The north-eastern part of Germany is characterized by a generally flat topography consisting of unconsolidated glacial sediments, humid climate conditions, urbanized cities, and often agricultural land use in the catchment basin. The present study was carried out in the WB (Figure 1), which is a coastal embayment in the southern BS, separated from the open BS by two shallow banks (Prena, 1995). Sediments in the bay are composed of quaternary glaciofluvial sands and intercalated tills (Schafmeister and Darsow, 2004). Maximum water depths of 10 m are in the western part of WB where muddy sediments are found, with sedimentary organic matter of up to 10% (Prena, 1995). Sandy sediments occur in the shallower areas.

The port of the town of Wismar is located on the east side of the bay, and ship navigation to the open BS is via an about 27 km long channel (Figure 1) (VSW, 2018).

The surrounding area of WB has three, each approximately 10 m thick aquifers in glaciofluvial sands of Saalian and Weichselian ages. The uppermost and partly phreatic aquifer is separated from the deeper aquifer by up to 20 m thick glacial till layers, which locally pinch out. Hydraulic gradients of up to 0.5% are slightly steeper than reported for other NE German areas (Figure 1, Schafmeister and Darsow, 2004, Jordan and Weder, 1995; Jurasinski et al., 2018).

Based on the regional flow dynamics, ground water is entering the WB area from 3 hydraulic height maxima, situated at least 10 km away from the WB (Hennig and Hilgert, 2007). Within the catchment area (about 200 km²), a lake Mühlenteich and other small streams act as routes of surface water flow into the WB.

The Mecklenburg Western-Pomeranian mean annual ground water recharge is about 123 mm/a, estimated by Hilgert (2009) and Hennig and Hilgert (2007). In combination with a mass balance approach, this leads to a rough daily SGD estimation of 60.000 m³ to the WB (Schafmeister and Darsow, 2004). Ground water recharge rates in the western part of the WB are lower compared to the SE part (Hilgert, 2009; LUNG, 2009).

Ship and boat based fieldwork took place during the spring and summer of 2019. Meteoric water inputs to WB during this period made up 6% of the annual precipitation 548 mm - Germany Weather Station - Wismar (DWD, 2020).

No large rivers drain into WB, but streams such as the Wallenstein located in the eastern part of the coast and the Tarnewitzer on the western part contribute to the fresh water inputs. The average discharge of both streams based on 10-years data are 0.76 and 0.39 m³ s⁻¹ respectively (Stalu-MV, 2019). In addition to these streams, smaller ones are also present, however, no hydrologic data are available for them.

Sampling in the WB was performed using RV LITTORINA (L19-06) between 20 and 25 May, 2019. Surface water samples were collected at 47 sites spanning the whole bay (Figure 1), and additional bottom water samples at four sites (Supplementary Figure S1). For sampling in shallow waters (<5 m) a rubber boat was used (20 sites). At six sites, short sediment cores were retrieved for pore water and sediment analyses (Figure 1). The coordinates of all sites in the WB are compiled in Supplementary Table S1.

At sites exceeding water depths >3 m, CTD profiles were obtained and surface water samples (1 m) were sampled using a submersible pump. The water was pumped through a filter cartridge (1 µm pore size) into 100 L barrels. Water subsamples were taken via syringe and filtered (0.45 µm, cellulose acetate disposable filters, Carl Roth) for analyses of major and trace elements, dissolved inorganic carbon (DIC), δ¹³C_DIC, and stable isotopes of water (δ¹⁸O and δ²H). Samples for DIC and δ¹³C_DIC were filled without headspace into 12 ml Exetainer^® tubes, previously cleaned with 2% HNO₃ washed, dried, and pre-filled with 100 µl saturated HgCl₂ solution. Samples for major and trace element analysis were filled into acid cleaned 2 ml reaction tubes and acidified with concentrated HNO₃ to 2 vol.% and samples for δ¹⁸O and δ²H analyses were collected in 1.5 ml vials glass sealed with a PTFE-coated septum cap. All samples were stored dark and cool until further analyses.

Water samples in the barrels were pumped, using a submersible pump through manganese-coated acrylic fibers at a flow rate of 1 L min⁻¹ to quantitatively extract radium (Ra) isotopes. The fibers were washed to remove salts and partly dried for further measurements.

