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Subarctic populations of blue mussels represented by “cryptic” speciesMytilus edulis(ME) andM. trossulus(MT) have been studied less intensively than Arctic and boreal populations. Ecological features ofMEandMTin sympatry are poorly known everywhere. The knowledge about mussels at the northeasternmost boundary of the Atlantic littoral communities on Murman coast of the Barents Sea is based on data obtained 50-100 years ago. Our study provides the first insight into the long-term dynamics of the Barents Sea mussels, the habitat segregation ofMEandMT, and the interannual dynamics of their mixed settlements. The Tyuva Inlet (Kola Bay), which is 3 km long, was used as the study site. Mussels were found everywhere in the littoral and the sublittoral down to a depth of 4 m. Their characteristic habitats were sandbanks, littoral rocks, sublittoral kelp forests and “the habitat of the mussel bed” in the freshened top of the inlet. The main spatial gradients explaining the variability of demographics of the settlements (abundance, age structure, size) were associated with the depth and the distance from the inlet top.MEandMTwere partially segregated by depth:MEdominated in the sublittoral andMT, in the littoral. In addition,MEdominated both in the littoral and in the sublittoral parts of the mussel bed. The ratio of species in the mixed settlements varied over time: between 2004 and 2010 the proportions ofMTdecreased everywhere, by 22 % on average. Historical data indicate that the abundance of the Murman mussels declined sharply between the 1960s and the 1970s, which coincided with the cooling of the Arctic. It seems that the populations have not completely recovered in the abnormally warm recent decades. The habitat distribution of mussels apparently changed with time, too: unlike today, in the 20th century mussels were rarely observed in kelps. We suggest that the spatial and temporal dynamics of subarctic mussels can be partly explained by the competition betweenMEandMTcombined with their differing sensitivity to environmental factors.

The process of climate change by which global temperatures increase and seasonal shifts occur is more pronounced at higher latitudes. These changes induce shifts in the phenology of biota, including zooplankton. Regression analysis revealed significant advance of the spring–summer water warming in the sub-Arctic White Sea (Chupa Inlet, Kandalaksha Bay), which occurs 3 weeks earlier in the last decade compared with 1961. The shift of timing of phenological events of Calanus glacialis, Pseudocalanus spp., Microsetella norvegica, Oithona similis, Acartia spp., Centropages hamatus, and Temora longicornis is particularly pronounced. These shifts were accompanied by a significant increase in Calanus and Microsetella abundance with an observed decrease in Acartia abundance. No correlation between population abundance and year-to-year changes in the species phenology was determined. We propose hypotheses to explain species adaptation to these phenological shifts in the ecosystem: (1) flexibility of life cycle of planktonic copepods, which ensures synchronization of life cycle phases with key events in the environment; and (2) copepod ability to switch to carnivory when algal sources are scarce. Interspecific interactions between ubiquitous species influenced their phenology and abundance, while early development of Microsetella caused later development of Oithona and higher abundance of the latter. We speculate that competition for food resources is a driving factor in the interactions, since the trophic niches of these species overlap.

Cryptic and hybridizing species may lack diagnostic taxonomic characters leaving researchers with semi-diagnostic ones. Identification based on such characters is probabilistic, the probability of correct identification depending on the species composition in a mixed population. Here we test the possibilities of applying a semi-diagnostic conchological character for distinguishing two cryptic species of blue mussels, Mytilus edulis and M. trossulus. These ecologically, stratigraphically and economically important molluscs co-occur and hybridize in many areas of the North Atlantic and the neighboring Arctic. Any cues for distinguishing them in sympatry without genotyping would save much research effort. Recently these species have been shown to statistically differ in the White Sea, where a simple character of the shell was used to distinguish two mussel morphotypes. In this paper, we analyzed the associations between morphotypes and species-specific genotypes based on an abundant material from the waters of the Kola Peninsula (White Sea, Barents Sea) and a more limited material from Norway, the Baltic Sea, Scotland and the Gulf of Maine. The performance of the “morphotype test” for species identification was formally evaluated using approaches from evidence-based medicine. Interspecific differences in the morphotype frequencies were ubiquitous and unidirectional, but their scale varied geographically (from 75% in the White Sea to 15% in the Baltic Sea). In addition, salinity-related variation of this character within M. edulis was revealed in the Arctic Barents Sea. For every studied region, we established relationships between the proportions of the morphotypes in the populations as well as between the proportions of the morphotypes in samples and the probabilities of mussels of different morphotypes being M. trossulus and M. edulis. We provide recommendations for the application of the morphotype test to mussels from unstudied contact zones and note that they may apply equally well to other taxa identified by semi-diagnostic traits.

