Bornholm Basin Research Articles

Biological and optical properties of Baltic surface waters and sea-land interaction – searching for interdependencies

In June 2015, aboard the r/v Akademik Ioffe, a study was conducted on the surface zooplankton community in selected Baltic Sea basins along the Arkona Basin - Gulf of Gdansk route. Samples were collected using a 100 μm mesh plankton net at depths of 2–0 m. Additionally, optical and physical properties were assessed through remote (lidar) and in-situ (CTD and surface microlayer seawater sampling) measurements to provide a foundation for biological analysis.The zooplankton included 19 taxa from holoplanktonic Copepoda, Diplostraca, Rotifera, and meroplanktonic stages of Bivalvia, Gastropoda, Polychaeta, Cirripedia, and Pisces. Zooplankton abundance ranged from 37399 ind. m−3 (Slupsk Furrow) to 267744 ind. m−3 (Bornholm Basin), with copepods being the most numerous groups. While the zooplankton community composition was relatively stable across the study area, their distribution varied. The most diverse station was Slupsk Furrow, with Copepoda, Diplostraca, Rotifera, and meroplankton making up approximately 40%, 25%, 20%, and 5%, respectively. In contrast, Bornholm Basin had the highest zooplankton numbers, mainly dominated by copepods (90%) with minor contributions from other groups.Fluorescence properties of surface microlayer organic matter were assessed by measuring the intensity ratio of the primary fluorophores (A, C, M, and T) of dissolved organic matter molecules, represented as (M + T)/(A + C). This indicated a mixed marine-terrestrial nature of organic matter in stations west of the Slupsk Furrow. Stations at the Arkona Basin and the Slupsk Furrow had the highest ratio values, suggesting a significant marine organic matter source. In contrast, stations east of the Slupsk Furrow exhibited lower (M + T)/(A + C) ratios, indicating a dominant terrestrial origin for organic matter. Lidar results further supported the division of the study area into two regions based on bio-optical properties: a western region (Arkona Basin, Bornholm Basin, and Slupsk Furrow) and an eastern region (Gotland Basin and Gdansk Basin). Moreover, the variability in zooplankton community structure and distribution is closely correlated with the water hydrographic and optical characteristics. We can therefore conclude that all the water properties that we have studied are a derivative of the interaction of sea and land.

Qualitative variability of dissolved organic matter in the Baltic Sea sediments apparent from fluxes and optical properties

IntroductionThe release of dissolved organic matter (DOM) from sediments serves as an important part of the carbon cycle.MethodsHere, we address pore water DOM quality and its release from shallow sediments (0–10 cm) of the central and southern Baltic Sea - Gdansk, Bornholm, and Eastern Gotland Basins - using excitation–emission matrix spectroscopy and size-exclusion chromatography.ResultsDOM release from sediments displayed spatial variability, with diffusive DOC fluxes ranging from 0.02 to 1.06 mmol m−2 d−1, and basin-averaged fluxes increased in the order Gotland < Bornholm < Gdansk. DOM qualitative characteristics also varied among investigated basins. In the oxygen-limited study sites from the Gdansk Basin, we observed elevated abundances of low apparent molecular weight DOM fraction and proteinaceous-like fluorescent DOM, while in the Bornholm Basin, pore water DOM was generally more humic-like and of higher apparent molecular weight. Pore waters from the deepest study sites in the Gotland Basin were qualitatively very similar to those of the pore waters from the upper sediment layers (0–4 cm) from all other investigated sites, suggesting little organic matter reworking at depth at those stations.DiscussionOur results suggest that the spatial variability in the magnitude of DOM release may be linked to the qualitative differences of DOM in sediments.

