This paper provides results of marine litter surveys carried out on 14 small uninhabited islands located in the coastal waters of Estonia, northeastern Baltic Sea. The islands were visited four times in total during 2019–2020. On each island, a litter survey was conducted on the beach and in terrestrial vegetation with a focus on macrolitter. Calculated over all surveys, the median value of macrolitter items per 100 m long beach section was 10.65, and the median density was 0.006 items m–2. At the sub-basin level, the three islands located in the Gulf of Finland had the highest number of beach litter items per 100 m and the highest density (items m–2), 38.05 and 0.017, respectively. The main litter material, representing 57.3% of all findings, was plastic; however, there were some variances across islands due to local conditions. The environmental variables most strongly correlated with differences in the composition of macrolitter were related to water movement and depth. Microlitter was found in low amounts (up to 60 items kg–1) in the sediment of all studied islands. Litter items used as nest material were noted on all the islands with seabird colonies.

Coastal areas are unique socio-ecological systems that are characterised by especially high provision of ecosystem services and sociocultural significance. This study investigates cultural ecosystem services (CES) provided by coastal areas in Latvia and Estonia, with a focus on their contribution to human well-being and factors that shape recreational preferences. By using a public participatory GIS (PPGIS) survey, data from 1381 recreational users were collected. The PPGIS data were utilized to map spatial patterns of CES use, assess the perceived suitability of various coastal ecosystem related factors for recreational activities, and link these activities to human well-being benefits. Results reveal that the diversity of coastal ecosystem shapes recreational uses, with Estonia’s highly indented and diverse coastline supporting a wider array of activities compared to Latvia’s predominantly sandy shores. Coastal CES provide diverse range of perceived well-being benefits, with both passive and active interactions with the coast contributing to mental and physical health, though the benefits vary across different CES. The study highlights the need to integrate participatory CES assessments into coastal planning to balance social and cultural values with other development interests, supporting holistic, evidence-based management strategies for Baltic Sea ecosystems and beyond.

Figures S1–S4 and Tables S1 and S2 detailing the different aspects of marine litter dis tribution in the Estonian coastal sea region.

This study applies a regional Dynamic Energy Budget (DEB) model, enhanced to include biocalcification processes, to evaluate the carbon capture potential of farmed blue mussels (Mytilus edulis/trossulus) in the Baltic Sea. The research emphasises the long-term capture of carbon associated with shell formation, crucial for mitigating global warming effects. The model was built using a comprehensive pan-Baltic dataset that includes information on mussel growth, filtration and biodeposition rates, and nutrient content. The study also examined salinity, temperature, and chlorophyll a as key environmental factors influencing carbon capture in farmed mussels. Our findings revealed significant spatial and temporal variability in carbon dynamics under current and future environmental conditions. The tested future predictions are grounded in current scientific understanding and projections of climate change effects on the Baltic Sea. Notably, the outer Baltic Sea subbasins exhibited the highest carbon capture capacity with an average of 55 t (in the present scenario) and 65 t (under future environmental conditions) of carbon sequestrated per farm (0.25 ha) over a cultivation cycle - 17 months. Salinity was the main driver of predicted regional changes in carbon capture, while temperature and chlorophyll a had more pronounced local effects. This research advances our understanding of the role low trophic aquaculture plays in mitigating climate change. It highlights the importance of developing location-specific strategies for mussel farming that consider both local and regional environmental conditions. The results contribute to the wider discourse on sustainable aquaculture development and environmental conservation.

The extracellular release of dissolved organic carbon (DOC) from marine macroalgae supports coastal ecosystem function by supplying photosynthetically fixed carbon to higher trophic levels via the microbial loop. Despite its widely acknowledged biogeochemical importance, DOC is not typically included in primary production estimates of coastal systems. Additionally, little is known about how changes in species composition and coverage will affect the supply of DOC to coastal systems. Within the context of the Baltic Sea, anthropogenic forces are driving a decline in habitat forming kelp species (Fucus vesiculosus) which is superseded by filamentous/turfing algal species, a pattern of change observed globally in numerous other aquatic systems. To evaluate how the drivers of this change may impact the flow of carbon within the Baltic Sea coastal system, the production of DOC by the filamentous algae (Ectocarpus siliculosus) was examined and its rate of release determined (0.095 mg C · g DW−1 · h−1 in light and 0.070 mg C · g DW−1 · h−1 in dark). In addition, bioassays were used to assess the short-term DOC use by bacteria as a proxy of the lability of the released products, with the majority (28.7% released in light and 18.6% released in dark) of the products remaining after 120 hours. This data was linked with long term macroalgae biomass and coverage surveys in order to assess changes in macroalgae community structure through time and to produce standing stock estimates of F. vesiculosus. DOC production as a metric of algal coverage was used to make a preliminary estimation of how changes in community structure may impact the flow of carbon within the system. Our results suggest that decreased levels of DOC released by filamentous algae relative to F. vesiculosus will act to reduce microbial activity. Our model estimates the presence of approximately 150,000 tonnes of F. vesiculosus (dry weight) within the Estonian coastal system translating to an annual release of 7,391 tonnes of DOC. Our study indicates that filamentous-dominated systems have likely altered carbon flow, impacting the broader ecology of the Baltic Sea. Consequently, the loss of kelp species and the expansion of filamentous algae may alter carbon dynamics, with important ecological consequences for other coastal systems globally.

