Bornholm Basin Research Articles

Spatio-temporal dynamics of cod nursery areas in the Baltic Sea

In this study the drift of eastern Baltic cod larvae and juveniles spawned within the historical eastern Baltic cod spawning grounds was investigated by detailed drift model simulations for the years 1971–2010, to examine the spatio-temporal dynamics of environmental suitability in the nursery areas of juvenile cod settlement. The results of the long-term model scenario runs, where juvenile cod were treated as simulated passively drifting particles, enabled us to find strong indications for long-term variations of settlement and potentially the reproduction success of the historically important eastern Baltic cod nursery grounds. Only low proportions of juveniles hatched in the Arkona Basin and in the Gotland Basin were able to settle in their respective spawning ground. Ocean currents were either unfavorable for the juveniles to reach suitable habitats or transported the juveniles to nursery grounds of neighboring subdivisions. Juveniles which hatched in the Bornholm Basin were most widely dispersed and showed the highest settlement probability, while the second highest settlement probability and horizontal dispersal was observed for juveniles originating from the Gdansk Deep. In a long-term perspective, wind-driven transport of larvae/juveniles positively affected the settlement success predominately in the Bornholm Basin and in the Bay of Gdansk. The Bornholm Basin has the potential to contribute on average 54% and the Bay of Gdansk 11% to the production of juveniles in the Baltic Sea. Furthermore, transport of juveniles surviving to the age of settlement with origin in the Bornholm Basin contributed on average 13 and 11% to the total settlement in the Arkona Basin and in the Gdansk Deep, respectively. The time-series of the simulated occupied juvenile cod habitat in the Bornholm Basin and in the Gdansk Deep showed a similar declining trend as the Fulton’s K condition factor of demersal 1-group cod, which may confirm the importance of oxygen-dependent habitat availability and its effect on density dependence as a process relevant for recruitment success.

Tracer Transport and Exchange Processes in the Baltic Sea 2000-2009

In this work we study the exchange processes in the Baltic Sea during a 10-year period (2000-2009). The study is based on numerical simulations of the hydrodynamics that are verified by salinity and temperature data. A particle tracking approach is used for characterising transport and exchange processes. Inert particles are released at different rates ranging from 100 to 5000 particles at durations ranging from 10 days to 10 years with the purpose of quantifying the inter-exchange processes between the Baltic Sea sub-basins and to estimate arrival times and pathways of particles released from the major rivers. The Gotland basins are the major contributors to the baroclinic exchange process in the Baltic Sea. There is evidence in support of notable mixing of the deeper waters of the Western Gotland Basin with the sub-surface waters of the Gulf of Finland. The transport processes in the Bornholm and Arkona basins are both advective and diffusive driven by the interacting flows of the northern basins of the Baltic Sea and the North Sea. The various sills and channels play a significant role in controlling the exchanging processes of the sub basins of the Baltic Sea. Two relative measures (indicators) are introduced for quantifying internal exchange between major basins and internal distribution between basins of tracers discharged by eleven major rivers. The Sea of Bothnia is found to be the major collector basin and the Bornholm Basin the major diffusor basin of the Baltic Sea. The surface waters of the rivers mix internally with the mixing and pycnocline layers. The Gulfs of Finland and Riga, and the islands of the Archipelago Sea are probably the most vulnerable regions from an ecological point of view. The worst scenario for river-borne pollutants is a combination of short travel time and maximum travelled path; rivers of Vistula and Oder fall into this category.

The distribution of heavy metals and 137Cs in the central part of the Polish maritime zone (Baltic Sea) – the area selected for wind farm acquisition