Short sediment cores were collected at six different sites in the WB (Figure 1) using a Rumohr-Lot (60 cm length, inner diameter: 10 cm). For pore water sampling, Rhizons (Rhizosphere Research Products, 0.12 µm pore size (Seeberg-Elverfeldt et al., 2005)) were inserted in pre-drilled holes of the plastic liner and attached to 10 ml plastic syringes. Salinity and pH of pore waters were measured immediately after recovery using a refractometer and hand-held pH meter (Schott handylab pH meter 11—calibrated with Mettler Toledo Buffer solutions), respectively. Pore water was fixed with 5% Zn-acetate solution for later analysis of total sulphide (H₂S) concentrations. Additional pore water samples were taken for major and trace elements, DIC, δ¹⁸O, and δ²H of water, and preserved and stored as described above. Parallel sediment cores were (sites 10, 11, 15, 20, and 31), sliced at 2–4 cm intervals for geochemical analyses of sediments. Sediments at site 2 were sampled on the same core after pore water extraction. Sediment aliquots were transferred into 50 ml centrifuge tubes (Sarstedt) and kept frozen (−20°C) until further preparation.

Between July and August 2019 along the shoreline pore water samples were collected at five different sites in 0.4–1.8 m depth using push-point lances (MHE products) (Figure 1). In addition, four streams draining to the WB were sampled at the mixing zone with BS water. For major and trace elements, and stable isotopes analyses using the same methods as described for surface water of WB. In addition, samples for chloride (Cl) analysis were filled into 500 ml plastic bottles. pH, conductivity, and temperature were measured in situ using Schott handylab pH 11 and Schott handylab LF 11 devices. For Ra isotope measurements, 25 L of pore water was collected using a peristaltic pump, while 60 L of stream surface waters were collected and filtered through a 25 µm filter into pre-cleaned gallon containers for further filtration through the Mn fibers.

To identify the potential impact of SGD on coastal waters a pre-survey of Ra isotope distribution in surface water was carried in April 2021 along the shoreline between WB and Warnemünde (∼50 km east of WB, Figure 3). Water samples for Ra measurements were taken at 23 sites, with 6 sites within WB and the remainder between WB and further east along the coastline (Figure 3). The sampling method is the same as described above.

The isotope-hydrochemical composition of ground waters emerging on a nearby coastal beach (Site Meschendorf–Figure 1) was considered for comparative purposes in the discussion of this study. The sampling was conducted in the same way as for the surface water campaign in the WB and took place in September 2018.

In addition, fresh ground water samples collected from monitoring wells located adjacent to the WB are presented for comparison purposes. Ground water sampling was carried out in 2014 and followed the protocol described by Jenner (2018). The ground water well at Poel Island (Figure 1) and the ground water well 2 are located in a confined aquifer with more than 10 m of soil layer above the filter screen. The other 3 ground water wells (1, 3, and 4) are located in an unconfined aquifer with sampling depths less than 5 m below the surface (Jenner, 2018).

Geophysical acoustic investigations of the subsurface sediments were carried out from July 29th to 31st 2019 using an Innomar SES96 parametric echo sounder mounted on the IOW research catamaran Klaashahn. Seismic data were collected at three frequencies, of which only the 8 kHz dataset is presented here. Results were corrected for ship movements using a motion sensor during data acquisition. The processing of the seismic data was done using Seismic Unix, which involved threefold stacking of the data for the signal to noise ratio improvement, and application of a time-varied gain. The time-depth conversion was calculated using a constant sound velocity of 1,500 m s⁻¹.

Water contents of the sediment samples were determined gravimetrically after freeze-drying (considering the water loss due to pore water extractions at Site 2). About 1 g of each sediment sample was pre-treated for grain size analysis by removing carbonate and organic matter using hydrochloric acid (HCl) and hydrogen peroxide, respectively to prevent the binding of the small particles. Grain size measurements in the range from 0.01 to 3.500 µm were performed using a Mastersizer 3,000 with a wet dispersion unit (Hydro EV; Malvern Panalytical). Grain size statistics (including the first mode) were calculated using the program GRADISTAT (Blott and Pye, 2001), applying the volume percentage data and a geometric graphical method modified after Folk and Ward (1957).

