Bothnian Bay Research Articles

Interannual sea level variability in the North and Baltic seas and net flux through the Danish straits

The paper presents the reconstruction of sea levels in the North Sea and Baltic Sea using Kalman filter approach. Based on the statistical characteristics of one year of daily maps of sea level from the Geesthacht COAstal model SysTem (GCOAST) and daily data at tide gauges along the coastline of two basins, the method can reconstruct effectively and accurately the multidecadal sea level anomalies. The high accuracy reconstruction data were then used to investigate the interannual variability in both basins and to estimate the difference between outflows and inflows (net flux) through the Danish Straits. The highest mean, standard deviation, and extreme values of sea level anomalies appear in winter and are well reproduced in different regions, such as the German Bight, the Southern North Sea, the Bothnian Bay, the Gulfs of Finland and Riga. The sea level variability is highly correlated with the mean sea level pressure and the zonal wind, particularly in the German Bight and in the winter months. The contributions of river runoff and net precipitation on the net flux are significant in the spring. The local wind has a greater influence on the net flux than the remote drivers.

Four Decades of Spatiotemporal Variability of Per- and Polyfluoroalkyl Substances (PFASs) in the Baltic Sea.

Temporal and spatial variability of per- and polyfluoroalkyl substances (PFASs) in herring, cod, eelpout, and guillemot covering four decades and more than 1000 km in the Baltic Sea was investigated to evaluate the effect of PFAS regulations and residence times of PFASs. Overall, PFAS concentrations responded rapidly to recent regulations but with some notable basin- and homologue-specific variability. The well-ventilated Kattegat and Bothnian Bay showed a faster log-linear decrease for most PFASs than the Baltic Proper, which lacks a significant loss mechanism. PFOS and FOSA, for example, have decreased with 0-7% y-1 in the Baltic Proper and 6-16% y-1 in other basins. PFNA and partly PFOA are exceptions and continue to show stagnant or increasing concentrations. Further, we found that Bothnian Bay herring contained the highest concentrations of >C12 perfluoroalkyl carboxylic acids (PFCAs), likely from rivers with high loads of dissolved organic carbon. In the Kattegat, low PFAS concentrations, but a high FOSA fraction, could be due to influence from the North Sea inflow below the halocline and possibly a local source of FOSA and/or isomer-specific biotransformation. This study represents the most comprehensive spatial and temporal investigation of PFASs in Baltic wildlife while providing new insights into cycling of PFASs within the Baltic Sea ecosystem.

Simulating sea level extremes from synthetic low-pressure systems

Abstract. In this article we present a method for numerical simulations of extreme sea levels using synthetic low-pressure systems as atmospheric forcing. Our simulations can be considered to be estimates of the high sea levels that may be reached when a low-pressure system of high intensity and optimal track passes the studied region. We test the method using sites located along the Baltic Sea coast and simulate synthetic cyclones with various tracks. To model the effects of the cyclone properties on sea level, we simulate internal Baltic Sea water level variations with a numerical two-dimensional hydrodynamic model, forced by an ensemble of time-dependent wind and air-pressure fields from synthetic cyclones. The storm surges caused by the synthetic cyclones come on top of the mean water level of the Baltic Sea, for which we used a fixed upper estimate of 100 cm. We find high extremes in the northern Bothnian Bay and in the eastern Gulf of Finland, where the sea level extreme due to the synthetic cyclone reaches up to 3.5 m. In the event that the mean water level of the Baltic Sea has a maximal value (1 m) during the cyclone, the highest sea levels of 4.5 m could thus be reached. We find our method to be suitable for use in further studies of sea level extremes.

Fatty acid profiles reveal dietary variability of a large calanoid copepod Limnocalanus macrurus in the northern Baltic Sea

