Belt Sea Research Articles

Observation of a moderate major Baltic Sea inflow in December 2023

Just before Christmas 2023, the low-pressure system storm “Zoltan” struck Germany, resulting in widespread damage and two consecutive large storm surges on the North Sea coast in the night from Thursday 21 December 2023 to Friday 22 December 2023. Storm Zoltan brought heavy rainfall, accompanied by thunderstorms and winds ranging between 90 and 115 kmh-1\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\mathrm {km\\,h^{-1}}$$\\end{document}, with gusts reaching up to 140 kmh-1\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\mathrm {km\\,h^{-1}}$$\\end{document} along the coast which caused severe damage, particularly in northern Germany. Characteristics of the inflowing water at the Fehmarn Belt buoy (FEB), Darss Sill station (DAR) and the Arkona Basin buoy (ARK), including salinity, temperature, dissolved oxygen and ocean currents properties, were analysed to understand the impact of storm Zoltan in the western Baltic Sea. In addition to the damage along its path, following the onset of strong westerly winds associated with storm Zoltan, a large volume of water, containing saline (17–22 psu), cold (5–6 °C), and oxygen-rich 7–8 (mll-1\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\,\\mathrm { ml\\,l^{-1}}$$\\end{document}) water from the Kattegat and the North Sea reached into the western Baltic Sea. The sea level at Landsort Norra increased by +57 cm over a period of 14 days, from 15 December to 29 December 2023. This resulted in a total volume change of 198 km3\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\mathrm {km^3}$$\\end{document} in the entire Baltic Sea, with 169 km3\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\mathrm {km^3}$$\\end{document} and 29 km3\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\mathrm {km^3}$$\\end{document} provided via the Belt Sea and the Sound Sea, respectively. Observations at the DAR indicated a significant inflow between 19 December 2023 and 1 January 2024 with salinity above 13 psu, temperature below 5.5 °C and dissolved oxygen of about 7.5–8 mll-1\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\mathrm { ml\\,l^{-1}}$$\\end{document}. While the maximum salinity of the bottom layer at the DAR was about 17 psu, the ARK exhibited significantly higher salinities, reaching up to 22 psu at the bottom layer. During the main inflow period, 75 km3\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\mathrm {km^3}$$\\end{document} of highly saline water entered the western Baltic Sea. This corresponds to an average salt transport of 1.75 Gt into the western Baltic Sea (1.39 Gt from the Belt Sea and 0.36 Gt from the Sound Sea), representing more than 20% of the total annual salt import into the Baltic Sea), which places the event in the moderate range of major Baltic inflows. This event brought an amount of about 0.8 106\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\mathrm {10^6}$$\\end{document} t oxygen into the Baltic Sea. This was the strongest inflow into the Baltic Sea since 2016.

A negative trend in abundance and an exceeded mortality limit call for conservation action for the Vulnerable Belt Sea harbour porpoise population

The management and conservation of biodiversity relies on information on both the abundance of species and the potential impact of threats. Globally, one of the largest threats towards marine biodiversity is bycatch in fisheries. Under the Marine Strategy Framework Directive (MSFD), EU Member States are required to assess the status of species, such as the harbour porpoise (Phocoena phocoena), in relation to their abundance and mortality due to bycatch every six years. The Vulnerable (HELCOM) Belt Sea population of harbour porpoise has been surveyed to determine its abundance six times using dedicated aerial or ship-based line-transect distance sampling surveys. Here, we estimated the first trend in population abundance over an 18 year period (2005-2022). Using the most recent abundance estimate, we computed a mortality limit applying the modified Potential Biological Removal (mPBR) method based on the regionally agreed conservation objective to restore or maintain 80% of carrying capacity over 100 years with an 80% probability. Over the past 18 years there has been a strong negative trend (-2.7% p.a.; 95% CI: -4.1%; + 1.3%) in abundance, with a 90.5% probability. The mortality limit was estimated to be 24 animals, which the current bycatch estimates (~900 porpoises/year from the commercial Danish and Swedish set net fishery fleets, with no data from Germany and other fishery types) exceed by far. The frequency and quality of data available on abundance for this population are higher than those available for the majority of marine species. Given the observed population decline and likely unsustainable levels of bycatch, the results presented here provide a strong basis to make informed, evidence-based management decisions for action for this population. Such action is needed urgently, before the dire situation of other porpoise species and populations around the globe is repeated.