Polar shallow marginal seas are of high importance as they are the most productive regions of the Arctic Ocean and serve as filters for terrestrial runoff. Salinity and turbidity gradients create diverse habitats for planktonic organisms in coastal areas. In the present study we aimed at assessing the degree to which environmental gradients influence the abundance and community structure of the zooplankton in a shallow Arctic sea affected by terrestrial runoff. Zooplankton distribution was studied in a coastal zone in the southeastern part of the Barents Sea (Pechora Sea) in July 2014 and September 2016 along the archipelago that stretches from the continent towards the open sea. The ecosystem was in a spring state in July (2014) and in a summer state in September (2016). A clear positive gradient of salinity and a negative cline of turbidity were revealed, directed from the coast towards the open sea. A horizontal salinity gradient was detected in both seasons. The turbidity gradient was most pronounced during summer. Distribution of several species of marine zooplankton (e.g. Pseudocalanus spp., Temora longicornis, Microsetella norvegica) was associated with the salinity gradient. Parameters of community structure (species richness, diversity, evenness, total zooplankton abundance) correlated with turbidity while only diversity and evenness were influenced by salinity. A gradient was observed from a more diverse and less abundant zooplankton community in areas with high turbidity and low salinity towards a less diverse and more abundant community in the open sea. This heterogeneity influences higher trophic levels including commercial fishes and reflects how marginal filters function.

Marine bivalves are well known for their impressive lifespans. Like trees, bivalves grow by accretion and record age and size throughout ontogeny in their shell. Bivalves, however, can form growth increments at several different periodicities depending on their local environment. Thus, establishing lifespans and growth rates of marine bivalves requires a proper identification of annual growth increments. Here, we use isotope sclerochronology to decipher the accretionary growth record of modern Astarte borealis from the White Sea, Russia (N 67°05.70′; E 32°40.85′). Unlike winter growth increments observed in many other cold-temperate and boreal bivalve and limpet species, prominent growth increments in A. borealis corresponded to the most negative values in the oxygen isotope (δ18O) time series indicating that they formed during summer. Furthermore, summer growth increments do not coincide with the external concentric ridges on the shell making the latter feature an unreliable indicator of age. Similar to many other polar bivalves, A. borealis shows slow growth and long life. The von Bertalanffy growth equation for our sample is Ht = 29.39*(1 − e(− 0.11(t−(− 1.86))). Lifespans of individuals examined here (n = 18) range from 16 to 48 years. Given its impressive longevity and widespread polar distribution, A. borealis may be a potentially valuable skeletal archive for monitoring environmental conditions in the Arctic Ocean and boreal seas in the face of changing climate.

Dense blue mussel assemblages are unstable, their structure changing from year to year. Three types of models can be used to describe this instability: (1) “exogenous” model based on regional temperature fluctuations, (2) “endogenous” deterministic model associated with negative impact of adult mussels on juveniles and (3) “density-linked stochasticity” model based on positive feedbacks resulting in overcrowding and destabilizing the settlement. We compared predictions deduced from these models with a time series based on the results of long-term (18 years) monitoring of abundance and demographic structure of three mussel beds at the White Sea. Most of our findings agreed well with the predictions deduced from the endogenous model. In particular, (1) long-term changes in mussel abundance and demographic structure were strictly cyclic, with non-matching periods (5–9 years) at different sites; (2) stages with the dominance of old mussels alternated with those where juveniles dominated and (3) some signals of delayed density dependence were revealed. However, the time series also contained elements of long-term trends, which may testify to the involvement of some exogenous factors (probably long-term climate changes).