Eutrophication, climate-induced warming, and salinity fluctuations are altering the fatty acid profiles and the availability of essential polyunsaturated fatty acids (PUFAs) in marine zooplankton communities. Limnocalanus macrurus Sars G.O., 1863 is a large calanoid copepod inhabiting the low-salinity areas in the Baltic Sea, where it is a major source of docosahexaenoic acid (DHA) and eicosapentaenoic acid (EPA) to commercially important fish. L. macrurus is sensitive to warming, eutrophication and hypoxia. As an opportunistic feeder, it is capable of dietary shifts, which affects its fatty acid profiles. Although much studied in boreal lakes, there are only a few studies on the fatty acid profiles of the Baltic Sea populations. This study aimed to compare the fatty acid profiles of L. macrurus in three basins of the Baltic Sea, in relation to the community fatty acids and environmental variables. We collected samples of L. macrurus and filtered plankton community for gas chromatographic fatty acid analyses in August 2021 on R/V Aranda. The nutritional quality of L. macrurus to consumers was lower in the Gulf of Finland (GoF) compared to the Gulf of Bothnia, indicated by the low levels of DHA and EPA, as well as the low n-3/n-6 ratio of PUFAs. The lower ratio of 18:1n-7 to 18:1n-9 implied higher degree of omnivory in GoF. In contrast, a diatom marker 16:1n-7 had high proportion in the Bothnian Bay. High temperatures in GoF may have restricted feeding in the upper water column, possibly forcing a shift towards cyanobacteria or seston-based diet, as interpreted from a high proportion of 18:2n-6 and 18:3n-3. We conclude that the ability of L. macrurus to utilize multiple food sources increases its resilience to environmental change, while the consequences on the nutritional quality may have further cascading effects on the food webs.

Links between Land Cover and In-Water Optical Properties in Four Optically Contrasting Swedish Bays

The optical complexity of coastal waters is mostly caused by the water discharged from land carrying optical components (such as dissolved and particulate matter) into coastal bays and estuaries, and increasing the attenuation of light. This paper aims to investigate the links between in-water optical properties in four Swedish bays (from the northern Baltic proper up to the Bothnian bay) and the land use and land cover (LULC) in the respective catchment of each bay. The optical properties were measured in situ over the last decade by various research and monitoring groups while the LULC in each bay was classified using the Copernicus Land Monitoring Service based on Landsat 8/OLI data. The absorption coefficient of colored dissolve organic matter (CDOM) at 440 nm, aCDOM (440), was significantly correlated to Wetlands which may act as sources of CDOM, while Developed areas (Agricultural and Urban classes) were negatively correlated. The Agriculture class was also negatively related to suspended particulate organic matter (SPOM), whilst Coniferous Forests and Mixed Forests as well as Meadows were positively correlated. SPOM seems thus to mostly originate from Natural classes, possibly due to the release of pollen and other organic matter. Overall, the methods applied here allow for a better understanding of effects of land use and land cover on the bio-optical properties, and thus coastal water quality, on a macroscopic scale.

National-scale nitrogen loading from the Finnish agricultural fields has decreased since the 1990s

The national scale nutrient load modelling system VEMALA-ICECREAM was used to simulate agricultural total nitrogen(TN) loading and its trends for all Finnish watersheds for the period from 1990–2019. Across Finland, agricultural TN loading (ATNL) has decreased from 17.4 kg ha-1 a-1 to 14.4 kg ha-1 a-1 (moving 10-year averages) since the 1990s. The main driver of the decrease in simulated ATNL is a reduction in mineral fertilizer use, which has decreased the N surplus in the soils. The TN leached fraction, however, did not show a trend but did have high annual variability due to variations in runoff; this corresponds to an average of 14.4% of the TN applied. The ATNL was considerably higher in the Archipelago Sea catchment compared to other Finnish Baltic Sea sub-catchments, with the lowest ATNL found in the Vuoksi catchment in Eastern Finland. The highest decrease of ATNL was simulated for Vuoksi and Gulf of Finland catchments. In the Bothnian Sea, Bothnian Bay and Archipelago Sea catchments, the decreasing trend of ATNL was smaller but still significant, with the exception of the Quark catchment, where there was no significant change. The differences in decreasing trends between regions can be explained by the heterogeneity of catchment characteristics, hydrology and agricultural practices in different regions.

