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The climate on our planet is changing and the range distributions of organisms are shifting in response. In aquatic environments, species might not be able to redistribute poleward or into deeper water when temperatures rise because of barriers, reduced light availability, altered water chemistry or any combination of these. How species respond to climate change may depend on physiological adaptability, but also on the population dynamics of the species. Density dependence is a ubiquitous force that governs population dynamics and regulates population growth, yet its connections to the impacts of climate change remain little known, especially in marine studies. Reductions in density below an environmental carrying capacity may cause compensatory increases in demographic parameters and population growth rate, hence masking the impacts of climate change on populations. On the other hand, climate-driven deterioration of conditions may reduce environmental carrying capacities, making compensation less likely and populations more susceptible to the effects of stochastic processes. Here we investigate the effects of climate change on Baltic blue mussels using a 17-year dataset on population density. Using a Bayesian modelling framework, we investigate the impacts of climate change, assess the magnitude and effects of density dependence, and project the likelihood of population decline by the year 2030. Our findings show negative impacts of warmer and less saline waters, both outcomes of climate change. We also show that density dependence increases the likelihood of population decline by subjecting the population to the detrimental effects of stochastic processes (i.e. low densities where random bad years can cause local extinction, negating the possibility for random good years to offset bad years). We highlight the importance of understanding, and accounting for both density dependence and climate variation when predicting the impact of climate change on keystone species, such as the Baltic blue mussel.

Advancing our understanding of how environmental variability affects the distribution of organisms is crucial for ecology and conservation. The exploration of changes in demographic patterns close to species distribution margins is important as populations here may provide a window into future population changes also elsewhere. However, the knowledge of factors causing recruitment variation is still inadequate in many systems and this deficiency is particularly evident close to species’ distribution borders. We studied the spatiotemporal variability in recruit-adult dynamics in a blue mussel, Mytilus trossulus, population to get insights into how environmental variables drive variation in recruitment and how this variability affects adult population growth. Thirty sites along a wave exposure gradient were monitored during four consecutive years. From each site, mussels were collected both from artificial recruitment units and from natural mussel beds. Our results showed high year-to-year variation in recruitment strength with high spatial variation. Mussel recruitment to artificial units and later recruitment to the benthos correlated highly. Juvenile abundances 1 year later paralleled prior recruitment strengths and caused synchronous but time-lagged changes in adult cohorts. Seawater salinity was the strongest predictor for recruitment variation, whereas sea temperature and wave exposure had low predictive power for this early life stage. For juveniles and for adults in the benthos, wave exposure explained the variation best, whereas temperature and especially salinity explained less. The results indicate that (a) the studied blue mussel population is strongly driven by variation in recruitment strength that (b) drives the size of the later cohorts, and the population is possibly even (c) recruitment limited in some years. Our study predicts a challenging future for this range population, resulting from a higher frequency of recruitment failure caused by a deteriorating sea climate. Knowledge about factors underlying variation in recruitment is thus essential for forecasting the future of this range population and for conserving its future state.

Mytilid mussels form abundant, species-rich reefs on rocky substrates, but the role of this key habitat in carbon (C) cycling remains poorly understood. We performed a seasonal study on a 5 m deep photicMytilus trossulusreef in the Central Baltic Sea to investigate pathways and rates of organic C flow. Reef gross primary production (GPP) and respiration (R) were estimated seasonally using underwater O2eddy covariance on hourly and daily timescales. Photogrammetry and biotic sampling were used to quantify reef rugosity and mussel coverage, and to derive mussel filtration and biodeposition. Mussels were highly abundant, reaching ~50000 ind. m-2, and the reef structure increased the seabed surface area by 44%. GPPhourlywas up to 20 mmol O2m-2h-1and GPPdailywas up to 107 mmol O2m-2d-1, comparable to a nearby seagrass canopy. Hourly eddy fluxes responded linearly to light intensity and flow velocity, with higher velocities enhancing reef O2uptake at night. ReefRdailyexceeded GPPdailyon 12 of 13 measurement days, andRannual(29 mol O2m-2yr-1) was 3-fold larger than GPPannual. The reef sustained a productive community of microbes and fauna whose activities accounted for ~50% ofRannual. Horizontal water advection promoted food supply to the reef and likely facilitated substantial lateral C export of mussel biodeposits. Our analyses suggest that a reduction in mussel reef extent due to ongoing environmental change will have major implications for the transport and transformation of C and nutrients within the coastal Baltic Sea.