Increase in marginal sea alkalinity may impact air–sea carbon dioxide exchange and buffer acidification

AbstractTotal alkalinity (TA) has increased in the Baltic Sea, with implications for atmospheric CO2‐induced acidification and CO2 uptake. We compiled extensive data of TA in surface waters of the Baltic Sea, aiming to (i) identify new tendencies in the relationship between TA and salinity (TA–S relationship), (ii) update the TA trend analysis, (iii) investigate spatial–temporal patterns, and (iv) discuss potential drivers and implications. We observed a progressive decrease in the slopes and increase in the intercepts of the TA–S overtime due to the persistent process of TA enhancement. A weak seasonal pattern was identified, with warmer months presenting lower salinity and TA. Lower rates of TA increase were observed in high salinities (Skagerrak–Kattegat; +1.00 to +2.20 μmol kg−1 yr−1), intermediate trends in low salinities (Gulf of Bothnia; +3.28 to +3.57 μmol kg−1 yr−1), and maximal trends in the Central Baltic Sea (+3.70 to +4.57 μmol kg−1 yr−1) and Bornholm Basin (+4.82 to +5.32 μmol kg−1 yr−1). The increase in the intercept of the TA–S in the Gulf of Bothnia suggests a progressive increase in the external supply of TA, although lower than previously thought. The maximum trend in the Bornholm Basin suggests an increase in external supply from the Southern catchment and/or the accumulation of internal production. The positive TA–phosphorus correlations underscore a significant internal source. The TA increase amplifies the CO2 uptake by 1.8–7.8% during spring/summer and reduces the CO2 outgassing by 3.4–7.7% in autumn/winter. The TA enhancement has the potential to buffer CO2‐induced acidification by 39–60% by 2050.

Spatio-temporal larval abundance dynamics of a depleted Baltic Sea herring ecotype

Abstract Fish stocks composed of several populations are considered to have more stable productivity than stocks containing only one or few populations. This stability is attributed to complementary or independent dynamics among the populations within the stock—the so-called portfolio effect. Declines of populations within stocks that have delayed recovery potentially increase risk to productivity and local extinctions. Portfolio effects may be relevant in the Baltic Sea, where autumn-spawning herring was the dominant herring ecotype until the early/mid-20th century. The current fishery assessment and management in the area assumes that the abundance of autumn spawning herring has been negligible since the 1970s; the share of spring spawning herring has become dominant and is considered to be the only ecotype. Data from a multi-annual ichthyoplankton survey in the southern Baltic Sea (Bornholm Basin) during autumn 2002–2019 show that abundance and spatial distribution of the smallest larval stages have increased significantly. This pattern could be due to an increase in autumn spawners in the area. Genetic studies confirm that these larval herring are offspring from true autumn spawners. Preserving intraspecific diversity is critical for a species’ future abilities to adapt to and survive in changing environmental conditions. Thus, our results not only provide new insights to the temporal dynamics of herring ecotypes and challenge the current understanding of central Baltic herring stock processes in general, but are also important in the context of monitoring, assessment, and spatial management of herring in the Baltic Sea.

The influence of seabed geology on the nature and preservation of bottom trawl marks in the Bornholm Basin, southern Baltic Sea

This study investigates six areas in a historically heavily trawled region of the southern Baltic Sea. Using acoustic geophysical mapping data and sediment cores from three field campaigns (2019, 2020, 2023), we evaluate and quantify the cumulative physical impacts from bottom trawling and the influence of seabed geology on mapped trawl tracks. The results are compared with fishing intensity data over three periods; 2012–2016, 2017–2019 and after the fishery closed. A correlation between fishing intensity and density of mapped trawl tracks exists in the soft sediments of the northern part of the area, while this link is weak in the less trawled southern part, where the seabed is characterized by more consolidated glacial clays and the high density of mapped trawl tracks reflects the preservation of tracks >8 years old. Four years after the closure of the fishery there were no signs of trawl-track degradation in any of the areas. In summary, mapped track densities alone are not a suitable measure of trawling intensity, considering the influence of seabed geology and the persistence of trawl tracks over time. Sediment deformation, observed by CT-scanning, indicates extensive remoulding and coarsening of the upper 20–40 cm of sediments in the trawled areas.