Disturbances of the marine sediments that may be induced by offshore wind energy (OWE) construction and functioning may cause reintroduction of contaminants accumulated in the sea bottom over last century. It is thus very important to check in which offshore region of the Polish coast the littlest consequences of the OWE construction may be expected. The area of 9000 km2 in the central part of the Polish economic zone was selected for sediments sampling. In total, sediments from 46 stations were collected by gravity twin corer or geo-vibro corer. Heavy metal (Hg, Pb, Cd, As, Zn, Cu, Cr, Ni, Fe, Mn) concentrations were measured by AAS and ICP-MS while 137 Cs activity concentrations were measured by gamma spectrometer. Sediment accumulation rates were quantified by 210Pb geochronology at 7 stations to follow the history of contaminants accumulation. Study areas located between isobaths of 20 m and 40 m were characterized by the lowest concentration of heavy metals, often below natural environmental background (e.g. Pb = 6.7 − 11.4 μg·g−1, As = 0.3 − 8.4 μg·g−1). The largest contamination by heavy metals (e.g. Pb reached 86.1 μg·g−1 while As = 21.0 μg·g−1) was visible at the outer part of Bornholm Basin and in Słupsk Furrow, the deepest studied regions. Highest accumulation of heavy metals in this area is connected to large fraction of pelite sediments and large proportion of organic matter at those stations. It may be concluded that shallow sandy sediments of the Southern Baltic are unpolluted and appropriate for OWE construction. Deeper sea regions e.g Słupsk Furrow and Bornholm Basin may be regarded as moderately polluted and therefore not recommended for OWE acquisition.

Survival and dispersal variability of pelagic eggs and yolk-sac larvae of central and eastern baltic flounder (Platichthys flesus): application of biophysical models

A hydrodynamic model coupled with a Lagrangian particle tracking technique was utilized to simulate spatially and temporally resolved long-term environmentally related (i) size of habitat suitable for reproduction, (ii) egg/yolk-sac larval survival, (iii) separation of causes of mortality, and (iv) connectivity between spawning areas of Baltic flounder with pelagic eggs. Information on reproduction habitat requirements and mortality sources were obtained from field or laboratory studies. In our modelling study we only quantified physical processes generating heterogeneity in spatial distribution of eggs and yolk-sac larvae, as e.g. predation is not accounted for. The spatial extent of eggs and larvae represented as modelled particles is primarily determined by oxygen and salinity conditions. The reproduction habitat most suitable was determined for the Gdansk Deep, followed by the Bornholm Basin. Relatively low habitat suitability was obtained for the Arkona Basin and the Gotland Basin. The model runs also showed yolk-sac larval survival to be to a large extent affected by sedimentation. Eggs initially released in the Arkona Basin and Bornholm Basin are strongly affected by sedimentation compared with those released in the Gdansk Deep and Gotland Basin. Highest relative survival of eggs occurred in the Gdansk Deep and in the Bornholm Basin. Relatively low survival rates in the Gotland Basin were attributable to oxygen-dependent mortality. Oxygen content had almost no impact on survival in the Arkona Basin. For all spawning areas mortality caused by lethally low temperatures was only evident after severe winters. Buoyancy of eggs and yolk-sac larvae in relation to topographic features appear as a barrier for the transport of eggs and yolk-sac larvae and potentially limits the connectivity of early life stages between the different spawning areas.

Estimating the effective nitrogen import: An example for the North Sea‐Baltic Sea boundary

AbstractSemienclosed water bodies such as the Baltic Sea are prone to eutrophication problems. If local nutrient abatement measures are taken to tackle these problems, their success may be limited if a strong nutrient exchange with the adjacent waters exists. The quantification of this exchange is therefore essential to estimate its impact on the ecosystem status. At the example of the Baltic Sea and the North Sea, we illustrate that neither gross transports nor net transports of nutrients have a strong informative value in this context. Instead, we define an “effective import” as the import of nutrients which have not been inside the Baltic Sea before and estimate it in an ecological model with a nutrient‐tagging technique. This effective import of bioreactive nitrogen from the Skagerrak to the Kattegat amounts to 103 kt/yr; from Kattegat to Belt Sea it is 54 kt/yr. The nitrogen exchange is therefore 30% stronger than other estimates, e.g., based on import in the deep water, suggest. An isolated view on the Baltic Sea and the North Sea in terms of eutrophication, as it is practiced in management today, is therefore questionable. Nitrogen imported from the North Sea typically spreads eastward up to the Bornholm Basin but can be transported into the deep waters of the Gotland Basin during Major Baltic Inflows in a significant amount.