Evolutionary history and seascape genomics of Harbour porpoises (Phocoena phocoena) across environmental gradients in the North Atlantic and adjacent waters.

The Harbour porpoise (Phocoena phocoena) is a highly mobile cetacean species primarily occurring in coastal and shelf waters across the Northern hemisphere. It inhabits heterogeneous seascapes broadly varying in salinity and temperature. Here, we produced 74 whole genomes at intermediate coverage to study Harbour porpoise's evolutionary history and investigate the role of local adaptation in the diversification into subspecies and populations. We identified ~6 million high quality SNPs sampled at eight localities across the North Atlantic and adjacent waters, which we used for population structure, demographic and genotype-environment association analyses. Our results suggest a genetic differentiation between three subspecies (P.p. relicta, P.p. phocoena and P.p. meridionalis), and three distinct populations within P.p. phocoena: Atlantic, Belt Sea and Proper Baltic Sea. Effective population size and Tajima's D suggest population contraction in Black Sea and Iberian porpoises, but expansion in the P.p. phocoena populations. Phylogenetic trees indicate post-glacial colonization from a southern refugium. Genotype-environment association analysis identified salinity as major driver in genomic variation and we identified candidate genes putatively underlying adaptation to different salinity. Our study highlights the value of whole genome resequencing to unravel subtle population structure in highly mobile species, shows how strong environmental gradients and local adaptation may lead to population differentiation, and how neutral and adaptive markers can give different perspectives on population subdivision. The results have great conservation implications as we found inbreeding and low genetic diversity in the endangered Black Sea subspecies and identified the critically endangered Proper Baltic Sea porpoises as a separate population.

Distribution and speed of recreational boats in Danish waters based on coastal observations and satellite images: Predicting where boats may affect harbour porpoises

Danish coastal waters are a hotspot for recreational boating and harbour porpoises (Phocoena phocoena), but as there is little data on the boats’ speeds and distributions, it is difficult to determine where porpoises are likely to be affected by boat noise. To gain knowledge on the potential impacts of small boats we 1) used coastal observations to assess how boat speed is related to boat size, type, distance to marinas, and wave height, and 2) used satellite images to assess the spatial distribution of small boats in Kattegat, the Sound, the Belt Sea and the Western Baltic areas (see map in Sveegaard et al., 2015). This made it possible to determine how the probability of observing boats was related to distance to the coast and number of people living within a radius of 5 km from a given area. We found that boat speeds varied among marinas and that speed was positively correlated with distance to the marina. The relationship between boat speed and size depended on boat type, with motorboats being the fastest. The satellite image analysis showed that motorboats were the dominant type in coastal waters, constituting 61.4% of the observed boats. The probability of observing boats on a satellite image covering 0.25 km2 dropped from 0.13 by the shore to 0.04 at a distance of 5 km from shore. The probability of observing boats was >10% in 6.6% of the study area, and the probability of observing boats and porpoises in the same area was >10% in 3.0% of the study area. Coastal waters are important habitats for many aquatic species, e.g., fish, invertebrates and porpoises, that use sound for communication or navigation. Our study uses a combination of observation and satellite data to identity areas where boats overlap with harbour porpoise presence and thus where the animals may be affected by boat noise. The study also illustrates how combining these two methods can yield important information on the potential impacts of boats on wildlife.