Seasonal variability and long-term winter shoaling of the upper mixed layer in the southern Baltic Sea

The upper ocean mixed layer plays a crucial role in regulating the exchange between the ocean and the atmosphere. The Mixed Layer Depth (MLD) is a key parameter affecting the air-sea exchanges of momentum and heat and determining the upper ocean temperature. Numerous previous studies have investigated MLD variability in the global ocean or regional seas but no such studies were carried in the Baltic Sea. In this study, we present the first observational assessment of the MLD and its properties in the Southern Baltic Sea including quantification of its seasonal and long-term changes and identification of the multi-year winter shoaling. We calculated monthly maps of MLD in the southern Baltic Sea using a large number of historical CTD profiles collected in 1995–2021 from a combination of different data sets. To test the robustness of the results we compared the MLDs calculated using different threshold methods. Throughout the southern Baltic Sea, across its three basins, a distinct seasonality is evident in the MLD, with values varying from 12 m in July to 60 m in December–March. During winter the water column is well mixed down to the upper halocline depth and the MLD reaches about 45 m in the Bornholm Basin, 50 m in the Slupsk Furrow, and 60 m in the Gdansk Basin. The observed global warming and decadal changes in the salty inflows from the North Sea to the Baltic have had an impact on stratification by increasing water densities in the intermediate and deep layers. Consequently, density gradients have strengthened with depth while the upper ocean mixing has weakened during the winter season. The results reveal a significant winter shoaling of the mixed layer by 4 m per decade, driven by the increased stratification due to rising temperatures and salinity. These changes could have significant impacts on the dynamics and productivity of marine ecosystems.

Numerical Modelling and Prediction of Oil Slick Dispersion and Horizontal Movement at Bornholm Basin in Baltic Sea

This paper presents an original approach to predicting oil slick movement and dispersion at the water surface. Special emphasis is placed on the impact of evolving hydro-meteorological conditions and the thickness of the oil spill layer. The main gap addressed by this study lies in the need for a comprehensive understanding of how changing environmental conditions and oil thickness interact to influence the movement and dispersion of oil slicks. By focusing on this aspect, this study aims to provide valuable insights into the complex dynamics of oil spill behaviour, enhancing the ability to predict and mitigate the environmental impacts of such incidents. Self-designed software was applied to develop and modify previously established mathematical probabilistic models for predicting changes in the shape of the oil trajectory. First, a semi-Markov model of the process is constructed, and the oil thickness is analysed at the sea surface over time. Next, a stochastic-based procedure to forecast the horizontal movement and dispersion of an oil slick in diverse hydro-meteorological conditions considering a varying oil layer thickness is presented. This involves determining the trajectory and movement of a slick domain, which consists of an elliptical combination of domains undergoing temporal changes. By applying the procedure and program, a short-term forecast of the horizontal movement and dispersion of an oil slick provided its trajectory at the Bornholm Basin of the Baltic Sea within two days. The research results obtained are preliminary prediction results, although the approach considered in this paper can help responders understand the scope of the problem and mitigate the effects of environmental damage if the oil discharge reaches sensitive ecosystems. Finally, further perspectives of this research are given.

Time-compensated sun compass in juvenile sprat (Sprattus sprattus) reveals the onset of migratory readiness.

Sprat (Sprattus sprattus) is one of the most commercially exploited fish species in the Baltic Sea and expresses a pronounced seasonal migration pattern. Spawning takes place, among other places, in the Kiel Bight and Kiel Fjord in early summer. Juvenile sprat leave the nursery areas in late summer/early autumn to move to their feeding and overwintering grounds. What kind of orientation mechanisms sprat use for migration is not known yet. This study shows that juvenile sprat can use a time-compensated sun compass, heading towards the northeast, in the direction of their proposed overwintering grounds in Bornholm Basin. The sprats tested at the end of August oriented themselves in the predicted direction, whereas the sprats tested at the beginning of August only showed a random orientation. For the first time, this demonstrates the onset of migratory readiness in juvenile sprat, indicating the preparation for starting their migration.