Processes and factors influencing the through-flow of new deepwater in the Bornholm Basin

Summary This paper is based on the idea that the hydrographical conditions in the Bornholm Basin, and any other basin, can be understood from knowledge of general hydromechanical principles and basin-specific factors. Published results on the variability of the vertical stratification are shown and discussed. Such analyses demonstrate the residence time of water at different depth levels. Different modes of currents forced by winds and by stratification gradients at open vertical boundaries are presented. Vertical mixing is discussed and published results for the Bornholm Basin are shown. An experiment demonstrates that the diffusive properties of the enclosed basin, i.e. below the sill depth of the Slupsk Furrow, can be computed quite well from the horizontal mean vertical diffusivity obtained from historical hydrographical observations. A published two decades long simulation of the vertical stratification shows that the through flow and modification of new deepwater in the Bornholm Basin can be well described based on existing knowledge regarding crucial hydromechanical processes. It also suggests, indirectly, that there should be a weak anticyclonic circulation above the sill depth, which is supported by current measurements.

The Bornholm Basin, southern Scandinavia: a complex history from Late Cretaceous structural developments to recent sedimentation

The Bornholm Basin has experienced a complex history linked closely to the structural development of the Tornquist Zone strike‐slip fault system. The latest major tectonic development was late Cretaceous and early Tertiary Rønne Graben inversion caused by dextral transpressional strike‐slip movements focused around rigid blocks such as Bornholm. By applying the strike‐slip concept, it is possible to show that the Rønne Graben inversion push‐up structure was created by restraining left step‐over. The fault system continues northwest and north of Bornholm where the strike‐slip movement resulted in right step‐over and the formation of a pull‐apart basin: the northern part of the Bornholm Basin. Detailed seismo‐acoustic and sediment core studies show that, despite a rather uniform present bathymetry in the central part of the Bornholm Basin, the Quaternary deposits mirror the pull‐apart basin synsedimentary depositional pattern with depocentres close to the major normal faults, bounding the Christiansø Ridge. IODP Site M0065 penetrated a secondary fault zone and provided detailed information on neotectonic events and on environmental changes during the Holocene. The generally rather homogeneous sediments in the cores indicate a dominance of oxic conditions in the Bornholm Basin during the Holocene. However, strong internal seismic reflectors, indirectly reflecting variations in the sediment density and the reflection coefficient, were caused by a shift to stronger laminated, organic‐rich sediments that were deposited during the Medieval Warm Period (MwP). The variable depth positions of the MwP‐related strong reflectors indicate large differences in the spatial and temporal sedimentation pattern in the Bornholm Basin. These differences in the depositional environment may have been caused by local neotectonic events in the basin.

The inflow in the Baltic Proper as recorded in January–February 2015

Summary Inflowing saline waters of the Major Baltic Inflow (MBI) in 2014 were recorded in the Baltic Proper in January 2015. After 12 years of stagnation, this inflow brought highly saline (about 20) waters into the Bornholm Basin. As in the previous inflow in January 2003, saltwater moved in the near-bottom layer with a current speed of approx. 25 cm s−1. This paper presents data collected in January and February 2015 and compares them to earlier records from 2000 to 2014.

Spawning areas of eastern Baltic cod revisited: Using hydrodynamic modelling to reveal spawning habitat suitability, egg survival probability, and connectivity patterns

