Baltic Sea ice coverage was modelled using a sea-ice thermodynamics and dynamics model coupled with a three-dimensional (3-D) PM3D hydrodynamic model. The validation for 1958–2007 showed the modelled maximum ice extents (MIEs) agree well with observations (r = 0.97) and the ice thickness less so, but satisfactory for most stations (r > 0.8). This enabled the production of cumulative ice thickness (CIT) maps and the determination of the spatial variation in sea-ice extent in the Baltic over the analysed period for four air temperature scenarios with a constant value reduction. This showed the spatial sensitivity of ice cover dynamics to temperature changes and allowed to distinct regions with different impact of change in temperature on CIT. The simulation for temperature of 2°C lower than 1958–2007 was consistent with the reconstruction of MIEs in the entire Baltic Sea for the end of the Little Ice Age (LIA) (1721–1860). For the western Baltic, the compliance was highest for temperature reduced by 3°C and 4°C. This indicates that climatic conditions may have differed between individual regions of the Baltic during the LIA, and the air temperature anomaly in the western Baltic may have been greater than indicated by previous studies

This study presents a novel approach using machine learning, specifically Random Forest modelling, to create a sensitivity map that addresses the conflict between offshore wind farms (OWFs) and wintering waterbirds in the southern Baltic Sea. This region is crucial or of great importance for wintering species such as the Long-tailed Duck Clangula hyemalis and Velvet Scoter Melanitta fusca. In 2020, the southern and central Baltic Sea hosted approximately one million wintering birds, including over 560,000 Long-tailed Ducks (35% of the species’ biogeographical population), 230,000 Velvet Scoters (41% of the global population), and 4,500 Black Guillemot Cepphus grylle (8% of the Baltic population). Within the Polish Exclusive Economic Zone (PEEZ), 21 wind farms are planned, totalling a capacity of 17.6 GW with an estimated 1,164 turbines. To assess potential risks, a Normalised Overlap Index (NOI) was calculated for each wind farm, quantifying the level of potential threat to wintering birds. High conflict zones, such as the Southern Middle Bank and Pomeranian Bay, were identified, where wind farms significantly overlap with areas of high bird density. Although the wind farms do not intersect with Natura 2000 sites, partial overlaps with Important Bird Areas (IBAs) were noted. The method developed in this study not only addresses the challenges in the Baltic Sea but also offers potential applications in other marine and terrestrial environments. This framework provides a novel tool for evaluating and mitigating the impacts of renewable energy development on wildlife, contributing to the sustainable expansion of green energy solutions globally.

The capacity of river mouths to reduce storm surge water levels upstream, referred to as along-estuary attenuation, has been assessed by several studies. The coastal protection function of semi-enclosed water bodies such as lagoons and channels with narrow inlets remains less explored and generalization is hampered by differences in morphology and hydrodynamic forcing. Here we use a hydrodynamic model to investigate surge attenuation along a microtidal channel with a narrow inlet at the Baltic Sea coast of Germany called The Schlei. We quantify the importance of wind and the contribution of the barrier spit system, which is constricting the inlet, to the reduction of water levels at the landward end of the channel. In addition, we explore the role of dikes in the region for the reduction of peak water levels and coastal flooding. We find effective along-channel attenuation inside The Schlei in its current state, which is mostly a result of the channel’s narrows. However, reduction rates decrease under simplified sea-level rise scenarios. Furthermore, along-channel attenuation is highly variable and can change to substantial amplification depending on hydrometeorological forcing. The barrier spit contributes to along-channel attenuation whereas the effect of existing dikes (or their removal) for along-channel attenuation is negligible.

This study identifies the reasons for geodynamics variability of the coastal system within two cliff-shore sections of the southern Baltic Sea (SBS). The comparative analysis included distinct moraines and their foregrounds near the open sea (S1) and within the Gulf of Gdańsk (S2). Short-term trends indicate a direct link between landslide occurrence and increased cliff retreat. Long-term (total) values were obtained by developing the 4F MODEL for large-scale applications, based on the analysis of remote sensing and hydroacoustic data (to determine the extent of shore platforms), the modelling of higher-order polynomial functions describing their extent, followed by the integral calculus of the indicated functions within the open-source Desmos environment. The retreat dynamics for individual landslides (S1) was an order of magnitude higher (m/yr) than the average for the whole cliff section (0.17 ± 0.008 m/yr), which correlates well with medium- and long-term development tendencies and recession dynamics, revealed by the numerical modelling method, since approximately 8 ka b2k, years before 2000 CE (at S1 = 0.17 ± 0.020 m/yr, at S2 = 0.11 ± 0.005 m/yr). While the approach described in this paper can reveal, project, and simulate the dynamics of past and future trends within other cliffed coasts shaped in tideless conditions, it also proves stable moraine erosional responses to sea-level rise since the Mid-Holocene.