Annual, seasonal and spatial differences in the growth rate of Baltic cod larvae and early juveniles in relation to zooplankton biomass fluctuations in 2006-2014

It is an acknowledged theory that a faster growth rate, determined by various environmental drivers, may boost the survival of larval and juvenile fish. In this study we examined the differences in the growth rate of larval and juvenile cod (age 2 – 136 d; SL: 4.1 – 39.2 mm) between the years 2006 and 2014, sub-areas of the Baltic Sea (Bornholm Basin, BB; Słupsk Furrow, SF; Gdańsk Basin, GB), and seasons (spring and summer). The average growth rate for all specimens was 0.25 mm/d, with significantly lower values between 2012 and 2014 than between 2006 and 2011. A reduction in zooplankton biomass, especially the large zooplankton fraction, was observed after the 2006–2008 period, which was related to the prevailing temperature conditions in the surface layers. The reduction in zooplankton biomass was accompanied by changes in the structure of zooplankton: less T. longicornis, C. hamatus, and Pseudocalanus spp., and more Acartia spp. The results suggest that the inter-annual differences observed in zooplankton biomass and structure are likely responsible for the observed reduction in the growth rate of cod larvae between 2012 and 2014 compared to the years between 2006 and 2011. The growth rate reduction could be one of the reasons for the decline in the recruitment of eastern Baltic cod between the years 2013 and 2015, after the high recruitment years of the period 2011-2012.

Acclimatizing laboratory-reared hatchling cod (Gadus morhua) to salinity conditions in the Baltic Sea

Decades of overfishing and unsustainable management, together with habitat degradation and eutrophication, depleted the cod (Gadus morhua) stocks in the Baltic Sea. Accompanying severe oxygen deficiency and decreased salinity in their spawning grounds restricted successful spawning today to the Bornholm basin, resulting in decreased recruitment. In order for the species to recover, several different measures are required and proposed, among others restocking. We therefore investigated the possibility of producing laboratory-reared cod larvae acclimatized to the current environment in the Baltic Sea. For this, cod were reared from newly fertilized eggs to non-feeding yolk-sac larvae, testing the effect of different salinity reduction treatments during early development on mortality, hatching success, and neutral buoyancy. The results show that a sudden ambient salinity decrease after hatching has no strong effect on survival or hatching (around 60% and 95%, respectively), while it decreased neutral buoyancy of larvae from 18 to minimum 12.5 psu. Lowest buoyancy was reached in treatments with a salinity change in the early egg stage. Gradual salinity decrease starting early in the egg stage yielded to significantly increased mortality and reduced hatching success, but also lowest buoyancy of 12 psu. We showed that a decrease of ambient salinity enables the production of yolk-sac cod larvae with reduced buoyancy, which are potentially better acclimatized to survive in current environmental conditions in the Baltic Sea.

Limited ventilation of the central Baltic Sea due to elevated oxygen consumption

The Baltic Sea is known as the world’s largest marine system suffering from accelerating, man-made hypoxia. Notably, despite the nutrient load reduction policy adopted in the 1980s, the oxygen conditions of the Baltic Sea’s deep waters are still worsening. This study disentangles oxygen and hydrogen sulfide sources and sinks using the results from the 3-dimensional coupled MOM-ERGOM numerical model and investigates ventilation of the deep central Baltic Sea by the 29 biggest oxygen inflows from 1948 to 2018 utilizing the element tagging technic. Everywhere across the central Baltic Sea, except in the Bornholm Basin, a shift in oxygen consumption from sediments to water column and a significant positive trend in hydrogen sulfide content were observed. The most notable changes happened in the northern and western Gotland basins. Mineralization of organic matter, both in the water column and sediments, was identified as the primary driver of the observed changes. A significant negative trend in the lifetime of inflowing oxygen was found everywhere in the central Baltic Sea. It leads to the reduced efficiency of natural ventilation of the central Baltic Sea via the saltwater inflows, especially in the northern and western Gotland basins.

Temporal and spatial changes in benthic invertebrate trophic networks along a taxonomic richness gradient.