In the highly variable environment of the Baltic Sea two genetically distinct cod stocks exist, one west of the island of Bornholm, which is referred to as the western stock, and one to the east of Bornholm, the eastern stock. A hydrodynamic model combined with a Lagrangian particle tracking technique was utilised to provide spatially and temporally resolved long-term information on environmentally-related (i) spawning habitat size, (ii) egg/yolk-sac larval survival, (iii) separation of causes of mortality, and (iv) connectivity between spawning areas of eastern Baltic cod. Simulations were performed to quantify processes generating heterogeneity in spatial distribution of cod eggs and yolk sac larvae up to the first-feeding stage. The spatial extent of cod eggs represented as virtual drifters is primarily determined by oxygen and salinity conditions at spawning, which define the habitat requirement to which cod’s physiology is suited for egg development. The highest habitat suitability occurred in the Bornholm Basin, followed by the Gdansk Deep, while relatively low habitat suitability was obtained for the Arkona and the Gotland Basin. During drift egg and yolk sac larval survival is to a large extent affected by sedimentation. Eggs initially released in the western spawning grounds (Arkona and Bornholm Basin) were more affected by sedimentation than those released in the eastern spawning grounds (Gdansk Deep and Gotland Basin). Highest relative survival of eastern Baltic cod eggs occurred in the Bornholm Basin, with a pronounced decrease towards the Gdansk Deep and the Gotland Basin. Relatively low survival rates in the Gdansk Deep and in the Gotland Basin were attributable to oxygen-dependent mortality. Low oxygen content had almost no impact on survival in the Arkona Basin. For all spawning areas temperature dependent mortality was only evident after severe winters. Egg buoyancy in relation to topographic features like bottom sills and strong bottom slopes could appear as a barrier for the transport of Baltic cod eggs and yolk sac larvae and could potentially limit the connectivity of Baltic cod early life stages between the different basins in the western and eastern Baltic Sea. The possibility of an eastward directed transport up to the first-feeding larval stage exists only for eggs and yolk sac larvae at high buoyancy levels, suggesting that dispersal of early life stages between these spawning areas is limited.

On the nonlinear internal waves propagating in an inhomogeneous shallow sea

Summary A concept of conservation of energy flux for the internal waves propagating in an inhomogeneous shallow water is examined. The emphasis is put on an application of solution of the Korteweg–de Vries (KdV) equation in a prescribed form of the cnoidal and solitary waves. Numerical simulations were applied for the southern Baltic Sea, along a transect from the Bornholm Basin, through the Slupsk Sill and Slupsk Furrow to the Gdansk Basin. Three-layer density structure typical for the Baltic Sea has been considered. An increase of wave height and decrease of phase speed with shallowing water depth was clearly demonstrated. The internal wave dynamics on both sides of the Slupsk Sill was found to be different due to different vertical density stratification in these areas. The bottom friction has only negligible influence on dynamics of internal waves, while shearing instability may be important only for very high waves. Area of possible instability, expressed in terms of the Richardson number Ri , is very small, and located within the non-uniform density layer, close to the interface with upper uniform layer. Kinematic breaking criteria have been examined and critical internal wave heights have been determined.

On the Role of Temperature and Salinity Data Assimilation to Constrain a Coupled Physical–Biogeochemical Model in the Baltic Sea

AbstractA three-dimensional variational data assimilation (3DVAR) method is implemented in a coupled physical–biogeochemical (CPB) model in the Baltic Sea. This study carries out a 10-yr assimilation experiment with satellite sea surface temperature (SST) and observed in situ temperature (T) and salinity (S) profiles. The impact of the assimilation is assessed with the focus on how the biogeochemical model responds to the improved hydrodynamics. The assimilation of temperature and salinity data yields considerable improvements in the physical model. On a basin scale, the mean bias of SST, T, S, and mixed layer depth (MLD) is decreased by 0.18°C (57%), 0.31°C (49%), 0.34 psu (43%), and 1.8 m (43%), respectively. More importantly, the biogeochemical simulation is improved in response to the physical data assimilation. Compared with in situ observations, the mean biases of chlorophyll a (Chl), dissolved inorganic nitrogen (DIN) and phosphorus (DIP) are decreased by 0.09 mg m−3 (15.5%), 0.19 mmol m−3 (9%), and 0.15 mmol m−3 (23%). Physical data assimilation also improves the simulated variability of Chl, DIN, and DIP and their correlations with observation. Compared with satellite observations, the mean bias of surface chlorophyll is reduced by 0.10–0.32 mg m−3 especially in the Skagerrak–Kattegat area and Bornholm basin. The decrease of total Chl change is caused by different mechanisms for winter and summer. While the deepened mixed layer acts as a dilution factor in winter, strengthened stratification agrees well with the decrease of chlorophyll in summer. In the vertical, relatively large changes of DIN and DIP occur below 60 m, which corresponds to the mean permanent halocline depth (~60–80 m) of the Baltic Sea.