Time allocation and diving behaviour of harbour porpoises (Phocoena phocoena) in Danish and adjacent waters

To gain insight into the time allocation and diving behaviour of harbour porpoises in Danish and adjacent waters, satellite linked dive recorders were mounted on 14 harbour porpoises. The animals were incidentally caught alive by fishermen using pound nets during 1997- 99 in the Danish Belt seas. Information on diving behaviour was collected from April to November. Contact with individual porpoises remained for up to 130 days. The average number of dives per hour was 29 during April-August and 43 during October-November. Daily maximum dive depth corresponds to the depth of the Belt seas and Kattegat where depth generally does not exceed 50m. Maximum dive depth recorded was 132m from animals moving north into Skagerrak. Dives were frequently recorded in the category 10-15min, but could potentially be an artefact of the sampling regime. The diurnal pattern shows that harbour porpoises dive continuously both day and night, but with peak activity during daylight hours. On average they spent 55% of their time in the upper 2m during April-August. These values have implications for aerial abundance surveys when correcting for animals not visible. A mature female and its approximately 10 months old calf were both tagged and swam together for 43 days until contact was lost. The calf made more frequent but shorter dives than the mature female. The number of dives per hour decreased, while the dive depth and duration increased for both animals from May to June, suggesting a change in feeding behaviour. It is not known whether the female and calf synchronised their dives, but the diurnal dive pattern shows a correlated dive rhythm in May, but not in June. This change in mother-calf behaviour suggests that the calf foraged more independently, corresponding to the time of year when porpoise calves leave their mother.

Arctic Research at PMEL: From Sea Ice to the Stratosphere

The Arctic is warming twice as fast as the global mean, making Arctic research essential for understanding the global climate system. For 50 years, researchers at the NOAA Pacific Marine Environmental Laboratory have sought to detect and understand the numerous changes the Arctic is undergoing, the Arctic’s connections with the Earth system, and the impacts of climate change on the people who live in the Arctic. PMEL accomplishments in Arctic research include identifying future states and variability of sea ice, defining the ice-free Arctic threshold and initiating a climate model selection process by applying observational constraints, developing a Bering Sea conveyor belt sea ice model and a vessel spray-icing index, investigating internal versus forced response of Arctic temperature change, connecting the Arctic with mid-latitude weather, and rescuing historical data. Through continued study, improved understanding, and communication, PMEL research informs policymakers, managers, and the public to help ensure a sustainable future for the Arctic.

Can sporadic records of ocean sunfish (Mola mola) in the western Baltic Sea be linked to saline inflow events?

The Baltic Sea is one of the largest brackish water bodies in the world with salinity levels ranging from fresh water conditions in the northeast to full strength saline waters at its transition zone to the North Sea in the west. Most of the water exchange happens in the SW Baltic Sea, the Belt Sea and The Sound where less saline water exits the Baltic Sea at the surface, while higher saline water is entering the Baltic at depth. Thus, the species composition in the Baltic Sea is heavily influenced by the strong salinity gradient, and here several species occur at their limit of their physiological tolerance and preference. In this study, we focused on sightings of the ocean sunfish Mola mola recorded in the western Baltic Sea between 1978 and 2020. This species is regarded as vagrant in the Baltic Sea, i.e., it does not belong to the common species assemblage in this area. Hydrographic conditions, such as water temperature and salinity, were obtained from a highly spatio-temporally resolved hydrodynamic Baltic Sea model, covering a daily resolved 71-year time series. We investigated if the occurrence of M. mola correlates with the dynamics of water mass exchange between the Kattegat/Skagerrak and the SW Baltic Sea. Our analyses show that these occurrences could be related to the presence of anomalously high saline water masses. However, in autumn and winter water temperatures of the western Baltic Sea usually drop below 8 °C with further cooling in January and February to 4–5 °C and during strong winters even down to <2 °C. If M. mola will follow the same strategy as in the North Sea, i.e. migrating southward to avoid lethal temperatures, they will not be successful by entering the Baltic Sea, because during winter months temperature everywhere falls far below their thermal tolerance. As a consequence, southward transport or active migration of M. mola into the Baltic Sea will expose the respective specimens to adverse environmental conditions finally precluding survival. • Linking sightings of Mola mola to physical processes • Novel approach for classification of environmental conditions • Occurrence of Mola mola linked to inflows conditions • Main occurrence of warm water species Mola mola during autumn and winter