Freeze-dried sediments were analyzed for total carbon (TC), total nitrogen (TN), and total sulphur (TS) content with a CHNS Elemental Analyzer (Euro Vector EuroEA 3,052). The combustion was catalyzed by V₂O₅, the resulting gaseous products were chromatographically separated and quantified via infrared spectrophotometry. Total inorganic carbon (TIC) was determined with an Elemental Analyzer multi-EA (Analytik Jena) after reaction with phosphoric acid 40% followed infrared spectrophotometry quantification of CO₂. The precision of the method were ±3, ±10, ±7.6, and ±4.3% for TC, TN, TS, and TIC respectively, using MBSS- (CNS) and OBSS-CaCO₃ (TIC) as in-house standards. The content of total organic carbon (TOC) was calculated from the difference of TC and TIC. The contents of total Mercury (Hg) were analyzed using a DMA-80 analyzer with a detection limit of about 0.15 μg kg⁻¹, as described by Leipe et al. (2013). The precision and accuracy of the measurement were ±8 and ±1.3%, respectively. The calibration was carried out with the 142R MBSS standards.

For the analysis of the acid extractable fraction of several metals (identified in the text with *) about 200 mg of freeze-dried and homogenized sediment was treated with 10 ml 0.5 M HCl agitated for 1 h (Kostka and Luther, 1994). The extracts were filtered with 0.45 µm disposable surfactant-free cellulose acetate membrane filters (Carl Roth), and analyzed by inductively coupled plasma optical emission spectrometry, ICP-OES (Thermo, iCAP, 7,400 Duo Thermo Fischer Scientific), after 10-fold dilution with 2% HNO₃ using external calibration and Sc as an internal standard. The international reference material SGR-1b (USGS) revealed precision and accuracy of better than 2.1 and 3.8%, respectively (Dellwig et al., 2019).

Major ions (Na, Mg, Ca, K, S) and trace elements (Ba, Fe, Li, Mn, Sr) in the water samples were analyzed by ICP-OES using matrix-matched external calibration (diluted OSIL Atlantic Sea water standard spiked with Ba, Fe, Li, Mn, and Sr) and Sc as internal standard. Precision and accuracy were checked with spiked SLEW-3 (NRCC) and were better than 4.7 and 7.6%, respectively.

Concentrations of H₂S were determined by the methylene blue method (Cline, 1969) using a Spekord 40 spectrophotometer (Analytik Jena). The chloride concentration of the water samples was determined by an electric potentiometric precipitation titration (Schott instruments), which utilizes the changing electrical potential difference of the solution by quantitative precipitation of Cl ions due to titration with 0.05 M AgNO₃. Analytical accuracy was constantly checked with the international reference standard OSIL Atlantic Seawater 35.

δ¹³C_DIC values were determined as described by Winde et al. (2014) by means of continuous-flow isotope-ratio-monitoring mass spectrometry (CF-irmMS) using a Thermo Finnigan MAT253 gas mass spectrometer attached to a Thermo Electron Gas Bench II via a Thermo Electron Conflo IV split interface. Solutions were allowed to react for at least 18 h at room temperature before introduction into the mass spectrometer. The international calibration materials IAEA Ca carbonate standard and Solnhofer Plattenkalk were used for calibration of measured isotope ratios towards the V-PDB scale. δ¹⁸O and δ²H values of water were analysed with a CRDS system (laser cavity-ring-down-spectroscopy; Picarro L2140-I; Böttcher and Schmiedinger, 2021). The reference materials SLAP and VSMOW were used for calibration of measured isotope ratios towards the V-SMOW scale. All stable isotope results are giving in the δ-notation. The given ‘‰’ values are equivalent to mUr (milli Urey; Brand and Coplen, 2012).

Ra isotopes (²²³Ra, t_1/2 = 11.4d, and ²²⁴Ra t_1/2 = 3.7d) were measured with radium-delayed coincidence counters (RaDeCC) (Moore and Arnold, 1996) within 3 days after sampling. After about a month, a further measurement was conducted for the determination of ²²⁴Ra supported by ²²⁸Th (t_1/2:1.9 years), this measurement is then subtracted from the initial ²²⁴Ra to obtain the excess of ²²⁴Ra activities (²²⁴Ra_ex). Activities of ²²³Ra, ²²⁴Ra, and ²²⁴Ra_ex were calculated and the expected error of the measurement is 12 and 7% for ²²³Ra and ²²⁴Ra respectively (Garcia-Solsona et al., 2008). The calibration of the detectors is done once a month using ²³²Th with certificate activities. In this study, we do not discuss ²²³Ra, but the data can be found in the Supplementary Table S2.