Acute toxicity of sea-dumped chemical munitions: luminating the environmental toxicity of legacy compounds

As a result of the disarmament of Germany after the Second World War, 65,000 tons of chemical munitions were dumped in the Baltic Sea. Approximately 13,000 tons containing chemical warfare agents (CWAs) of which 11,000 tons were dumped in the Bornholm Basin east of Bornholm. This paper addresses the ecotoxicity of compounds actually present in the Bornholm dumpsite by obtaining novel acute ecotoxicity data. EC50 values were successfully obtained for 12 CWAs from acute tests using Allivibrio fischeri (Microtox™). The three most toxic compounds were α-chloroacetophenone, 2-chlorovinylarsonic acid and 1,2,5-trithiepane having EC50 values of 11.20, 31.20 and 1170 μg L−1, respectively. A. fischeri demonstrated hormesis when exposed to triphenylarsine and triphenylarsine oxide at concentrations of 100 and 50 mg L−1, respectively. Four different mixtures were assessed including compounds which were dissolvable; a mixture of sulphur mustard degradation products, a mixture of the three most toxic sulphur mustard compounds, a mixture of organoarsenical degradation products and a mixture of all compounds. The mixtures deviate by a factor of 1.5–2.5 from the prediction of the concentration addition model and hence, the mixtures demonstrate no sign of synergism or antagonism. The compounds presented in this study are mainly CWA.

Frequency dependence in seismoacoustic imaging of shallow free gas due to gas bubble resonance

AbstractShallow free gas is investigated in seismoacoustic data in 10 frequency bands covering a frequency range between 0.2 and 43 kHz. At the edge of a gassy patch in the Bornholm Basin (Baltic Sea), compressional wave attenuation caused by free gas is estimated from reflection amplitudes beneath the gassy sediment layer. Imaging of shallow free gas is considerably influenced by gas bubble resonance, because in the resonance frequency range attenuation is significantly increased. At the resonance frequency of the largest bubbles between 3 and 5 kHz, high scattering causes complete acoustic blanking beneath the top of the gassy sediment layer. In the wider resonance frequency range between 3 and 15 kHz, the effect of smaller bubbles becomes dominant and the attenuation slightly decreases. This allows acoustic waves to be transmitted and reflections can be observed beneath the gassy sediment layer for higher frequencies. Above resonance beginning at ∼19 kHz, attenuation is low and the presence of free gas can be inferred from the decreased reflection amplitudes beneath the gassy layer. Below the resonance frequency range (<1 kHz), attenuation is generally very low and not dependent on frequency. Using the geoacoustic model of Anderson and Hampton, the observed frequency boundaries suggest gas bubble sizes between 1 and 4–6 mm, and gas volume fractions up to 0.02% in a ∼2 m thick sediment layer, whose upper boundary is the gas front. With the multifrequency acoustic approach and the Anderson and Hampton model, quantification of free gas in shallow marine environments is possible if the measurement frequency range allows the identification of the resonance frequency peak. The method presented is limited to places with only moderate attenuation, where the amplitudes of a reflection can be analyzed beneath the gassy sediment layer.

Observations of near-bottom currents in Bornholm Basin, Slupsk Furrow and Gdansk Deep

Dense bottom currents are responsible for transport of the salty inflow waters from the North Sea driving ventilation and renewal of Baltic deep waters. This study characterises dense currents in three deep locations of the Baltic Proper: Bornholm Basin (BB), Gdansk Basin (GB) and Slupsk Furrow (SF). These locations are of fundamental importance for the transport and pollution associated with chemical munitions deposited in BB and GB after 2nd World War. Of further importance the sub-basins are situated along the pathway of dense inflowing water.Current velocities were measured in the majority of the water column during regular cruises of r/v Oceania and r/v Baltica in 2001–2012 (38 cruises) by 307kHz vessel mounted (VM), downlooking ADCP. Additionally, the high-resolution CTD and oxygen profiles were collected. Three moorings measured current velocity profiles in SF and GB over the summer 2012. In addition, temperature, salinity, oxygen and turbidity were measured at about 1m above the bottom in GB. The results showed that mean current speed across the Baltic Proper was around 12cms−1 and the stronger flow was characteristic to the regions located above the sills, in the Bornholm and Slupsk Channels, reaching on average about 20cms−1. The results suggest that these regions are important for the inflow of saline waters into the eastern Baltic and are the areas of intense vertical mixing. The VM ADCP observations indicate that the average near-bottom flow across the basin can reach 35±6cms−1. The mooring observations also showed similar near-bottom flow velocities. However, they showed that the increased speed of the near-bottom layer occurred frequently in SF and GB during short time periods lasting for about few to several days or 10–20% of time. The observations showed that the bottom mixed layer occupies at least 10% of the water column and the turbulent mixing induced by near-bottom currents is likely to produce sediment resuspension and transport within the layer in all three sub-basins. The turbidity measurements, performed for 5-month-long time period over the summer 2012 in GB show that increased sediment resuspension is associated with a faster near-bottom flow.