Catches, bycatches and stock indicators of fisheries targeting cyprinids along the Swedish Baltic Sea coast

Decreasing abundance of many traditionally exploited fish stocks in the Baltic Sea force small-scale fisheries to find new ways to make a living. In line with Swedish national strategies on food supply there is an interest to develop commercial cyprinid fisheries. In the Bothnian Bay in the northern part of the Baltic Sea, annual catches have increased from zero-catches 2018–30 tonnes 2021. To aid a sustainable development of these cyprinid fisheries that target mainly bream (Abramis brama) and ide (Leuciscus idus), we study catch efficiency of target species and bycatch in different gears and seasons using logbook data from the Bothnian Bay. Using cameras, we also assessed bycatch rates. To assist the sustainability of the fishery we develop potential stock indicators. Our results suggests that larger gear (pound-nets) are more effective in catching bream, and that the proportion of bycatch decreased with gear size, being < 10% in the largest gear, which is similar or lower than many other Baltic Sea fisheries. By-catches of salmon is of concern in the Bothnian Bay, but the camera study indicates that salmon bycatches are sporadic. Catch per unit effort (CPUE) of bream was highest in spring and fall, and we conclude that site specific median CPUE is the most suitable stock abundance indicator. The size indicator L90, the 90th percentile of the length distribution, was similar among areas and we propose it as a suitable indicator of the demographic structure of the targeted bream stocks. Our results provide reference points for relatively unfished conditions, but as the study was based on mainly fishery dependent data, it is important to also include fishery independent data to assess ecosystem effects of a future and intensified cyprinid fishery.

Observations of Snow–Slush–Snow Ice Transformation and Properties of Brash Ice in Ship Channels

The thickness and properties of brash ice are usually compared with the properties of the surrounding level ice. The differences between these ice types are important to understand since the consolidated brash ice layer is typically assumed to have the same properties as level ice. Therefore, significant effort in the measurement campaign during the winters of 2020–2021, 2021–2022, and 2023 was made to develop a better understanding of the full-scale brash ice channel development. The channels were located near the shore in the Bay of Bothnia, Luleå, Sweden. The main parameters investigated were the snow, slush, and total ice thicknesses, including ice formed from freezing water and from freezing slush as well as the ice microstructure and strength. To our knowledge, this is the first paper to report the influence of snow in brash ice channels. It was observed that a significant amount of snow covered the brash ice channels between the ship passages. After each ship passage, the snow was submerged and formed slush-filled voids, which thereafter transformed into snow ice (SI) clusters frozen together with columnar ice. The SI content in the brash ice and side ridges was estimated from image analyses. The analyses showed that the snow ice content was 73% in level ice in the vicinity of the ship channel, 58% in the side ridges of the channel, and 21% in the middle of the test channel, whereas in the main channel, the SI contents were 54%, 43%, and 41% in each location, respectively.

A comparison of Baltic Sea wave properties simulated using two modelled wind data sets

We calculated two sets of idealised ice-free wave hindcast data on a high-resolution, three-layer nested grid covering the Baltic Sea, using two different wind datasets (ERA5 and BaltAn65+), to evaluate the performance of the model in the open and sheltered parts of the sea. The highest resolution of the grids is 260 m. The outcome of simulations is compared against the available in-situ measurements. The main statistical properties of the Baltic Sea wave climate as well as the timing and maximum values of extreme significant wave heights are well replicated in the Baltic proper using the 3 nautical mile (nmi) grid. Simulations using BaltAn65+ have a better match with recorded average wave properties while the ERA5 forcing better replicates wave extremes. The largest differences between the two simulations occur in the Sea of Bothnia, Bay of Bothnia and to the east of large islands. Wave climate in the Gulf of Finland is replicated with a resolution of 1 nmi in the open part of the gulf a down to a resolution of 260 m in selected coastal areas. The wave intensity is the largest in the central part of the gulf and decreases towards the coast and from the West to the East. The use of ERA5 winds yields higher wave heights at the entrance of the gulf and in the central part of the gulf along its axis. Simulations using BaltAn65+ winds show higher waves in the nearshore regions.

Probabilistic projections and past trends of sea level rise in Finland

Abstract. We explore past trends and future projections of mean sea level (MSL) at the Finnish coast, in the northeastern Baltic Sea, during the period 1901–2100. We decompose the relative MSL change into three components: regional sea level rise (SLR), postglacial land uplift, and the effect of changes in wind climate. Past trends of regional SLR can be calculated after subtracting the other two components from the MSL trends observed by tide gauges, as the land uplift rates obtained from the semi-empirical model NKG2016LU are independent of tide gauge observations. According to the results, local absolute SLR trends are close to global mean rates. To construct future projections, we combine an ensemble of global SLR projections in a probabilistic framework. In addition, we use climate model results to estimate future changes in wind climate and their effect on MSL in the semi-enclosed Baltic Sea. This yields probability distributions of MSL change for three scenarios representing different future emission pathways. Spatial variations in the MSL projections result primarily from different local land uplift rates: under the medium-emission scenario RCP4.5/SSP2-4.5, for example, the projected MSL change (5 % to 95 % range) over the 21st century varies from −28 (−54 to 24) cm in the Bothnian Bay to 31 (5 to 83) cm in the eastern Gulf of Finland.