Effective sustainable marine management requires the integration of cultural and natural assets, a practice that is rarely attempted. This study developed and tested a new method for Marine Spatial Planning (MSP) that uses interdisciplinary knowledge of the spatial distribution of cultural and natural assets in marine ecosystems along environmental and human pressure gradients and seeks to understand their interrelationships. The study focused on Estonian marine areas in the north-eastern Baltic Sea. Environmental conditions and threats were assessed, and the extent of spatial overlap between underwater cultural heritage (UCH), natural assets and conservation measures was identified. Although nature reserves and UCH sites were generally exposed to different ranges of environmental and pressure gradients, significant overlap with UCH sites was observed for certain natural assets, as well as along gradients of current velocity, oxygen concentration, bottom sediment, and human pressure. These results highlight the value of joint archaeological and ecological analyses to identify natural assets specific to UCH regions that require protection from both a natural and cultural heritage perspective.

The distribution data of 11 soft substrate charophyte and angiosperm species were analyzed. Our study aimed to elucidate the co-occurrence patterns among these sympatric macrophyte species and quantify their distribution areas. The central hypothesis of this study proposed that the observed co-occurrence patterns among the studied species deviate from what would be expected by random chance. Macrophyte occurrence data was derived from an extensive field sampling database. Environmental variables available as georeferenced raster layers including topographical, hydrodynamic, geological, physical, chemical, and biological variables were used as predictor variables in the random forest models to predict the spatial distribution of the species. Permutation tests revealed statistically significant deviations from random co-occurrence patterns. The analysis demonstrated that species tended to co-occur more frequently within their taxonomic groups (i.e., within charophytes and within angiosperms) than between these groups. The most extensive distribution overlap was observed between Chara aspera Willd. and Chara canescens Loisel., while Zostera marina L. exhibited the least overlap with the other species. The mean number of co-occurring species was the highest in Chara baltica (Hartman) Bruzelius while Z. marina had the largest share of single-species occurrences. Based on the distribution models, Stuckenia pectinata (L.) Börner had the largest distribution area.

Blue mussels of the genus Mytilus are important ecosystem engineers in intertidal and subtidal communities. The distribution of Mytilus mussels is influenced by a number of benthic and pelagic environmental variables (e.g., substratum type and availability, water movement, phytoplankton production, physical disturbance) as well as interactions between these variables. Because of its broad tolerance of environmental variation the Mediterranean species, Mytilus galloprovincialis, has the greatest ability of all blue mussels to colonise new geographic regions. Understanding how population genetic variation is related to, or caused by, environmental variation is important but has long been a challenge. The present study examined the genetic differentiation of native populations of M. galloprovincialis throughout its entire geographic range in the Mediterranean Sea, the Black Sea and the Sea of Azov using 53 single nucleotide polymorphisms (SNP loci). Mussels, in total 1004 individuals collected from 36 locations, were genotyped and combined with existing SNP data for mussels from 11 reference sites. Pairwise comparisons of FST values, correspondence analysis (CA) and STRUCTURE analysis all revealed four groups of populations: the Atlantic Ocean; the western Mediterranean; the Aegean Sea; and the Azov, Black and Marmara Seas. One population – from Algeria (Oran West) – was intermediate between the two main groups of the Mediterranean Sea and Atlantic Ocean. Seascape genetic analyses using GLM and DistLM analyses were employed to test site-specific genetic variation as a function of 13 environmental variables. The GLM identified five environmental variables that explained variation in site-specific FST values, whilst in the DistLM best-fit model only four were significant. These analyses suggest that a complex mix of environmental variables contribute to explaining genetic variation of M. galloprovincialis populations within the Mediterranean Sea, which most likely reflects the complex geological history of formation, isolation and reconnection among the regional sub-basins of the Sea.