Species interactions underlie most ecosystem functions and are important for understanding ecosystem changes. Representing one type of species interaction, trophic networks were constructed from biodiversity monitoring data and known trophic links to assess how ecosystems have changed over time. The Baltic Sea is subject to many anthropogenic pressures, and low species diversity makes it an ideal candidate for determining how pressures change food webs. In this study, we used benthic monitoring data for 20 years (1980–1989 and 2010–2019) from the Swedish coast of the Baltic Sea and Skagerrak to investigate changes in benthic invertebrate trophic interactions. We constructed food webs and calculated fundamental food web metrics evaluating network horizontal and vertical diversity, as well as stability that were compared over space and time. Our results show that the west coast of Sweden (Skagerrak) suffered a reduction in benthic invertebrate biodiversity by 32% between the 1980s and 2010s, and that the number of links, generality of predators, and vulnerability of prey have been significantly reduced. The other basins (Bothnian Sea, Baltic Proper, and Bornholm Basin) do not show any significant changes in species richness or consistent significant trends in any food web metrics investigated, demonstrating resilience at a lower species diversity. The decreased complexity of the Skagerrak food webs indicates vulnerability to further perturbations and pressures should be limited as much as possible to ensure continued ecosystem functions.

First SMOS Sea Surface Salinity dedicated products over the Baltic Sea

Abstract. This paper presents the first Soil Moisture and Ocean Salinity (SMOS) Sea Surface Salinity (SSS) dedicated products over the Baltic Sea. The SSS retrieval from L-band brightness temperature (TB) measurements over this basin is really challenging due to important technical issues, such as the land–sea and ice–sea contamination, the high contamination by radio-frequency interference (RFI) sources, the low sensitivity of L-band TB at SSS changes in cold waters, and the poor characterization of dielectric constant models for the low SSS range in the basin. For these reasons, exploratory research in the algorithms used from the level 0 up to level 4 has been required to develop these dedicated products. This work has been performed in the framework of the European Space Agency regional initiative Baltic+ Salinity Dynamics. Two Baltic+ SSS products have been generated for the period 2011–2019 and are freely distributed: the Level 3 (L3) product (daily generated 9 d maps in a 0.25∘ grid; https://doi.org/10.20350/digitalCSIC/13859, González-Gambau et al., 2021a) and the Level 4 (L4) product (daily maps in a 0.05∘ grid; https://doi.org/10.20350/digitalCSIC/13860, González-Gambau et al., 2021b)​​​​​​​, which are computed by applying multifractal fusion to L3 SSS with SST maps. The accuracy of L3 SSS products is typically around 0.7–0.8 psu. The L4 product has an improved spatiotemporal resolution with respect to the L3 and the accuracy is typically around 0.4 psu. Regions with the highest errors and limited coverage are located in Arkona and Bornholm basins and Gulfs of Finland and Riga. The impact assessment of Baltic+ SSS products has shown that they can help in the understanding of salinity dynamics in the basin. They complement the temporally and spatially very sparse in situ measurements, covering data gaps in the region, and they can also be useful for the validation of numerical models, particularly in areas where in situ data are very sparse.

Support for the hypothesis that growth of eastern Baltic cod is affected by mild hypoxia. A comment on Svedäng et al. (2022)

Abstract Stock biomass and size composition of eastern Baltic cod (EBC) has declined, with low recent stock productivity attributed to loss of major spawning areas, reduced growth rates, and possibly increased natural mortality. Svedäng et al. present valuable new data for evaluating the causes of these changes. However, contrary to their own conclusions, those new data actually provide both direct and indirect support for the hypothesis that the reduced growth rate observed may be due to a decline in ambient oxygen for small cod in the Bornholm Basin area (SD25). Their long time series of reproductive volume (RV) extends our knowledge of historic changes in the limits to spawning imposed by salinity and oxygen. However, the causes and consequences of such spawning area changes are well established. Therefore, this Comment only addresses the contentious debate about causes of decline in growth.

Salinity as a key control on the diazotrophic community composition in the southern Baltic Sea