Long‐term monitoring of pikeperch (Sander lucioperca) populations under increasing temperatures and predator abundances in the Finnish coastal waters of the Baltic Sea

AbstractThe Baltic Sea has been under intense environmental changes in the recent decades, such as climate change, eutrophication and increasing abundance of top‐predators, which pose serious challenges to its aquatic life. For informed conservation measures and sustainable yields, we need to understand how the populations are being affected. Accordingly, we used long‐term data series (covering the period between 1980 and 2021) to assess how these changes have affected populations of an ecologically and economically important predatory fish, the pikeperch (Sander lucioperca), in the coastal waters of Finland in the northern Baltic Sea. We investigated the estimates of abundance and recruitment, commercial and recreational catch statistics and growth and mortality rates. We found a clear increase in the total catches in the northernmost part of the Finnish coast (Bothnian Bay) that were not explained by changes in fishing effort, indicating increased abundance, most likely due to higher water temperature. In the southern part of the study area (Archipelago Sea), density‐dependent factors prevented the development of particularly strong year classes, despite the beneficial conditions of warming seawater and consecutive warm summers. Individual growth has increased in younger age groups, contributing to an upward trend in the spawning population biomass. We also uncovered a declining trend in the total mortality in the southern area, despite increased abundances of cormorants and seals, explained by reduced total fishing mortality. These results show that the pikeperch is one of the species that has, thus far, benefited from the environmental change in the northern Baltic Sea, strengthening its role in the ecosystem.

Contrasting distribution and speciation of sedimentary organic phosphorus among different basins of the Baltic Sea

AbstractRecycling of phosphorus (P) from deoxygenated sediments perpetuates eutrophic conditions in parts of the Baltic Sea. Sedimentary organic P is a major source of dissolved P to the water column, but also a sink for permanent P burial. The mechanisms behind these two pathways are, however, largely unknown. Using new methods, we determined P in DNA and phospholipids, which are both found in all organisms. We also identified inositol phosphates that are particularly important in eukaryotes. Sediment cores were collected from contrasting basins in the Baltic Sea to study their relative contribution to the total P pool. We found high DNA‐P/phospholipid‐P ratios in surface sediments from the Bothnian Bay and Bothnian Sea. However, these ratios were low throughout profiles in euxinic Baltic Proper sediments. The elevated ratios present in sediments overlain by oxic bottom waters might indicate the presence of a microbial community stimulated by bioturbation, whereas the low DNA‐P/phospholipid‐P ratios in Baltic Proper sediments likely indicate an energy‐limited microbial community, typical to the “deep biosphere” environment. Inositol‐P was almost absent in euxinic Baltic Proper sediments that had a low total P amount compared to those in the other basins. We suggest that variability in the composition of sedimentary microbial communities among the Baltic Sea basins might cause differences in organic P forms that in turn affects its turnover.

Association between Legionella species and humic substances during early summer in the northern Baltic Sea