North-American Gammarus tigrinus and Ponto-Caspian Chelicorophium curvispinum are the most recently arrived exotic species in the benthic fauna of the Estonian coastal sea. Gammarus tigrinus was found for the first time in Kõiguste Bay, northern Gulf of Riga, in 2003. The range expansion of G. tigrinus was extremely rapid and by 2005 the species was found all over the northern Gulf of Riga. Chelicorophium curvispinum was found for the first time near Sillamäe, eastern Gulf of Finland, in 2005. The nearest established populations of these species are located in the Curonian Lagoon, Lithuania. In connection with the range expansion survey of G. tigrinus the rare gammarids of the Estonian coastal sea G. duebeni and G. lacustris were found in marginal coastal habitats, e.g. isolated bays and small coastal pools of the northern Gulf of Riga in 2005.

Ecosystems are simultaneously regulated by bottom-up (resources) and top-down forces (predators). However, because predator-prey interactions operate on spatial scales beyond the reach of manipulative experimentation, the actual roles of predators remain poorly understood. In ecosystems where predators are naturally absent, biological invasions provide a unique experiment to shed light on the large-scale and long-term effects of predators in the recipient ecosystem. We combined data from long-term benthic monitoring, environmental conditions, and a census of round goby population to identify changes in the dominant benthic bivalve population (Mytilus trossulus and Macoma balthica) following round goby (Neogobius melanostomus) invasion. This study demonstrates a substantial change in the food web, with earlier primarily bottom-up regulated communities facing strong top-down control effects. The introduction of a novel predator to a species-poor ecosystem substantially reduced dominant invertebrate populations in a large part of the study area, sometimes with a time lag of 1−3 years. In general, the effects were less in high productivity areas. In some cases, community recovery was observed but rarely to pre-invasion levels, raising the question of the ability of bivalve communities to act as a natural filter. This study demonstrates long-term trends in benthic communities and by excluding possible effects due to the changing environment provides more evidence that invasive species transform coastal ecosystems over time and space.

The current study focuses on trends in hydrophysical and -chemical parameters (e.g. temperature, salinity, dissolved oxygen, chlorophyll a (Chl a), pH and nutrients) in the Estonian coastal sea and offshore areas in relation to the biogeochemical processes and marine carbon dioxide system of the Baltic Sea. Analysis of 586 time series of these parameters, retrieved during national monitoring activities in 1993-2017, revealed a number of significant trends, which characterize the changes in the northeastern (NE) Baltic Sea. The number of significant trends in the surface layer was slightly higher in the coastal sea area than in the offshore area. No significant (e.g. climate change-related) temperature trends were revealed in the surface layers of the Estonian offshore area. Over a longer time frame (since the 1970s-1980s), the trends in hydrochemical parameters have shown improved ecological conditions in the Estonian coastal waters, however, further improvement is not so obvious. In fact, most nutrient trends were positive over the last two decades. A positive Chl a trend was detected in the offshore area of the Baltic Proper. Dissolved oxygen trends in the bottom layers were all negative. So far, not enough parameters have been monitored for the evaluation of marine acidification processes. Several important recommendations for further improvement of monitoring programmes are suggested.

Macrophytes provide food, shelter and habitat for a multitude of other species and are therefore considered as important habitat-forming species. Loss or decrease of habitat-forming species severely affects biodiversity and functioning of coastal marine ecosystems. In the brackish Baltic Sea, such special, structuring species are large perennial macroalgae Fucus vesiculosus and Furcellaria lumbricalis on hard seabed and eelgrass Zostera marina and charophytes (Chara spp.) on soft substrates. The Baltic Sea is expected to face severe changes in environmental conditions due to climate change by the end of the 21st century, e.g. decrease in salinity and increase in temperature, wind speed, and storminess. It is essential to forecast changes in the distribution of valuable species in order to provide data for marine environmental protection and management decisions. Boosted regression trees modelling method was used to produce current species distribution models and predict the potential changes based on future climate scenario. Data from over 10 000 benthic sampling sites were used as an input for distribution models. Following the influence of the water depth, the next major drivers of species distribution were substrate type for Fucus, temperature for the charophytes and Furcellaria, and salinity for Zostera. Based on the model predictions, the climate change may cause a significant reduction of the distributional range of Zostera and Furcellaria. Slight decline of Fucus was also detected. Unlike the other habitat-forming species, charophytes are potential winners by probably increasing their distribution in the future. However, charophytes are not able to replace the niche of the other key habitat-forming species due to different substrate, wave exposure and salinity preferences.

