the succession of Calanus development and that the descent to overwintering depth had started at time of sampling. The vertical distribution patterns of C. finmarchicus and M. longa were found to be consistent with the hypothesis that the developmental stages distributed according to preferences for light intensity. This thesis includes one of few year-round studies on Calanus population dynamics from the high Arctic. The three Calanus species co-existed in the studied fjord. The estimated length of the life cycle of C. glacialis (1-2 years) and C. finmarchicus (1 year) were in agreement with previous studies in Svalbard and the Arctic in general. For C. hyperboreus a one year life cycle was observed which is among the shortest life cycle duration reported for this species. Differences in winter mortality rates for the three Calanus populations indicate that these affect the species composition in the study area. The observed statistical relationships between Calanus abundance and hydrography indicate that changes in ocean climate as a consequence of global warming may alter the relative comp...