This paper reveals deglaciation palaeodynamics (Marine Oxygen Isotope Stage 2 [MIS 2]) in Poland and the southern Baltic Sea (SBS) development during marine transgression/regression phases (MIS 1) determined by a numerical modelling method. The introduced approach uses a high-level polynomial regression followed by the integral calculus of successive functions and an application of formulae. As a result, palaeogeographic relations from primary matrix transform instantly into palaeodynamics within a nested matrix. Accordingly, within 9 ka of the late Pleistocene, glacial recession dynamics increased by two orders of magnitude, from −8.5 m/yr between Leszno (L, 24 ka BP) and Poznań (Poz, 20–19 ka BP) phases, through several dozen (−37.2 m/yr, −60.6 m/yr, −90.7 m/yr) to the maximum average equalling −427.3 m/yr (max. −861.4 m/yr) between the Pomeranian (Pom, 17–16 ka BP) and the Gardno (G, 16.8–16.6 ka BP) phases. In turn, SBS coastline transgression and regression dynamics varied by three orders of magnitude. Since the Baltic Ice Lake (BIL, 10.5–10.3 ka BP) up to the Yoldia Sea (YS, 10–9.9 ka BP) regression was intense and equalled −56.8 m/yr (max. −128.7 m/yr), followed by marine transgression towards the Ancylus Lake (AL, 8.7–8.5 ka BP) at 21.43 m/yr through 9.30–2.20 m/yr during the Littorina Sea 1 and Littorina Sea 2 stages (LS1 and LS2, since 7.7 ka BP), eventually 0.51 m/yr in the last 6.05 ka. The 2 m sea-level rise scenario projections indicate approx. 3400 km2 of land and 684,000 inhabitants face flood risk around 2150–2240 CE, with marine transgression dynamics expected to range from 23.9–38.2 m/yr.

Cod was one of the most important fish species in the Baltic Sea, but its condition is deteriorating for several reasons, including an increasing parasite burden. The aim of this study was to determine the source of infection of Baltic cod with parasites by examination of invertebrates found in situ in the cod stomach. A total of 1681 cod were sampled during four research cruises in the southern Baltic Sea in 2012, 2013 and 2014 and the composition of their diet was analysed. Each prey item from cod stomach was identified to the lowest possible taxonomic level and a parasitological analysis of all invertebrates collected was performed. Crangon crangon, Saduria entomon and Mysis mixta were the most commonly represented invertebrates among food items. Hysterothylacium aduncum was found only in C. crangon. This host–parasite system is reported here for the first time in situ in the stomach of cod from the Baltic Sea, confirming the role of C. crangon in cod infection with H. aduncum.

The cemetery at Zvejnieki in Latvia was in use from c. 7500–2600 BC, spanning part of the regional Mesolithic and Neolithic. This article presents a reanalysis of finds from a double inhumation burial of a male and a female dating to 3786–3521 BC. A unique leg ornament associated with the female is composed of tubular beads. Previously believed to have been made of bird bone, reanalysis of 68 of these beads now demonstrates that they were produced from fossilised sea lilies (Crinoidea). This new identification of a rarely recognised raw material is discussed in the context of other hunter-gatherer encounters with unusual materials and their environments.

We suggest helminthological investigations of cod as a supplement to traditional biological and hydrographical methods for elucidation of ecological changes in the Baltic Sea. It is under discussion if oxygen deficit or seal abundance should explain the present critical situation of Baltic cod. A comparative investigation of endoparasitic helminths in Baltic cod (Gadus morhua), captured in the same marine habitat with an interval of 35 years (1983/2018) recorded 11 species of helminths comprising trematodes (Hemiurus luehei, Podocotyle atomon, Lepidapedon elongatum), nematodes (Contracaecum osculatum, Hysterothylacium aduncum, Capillaria gracilis, Cucullanus cirratus), cestodes (Bothriocephalus sp.) and acanthocephalans (Echinorhynchus gadi, Pomphorhynchus laevis, Corynosoma semerme). Significant prevalence and intensity increases were recorded for third-stage larvae of the nematode C. osculatum (liver location) and larvae of C. semerme (encapsulated in viscera). Both parasite species use grey seal as their final host, indicating the recent expansion of the Baltic seal population. A lower E. gadi intensity and an increased prevalence of L. elongatum of small cod (31–40 cm body length) suggest a lowered intake of amphipods (intermediate host) and elevated ingestion of polychaetes, respectively, but no significant changes were seen for other helminths.