Large-scale climate changes influence the geographic distribution of biodiversity. Many taxa have been reported to extend or reduce their geographic range, move poleward or displace other species. However, for closely related species that can hybridize in the natural environment, displacement is not the only effect of changes of environmental variables. Another option is subtler, hidden expansion, which can be found using genetic methods only. The marine blue mussels Mytilus are known to change their geographic distribution despite being sessile animals. In addition to natural dissemination at larval phase—enhanced by intentional or accidental introductions and rafting—they can spread through hybridization and introgression with local congeners, which can create mixed populations sustaining in environmental conditions that are marginal for pure taxa. The Mytilus species have a wide distribution in coastal regions of the Northern and Southern Hemisphere. In this study, we investigated the inter-regional genetic differentiation of the Mytilus species complex at 53 locations in the North Atlantic and adjacent Arctic waters and linked this genetic variability to key local environmental drivers. Of seventy-nine candidate single nucleotide polymorphisms (SNPs), all samples were successfully genotyped with a subset of 54 SNPs. There was a clear interregional separation of Mytilus species. However, all three Mytilus species hybridized in the contact area and created hybrid zones with mixed populations. Boosted regression trees (BRT) models showed that inter-regional variability was important in many allele models but did not prevail over variability in local environmental factors. Local environmental variables described over 40% of variability in about 30% of the allele frequencies of Mytilus spp. For the 30% of alleles, variability in their frequencies was only weakly coupled with local environmental conditions. For most studied alleles the linkages between environmental drivers and the genetic variability of Mytilus spp. were random in respect to “coding” and “non-coding” regions. An analysis of the subset of data involving functional genes only showed that two SNPs at Hsp70 and ATPase genes correlated with environmental variables. Total predictive ability of the highest performing models (r2 between 0.550 and 0.801) were for alleles that discriminated most effectively M. trossulus from M. edulis and M. galloprovincialis, whereas the best performing allele model (BM101A) did the best at discriminating M. galloprovincialis from M. edulis and M. trossulus. Among the local environmental variables, salinity, water temperature, ice cover and chlorophyll a concentration were by far the greatest predictors, but their predictive performance varied among different allele models. In most cases changes in the allele frequencies along these environmental gradients were abrupt and occurred at a very narrow range of environmental variables. In general, regions of change in allele frequencies for M. trossulus occurred at 8–11 psu, 0–10 °C, 60%–70% of ice cover and 0–2 mg m−3 of chlorophyll a, M. edulis at 8–11 and 30–35 psu, 10–14 °C and 60%–70% of ice cover and for M. galloprovincialis at 30–35 psu, 14–20 °C.

The ongoing climate change is expected to affect the distribution and vitality of marine aquatic species through various links and changes in environmental conditions.The aim of the study was to analyse and compare climate change related effects on native and non-native benthic species groups in the Baltic Sea.We analysed the impact of changes on the charophytes (native Chara aspera and non-native Chara connivens) and gammarid amphipods (native Gammarus salinus and non-native Gammarus tigrinus).Currently, C. aspera and G. salinus are the most widespread and frequent species among charophytes and gammarids in NE Baltic Sea. C. connivens has been recorded in single locations in the 1960s, following the continuous enlargement of the distribution area since 2005.G. tigrinus has showed significant occupation success in the region since their first finding in 2003.The random forest modelling method was used to produce current species distribution models and to predict the potential changes of distribution based on future climate scenarios.In the brackish Baltic Sea, species are often living close to their salinity tolerance limits and the decrease in salinity will probably cause profound changes in distribution of both marine and freshwater species.Moreover, the model predictions showed that, probably due to wider salinity and temperature tolerance, the non-native species will gain advantages over native species.Due to climate change, a significant increase of the distribution areas of non-native species is expected to occur on account of the native species.The distribution area of C. connivens is predicted to double and G. tigrinus to increase by 15%.Changes in environmental conditions also favour the distribution of native charophyte C. aspera due to the freshwater origin of the species.However, the marine species G. salinus is predicted to lose 65% of its distribution area.