Ecological patterns are inherently scale-dependent and driven by the interplay of abiotic gradients and biotic processes. Despite the fundamental importance of such gradients, there are many gaps in our understanding of how abiotic stress gradients interplay with biotic processes and how these collectively affect species distributions. Using a hierarchical design, we sampled two communities separated by depth along wave exposure and salinity gradients to elucidate how these two gradients affect species composition in habitats formed by the foundation species Mytilus trossulus and Fucus vesiculosus. Specifically, we looked at the impacts of regional salinity and temperature, local wave exposure, and site-dependent facilitation effects on the associated community composition. Wave exposure was the best predictor for species assembly structure, which was also affected by Mytilus biomass and by salinity and water temperature. While the tested variables provided robust explanations for community structure and density, they did not provide conclusive explanations for variation in species richness or evenness. Mytilus biomass had a stronger effect on the associated community with increasing wave exposure at the deeper depth, but the patterns were less obvious at the shallower depth. The latter was also the case for Fucus. These findings comply partly with theoretical predictions suggesting stronger facilitation effects in physically harsh environments. Our results indicate that environmental drivers are the main structuring forces that affect species assembly structure, but also foundation species are important. Thus, predicting changes in species distributions and biodiversity requires the simultaneous consideration of environmental gradients, as well as the structure and composition of foundation species and the interplay between these factors. This work advances our understanding of the processes that modulate species distributions in a marginal marine area and broadens the knowledge of how biological and environmental factors interplay and have an influence on hard-bottom community structure in brackish water seas. peerReviewed

The blue mussel Mytilus edulis is a foundation species with ecosystem engineering functions in the brackish, non-tidal Baltic Sea. In this study from the western Gulf of Finland, the relationship between the spatial patchiness of blue mussels and the diversity of associated macrofauna was examined across small scales (centimeters to meters) for the first time in subtidal habitats. It was demonstrated using geostatistical tools that blue mussel abundance and the diversity of associated macrofauna varied and interacted at 2 depths. Classic analyses (ANOVAs, correlations and multivariate techniques) detected no relationships between the abundance of blue mussels and their associated macrofaunal diversity, or differences in the abundance of mussels or the diversity of associated macrofauna between depths. Using semivariograms, differences in spatial heterogeneity between depths emerged: i.e. patchiness at 5 m and random patterns at 8 m depth. Cross-semivariograms detected negative spatial co-variation between blue mussel abundance and diversity of macrofauna at 5 m, but positive and neutral spatial relationships at 8 m depth. Combining the approaches suggested that high dislodgment of mussels in shallow environments causes this pattern. Dislodgement effects may be compensated for by increased turnover of small mussels in patches within mussel beds, which would result in reduced habitat space for associated macrofauna. On the basis of our results, it is suggested that patchiness of a foundation species is an ecological response, or result of a disturbance, that reduces the diversity of the associated macrofaunal community.

The majority of studies in metacommunity ecology have focused on systems other than marine benthic ecosystems, thereby providing an impetus to broaden the focus of metacommunity research to comprise marine systems. These systems are more open than many other systems and may thus exhibit relatively less discrete patterns in community structure across space. Metacommunity structure of soft-sediment benthic invertebrates was examined using a fine-grained (285 sites) data set collected during one summer across a large spatial extent (1700km(2)). We applied the elements of metacommunity structure (EMS) approach, allowing multiple hypothesis of variation in community structure to be tested. We demonstrated several patterns associated with environmental variation and associated processes that could simultaneously assemble species to occur at the sites. A quasi-Clementsian pattern was observed frequently, suggesting interdependent ecological relationships among species or similar response to an underlying environmental gradient across sites. A quasi-nested clumped species loss pattern was also observed, which suggests nested habitat specialization. Species richness declined with depth (from 0.5 to 44.8m). We argue that sensitive species may survive in shallower water, which are more stable with regard to oxygen conditions and present greater habitat complexity, in contrast to deeper waters, which may experience periodic disturbance due to hypoxia. Future studies should better integrate disturbance in terms of temporal dynamics and dispersal rates in the EMS approach. We highlight that shallow water sites may act as sources of recruitment to deeper water sites that are relatively more prone to periodic disturbances due to hypoxia. However, these shallow sites are not currently monitored and should be better prioritized in future conservation strategies in marine systems.