Appraisal of a novel fishery of whelks (Buccinum undatum) in Danish waters

In 2016, a fishery for whelks (Buccinum undatum) was initiated in the Danish part of Kattegat and the Belt Sea. Until then the whelk had been a pristine resource in Danish fisheries. The inner Danish waters are not covered by EU regulations and very little is known about the whelk population characteristics in the area. Thus, the Danish whelk fishery is at present carried out without any administrative regulations or specific biological knowledge. This work gives novel information about the Danish whelk populations in the Southern Kattegat and Belt Sea. The stock size (shell length > 40 mm) in Kattegat was estimated to be in the range of 230,250–921,000 tons. The overall meat percentage in the three studied fishing zones ranged from 58.2–65.6 %. The size at maturity (sexes pooled) ranged from 62.9–70.3 mm corresponding to 4–4.5 years of age. The average shell length of the studied populations ranged from 64.1–73.2 mm and the asymptotic shell length was estimated to be 77.6–83.1 mm. Test fishery showed the CPUE quickly to decrease in a southern direction in the Belt Sea due to the prevailing salinity gradient. Commercial fishermen achieved an average CPUE of 1.3 kg pot-1 in the Southern Kattegat (January 2017–November 2018). Although the three zones studied are adjacent, significant differences were found in several of the measured parameters. This underlines the need of local monitoring of population characteristics in order to advice authorities on possible regulation of whelk fishery. The ongoing whelk fishery in the Southern Kattegat has at present an intensity not believed to cause recruitment overfishing. However, the Danish fishing authority is encouraged to implement a minimum conservation reference sizes (MCRS) based on locally determined size of maturity (SOM).

Sightings of extraordinary marine species in the SW Baltic Sea linked to saline water inflows

The Baltic Sea, located in northern Europe, is one of the largest brackish water bodies in the world. Salinity levels range from fresh water conditions in the northeast to full strength saline waters at its transition zone to the North Sea in the west. Most of the water exchange happens in the SW Baltic Sea, the Belt Sea, where fresh water exits the Baltic Sea at the surface, while high saline, oxygen rich water is entering the Baltic at depth. Due to the extended salinity gradient of the Baltic Sea, a variety of species occur at the limit of their physiological tolerance and preference, i.e. in areas and habitats not representing their marine or fresh water origin. Additionally, the Baltic Sea is known for its high share of non-indigenous species, which have established. In this study, we compiled extraordinary sightings of transient, non-native or potentially range expanding species in the SW Baltic Sea for a period from 2001 to 2018. We focused on jellyfish, squid, fishes and marine mammals and linked their occurrences to the local hydrography. Hydrographic conditions, such as water temperature and salinity, were obtained from a high spatio-temporally resolved hydrodynamic Baltic Sea model, covering a daily resolved 40-year time series. We investigated that changes in the occurrence of exceptional species reflect the dynamics of water mass exchange between the Kattegat/Skagerrak and the SW Baltic Sea. Our analyses show that these changes could be related to the presence of anomalously high saline water masses. However, only a minor part of the sightings was caused by major Baltic inflow events, which are important to sustain oxygen rich deep water in the central Baltic Sea. This documents that the hydrographically highly dynamic SW Baltic Sea needs special attention for monitoring of non-indigenous species, as (i) high saline and warm water intrusions are more frequent than currently believed and ii) can be linked to sightings of exceptional species in the SW Baltic Sea. Additionally, most of the recent sightings occurred during anomalously warm periods. This supports the hypothesis, that the Baltic Sea is presently a predominant receiver area for non-indigenous species from warmer regions of the world.

Estimating the abundance of the critically endangered Baltic Proper harbour porpoise (Phocoena phocoena) population using passive acoustic monitoring.