For Greater Scaup Aythya marila, classified as ‘Vulnerable’ on the European Red List of Birds, the south-western Baltic Sea is one of the most important wintering sites in Europe. In this area, a large concentration of gillnet fishery temporally overlaps periods of the most abundant occurrence of foraging diving birds. The aim of the article is to show how bycatch can impact the population of a diving duck. To assess this, we calculate the Potential Biological Removal (PBR) for the studied Greater Scaup population and we model the population change according to age-structured matrix models. Summing all the available recent figures on Greater Scaup bycatch in north-west Europe yields an estimated mean annual total of 3,991 individuals (2% of the flyway population). For a baseline stable population, an age-structured matrix model indicates that at this bycatch level the Greater Scaup population that winters in north-west Europe will decrease by 36% over the next 30 years, qualifying the status of the population as ‘Vulnerable’ according to IUCN criteria. As this population also experiences decline prior to bycatch, this decrease will be 57%, which qualifies the status as ‘Endangered’. PBR as an indicator of population vitality does not work in our case because the PBR-informed allowable bycatch values have a significantly negative impact on the population. Our results indicate unambiguously that fishery bycatch is among the most important threats responsible for the Greater Scaup’s decline. While recent data suggest that some improvement has taken place in the species’ status over the last 10 years, measures to protect Greater Scaup from bycatch are required. The solution should involve the prohibition of gillnet fishing in selected key sites and the use of mitigation techniques in other areas.

The importance of fish for the medieval and early modern economy in the Baltic Sea is known through separate studies by historians and archaeologists. This article aims to combine zooarchaeological data with account books from castles in Kastelholm (Åland Islands) and Raseborg (south-western coast of Finland) in order to understand the processing and transport of fish in the area. Fish was paid as tax by the peasants but was also fished by the castle fishermen and brought to the castles to be consumed there. Here, the preserved fish products are primarily studied through pike and cod, which represent the main economically important larger fish species in the Baltic Sea. The study reveals some differences between the castles studied and the importance of fish for the castle economy.

Studies of cyanobacterial bloom dynamics show that the highest biomass accumulation of Nodularia spumigena is observed in the shallowest area of the Gulf of Gdańsk in summer. In the same region and time, the highest fish abundance is observed. Mostly young individuals of gobies, small sandeel, flounder, three-spine stickleback and young herring occur. In this work we compare how toxic blooms of cyanobacteria influence the number and structure of fish communities in a coastal zone. The results obtained in our study were rather unexpected. More fish species were caught and the biomass of fish was higher during a bloom than in a month following the sampling (no bloom).

A high-resolution, well-dated dinoflagellate cyst record from a lagoon of the southeastern Swedish Baltic Sea reveals climate and hydrological changes during the Holocene. Marine dinoflagellate cysts occurred initially at about 8600 cal yr BP, indicating the onset of the Littorina transgression in the southeastern Swedish lowland associated with global sea level rise, and thus the opening of the Danish straits. Both the species diversity and the total accumulation rates of dinoflagellate cysts continued to increase by 7000 cal yr BP and then decreased progressively. This pattern reveals the first-order change in local sea level as a function of ice-volume-equivalent sea level rise versus isostatic land uplift. Superimposed upon this local sea level trend, well-defined fluctuations of the total accumulation rates of dinoflagellate cysts occurred on quasi-1000- and 500-yr frequency bands particularly between 7500 and 4000 cal yr BP, when the connection between the Baltic basin and the North Atlantic was broader. A close correlation of the total accumulation rates of dinoflagellate cysts with GISP2 ice core sea-salt ions suggests that fluctuations of Baltic surface conditions during the middle Holocene might have been regulated by quasi-periodic variations of the prevailing southwesterly winds, most likely through a system similar to the dipole oscillation of the modern North Atlantic atmosphere.