Understanding large-scale spatial variation in ecosystem properties and associated functionality is key for successful conservation of ecosystems. This study provides a template for how to estimate differences in ecosystem functionality over large spatial scales by using groupings of biological traits. We focus on trait groupings that describe three important benthic ecosystem properties, namely bioturbation, community stability, and juvenile dispersal. Recognizing that groups of traits interact and are constrained within an organism, we statistically define important functional trait subgroups that describe each ecosystem property. The sub-groups are scored according to their weighted ecological impact to gain an overall estimation of the cumulative expression of each ecosystem property at individual sites. Furthermore, by assigning each property a value relative to its observed maximum, and by summing up the individual property values, we offer an estimate of benthic ecosystem multifunctionality. Based on a spatially extensive benthic data set, we were able to identify coastal areas with high and low potential for the considered benthic ecosystem properties and the measure of ecosystem multifunctionality. Importantly, we show that a large part of the spatial variation in functional trait sub-groups and in benthic ecosystem multifunctionality was explained by environmental change. Our results indicate that through this simplification it is possible to estimate the functionality of the seafloor. Such information is vital in marine spatial planning efforts striving to balance the utilization with the preservation of natural resources.

Climate change is predicted to cause a freshening of the Baltic Sea, facilitating range expansions of freshwater species and contractions of marine. Resident marine flounders (Platichthys flesus) and expansive freshwater roach (Rutilus rutilus) are dominant consumers in the Baltic Sea sublittoral where they occur in partial sympatry. By comparing patterns of resource use by flounders and roach along a declining resource gradient of blue mussels (Mytilus trossulus) our aim was to explore predator functional responses and the degree of trophic overlap. Understanding the nature of density-dependent prey acquisition has important implications for predicting population dynamics of both predators and their shared prey. Results showed a highly specialized diet for both species, high reliance on blue mussels throughout the range, similar prey size preference and high trophic overlap. Highest overlap occurred where blue mussels were abundant but overlap was also high where they were scarce. Our results highlight the importance of a single food item - the blue mussel - for both species, likely promoting high population size and range expansion of roach. Findings also suggest that range expansion of roach may have a top-down structuring force on mussels that differ in severity and location from that originating from resident flounders.

This study examined the relationship of seascape structure, prey availability and sex on the post-spawning distribution and diet of European flounder Platichthys flesus in the northern Baltic Sea. The objectives were to determine whether: (1) wave exposure and substratum affect abundance and distribution of P. flesus, (2) diet reflects the benthic prey composition and (3) sex affects the distribution or diet of P. flesus. The results showed that P. flesus was evenly spread in the archipelago with no correlation to wave exposure. The distribution was, however, sex specific; reproductive males dominated the exposed zone and mainly post-reproductive females dominated the intermediate and sheltered zones. Platichthys flesus fed mainly on two bivalve prey species: blue mussels Mytilus edulis and Baltic tellins Macoma balthica. Hard substratum invertebrates dominated the diet in all habitats and apart from some typical soft substratum species, there was no clear link between fish feeding and the dominance structure of benthic prey. Diet was further sex specific, with females showing a broader range of diet than males. Results suggest that P. flesus is a specialist molluscivore found commonly and equally in soft- and hard-substratum habitats throughout the archipelago area. Previous studies on P. flesus in the Baltic Sea have yielded inconsistent results regarding diet and it has commonly been believed that the distribution of Baltic Sea P. flesus is linked to sand and soft substrata. The present findings emphasize the importance of including the entire range of habitats when diet and regional species distributions are assessed.

The long-term impact of natural and human-induced large-scale changes on aquatic phanerogams and charophytes has remained mostly unstudied in the Baltic Sea, largely because of a lack of historical data. A vegetation data set based on transects in an estuary in the western part of the Gulf of Finland studied in the 1930s–1940s, however, exists. We re-surveyed 91 of these transects in 2005 and 2007. The historical data set included 60 species and the contemporary 52 species. The species were classified into increasing, decreasing or unchanged according to the change in their occurrence frequencies. A significant frequency change was found for 31 of the 50 (62%) species which were present in both surveys. The proportion of species/growth forms with high-nitrogen and low-light preferences increased, whereas that of species/growth forms preferring oligotrophic conditions and species of shallow waters decreased. In the area, the species expanded their distribution ranges mainly towards the open sea. Eutrophication, reduced grazing pressure and shore overgrowth by Phragmites australis are suggested as the main reasons for the observed changes. In addition, increased boating and shore-construction activities contributed to the long-term floristic changes.