Climate change is projected to cause alterations in northern coastal systems, including humification and intensified nutrient loads, which can lead to ecosystem imbalances and establishment of new bacterial species. Several potential pathogens, such as different species of Legionella, hide in the environment between infections, some by living inside protozoan host cells. Knowledge about the occurrence of Legionella in natural waters is missing, which disable risk assessments of exposure. We performed a study of the species diversity of Legionella in the northern Baltic Sea (Gulf of Bothnia) during early summer to map their occurrence and to identify possible environmental drivers. We detected Legionella and potential protozoan hosts along gradients of the Gulf of Bothnia. We also for the first time present third generation full-length 16S rRNA amplicon sequencing (Nanopore) to resolve environmental species classification of Legionella, with a method suitable to study all bacteria. Our data show that full length 16S rRNA sequences is sufficient to resolve Legionella while the standard short Illumina sequences did not capture the entire diversity. For accurate species classification of Legionella, harmonization between the Nanopore classification methods is still needed and the bias toward the well-studied Legionella pneumophila need to be resolved. Different Legionella species occurred both in the Bothnian Sea and in the Bothnian Bay and their abundance were linked to humic substances and low salinity. The relative abundance of Legionella was higher in the humic-rich northern waters of the Bothnian Bay. The link between Legionella species and humic substances may be indirect via promotion of the heterotrophic microbial food web, allowing Legionella species and similar bacteria to establish. Humic substances are rich in iron, which has been shown crucial for growth of Legionella species and other pathogens. Considering climate change projections in this regional area, with increased humification and freshwater inflow, this bacterial niche containing potential pathogens might become more widespread in the future Baltic Sea. This study demonstrates the significance of DNA sequencing to monitor public health relevant bacteria like Legionella species in the environment. Including sequencing of bacteria and protozoa in the environmental monitoring programs could be used to identify ecosystem imbalances, which enable appropriate responses to emerging diseases.

Integrated risk assessment approach for eutrophication in coastal waters: Case of Baltic Sea

Eutrophication of coastal ecosystems is a global and rapidly intensifying environmental crisis. Efficient strategies are in great demand to adapt changes to coastal ecosystems and enhance the management. This study proposes a flexible methodology for quantifying the risk status of coastal subbasins. The proposed method integrates the characteristics of both a strong multi-criteria analysis and fuzzy set theory. Central to this methodology is the construction of a risk index for revealing the extent of cumulated nutrients discharges. The proposed model is applied to assess risk status in the Baltic Sea, showing an increasing trend of risk index over the years. Results also show good agreement with field observations and Helsinki Commission (HELCOM) predictions ─ the Bothnian Bay is ranked as a ‘Medium’ risk area while the Bornholm Bay shows a ‘Very High’ risk of eutrophication. The proposed approach not only evaluates the risk degree of nutrients enrichment, but also captures the extent of exogeneous contributions for the given area, leading to better management recommendations to reduce eutrophication risk.

Hydrodynamic Impacts on the Fate of Polychlorinated Biphenyl 153 in the Marine Environment

Due to their long half-life, polychlorinated biphenyls (PCBs) tend to contaminate not only coastal areas, but they travel over long distances, eventually reaching remote areas such the Arctic. The physical and biogeochemical features of every coastal area govern the main distribution patterns of freshly introduced PCBs into the marine system. Some of these processes are determined by chemical properties of the individual PCB congener. Thus, atmospheric influx along with ad/absorption on non-living organic material, photolytical and biological degradation processes vary from one PCB congener to another. For a detailed fate analysis of individual congeners, we developed a new chemical model, based on the GOTM-ECOSMO-FABM model framework. Here, we exemplarily present results for PCB153 based on 1D simulations of four regions in the North-Baltic Sea. The study area is characterized by different hydrodynamic and biogeochemical conditions. We investigate the impact of resuspension, mixing and the biological pump, sea ice and tides on the final phasal distribution of PCB153. Different combinations of these factors lead to the development of different areas of PCB153 accumulation, with the formation of hotspot areas, and influence the total uptake and concentration of PCB153 in the water column. As a result, two major dynamics determine the fate of PCB153 in the coastal ocean: (i) Primary production leads to PCB153 being adsorbed by organic material. Partitioning to organic material and sedimentation of organic particles removes dissolved PCB153 from the surface ocean and increases atmospheric influx. (ii) Tidal-induced resuspension and mixing control the benthic–pelagic exchange of PCB153 and its distribution in the water column. Depending on the resuspension regime and stratification, sediments can become a permanent (Gotland Deep, the Baltic Sea) or seasonal sink for PCB153. In regions with seasonal stratification and high near bottom turbulence (Northern North Sea), resuspension events can lead to pronounced peaks in PCB153 concentrations and can therefore have a major impact on bioaccumulation. Under the conditions of permanent mixing and high bottom turbulence (Southern North Sea, Bothnian Bay), pollutants are hardly accumulating in sediments.

