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.

Coastal marine species with pelagic egg and larval stages, such as the Atlantic cod Gadus morhua, can be structured into genetically distinct local populations on a surprisingly small geographic scale considering their dispersal potential. Mechanisms responsible for such small-scale genetic structure may involve homing of adults to their natal spawning grounds, but also local retention of pelagic eggs and larvae. For example, spawning within sheltered fjord habitats is expected to favour local retention of early life stages. Here, we studied the distribution of cod eggs along inshoreoffshore transects in 20 Norwegian fjords. The general pattern exhibited across all fjords was a higher concentration of cod eggs inside the fjords than further offshore. In particular, fjords with shallow sills (model threshold 37 m) show an abrupt reduction in egg density over the sill. This study provides empirical support for an offspring retention hypothesis, which may help to explain the maintenance of local population structure in pelagic marine systems.

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The study addresses how local abiotic variability links zooplankton stocks to climate systems. Four zooplankton species and abiotic environmental data were sampled annually during October and February 1983-2005 in two northern Norwegian fjord basins. Inter-decadal change in abundance occurred in both fjords but differed between the two communities, presumably resulting from combinations of topography, specific life history strategies and preference for ambient water qualities before advective migration into the habitats. The shallow Mistfjord sill led to accumulation of coastal water in the fjord basin, while the deeper Saltfjord sill allowed frequent seasonal exchange of Atlantic basin water. Changes in basin water temperature and salinity were decadal in the Mistfjord basin, and inter-annual in the Saltfjord basin. Several biotic and abiotic variables covaried with climate indices, which indicates causal relationships between hemispheric climate variability and abundance of wintering zooplankton, but the processes involved are not fully understood. North Atlantic Oscillation apparently forced vernal freshwater discharge from Norway, summer reproduction in neritic waters and geographic distribution of basin-scale population systems. Stratospheric forcing as indicated by Arctic Oscillation seems to be involved in geophysical relationships which influence dynamic processes in zooplankton population systems within the Arctic Mediterranean. arctic regions of the Atlantic sector. NAO above the long-term average (positive NAO) generates southwesterly winds towards Scandinavia (Hurrell, 1995) which discharges latent heat and precipitation over Norwegian coastal landscapes while NAO below average (negative NAO) is associated with dryer climate. Seasonal accumulation of snow and vernal meltwater runoff causes large river discharge in May-July (Tollan, 1976) which generates pronounced geophysical changes in coastal recipients (Saetre, 2007). Several authors have discussed how natural freshwater discharge and lowered salinity during summer may influence neritic biological production (Helland-Hansen

In the North Atlantic and the Arctic Ocean, four species of the copepod genus Calanus dominate the zooplankton biomass. Because of their morphological resemblance, knowledge of their respective distribution range has long been biased by misidentification, until the recent use of molecular tools uncovered numerous areas of sympatry. As hybridization between Calanus finmarchicus and Calanus glacialis has been claimed in the East-Canadian Arctic based on microsatellites, we investigated further the potential for interbreeding in newly uncovered areas of sympatry. Calanus species and stage composition were analyzed during winter in two Norwegian subarctic fjords, using molecular markers developed specifically for species identification and hybrid detection between C. finmarchicus and C. glacialis. Overall, C. glacialis were the most abundant throughout the winter, followed by C. finmarchicus and Calanus hyperboreus with only a few records of Calanus helgolandicus. The presence of C. glacialis, C. hyperboreus, and C. finmarchicus' nauplii was recorded, indicating that these species reproduce locally. In January and February, the simultaneous occurrence of males and females of both C. finmarchicus and C. glacialis suggested a potential for interspecies mating. However, genetic admixture tests performed on all 1126 individuals revealed no signal of hybridization, implying a strong reproductive isolation mechanism. We conclude that no evidence supports a potential for hybridization between C. finmarchicus and C. glacialis.