The isotope-hydrochemical analysis of the ground water from the wells was carried out by the department of “Strahlen und Umweltschutz” at the “Landesamt für Umwelt, Naturschutz und Geologie”. As for methods used, all methods and DIN standards are given in Supplementary Table S3. The data was kindly provided by LUNG Güstrow and further details can be found at Jenner (2018).

Steady-state modeling of pore water profiles was performed to assess the transport mechanism, i.e., diffusive or advective fluxes. The model further evaluates the essential geochemical reactions and the transformations rates. Net-release of a solute into the bottom water leads to positive transformation rates while net consumption results in negative transformation rates (Schultz, 2006).

Pore water sulphates (SO₄) and DIC concentration profiles were computed as a function of sedimentation rate, organic matter degradation, and diffusion rates using the reactive transport model described in (Meister et al., 2013). The following general equation was used: ∂ ( φ C ) ∂ t = − ∂ ( ω φ c ) ∂ z + ∂ ∂ z ( D 0 τ 2 φ ∂ C ∂ Z ) + φ ∑ R i (1) where C is the concentration, t is time, ω is the sedimentation rate, z is the depth below seafloor, φ is the porosity, and D₀ is the diffusion coefficients corrected by tortuosity (τ²), which was calculated according to Boudreau (1997) as τ ² = 1–2·ln φ. The mean sedimentation rate ω was taken as 2 mm yr⁻¹ (Lampe et al., 2013) and an average porosity was taken as 0.88 derived from the sediment water content. The diffusion coefficients for SO₄ and DIC were calculated as 0.0236 and 0.0257 m² yr⁻¹ respectively, based on the average temperature of 12°C and an average salinity of 13 (Boudreau, 1997).

Sources and sinks of SO₄ and DIC are stoichiometrically coupled to rates of organic carbon decay via the following simplified reaction for organoclastic sulphate reduction: S O 4 2 − + 2 C H 2 O → H S − + 2 H C O 3 − + H + (2)

The organic matter decay rate R_G was followed by the reactive continuum (RC) model of Boudreau and Ruddick (1991). R G ( t ) = − v ( a + t ) − 1 G ( t ) = − v a v ( a + t ) v + 1 ρ s ( 1 − φ ) G 0 100 φ M C (3) where the free parameters and a ν determine the shape of the initial distribution of organic matter reactivity. The parameter a describes the average life-time of the more reactive components of the spectrum whereas ν defines the shape of the distribution. The dry density of the sediment is ρ _s (ρ _s = 2.6 g cm⁻³), M_C is the molecular weight of carbon (M_C = 12) and G₀ is the initial TOC upon sedimentation (wt%). Thus, the SO₄ reduction and DIC production rate can be given by 0.5R_G and R_G, respectively.

The isotopic composition was calculated from the measured ratios R and referred to the VPDB according to Hoefs (2018): δ 13 C = ( R s a m p l e R V P D B − 1 ) · 1000 (4)

The absolute concentrations of [¹³DIC] and [¹²DIC] were computed by separated reaction-transport equations (Eq. 1) for each isotope. Negligible carbon isotope fractionation was assumed during organoclastic sulphate reduction (cf. Claypool and Kaplan, 1974). Therefore, this source of inorganic carbon was assumed to show the same isotopic composition as the organic source.

Initial conditions were 0 mM SO₄ and DIC at all depths. The computer domain was set to be 50 cm. The Dirichlet boundary condition was applied at both the sediment/water interface (z = 0 cm) and bottom (z = 50 cm) according to the observed data. All parameters used in the model are listed in Supplementary Table S4.

The figures presenting the vertical profiles were created using Sigma Plot 10.0 software (Systat Software, Inc., United States). The concentration maps were plotted using Ocean Data View (Schlitzer, 2001). The study area map was created using the Qgis development Team.