Knowing the abundance of a population is a crucial component to assess its conservation status and develop effective conservation plans. For most cetaceans, abundance estimation is difficult given their cryptic and mobile nature, especially when the population is small and has a transnational distribution. In the Baltic Sea, the number of harbour porpoises (Phocoena phocoena) has collapsed since the mid‐20th century and the Baltic Proper harbour porpoise is listed as Critically Endangered by the IUCN and HELCOM; however, its abundance remains unknown. Here, one of the largest ever passive acoustic monitoring studies was carried out by eight Baltic Sea nations to estimate the abundance of the Baltic Proper harbour porpoise for the first time. By logging porpoise echolocation signals at 298 stations during May 2011–April 2013, calibrating the loggers’ spatial detection performance at sea, and measuring the click rate of tagged individuals, we estimated an abundance of 71–1105 individuals (95% CI, point estimate 491) during May–October within the population's proposed management border. The small abundance estimate strongly supports that the Baltic Proper harbour porpoise is facing an extremely high risk of extinction, and highlights the need for immediate and efficient conservation actions through international cooperation. It also provides a starting point in monitoring the trend of the population abundance to evaluate the effectiveness of management measures and determine its interactions with the larger neighboring Belt Sea population. Further, we offer evidence that design‐based passive acoustic monitoring can generate reliable estimates of the abundance of rare and cryptic animal populations across large spatial scales.

In the Search of Marine Pestiviruses: First Case of Phocoena Pestivirus in a Belt Sea Harbour Porpoise.

Pestiviruses are widespread pathogens causing severe acute and chronic diseases among terrestrial mammals. Recently, Phocoena pestivirus (PhoPeV) was described in harbour porpoises (Phocoena phocoena) of the North Sea, expanding the host range to marine mammals. While the role of the virus is unknown, intrauterine infections with the most closely related pestiviruses— Bungowannah pestivirus (BuPV) and Linda virus (LindaV)—can cause increased rates of abortions and deaths in young piglets. Such diseases could severely impact already vulnerable harbour porpoise populations. Here, we investigated the presence of PhoPeV in 77 harbour porpoises, 277 harbour seals (Phoca vitulina), grey seals (Halichoerus grypus) and ringed seals (Pusa hispida) collected in the Baltic Sea region between 2002 and 2019. The full genome sequence of a pestivirus was obtained from a juvenile female porpoise collected along the coast of Zealand in Denmark in 2011. The comparative Bayesian phylogenetic analyses revealed a close relationship between the new PhoPeV sequence and previously published North Sea sequences with a recent divergence from genotype 1 sequences between 2005 and 2009. Our findings provide further insight into the circulation of PhoPeV and expand the distribution from the North Sea to the Baltic Sea region with possible implications for the vulnerable Belt Sea and endangered Baltic Proper harbour porpoise populations.

Distribution and bioavailability of mercury in the surface sediments of the Baltic Sea

The study aimed to determine the level of mercury (Hg) and its labile and stable forms in the surface sediments of the Baltic Sea. The work considers the impact of current and historical sources of Hg on sediment pollution, together with the influence of different environmental parameters, including water inflows from the North Sea. Surface sediments (top 5 cm) were collected in 2016–2017 at 91 stations located in different areas of the Baltic Sea, including Belt Sea, Arkona Basin, Bornholm Basin, Gdańsk Basin, West Gotland Basin, East Gotland Basin, and the Bothnian Sea. Besides, the particulate matter suspended in the surface and near-bottom water was also collected. The analysis of total Hg concentration and individual Hg forms in collected samples was carried out using a 5-step thermodesorption method. This method allows for the identification of three labile and thus biologically available, fractions of Hg, which are mercury halides, organic Hg, mercury oxide and sulphate. Two stable fractions, mercury sulphide and residual Hg, were also determined. The highest Hg concentrations, reaching 341 ng g−1, were measured in the highly industrialised Kiel Bay, which was additionally a munition dumping site during and after World War II. High Hg level, ranging from 228 to 255 ng g−1, was also recorded in the surface sediments of the Arkona Basin, which was a result of the cumulative effect of several factors, such as deposition of Hg-rich riverine matter, favourable hydrodynamic conditions and military activities in the past. The relatively elevated Hg concentrations, varying from 60 to 264 ng g−1, were found in the Gdańsk Basin, a region under strong anthropopressure and dominated by soft sediments. The sum of labile Hg in sediments was high and averaged 67% (with the domination of organic Hg compounds), which means that a large part of Hg can be released to the water column. It was found that the water inflows from the North Sea intensify the remobilisation of Hg and its transformation into bioavailable labile forms. As a consequence, the load of Hg introduced into the trophic chain can increase. Despite the significant reduction of Hg emission into the Baltic in the last decades, surface sediments can be an important secondary Hg source in the marine ecosystem. This is especially dangerous in the case of the western Baltic Sea.