Abstract. Over the next decade, the Baltic Sea is predicted to undergo severe changes including decreased salinity due to altered precipitation related to climate changes. This will likely impact the distribution and community composition of Baltic Sea dinitrogen-fixing (N2-fixing) microbes, among which heterocystous cyanobacteria are especially adapted to low salinities and may expand to waters with currently higher salinity, including the Danish Strait and Kattegat, while other high-salinity-adapted N2 fixers might decrease in abundance. In order to explore the impact of salinity on the distribution and activity of different diazotrophic clades, we followed the natural salinity gradient from the eastern Gotland and Bornholm basins through the Arkona Basin to the Kiel Bight and combined N2 fixation rate measurements with a molecular analysis of the diazotrophic community using the key functional marker gene for N2 fixation nifH, as well as the key functional marker genes anfD and vnfD, encoding for the two alternative nitrogenases. We detected N2 fixation rates between 0.7 and 6 nmol N L−1 d−1, and the diazotrophic community was dominated by the cyanobacterium related to Nodularia spumigena and the small unicellular, cosmopolitan cyanobacterium UCYN-A. Nodularia was present in gene abundances between 8.07 × 105 and 1.6 × 107 copies L−1 in waters with salinities of 10 and below, while UCYN-A reached gene abundances of up to 4.5 × 107 copies L−1 in waters with salinity above 10. Besides those two cyanobacterial diazotrophs, we found several clades of proteobacterial N2 fixers and alternative nitrogenase genes associated with Rhodopseudomonas palustris, a purple non-sulfur bacterium. Based on principal component analysis (PCA), salinity was identified as the primary parameter describing the diazotrophic distribution, while pH and temperature did not have a significant influence on the diazotrophic distribution. While this statistical analysis will need to be explored in direct experiments, it gives an indication for the future development of diazotrophy in a freshening Baltic Sea with UCYN-A retracting to more saline North Sea waters and heterocystous cyanobacteria expanding as salinity decreases.

Ten years of remote sensing and analyses of the Baltic Sea primary production (2010–2019)

The aim of the work is to present the possibilities of using advanced systems combining data obtained by remote sensing methods with data from other sources and bio-optical models for tracking and analyzing long-term changes in marine ecosystems. For this purpose, the 2010–2019 primary production data set provided by the SatBałtyk System, a platform that monitors the Baltic Sea based on satellite data, was used. As a result, the paper provides a quantitative description of the variability of the organic matter primary production (PP) in the Baltic Sea. In the individual months, the mean daily PP for the entire Baltic Sea, ranges from ca. 5 mgC m-2day-1 in winter (December and January) to over 700 mgC m-2day-1 in July. The overall annual PP of this basin ranges from 37 to 45 *106 tC year-1, but it is clearly noticeable that the yearly distributions of PP vary significantly from year to year over the 2010–2019 period and different regions. On the basis of the presented results for selected regions, it can be also seen that the maximum values of PP characteristic for the Baltic Sea in the summer months in the Gdańsk Basin can be observed from June to July. However, in the Bornholm Basin and the East Gotland Basin, PP does not peak until July and occasionally in August. The obtained results indicate a slight increase in the productivity of the Baltic Sea (ca. 11%) over the 10 years’ period and it also varies regionally.

Re-thinking the “ecological envelope” of Eastern Baltic cod (Gadus morhua): conditions for productivity, reproduction, and feeding over time

Abstract Hypoxia is presently seen as the principal driver behind the decline of the former dominating Eastern Baltic cod stock (EBC; Gadus morhua). It has been proposed that both worsening conditions for reproduction and lower individual growth, condition, and survival are linked to hypoxia. Here, we elucidate the ecological envelope of EBC in terms of salinity stratification, oxygen content, and benthic animal biomasses, and how it has affected EBC productivity over time. The spawning conditions started deteriorating in the Gotland Deep in the 1950s due to oxygen depletion. In contrast, in the Bornholm Basin, hydrographic conditions have remained unchanged over the last 60 years. Indeed, the current extent of both well-oxygenated areas and the frequency of hypoxia events do not differ substantially from periods with high EBC productivity in the 1970s–1980s. Furthermore, oxygenated and therefore potentially suitable feeding areas are abundant in all parts of the Baltic Sea, and our novel analysis provides no evidence of a reduction in benthic food sources for EBC over the last 30 years. We find that while reproduction failure is intricately linked to hydrographic dynamics, a relationship between the spread of hypoxia and the decline in EBC productivity during the last decades cannot be substantiated.