It is widely accepted that blue mussel Mytilus edulis L. beds support a high macrofaunal diversity. Nonetheless, sublittoral mussel beds on rocky shores have rarely been examined in a patch context to determine the processes creating and maintaining the diversity and assemblages of organ- isms occupying them. Whereas the species-area relationship is a well-established theory in ecology, less is known about its effect on faunal compositions in aquatic habitats. Also, the effect of patch shape and relative patch edge are poorly studied within a rocky shore mussel framework. The pre- sent study examined the effects of blue mussel patch size, patch shape, and the presence of algae and sand in mussel patches on the composition and diversity of associated macrofaunal communities in sublittoral habitats. In line with the species-area relationship, we found that the total number of taxa increased with patch size up to ca. 500 cm 2 , thereafter declining. Small patches largely lacked annelid worms and showed a lower diversity of gastropod species. We also found that the diversity was further enhanced by a high biomass of blue mussels, the presence of algae and the weight of interstitial sand. The shape of the patch had no effect on species richness, but affected the species composition. We conclude that structural components of sublittoral blue mussel habitats, such as patch content and shape, greatly affect the diversity and faunal assemblage. This has important impli- cations in conservation biology, since habitats are becoming more fragmented. Our study demon- strates that not only habitat quantity, but also quality has a large impact on species communities in marine ecosystems.

Whereas it is well known that ecosystem engineers can have positive effects on biodiversity, fundamental gaps still remain in our understanding of the underlying mechanisms in many marine systems. Individual- and population-level traits of engineering species are likely to determine their ability to modify the environment. Blue mussels function as important ecosystem engineer species in the Baltic Sea where they facilitate the existence of distinct associated communities. The population structure of these blue mussel beds fluctuates widely, but ecosystem effects of these fluctuations are poorly known. In this study, we experimentally and descriptively measured the effect of mussel succession stage with differing biomass, density and size structure on macrofaunal diversity and body size at 5 m depth. We found that mussel succession stage is an important determinant of faunal abundance, species richness and species assemblage. Our results demonstrate that mussel succession stage (size structure, density and biomass) highly affect the associated macrofaunal communities and the strength of facilitation in these systems. We also found that the body size of crustaceans and clams was positively correlated with mussel body size, increasing with a growing mussel body size. We suggest that it is of high importance to consider individual traits of ecosystem engineers and the types of environments in which they play critical roles in order to increase the value of the ecosystem engineering concept in ecology and conservation biology.

Vegetation communities, morphometric and water quality variables were sampled in 62 undisturbed coastal lagoons along a natural land-uplift gradient in the northern Baltic Sea. The lagoons had a morphological inlet threshold reflecting habitat isolation (i.e. the diminished connectedness of the lagoon to the sea) and located in different parts of the archipelago (corresponding to altering wave exposure). We used indirect multivariate methods to find changes in vegetation composition, and axes derived from the analyses were used in correlation and stepwise multiple regression analyses together with environmental variables. Habitat isolation proved to be the strongest predictor for vegetation composition. To identify significant change-points we used a method, which detects regime shifts by accounting for differences in chronological series and gives a regime shift index. Significant shifts in habitat isolation, average depth and total nitrogen were followed by shifts in species variables (i.e. dominant species, cover and species richness) and salinity, total phosphorus and turbidity. Of the species, especially Chara tomentosa showed a shift of high magnitude. Also a shift in wave exposure mirroring position of lagoons in the archipelago structured the species, but this division was seen only for less isolated lagoons. Our results suggest that lagoons are primarily structured by habitat isolation forming two structurally different regimes: one of less isolated lagoons dominated by a diverse array of vascular plants and marine algae of both outer and inner archipelago characters, and one with isolated lagoons dominated by high vegetation cover and dominance of Chara. In addition, vegetation seemed to stabilize water quality in the isolated regime by reducing turbidity and phosphorus, indicating a potential feed-back mechanism and a shift in structuring disturbances from abiotic to biotic factors with habitat isolation.

Habitat-forming, ecosystem engineer species are common in most marine systems. Still, much uncertainty exists about how individual and population-level traits of these species contribute to ecosystem processes and how engineering species jointly affect biodiversity. In this manipulative field experiment, we examined how biodiversity in marginal blue mussel beds is affected by blue mussel (1) body size, density and patch context and (2) presence of fucoid and algal structures. In the study area, bladder-wrack (Fucus vesiculosus), filamentous algae and blue mussels (Mytilus edulis) coexist at shallow depths in a variety of patch configurations and offer complex habitats with a high variability of resources. We hypothesized that complexity in terms of mussel bed structure and algal presence determines species composition and abundance. Results from the experiment were compared with macrofaunal communities found in natural populations of both engineering species. Results show that the physical structure and blue mussel patch context are important determinants for species composition and abundance. Results further show that the presence of algal structures positively affects diversity in blue mussel habitats due to increased surface availability and complexity that these algae offer. This study shows that blue mussel beds at the very margin of their distribution have an indisputable function for promoting and maintaining biodiversity and suggest that facilitative effects of habitat-modifying species are important on Baltic Sea rocky shores with fundamental importance to community structure.