Health assessment of harbour porpoises (PHOCOENA PHOCOENA) from Baltic area of Denmark, Germany, Poland and Latvia

Harbour porpoise (Phocoena phocoena), the only resident cetacean species of the Baltic Sea is formed of two subpopulations populations, occurring in the western Baltic, Belt Seas and Kattegat and the Baltic Proper, respectively. Harbour porpoises throughout these areas are exposed to a large number of human activities causing direct and indirect effects on individuals, that might also harm this species on a population level. From Latvia, Poland, Germany and Denmark 385 out of 1769 collected dead harbour porpoises were suitable for extensive necropsy. The animals were collected between 1990 and 2015 and were either by-caught or found dead on the coastline. Following necropsies, histopathological, microbiological, virological and parasitological investigations were conducted. Females and males were equally distributed among the 385 animals. Most animals from the different countries were juveniles between 3 months and 3 years old (varying between 46.5 and 100% of 385 animals per country). The respiratory tract had the highest number of morphological lesions, including lungworms in 25 to 58% and pneumonia in 21 to 58% of the investigated animals. Of those with pneumonia 8 to 33% were moderate or severe. The alimentary, hearing, and haematopoietic systems had inflammatory lesions and parasitic infections with limited health impact. 45.5 to 100% of the animals from the different countries were known by-caught individuals, of which 20 to 100% varying between countries had netmarks. Inflammatory lesions, especially in the respiratory tract were found in higher numbers when compared to control populations in areas with less human activities such as arctic waters. The high number of morphological changes in the respiratory tract and of bycatches especially among immature animals before reaching sexual maturity is of serious concern, as well as the low number of adult animals among the material. Data on health status and the causes of death are valuable for management. A next step in this regard will combine data from health and genetic investigations in order to detect differences between the two populations of the Baltic.

Changes of Nutrient Concentrations in the Western Baltic Sea in the Transition Between Inner Coastal Waters and the Central Basins: Time Series From 1995 to 2016 With Source Analysis

Anthropogenic nitrogen input to the western Baltic Sea has continued to decline, by about 30% since 1995, whereas phosphorus input, after a strong decrease of 25% from 1995 to 2000, remained almost on the same level. Despite this development, the Arkona Sea and Belt Sea apparently show no strong changes of nutrient concentrations in the last two decades. In this study, we investigate the concentrations of nitrate, nitrite, ammonium, DIN (dissolved inorganic nitrogen), dissolved inorganic phosphorus (DIP), Total nitrogen (TN) and Total phosphorus (TP), as the substances of major concern in respect to the eutrophication of the Baltic Sea. To link open sea nutrient concentrations with nutrient supply by freshwater drain, we used nutrient versus salinity linear fits, including data from coastal waters obtained during regular German monitoring campaigns. Therefore, the data set was grouped according to differently sized areas to identify regional differences and was subsequently analyzed for four time intervals 1995-1999, 2000-2004, 2005-2009 and 2010-2016. By extrapolation to a salinity of “zero” we obtained hypothetical freshwater end-members of the different N and P parameters. Most of these end-members indicated a decline during the four time intervals, supporting a reduction of freshwater borne N and P supply to the western Baltic Sea. For the period 1995-1999 the freshwater end-members of TN were determined between 200 µM and 400 µM. This improved to a TN range between 150 µM and about 300 µM in 2010-2016. TP ranged from about 2.3 µM to 5.4 µM in 2010-2016 with previous values of slightly above 4 µM and up to almost 15 µM in the period 1995-1999. The end-members determined by our method would fulfill limnic-marine target values of the EU Water Framework Directive of TN = 186 µM and river-type specific target values of TP between 3-5 µM for some areas of the western Baltic Sea in recent years, because the method includes removal processes in the transition zone. Finally, the end-member values were compared to area specific averages of measured values at the limnic-marine transition point.