Consequences of artificial deepwater ventilation in the Bornholm Basin for oxygen conditions, cod reproduction and benthic biomass – a model study

Abstract. We develop and use a circulation model to estimate hydrographical and ecological changes in the isolated basin water of the Bornholm Basin. By pumping well-oxygenated so-called winter water to the greatest depth, where it is forced to mix with the resident water, the rate of deepwater density reduction increases as well as the frequency of intrusions of new oxygen-rich deepwater. We show that pumping 1000 m3 s−1 should increase the rates of water exchange and oxygen supply by 2.5 and 3 times, respectively. The CRV (cod reproduction volume), the volume of water in the isolated basin meeting the requirements for successful cod reproduction (S > 11, O2 > 2 mL L−1), should every year be greater than 54 km3, which is an immense improvement, since it has been much less in certain years. Anoxic bottoms should no longer occur in the basin, and hypoxic events will become rare. This should permit extensive colonization of fauna on the earlier periodically anoxic bottoms. Increased biomass of benthic fauna should also mean increased food supply to economically valuable demersal fish like cod and flatfish. In addition, re-oxygenation of the sediments should lead to increased phosphorus retention by the sediments.

Environmental effects on sprat (Sprattus sprattus) physiology and growth at the distribution frontier: A bioenergetic modelling approach

Sprat, Sprattus sprattus, is a small pelagic fish species with a wide distribution along the European continental shelf, the Mediterranean Sea, the Black Sea and the Baltic Sea. The Baltic Sea is the coldest area of the species distribution range. Even here, sprat is still numerous and holds a key role in the ecosystem. However, the population is sensitive to small changes in environmental conditions. In this paper, we set up a bioenergetic model of Baltic sprat and evaluate how variability and seasonal changes in the physical environment affects the physiology and growth of individual sprat. The model is dynamic and seasonally resolved. It is parameterised based on sprat from the Bornholm Basin. Our model results suggest that the optimal temperature for sprat growth is 17.5°C, which is only observed in the Central Baltic Sea in the top water layer during a short period in summer. During 4½ winter months individual sprat do not grow and utilize stored energy from the previous growth season. We analysed the maximum attainable individual body size as a function of temperature. The model predicted reduced maximum body sizes with increasing temperature. The model can be used for studying climate change scenarios on individual growth, egg production and condition of Baltic sprat.

Estimating the free gas content in Baltic Sea sediments using compressional wave velocity from marine seismic data

AbstractA 2‐D high‐resolution velocity field was obtained from marine seismic data to quantify free gas content in shallow muddy sediments at in situ pressure and temperature. The velocities were acquired applying Migration Velocity Analysis on prestack time‐migrated data. Compressional wave velocities are highly sensitive to free gas as very small amounts of gas can cause a significant decrease in the medium velocity. The analyzed profile crosses a depression filled with organic‐rich Holocene mud in the Bornholm Basin, Baltic Sea. The interval velocity field reveals two low‐velocity patches, which extend from the reversed polarity reflections marking the top of the gassy sediment layer down to the base of the Holocene mud. Average interval velocities within the gassy mud are lower than the seafloor migration velocity by up to ∼500 m/s. This decrease, using a geoacoustic model, is caused by an average 0.046% gas volume fraction. The interval velocities in individual cells of the velocity field are reduced to ∼200 m/s predicting up to 3.4% gas content. The velocity field is limited in resolution due to velocity determination at and between reflections; however, together with the stratigraphic interpretation, geological units containing free gas could be identified. Shallow gas occurs vertically throughout most of the Holocene mud in the gassy area. Comparison with biogeochemical studies at other Baltic Sea sites suggests that the distribution of free gas is likely to be patchy in the sediment, but the gas concentration may peak below the sulfate‐methane transition zone and gradually decrease below.