Closely related species may occupy very similar niches but are often found to diverge by one or more traits when they share the same habitat. Five indigenous and sympatric Gammarus species are characteristic for the Baltic rocky littoral ecosystem. Yet, the species-specific distribution of these sympatric gammarids has not been well studied in the northern Baltic Sea. This study was undertaken to assess the spatial distribution of gammarid amphipods along wave exposure and depth gradients to study whether they show segregation in their microhabitat use. We sampled 12 rocky sublittoral shores along a wave exposure gradient over a period of 5 years. Samples differed with respect to depth and macroalgal type. Three of the five gammarid species occurred mainly in different depth zones and among different macroalgae at the exposed shores. In contrast, on protected shores, where algal zonation is weaker, a link to zonation and macroalgal type was almost absent. Moreover, the microhabitat use was strongest during the reproductive seasons of the species. The observed microhabitats of the three gammarid species fit well to their species-specific mean body sizes.

Physical and biological processes interact to produce pattern in nature. Pattern is scale dependent as processes generating pattern are heterogeneous in time and space. We tested some causes of variation in abundance and distribution of three marginal populations of sublittoral blue mussels, Mytilus edulis, in the non-tidal northeastern Baltic Sea. We studied the role of substrate inclination, perennial algae and siltation along local wave exposure gradients on mussel distribution over a regional salinity gradient. We found marked differences on regional scales (p < 0.001) with lower densities and biomasses of mussels with declining salinity. Along local gradients, mussel densities increased with increasing exposure (p < 0.001) and declining slope and sedimentation (p < 0.01). Site specifically, densities of blue mussels and the perennial red algae, Furcellaria lumbricalis, were positively related, results supported by a colonisation experiment. Also, young post-recruits showed significant relations to adult biomass, wave exposure, algal biomass, bottom slope and sediment cover. Findings showed that the relative importance of the determinants affecting blue mussels at the edge of their range vary with scale and are affected by the density and size structure of mussel populations. The study provides an indication of the types of factors that may be invoked as causes of spatial variation in marginal blue mussel populations and reinforces the need to consider multiple aspects when distributional patterns are assessed.

Predation is a key trophic component with a potentially large influence on rocky shore community organization. We studied prey size selection by roach Rutilus rutilus, feeding on blue mussels Mytilus edulis in the northeastern Baltic Sea. In this region, roach feed extensively on abun- dant populations of blue mussels living at the very edge of their range primarily set by low salinity conditions. The study area is characterised by a marked decline in mean mussel size from the saltier west to the less salty east. We predicted that average prey size of roach would decrease with decreas- ing availability of larger prey but increase with increasing size of the predator. The size of mussels ingested by roach ranged from 2 to 28 mm, largely covering the size distribution of blue mussels in the area. In accordance with our prediction, body size of the predator was the foremost factor deter- mining patterns of prey selection and mean prey size increased steadily with increasing size of the predator. Roach were size selective, preferring median and large mussels in proportions different from their accessibility in the habitat. Contrary to our expectations, no differences between the areas were seen regarding prey size selection. These results contrast with optimal foraging theory predict- ing that predators under higher prey densities decrease the proportion of less profitable prey. Our results suggest that roach have the potential to significantly affect the blue mussel dynamics in areas with poor prey availability and we predict that ongoing environmental change in the Baltic will likely increase its impact.