Assessing SNP-markers to study population mixing and ecological adaptation in Baltic cod.

Atlantic cod (Gadus morhua) is a species of great ecological and economical importance in the Baltic Sea. Here, two genetically differentiated stocks, the western and the eastern Baltic cod, display substantial mechanical mixing, hampering our understanding of cod ecology and impeding stock assessments and management. Based on whole-genome re-sequencing data from reference samples obtained from the study area, we designed two different panels of Single Nucleotide Polymorphisms markers (SNPs), which take into account the exceptional genome architecture of cod. A minimum panel of 20 diagnostic SNPs and an extended panel (20 diagnostic and 18 biologically informative SNPs, 38 in total) were developed and validated to distinguish unambiguously between the western and the eastern Baltic cod stocks and to enable studies of local adaptation to the specific environment in the Baltic Sea, respectively. We tested both panels on cod sampled from the southern Baltic Sea (n = 603) caught in 2015 and 2016. Genotyping results showed that catches from the mixing zone in the Arkona Sea, were composed of similar proportions of individuals of the western and the eastern stock. Catches from adjacent areas to the east, the Bornholm Basin and Gdańsk Deep, were exclusively composed of eastern Baltic cod, whereas catches from adjacent western areas (Belt Sea and Öresund) were composed of western Baltic cod. Interestingly, the two Baltic cod stocks showed strong genetic differences at loci associated with life-history trait candidate genes, highlighting the species’ potential for ecological adaptation even at small geographical scales. The minimum and the extended panel of SNP markers presented in this study provide powerful tools for future applications in research and fisheries management to further illuminate the mixing dynamics of cod in the Baltic Sea and to better understand Baltic cod ecology.

Major Baltic Inflow Statistics – Revised

Major Baltic inflow events (MBI) transport large amounts of saline water into the Baltic. They are the solely source for deep water ventilation in the central Baltic basins, and control to a large extent the environmental conditions below the permanent halocline. Available series of MBI frequency and intensity depict strong decrease of MBI frequency after the 1980ies, followed by long lasting stagnation periods in the central Baltic basins. However, the expected decrease in mean salinity of the Baltic was not observed. Also the frequency of large volume changes of the Baltic has not changed, and recent model studies predict a slight increase of MBI frequency with warming climate. Using long term data series of sea level, river discharge, and salinity from the Belt Sea and the Sound a continuous series of barotropic inflows was reconstructed for the period from 1887 till present. A comparison with the MBI series of Fischer and Matthaus (1996) revealed significant differences in the period since the 1980ies. The reasons for the deviations are mainly the lack of appropriate data between 1976 and 1991, and the change in observation methods afterwards, which caused a bias in the inflow statistics. In contrast to earlier investigations the revised MBI series depicts no significant long term trend in MBI frequency and intensity, contradicting the hypothesis that climate change caused a decreasing MBI frequency. There exists a decadal variability of MBI with a main period of 25 to 30 years. Periods with reduced MBI frequency were identified. The revised MBI series was proved with observations of dissolved oxygen and salinity in the bottom layer of the Bornholm basin. Until today climate change has no obvious impact on the MBI related oxygen supply to the central Baltic Sea. The increased eutrophication during the last century is most probably the main driver for temporal and spatial spreading of suboxic and anoxic conditions in the deep layer of the Baltic Sea.

Basin-scale distribution of harbour porpoises in the Baltic Sea provides basis for effective conservation actions