Leaching of Heavy Metals from Contaminated Soil Stabilised by Portland Cement and Slag Bremen

Abstract Leaching behaviour is an important evidence of soil quality. The assessment of leaching of heavy metals from the contaminated soil is vital for environmental applications. However, leaching may differ in soil stabilised by various ratios of binders. In this study we measured leaching behaviour of soil contaminated by As, Cd, Co, Cr, Cu, Hg, Ni, Pb, V, Zn, methyl Hg, aliphatic compounds of hydrocarbons, polycyclic aromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs) and dissolved organic carbon (DOC). To evaluate leaching of these substances we tested the effects of changed amount of binder (120 kg and 150 kg) and binder ratios (70/30 %, 50/50 % and 30/70 %) added to soil samples. Soil was dredged from several stations in Ostrand area, SCA Sundsvall Ortvikens Pappersbruk. The results demonstrated a systematically decreasing leaching with the increased slag. The contribution of this research include: (i) devising systematic approach to extract information on leaching from stabilised soil collected from the coastal area of Bothnian Bay, (ii) developing a workflow for stabilising soils by various combination of Portland cement Basement CEM II/A-V (SS EN 197-1) and ground granulated blast furnace slag (GGBFS), Bremen type (SS EN 15167-1), (iii) determining water ratio and density for the untreated and stabilised soil and performing comparative analysis, (iv) evaluating chemical content of pollutants and toxic elements in the aggregated soil samples. Treatment of the contaminated soil by binders improved its parameters by the increased strength and decreased leaching of heavy metals and toxic elements.

The Baltic Sea Model Intercomparison Project (BMIP) – a platform for model development, evaluation, and uncertainty assessment

Abstract. While advanced computational capabilities have enabled the development of complex ocean general circulation models (OGCMs) for marginal seas, systematic comparisons of regional ocean models and their setups are still rare. The Baltic Sea Model Intercomparison Project (BMIP), introduced herein, was therefore established as a platform for the scientific analysis and systematic comparison of Baltic Sea models. The inclusion of a physically consistent regional reanalysis data set for the period 1961–2018 allows for standardized meteorological forcing and river runoff. Protocols to harmonize model outputs and analyses are provided as well. An analysis of six simulations performed with four regional OGCMs differing in their resolution, grid coordinates, and numerical methods was carried out to explore intermodel differences despite harmonized forcing. Uncertainties in the modeled surface temperatures were shown to be larger at extreme than at moderate temperatures. In addition, a roughly linear increase in the temperature spread with increasing water depth was determined and indicated larger uncertainties in the near-bottom layer. On the seasonal scale, the model spread was larger in summer than in winter, likely due to differences in the models' thermocline dynamics. In winter, stronger air–sea heat fluxes and vigorous convective and wind mixing reduced the intermodel spread. Uncertainties were likewise reduced near the coasts, where the impact of meteorological forcing was stronger. The uncertainties were highest in the Bothnian Sea and Bothnian Bay, attributable to the differences between the models in the seasonal cycles of sea ice triggered by the ice–albedo feedback. However, despite the large spreads in the mean climatologies, high interannual correlations between the sea surface temperatures (SSTs) of all models and data derived from a satellite product were determined. The exceptions were the Bothnian Sea and Bothnian Bay, where the correlation dropped significantly, likely related to the effect of sea ice on air–sea heat exchange. The spread of water salinity across the models is generally larger compared to water temperature, which is most obvious in the long-term time series of deepwater salinity. The inflow dynamics of saline water from the North Sea is covered well by most models, but the magnitude, as inferred from salinity, differs as much as the simulated mean salinity of deepwater. Marine heat waves (MHWs), coastal upwelling, and stratification were also assessed. In all models, MHWs were more frequent in shallow areas and in regions with seasonal ice cover. An increase in the frequency (regionally varying between ∼50 % and 250 %) and duration (50 %–150 %) of MHWs during the last 3 decades in all models was found as well. The uncertainties were highest in the Bothnian Bay, likely due to the different trends in sea ice presence. All but one of the analyzed models overestimated upwelling frequencies along the Swedish coast, the Gulf of Finland, and around Gotland, while they underestimated upwelling in the Gulf of Riga. The onset and seasonal cycle of thermal stratification likewise differed among the models. Compared to observation-based estimates, in all models the thermocline in early spring was too deep, whereas a good match was obtained in June when the thermocline intensifies.