Knowledge of sea-level change in the southern Baltic Sea region is important for understanding the variations in late Pleistocene and Holocene sea-level change across northern Europe. These variations are a consequence of the response of the Earth's crust to the deglaciation of Fennoscandia and of the water added to the oceans from the melting of all Pleistocene ice sheets. The sedimentological and geochemical composition of five sediment cores from the lagoonal Oder Estuary offers new observational evidence for sea-level change and coastal development in the southern Baltic Sea region. The combined use of several geochemical proxies (organic carbon, nitrogen, calcium carbonate and biogenic opal contents, Corg/S and Corg/N ratios, δ13C values of organic matter, and δ15N values) is a new approach for the study area. The chemical evidence of this multiproxy approach allows clear identification of several stages in the development of the lagoonal environment: postglacial lake stages with sandy sedimentation during the Older Dryas and the Allerød stades, lacustrine phases with high autochthonous productivity during the Atlantic stade, terrestrial stages with peat formation at the beginning of the Subboreal stade, sedimentation as a result of marine transgression, and brackish sedimentation after the formation of sand spits and barrier islands during the Subatlantic stade. The stages are the result of regional sea-level change owing to complex shoreline development. They support the tentative sea-level curve proposed nearly 20 years ago for the region. In addition, changes in Oder River input in response to climate conditions is monitored. Whereas high terrigenous input of organic matter from the Oder River occurred during periods of humid climate during the Allerød, Atlantic, and Subatlantic stades, Oder River discharge decreased with drier and cooler climate conditions during the Subboreal stade. Furthermore, the geochemical evidence points to local anomalies such as the significance of river input and additional sulfate supply into the Oder lagoon for the composition of the sediments. Overall, the results provide a framework for future studies, which would allow for a more detailed comparison with other, similar environments.

Taphonomic analysis of Eemian marine mollusks and barnacles at Ristinge Klint on the island of Langeland (Denmark) provides a distinct record of a temporal succession in preservation states. Four different states of preservation are recognized and related to a decreasing hydrodynamic regime in the depositional setting of the Eemian Baltic Sea. The states show a deepening-upward transition from shallow bay environment towards deeper offshore environment. The depositional setting changed significantly in hydrodynamics about 620 and 1550 years into the Eemian (130,000 to 115,000 years BP), according to biostratigraphic correlation with the varves of the Bispingen succession. The taxonomic composition of the paleofauna supports such a deepening-upward interpretation with a contemporaneous change from brackish water to nearly full marine conditions. The sea bottom was affected by at least one period of oxygen deficiency. The analysis also shows that the preservation of shells varies according to differences in shell structures and life habits. Here we show how these differences should be considered in paleoenvironmental reconstructions based on taphonomic analyses. Taphonomy may play an important role in understanding the hydrodynamic conditions within the Eemian Baltic Sea.

The harbour porpoise is seriously depleted and threatened with extinction in the Baltic Sea. It is usually assumed that Baltic porpoises form a separate population unit, although the evidence for this has been disputed lately. Here, a 3-D geometric morphometric approach was employed to test a number of hypotheses regarding population structure of the harbour porpoise in the Baltic region. 277 porpoise skulls from Denmark, Sweden, Finland, Germany and Poland were measured with a suite of 3-D landmarks. Statistical analyses revealed highly significant shape differences between porpoises from the North Sea, Belt Sea and the inner Baltic Sea. A comparison of the directionalities of the shape vectors between these units found differences that cannot be attributed to a general, continual shape trend going from the North Sea to the inner Baltic Sea. These vectors indicate a morphological adaptation to the specific sub-areas. Such adaptation may be the result of the topographic peculiarities of the area with variable topography and shallow waters, e.g. in the Belt Sea porpoises, there may be a greater reliance on benthic and demersal prey. The present results show that isolation by distance alone is an unlikely explanation for the differences found within the Baltic region and thus support previously reported molecular indications of a separate population within the inner Baltic Sea.

The Japanese skeleton shrimp Caprella mutica, originally from the north-west Pacific, artificially introduced to many parts of the world, is recorded in Sweden for the first time. The new record is from Eastern Skagerrak, slightly north of the border between the North Sea and the Baltic Sea. A detailed morphological description of the specimens from Swedish waters with notes on habitat is presented. In comparison with the specimens from the native range, the large males of C. mutica from Sweden, and material earlier reported from elsewhere in Europe, are characterized by rather long pereonites 1 and 2, which are longer than the combined length of pereonites 3 and 4; and long segments of antenna 1 peduncle; among them segment 3 is longer than flagellum.