Fresh ground waters from the catchment in the southern part of the WB and from a well on Poel Island (Figure 1) display a water isotope composition positioned on the LMWL of Warnemünde (Table 1; Figure 4). These data are also close to the composition of ground waters escaping at beach spring in Meschendorf (Figure 1), which has been shown water of age about 30 years (Lipka et al., 2018a). The hydrogeochemical ground water composition of the majority of investigated wells and the springs fall into the field for relatively “young” ground waters (Figure 5).

The Wallenstein stream and the other western streams carry different isotope-hydrochemical signatures, thus representing different surface waters sources entering the southern WB.

The western streams showed a water isotopic composition and δ¹³C_DIC signatures similar to those found in the well waters of the catchment area (Table 1). The slight shift towards heavier isotope values is likely the result of an impact by evaporation and CO₂ degassing when stream waters get in contact with the atmosphere (Michaelis et al., 1985; Clark and Fritz, 1997).

The Wallenstein stream, on the other hand, is enriched in heavy water isotopes and dissolved ions when compared to well waters and western streams (Figure 4; Table 1). Besides the mixing with BS surface waters during the time of sampling, this could also be the result of surface water evaporation (e.g., Gat, 1996; Clark and Fritz, 1997) that took place during the origin of this stream in Lake Schwerin and a further crossing through Lake Mühlenteich before reaching the WB coastline (Figure 1).

The impact of stream waters on the composition of the WB surface waters is most pronounced in the western part of the bay. There the water isotopic composition decreased towards the coastline (Figures 3B,C), due to mixing between open BS water, containing heavier isotopes, and a fresh water component characterized by lighter isotopes. In addition, higher concentrations of DIC together with the depleted δ¹³C_DIC found near the western coastline (Figures 3E) are probably related to fresh water sources (Winde et al., 2014). The surface water of the WB in the western part showed water isotopes values decreasing close to the coastline (Figures 3B,C), probably due to mixing between open BS water containing heavier isotopes, and a fresh water component characterized by lighter isotopes coming from the small streams located in this region. These streams showed a water isotopic composition similar to those found in the ground water recovered from wells near Wismar (Table 1, GW) and fresh water springs at Meschendorf (Table 1–springs). The slight trend to heavier isotopes is likely the result of an impact by evaporation and CO₂ degassing when stream waters become in contact with the atmosphere (Clark and Fritz, 1997). The ¹³C_DIC isotopic signatures of the western streams are similar to the values in the ground water as well, which reinforces the idea that the streams are supplied by ground water from unconfined aquifers.

Within the western part of the WB, ²²⁴Ra activities showed a decrease towards the open BS (Figure 2B). Since the ²²⁴Ra activities in the streams were comparable to those measured in the surface water of the WB (Figure 2; Table 1), further sources may are contributing to the observed spatial patterns. Besides the ground water below the central part of the bay (see 4.3), which has not been analyzed for ²²⁴Ra, the permeable sediments along the bay may act as a further fresh water and ²²⁴Ra source to the WB. The pore water at the beach showed average ²²⁴Ra_ex activities that were about one order of magnitude higher than the activities found in the surface water of WB. These findings, therefore, suggest, that SGD can enter the WB from the boundaries and contributing as a further source to the water balance of the WB.

Beach pore water consist of a mixture between fresh water and a brackish water component. To estimate the fresh water endmember the beach pore water compositions were extrapolated based on actual measurements and using Na as conservative mixing tracer (boundary conditions: average WB and 1 mM for the fresh water, which is the mean concentration found in the ground water wells). These values can be found in Table 1—Beach SGD. For the majority of the dissolved compounds a composition close to ground waters was found (Table 1). Only some non-conservative constituents, like DIC and Ca were enhanced in the estimated fresh water component, likely due to corrosion processes of carbonates present in the coastal sands (e.g., Smellie et al., 2008).

Although a ²²⁴Ra gradient was not established in the eastern part of the WB (Figure 2), the generally higher activities in this part may indicate a source of ²²⁴Ra. This source may originate from the exchange with pore water or resuspended sedimentary particles or SGD, besides the Wallenstein stream. Moreover, enhancement of Mn and Ba compared to the open BS surface water confirms strong benthic pelagic fluxes.