Lack of panmixia of Bothnian Bay vendace - Implications for fisheries management

Overexploitation of fisheries is recognized as a major environmental and socioeconomic problem that threats biodiversity and ecosystem functioning. Inappropriate management policies of fish stocks have been applied as a consequence of inadequate characterization of subtle genetic structure in many fish species. In this study, we aim to assess the extent of genetic differentiation and structure of vendace (Coregonus albula) collected from eight locations in the Bothnian Bay, the northernmost part of the Baltic Sea. Specifically, we test if this species forms a single panmictic population or is divided into several genetically distinct units. We used restriction site-associated DNA sequencing (RAD-seq) to identify 21,792 SNPs based on 266 individuals. We identified a clear pattern of genetic differentiation between River Kalix and the other sampling locations, and a weak structuring between samples from Swedish and Finnish coast. Outlier analysis detected 41 SNPs putatively under divergent selection, mainly reflecting the divergence between River Kalix and the other samples. The outlier SNPs were located within or near 25 genes, including voltage-dependent calcium channel subunit alpha-2 (CACNA2D2), cadherin 26 (CDH26) and carbonic anhydrase 4-like (CA4) that have earlier been associated with salt-tolerance and salinity stress. Our study provides the first genome-wide perspective on genetic structuring of Baltic Sea vendace and rejects the hypothesis of panmixia in the Bothnian Bay. As such, our work demonstrates the power of RAD-sequencing to detect low but significant genetic structuring relevant for fisheries management.

Release mortality of wild Atlantic salmon in coastal pontoon-trap fishery in the northern Baltic Sea

The survival and injuries of wild Atlantic salmon released by coastal fishers from pontoon traps was examined in the early summer of 2020 in the Gulf of Bothnia in the northern Baltic Sea. A total of 491 salmon caught with pontoon traps was tagged with dart tags. By the end of December 2020, a total of 62 tag recoveries (13%) were received. Sixty-two per cent of the tag recoveries were obtained from the sea area, mainly from the commercial fishers in the Bay of Bothnia near northern river mouths. The remaining tags were received from river fishers. It took an average of 22 days from tagging to the capture (range 1–91 days), and the longest distance a tagged salmon travelled was more than 700 km. The mortality of salmon caused by capture and release was estimated to be a maximum of 25–31% for multi-sea-winter (MSW) salmon and a maximum of 24% for one-sea-winter (1SW) salmon. Most of the tagged fish were visually in good condition, although some injuries were observed. When two trap emptying techniques were compared, salmon caught in the Bothnian Sea with an emptying chute showed more injuries than salmon caught in the Bay of Bothnia with a lifting bag. Our results suggest relatively low release mortality for Baltic salmon. Furthermore, we show that disentangling release mortality from combined tagging and handling mortality is crucial in tagging studies.

The stable atmospheric boundary layer over snow‐covered sea ice: Model evaluation with fine‐scale ISOBAR18 observations

AbstractA realistic representation of the stable atmospheric boundary layer in numerical weather prediction (NWP) and climate models is still a challenge. We study the evolution of a stable boundary layer over snow‐covered sea ice in Bothnian Bay during wintertime in 2018. We perform high‐resolution model experiments with the Weather Research and Forecasting model in its single‐column model configuration and its default mesoscale configuration to assess which physical processes are essential to predict near‐surface variables correctly. We evaluate our model runs against the unique observational dataset collected during ISOBAR18, which combines novel, upper‐air measurements by an uncrewed aircraft system with wind lidar, sodar, and conventional meteorological mast data. By analysing surface fluxes in the single‐column model, we demonstrate how the atmospheric cooling at the ground can be modelled more realistically than in the mesoscale set‐up. We show that surface albedo and sea‐ice thickness are essential for the surface energy balance in the model, and we demonstrate how the surface fluxes in the mesoscale downscaling with default settings are subject to strong biases. We also show that the ERA5 reanalysis is not capable of representing the observed surface meteorology in the stable atmospheric boundary layer. Our study illustrates the importance of surface albedo and sea‐ice thickness for NWP models. Though a seasonal snow albedo is already in use in many NWP settings, the routine inclusion of sea‐ice thickness, in particular, would be a great step forward for weather forecasts and regional climate simulations.