Intermediate plumes of low oxygen in the southeastern Baltic Sea

Closely spaced CTDO profiling on the pathway of saltwater flow in the southeastern Baltic Sea in August and December 2019 revealed low oxygen intermediate layers-plumes with dissolved oxygen concentration (DOC) less than 2 mg/l under the upper boundary of permanent halocline. At the same time, DOC in the underlying layers was higher and reached 5.0–5.3 mg/l. In order to test the hypotheses about the origin of the intermediate hypoxic plumes, numerical hydrodynamic modelling was performed for time intervals including the measurement periods. The model was validated against bottom current velocity instrumental measurements by a TCM (Tilt Current Meter) moored on the track of the CTDO profiling. Numerical experiments with Lagrangian particles of neutral buoyancy coupled with the operational circulation model showed that sub-halocline hypoxic intermediate layers in the southern part of the Eastern Gotland Basin and in the Gdańsk Basin could be formed as a result of intrusion into the underlying bottom layer of denser, moderately oxygenated water from the Słupsk Furrow, which replaced the old anoxic and hypoxic water of Gotland and Gdańsk origin. It is assumed that cyclonic eddy activity above the saltwater flow along sloping bottom caused fragmentation of the hypoxic intermediate layer into separate plumes. The hypoxic intermediate layers in the Słupsk Furrow, in accordance with the results of monitoring measurements and modelling the motion of particles of neutral buoyancy, could be formed in the Bornholm Basin and moved eastward over the Słupsk Sill.

A Regime Shift Toward a More Anoxic Environment in a Eutrophic Sea in Northern Europe

Dissolved oxygen in the sea is essential for marine fauna and biogeochemical processes. Decline in the sea water oxygen concentration is considered to be an effect of eutrophication, also exacerbated by climate change. The Baltic Sea is one of the most eutrophic seas in the world and is located in northern Europe. It is a vulnerable, brackish, semi-enclosed sea, suffering from high pressures from human activity. This leads to increased hypoxic and anoxic areas, which can be used as a measure of the environmental state. In the present study the extent of anoxic (O2 < 0 ml l–1) and hypoxic (O2 < 2 ml l–1) areas were estimated for the autumns in 1960–2019 using vertical profiles of observed oxygen concentrations in the Baltic proper and four sub-areas of the Baltic proper: the Bornholm Basin, the western, northern and eastern Gotland basins. From vertical profiles of observed salinity, the annual average of the halocline depths in the four sub-basins were estimated. The results imply regime shifts toward increased anoxic area extents in the Gotland basins around the turn of the 20th century. In autumn 2018, the extent of anoxic bottom areas in the Baltic Sea was record high since the start of the data series. During the later part of the studied period the depths of the halocline coincide with the depth of the hypoxia in the Gotland basins. This implies that in these basins a worst-case scenario for the extent of hypoxic areas seems to be reached.

Differences in Carbon Acquisition Could Explain Adaptive Responses in a Baltic Sea Pico-Phytoplankton

Pico-phytoplankton have ample scope to react to environmental change. Nevertheless, we know little about the underlying physiological mechanisms that govern how evolutionary history may affect short-term responses to environmental change. We investigated growth rates and carbon uptake related traits at 15° and 22°C and at different times during the microbial growth curve (lag phase, mid and late exponential) of eight novel strains of Ostreococcus sp. (ca. 1 μm). The strains were isolated from two distinct regions of the Baltic Sea differing in salinity and temperature as well as variability therein from North-East (Bornholm Basin) to South-West (Kiel area). Strains from Kiel area had ca. 10% higher growth rates on average and showed more variation between strains compared to strains from the Bornholm Basin. While biomass increased throughout the experiment in both temperature, CUE (carbon use efficiency, indicative of photosynthetically derived carbon available for growth) was too low to explain positive growth throughout the entire growth curve at 15°C and during the early stages at 22°C. Throughout the growth curve CUE then increased enough to sustain growth, but only at 22°C. Consequently, we then tested whether Ostreococcus use organic carbon to supplement growth when light is not a limiting factor. We show that Ostreococcus qualitatively modulate their potential to grow on organic carbon sources throughout a single growth curve. Based on the differences between CUE and a potential to grow on organic carbon, we postulate a shift in carbon acquisition between inorganic and organic sources in Ostreococcus sp. with potential implications on ecological dynamics within microbial communities.