Sea stars Asterias rubens are important natural enemies of the blue mussel Mytilus in the North Atlantic. We asked whether these predators distinguish between the cryptic species M. edulis and M. trossulus that occur sympatrically in the White Sea. In mixed experimental stocks, the odds of being eaten by sea stars were about four times greater for M. trossulus. We also showed that A. rubens preferred smaller mussels to larger ones, irrespective of their species affinity. Our findings support earlier indirect observations showing that sea stars recognize M. trossulus as a more preferable prey than M. edulis. Dramatic differences in the vulnerability to sea star predation may explain the segregation of habitats between the two mussel species in contact zones; M. trossulus usually tends to occupy habitats where the sea star predators are scarce.

The paper describes changing patterns of commercial fish catch in the downstream part of the Neva River and the eastern Gulf of Finland and analyzes drivers of these changes for the period 1929–1995. We summarize catch data on 20 species and species groups of fishes and lamprey, as well as available abiotic data (salinity, temperature and water transparency). Water transparency gradually decreased during the 20th century being inseparable from a number of non-quantified anthropogenic factors, thus it can be used as an integral index of anthropogenic loading on the ecosystem. Because fisheries statistics were not published regularly, catch data were extracted from archives and various publications. Fishing locations, gear and target species changed over time in relation to each other, reflecting technological developments in fisheries, commercial demands for fishery products and the abundance of fish populations. Until the 18–19th centuries, fisheries took place mostly in rivers where weirs and set nets targeted sturgeon, salmon and whitefish. By the end of the 19th century, herring and smelt were the main targets of fixed nets in coastal areas. A century later, the main commercial species, herring, was harvested with pelagic trawls operating offshore in the Gulf. This evolution in fisheries, along with other anthropogenic activities, caused severe declines in diadromous species. Spawning migrations that make them easy to catch, and their high market value, make diadromous fish more vulnerable than other groups. Canonical correspondence analysis showed that catches of most diadromous species decreased with increasing transparency, which may reflect their response to anthropogenic pressure. Marine and freshwater fish suffered from anthropogenic pressure, but to a lesser extent probably because of a wider distribution and dispersal, and more capital-intensive fishing methods. Catches of marine species, except herring, significantly increased in the 1970–1980s when salinity was comparatively high. We found no correlation of fish catches with temperature.

Two blue mussel lineages of Pliocene origin, Mytilus edulis (ME) and M. trossulus (MT), co-occur and hybridize in several regions on the shores of the North Atlantic. The two species were distinguished from each other by molecular methods in the 1980s, and a large amount of comparative data on them has been accumulated since that time. However, while ME and MT are now routinely distinguished by various genetic markers, they tend to be overlooked in ecological studies since morphological characters for taxonomic identification have been lacking, and no consistent habitat differences between lineages have been reported. Surveying a recently discovered area of ME and MT co-occurrence in the White Sea and employing a set of allozyme markers for identification, we address the issue whether ME and MT are true biological species with distinct ecological characteristics or just virtual genetic entities with no matching morphological and ecological identities. We find that: (1) in the White Sea, the occurrence of MT is largely concentrated in harbors, in line with observations from other subarctic regions of Europe; (2) mixed populations of ME and MT are always dominated by purebred individuals, animals classified as hybrids constituting only ca. 18%; (3) in terms of shell morphology, 80% of MT bear a distinct uninterrupted dark prismatic strip under the ligament while 97% of ME lack this character; (4) at sites of sympatry MT is more common on algal substrates while ME mostly lives directly on the bottom. This segregation by the substrate may contribute to maintaining reproductive isolation and decreasing competition between taxa. We conclude that while ME and MT are not fully reproductively isolated, they do represent clearly distinguishable biological, ecological and morphological entities in the White Sea. It remains to be documented whether the observed morphological and ecological differences are of a local character, or whether they have simply been overlooked in other contact zones.