Exposure to wave action is an important factor determining the distribution and abun-dance of marine species on rocky shores. We studied the primary and secondary effects of waveexposure on sublittoral blue mussel Mytilus edulis populations in a fragmented archipelago in thetideless northeastern Baltic Sea. Triplicate bottom samples were collected by SCUBA divers from4 depths at 30 locations along a wave exposure gradient defined by an exposure index. From 15 ofthese locations, water samples were collected in order to measure physicochemical and biologicalconditions at each site. Results showed that mussel densities increased steadily with increasing waveexposure. Biomasses, however, were highest at areas with intermediate exposure. Water chemistry,seston and chlorophyll a concentrations did not differ between locations. Recruitment and growthexperiments on artificial substrata carried out at the same 30 locations showed that exposure neitherinfluenced the abundance of recruits on ropes nor markedly influenced the growth of adult mussels.This suggests that lack of mussels at less exposed localities may originate from problems in recruit-ment processes to adult populations or early post-recruitment mortality. We hypothesize that eventhin films of stagnant sediment on rocks may have a profoundly negative effect on mussel recruit-ment. Results suggest that increasing sedimentation of rocky bottoms may limit the spatial distri-bution of mussels, pushing stable mussel beds towards outermost areas of the archipelago wherestrong wave action keeps bottoms free of sediments.KEY WORDS: Physical stress · Predation · Recruitment · Rocky shore · Sedimentation · Artificialsubstrata · Wave exposure index · Depth distribution

Suspension-feeding bivalves are organisms of major functional importance in several aquatic environments around the world. They are also important food items for many fish and benthivorous seabirds. It has commonly been thought that predation pressure on blue mussel (Mytilus edulis) populations is negligible in the Baltic Sea, owing to the scarcity of major invertebrate predators such as starfish and crabs. It has recently been shown, however, that the blue mussel is the main food item for roach (Rutilus rutilus) in the archipelago areas of the western Gulf of Finland, where this freshwater fish species has become increasingly abundant, mainly due to increased eutrophication. To quantify the influence of roach predation on blue mussel populations we measured the standing biomass and size structure of the local blue mussel population and used a bioenergetic model to estimate mussel consumption by individual roach during two consecutive summers, 1997 and 1998. The results of the model were combined with existing data on roach abundance, giving annual consumption estimates of 75–105 kg blue mussel dry weight ha−1 in the study area, approximately two-thirds of these consumed mussels being >10 mm. This corresponds to approximately one-third of the standing population of mussels >10 mm in the area. Our results suggest that the predation effects of vertebrates on Baltic blue mussel populations are not insignificant, as previously believed. Predation by roach and other predators may have an important structuring effect on unstable blue mussel communities within the Gulf of Finland, where the species lives at the edge of its range.

The blue mussel (Mytilus edulis) is one of the key species in the Baltic Sea ecosystem and it is living at the edge of its range in the western Gulf of Finland. Roach (Rutilus rutilus) is a freshwater fish species that has benefited from recent coastal eutrophication and is at present highly abundant in the outer archipelago of the Gulf of Finland. In 2000 and 2001, a total of 516 roach were sampled for diet analysis in three study areas. Shelled molluscs formed over 95% of the diet of roach, blue mussels being the dominant single species. The proportion of this species in the food of roach in the three study areas ranged between 38–61% for smaller roach ( 225 mm) roach, indicating that blue mussel is a highly important food source for roach in outer archipelago areas of the western Gulf of Finland, in contrast to reports from other parts of the northern Baltic Sea. The scarcity of large blue mussels in mussel beds in the easternmost study area was reflected in the lower proportion of blue mussels in the diet of larger roach. However, the growth of roach was not affected by the availability of blue mussels. The twofold differences observed in the annual growth of roach between warm and cold years demonstrated that temperature is an important factor controlling the growth of roach in the western Gulf of Finland.

The blue mussel, Mytilus edulis, is the most conspicuous animal species in the northern Baltic rocky sublittoral. In the studied area the species lives at the margin of its salinity tolerance. Although dwarfed by the low-salinity conditions, blue mussels in the northern Baltic are very abundant and have a decisive role in the benthic and pelagic ecosystems. We studied abundance, size distribution, biomass and growth rate of blue mussels along a 270 km salinity gradient in the northern Baltic Sea. Samples (n=317, 1998–1999) from moderately exposed and exposed rocky shores at seven study areas were collected in the southern Archipelago Sea in the west and into the central Gulf of Finland in the east, where the species is becoming increasingly rare. The results show a marked decline in mean mussel size and biomass from the saline west to the less saline east. The growth rate also decreased with lower ambient salinity. However, abundance of small mussels was considerably higher in the central and eastern parts of the study area. Vertically, the highest biomass was recorded at intermediate depths (5 and 8 m), being lower at both shallower (3 m) and deeper bottoms (12 m). It is concluded that salinity is the foremost factor determining size structure and growth rate among populations within the area. The results suggest that predation further influences the population structure of blue mussels living at the edge of their range in the central Gulf of Finland ultimately set by their salinity tolerance.