Impact of saltwater inflows on phosphorus cycling and eutrophication in the Baltic Sea: a 3D model study

The impact of dense saltwater inflows on the phosphorus dynamics in the Baltic Sea is studied from tracer experiments with a three-dimensional physical model. Model simulations showed that the coasts of the North West Gotland Basin and the Gulf of Finland, the Estonian coast in the East Gotland Basin are regions where tracers from below the halocline are primarily lifted up above the halocline. After 1 yr tracers are accumulated at the surface along the Swedish east coast and at the western and southern sides of Gotland. Elevated concentrations are also found east and southeast of Gotland, in the northern Bornholm Basin and in the central parts of the East Gotland Basin. The annual supplies of phosphorus from the deeper waters to the productive surface layers are estimated to be of the same order of magnitude as the waterborne inputs of phosphorus to the entire Baltic Sea. The model results suggest that regionally the impact of these nutrients may be quite large, and the largest regional increases in surface concentrations are found after large inflows. However, the overall direct impact of major Baltic inflows on the annual uplift of nutrients from below the halocline to the surface waters is small because vertical transports are comparably large also during periods without major inflows. Our model results suggest that phosphorus released from the sediments between 60 and 100 m depth in the East Gotland Basin contributes to the eutrophication, especially in the coastal regions of the eastern Baltic Proper.

Seismo-acoustic signatures of shallow free gas in the Bornholm Basin, Baltic Sea

Research on gas-charged sediments is generally based on identification and mapping with hydroacoustic methods, and seismic sections are typically verbally described. The terminology, however, does often not properly account for different instrumentation, as the seismic response of gas-charged sediments is frequency-dependent, and seismic appearance may vary with the instrumentation used. We therefore combine and compare the expressions of shallow gas in high frequency Parasound data and lower frequency multichannel seismic data for the Bornholm Basin, Baltic Sea, to generalize the descriptional approach and to indicate distinct imaging differences. Gas bubbles strongly scatter the acoustic energy of a high frequency signal, shallow gas appears therefore in Parasound data as a cloud of point scatterers and acoustic blanking. The seismic waves undergo reflection from the gassy sediment, but with sufficient transmission, and the top of the gassy sediment appears as a reversed polarity reflector. In seismic data, free gas can modify the seafloor reflection in different ways depending on the proximity of the gas front to the seafloor. High reflectivity at this interface causes reverberation of the water bottom reflector. Gas signatures also reveal relative measures about the amount and concentration of gas bubbles, distribution of bubble sizes, and the depth distribution of gas.

Seasonal variation of methane in the water column of Arkona and Bornholm Basin, western Baltic Sea

Methane and the hydrographic parameters temperature, salinity and oxygen (T, Sal, O2) were surveyed at several stations and along selected transects in the Arkona and Bornholm Basin, western Baltic Sea, between 2009 and 2012. The methane distribution in the two adjacent basins show annually reoccurring as well as seasonal variations, governed by stratification of the water column during the summer period, enhanced vertical mixing during winter and the frequent inflow of oxygen-rich saline water from the North Sea. The Arkona Basin is characterized by low methane concentrations in the surface water with increasing values towards the bottom water layer. Elevated methane concentrations were found in the bottom layer during the summer period. The Bornholm Basin also shows low methane concentrations in the surface water, but high values in the bottom water layer throughout the year. For anoxic conditions, often prevailing in the Bornholm Basin in summer, a positive correlation between methane and hydrogen sulfide concentrations was observed. Strong depletion in the stable isotopic ratio of methane in the deeper waters of the Bornholm Basin reveals effective oxidation processes. The midwater region from 50 to 70m water depth in the Bornholm Basin is characterized by a methane-enriched water layer, persistent throughout the survey period. High-resolution hydrographic modelling of the physical driving forces suggest this finding to be caused by intrusion of methane-rich waters originating from the Arkona Basin into the water column of the Bornholm Basin and is shown to be a powerful tool for the interpretation of the development of the methane distribution in space and time.