Knowledge on spatial and seasonal distribution of species is crucial when designing protected areas and implementing management actions. The Baltic Proper harbour porpoise (Phocoena phocoena) population is critically endangered, and its distribution is virtually unknown. Here, we used passive acoustic monitoring and species distribution models to describe the spatial and seasonal distribution of harbour porpoises in the Baltic Proper. Porpoise click detectors were deployed over a systematic grid of 297 stations in eight countries from April 2011 through July 2013. Generalized additive models were used to describe the monthly probability of detecting porpoise clicks as a function of spatially-referenced covariates and time. During the reproductive season, two main areas of high probability of porpoise detection were identified. One of those areas, situated on and around the offshore banks in the Baltic Proper, is clearly separated from the known distribution range of the Belt Sea population during breeding season, suggesting this is an important breeding ground for the Baltic Proper population. We commend the designation of this area as a marine protected area and recommend Baltic Sea countries to also protect areas in the southern Baltic Sea and the Hanö Bight where additional important harbour porpoise habitats were identified. Further conservation measures should be carried out based on analyses of overlap between harbour porpoise distribution and potentially harmful anthropogenic activities. Our study shows that large-scale systematic monitoring using novel techniques can give important insights on the distribution of low-density populations, and that international cooperation is pivotal when studying transnationally migratory species.

First evidence of spawning of eastern Baltic cod (Gadus morhua callarias) in the Belt Sea, the main spawning area of western Baltic cod (Gadus morhuaL.)

Two cod stocks (western Baltic cod, WBC, and eastern Baltic cod, EBC) are managed in the Baltic Sea which is characterized by different main spawning areas and different main spawning periods. In this study we analyse the spatial and temporal occurrence of spawning individuals of both cod stocks in the main spawning grounds of the Baltic Sea based on eight microsatellite loci. Our results suggest that EBC (Gadus morhua callarias) has formed currently temporally stable, substantially homogeneous population not only in the Bornholm Sea (ICES SD: 25) but also in the Arkona Sea (ICES SD: 24). The presented analyses proved that EBC (G. m. callarias) can temporarily also spawn in the Belt Sea.

Invasion of Danish and Adjacent Waters by the Comb Jelly Mnemiopsis leidyi—10 Years After

The invasive ctenophore Mnemiopsis leidyi, which comes from North America’s east coast, was observed in Danish waters for the first time in 2007. Since then, the new invader has every summer spread in Danish and adjacent waters (i.e. North Sea, Limfjorden, Skagerrak, Kattegat, Belt Sea, Baltic Sea). The invasive comb jelly has apparently come to stay, as it has no effective enemies. Possible harmful effects of M. leidyi which feeds voraciously on zooplankton, fish eggs and larvae, have so far not been thoroughly studied in Danish waters, although dedicated attempts have been made in Limfjorden and in the central Baltic Sea. Over the last 10 years, the Danish national environmental monitoring program did not include gelatinous zooplankton, but new initiatives have been recently taken. A brief overview of our current knowledge on the impact of M. leidyi in Danish waters is given here.

Spatially Explicit Analysis of Genome-Wide SNPs Detects Subtle Population Structure in a Mobile Marine Mammal, the Harbor Porpoise.

The population structure of the highly mobile marine mammal, the harbor porpoise (Phocoena phocoena), in the Atlantic shelf waters follows a pattern of significant isolation-by-distance. The population structure of harbor porpoises from the Baltic Sea, which is connected with the North Sea through a series of basins separated by shallow underwater ridges, however, is more complex. Here, we investigated the population differentiation of harbor porpoises in European Seas with a special focus on the Baltic Sea and adjacent waters, using a population genomics approach. We used 2872 single nucleotide polymorphisms (SNPs), derived from double digest restriction-site associated DNA sequencing (ddRAD-seq), as well as 13 microsatellite loci and mitochondrial haplotypes for the same set of individuals. Spatial principal components analysis (sPCA), and Bayesian clustering on a subset of SNPs suggest three main groupings at the level of all studied regions: the Black Sea, the North Atlantic, and the Baltic Sea. Furthermore, we observed a distinct separation of the North Sea harbor porpoises from the Baltic Sea populations, and identified splits between porpoise populations within the Baltic Sea. We observed a notable distinction between the Belt Sea and the Inner Baltic Sea sub-regions. Improved delineation of harbor porpoise population assignments for the Baltic based on genomic evidence is important for conservation management of this endangered cetacean in threatened habitats, particularly in the Baltic Sea proper. In addition, we show that SNPs outperform microsatellite markers and demonstrate the utility of RAD-tags from a relatively small, opportunistically sampled cetacean sample set for population diversity and divergence analysis.