A mass balance is constructed, based on Rodellas et al. (2021), to estimate SGD in the WB. The Ra mass balance relies on the assumption that the most important Ra sources and sinks are in steady state, and then SGD can be evaluated by the difference of supply and removal according Eq. 5. F S G D ∗ A S G D + Q s t r C s t r + F s e d ∗ A W B = λ V C + Q o u t C (5) Where V and A_WB are the water volume (m³) and surface area (m²) of where samples were collected (Figure 1). We assume for the balance that SGD is taking place at 27 km of the shoreline and 10 m offshore (A_SGD). Q_str is the discharge from streams (m³ d⁻). We consider here the Wallenstein stream and the three western streams. Q_out is the water outflow to the open BS (m³ d⁻¹) determined by using the water mass ages, the activity difference between WB and open-sea water and the volume of the study area. The water ager was calculated following (Moore and de Oliveira, 2008). C is the weighted average of ²²⁴Ra_ex of the WB, determined by the ²²⁴Ra_ex inventory. C_str are the mean concentrations of ²²⁴Ra_ex in the surface water inlets (Bq m⁻³). F_sed is the ²²⁴Ra_ex diffusion from sediments. λ is the radioactive decay constant of ²²⁴Ra. _.F_SGD is the fluxes of ²²⁴Ra_ex per unit area associated with SGD (Bq m⁻² d⁻¹), respectively. Terms used in the mass balance together with the units are summarized in Table 3.

Solving Eq. 5 a ²²⁴Ra_ex flux from SGD (F_SGD—Table 3) of 261.7 Bq m⁻² d⁻¹ was obtained.

This flux was converted into a water flow by dividing it by the ²²⁴Ra_ex activities in different endmembers which were derived from four beach pore water samples with activities of 51, 65, 84, and 88 Bq m⁻³. The resulting estimated volumetric SGD flow is 3.8 ± 0.7 cm d⁻¹ (Table 3).

The estimated flow is higher compared to the flow obtained by (Schafmeister and Darsow, 2004). These authors applied a numeric ground water flow model (about 0.02 cm d⁻¹). This is due to the model only considering the fresh water component, whereas the Ra mass balance applied in the present study includes recirculated sea water. However, the SGD rate from WB is considered low when comparing to other SGD rates from different regions in the world, where SGD rates can reach up to 280 cm d⁻¹ (Santos et al., 2021).

The composition of sediments investigated in the WB is typical for silty/muddy deposits along the North-German coastline (e.g., Böttcher et al., 2000; Al-Raei et al., 2009; Lipka et al., 2018b).

Except for sites 10 and 11 (Figure 1), pore water salinities in the WB sediments were characterized by relatively constant values (Figures 6A). Slight maxima around 10 to 15 cmbsf were observed at all sites and are due to past changes of bottom water salinities which are consequence of intrusion of salty North Sea water intrusions into the southern BS (Matthäus and Lass, 1995; Mohrholz, 2018).

Pore water salinities at Site 10 and in particular at Site 11 show a different pattern. Below the surface maxima, they decrease, and at Site 11 the values even reach fresh water conditions at the lowermost depth (salinity of about 1). It suggests that the sediments below the surface muds may host an aquifer containing fresh water, situated above glacial deposits and potentially part of seismic unit 3 (Figure 11). The pore water gradients of Site 11 (Figure 6A-2) result from mixing between endmembers (e.g., ground water and BS), superimposed by diagenetic reactions leading to further accumulation of some elements in the top sediments.

The pore waters at sites not impacted by freshening (sites 2, 15, 20, and 31) are characterized by a continuous increase in metabolite concentrations and a decrease in SO₄ and δ¹³C_DIC values (Figures 6J–O). These vertical profiles are shaped by intense OM mineralization catalyzed by microorganisms using in particular SO₄ as terminal electron acceptors (Froelich et al., 1979; Jørgensen, 1982).

The maxima in Si, P, DIC, Mn, and Fe concentrations at Site 11 at around 10 cmbsf are in agreement with higher loads of top sediments in metabolizable organic matter leading to enhanced mineralization when compared to the other sites (Figures 6K-2–M-2,O-2). These observations also demonstrate that the fresh ground water is not substantially enriched in those substances. The pore water at Site 11 is depleted in SO₄ due to both, microbial sulphate reduction in the upper part of the sediment, but in particular due to the dilution with the underlying ground water that is free of SO₄.

The high dissolved Fe and Mn concentrations observed in the top sediments (Figures 6L-2,M-2) may accumulate due to the chemical reaction of metal ox (yhydrox)ides by biogenic sulphide (e.g., Moeslund et al., 1994; Thamdrup et al., 1994; Böttcher et al., 2000). At the sediment-water interface, Fe and in part Mn may are oxidized by NO₃ and/or O₂, whereas in deeper sediment layers Fe and Mn may react with H₂S and DIC to form Fe sulphides and MnCa carbonate solid-solutions, respectively (Froelich et al., 1979; Berner, 1980; Huckriede and Meischner, 1996; Böttcher and Dietzel, 2010). The observed near-surface maximum in dissolved P at Site 11 is due to the parallel release from OM and reduction of Fe ox(yhydrox)ides that acted previously as sorption substrate for P (e.g., Haese, 2006).

The surface sediments at all sites are low in accumulating TS (considered to reflect essentially pyrite contents) when compared to the predicted Holocene saturation line (Figure 8), indicating the impact of sulphate-independent mineralization processes in the top sediments and the impact of bioturbation-driven-re-oxidation of sedimentary sulphur species (e.g., Jørgensen and Kasten, 2006). This is in line with previous observations (Mossmann et al., 1991; Böttcher et al., 2000) that gave evidence that for modern sediments some burial at depth and associated time is necessary to establish a constant TOC/S ratio. However, the freshening of pore water at Site 11 establishes SO₄ limited conditions and caused the TOC/TS ratio to remain below 2.8 (Figure 8).

Overall, the modeled mineralization rates at the WB are at the lower end when compared to measured gross sulphate reduction rates in muddy sediments from German coastal areas (Böttcher et al., 2000; Llobet-Brossa et al., 2002; Al-Raei et al., 2009; Lipka et al., 2018b) which is likely due to some H₂S reoxidation by metal oxy (hydrox)ides taken place in the surface sediments (Jørgensen, 1982). The highest net anaerobic mineralization rates are found at Site 11 (Table 2). The excavation of sediment (Figure 11) formed a local sediment trap and led to a fast accumulation of sediments as reflected by the homogeneous distribution of sedimentary organic matter, reactive metals like Fe and Mn, and anthropogenic tracers (Hg, Pb*) (Figure 9). This accumulation further leads to enhanced availability of fresh organic matter fostering microbial activity (Aller and Blair, 1996; Jørgensen and Kasten, 2006), and therefore, higher diagenetic element fluxes across the sediment-water interface (Table 2).

Moreover, the modeled fluxes indicate that advective and diffuse components are taking place, however, the last one has been more pronounced. DIC is liberated from deeper sediments to the bottom waters, especially at Site 11 (Table 2). The pore water at Site 11 was physicochemically evaluated by the Phreeq-C package (Parkhurst and Appelo, 1999). Assuming an equilibrium with calcite at depth (Morse and Mackenzie, 1990) all solutions are characterized by an enhanced CO₂ partial pressure higher than atmospheric pressure. Therefore, the impact of fresh water at Site 11 on the bottom water of WB will enhance the capability of surface water to act as a potential CO₂ source for the atmosphere.

A stronger decrease in salinity, and therefore an indication of fresh water occurrence is observed at Site 11 compared to the other sites (Figure 6A-2). The main morphological difference between this sites is the presence of an excavated navigation channel. This is an about 3–4 m deep artificial sedimentary depression (Figure 11) which was built for ship navigation channel between the open BS and the port of Wismar (VSW, 2018).

Based on the geophysical profiles, the dredging removed the previously overlying muddy sediments (units S1 and S2), thereby touching the underlying aquifer in unit 3 (Figure 11). The removal of the mud cover at Site 11 promotes the exchange of solutes and water between the aquifer and the bottom water. A more continued excavation this structure in the future, may enhance the direct impact of SGD on the WB. On the other hand, depending on the hydrographic boundary conditions, the artificial channel may also temporarily allow brackish water to penetrate the aquifer.