Kathleen J Gosnell
email: kgosnell@geomar.de

Lars-Eric Heimbürger-Boavida

Aaron J Beck

Peter R Ukotije-Ikwut

Eric P Achterberg

World War Munitions as a source of Mercury in the Southwest Baltic Sea

Keywords: Kolberger Heide, Mercury fulminate, Mercury speciation, methylmercury, UXO

Mercury (Hg) fulminate was used as a primary fuse in World War (WW) munitions, and may consequently be a Hg source for impacted environments. Mercury is a conspicuous and persistent pollutant, with methylmercury (MeHg) acting as a notorious neurotoxin. Considerable amounts of munitions were intentionally dumped in the North Sea and Baltic Sea following the First and Second WWs. After more than 70 years on the seafloor many munitions have corroded and likely release explosive compounds, including Hg fulminate. The Germany coastal city of Kiel was a manufacturing centre for submarines, and accordingly a prominent target for bombing and post-war disarmament. We collected water and sediment samples around Kiel Bay to assess regional levels and quantify any Hg contamination. The munition dump site Kolberger Heide (KH) and a former anti-aircraft training center Dänisch-Nienhof are situated in Kiel Bay, and were targeted for sampling. Sediment Hg concentrations around KH were notably elevated.

Average Hg concentrations in KH sediments were 125 ± 76 ng/g, compared to 14 ± 18 ng/g at background (control) sites. In contrast, dissolved Hg in the water column exhibited no site variations, all ranging between 0.8 -2.1 pM. Methylmercury in sediments and waters did not have enhanced concentrations amongst sites (<30 pg/g and <50 fM, respectively). Sedimentwater exchange experiments showed elevated Hg and MeHg fluxes (i.e. >400 pmol m -2 d -1 MeHg) at one KH location, however remaining cores had low to no Hg and MeHg output (<0 to 27 pmol m -2 d -1 MeHg). Thus, sediments in Kiel Bay proximate to WW munitions could harbor and form a source of Hg, however water column mixing and removal processes attenuate any discharge from the seafloor to overlying waters.

Introduction

Mercury (Hg) is a naturally occurring element which is readily transferred between the earth's geological reservoirs [START_REF] Outridge | Updated global and oceanic mercury budgets for the United Nations Global Mercury Assessment 2018[END_REF][START_REF] Edwards | Fifty years of volcanic mercury emission research: Knowledge gaps and future directions[END_REF], yet also notoriously toxic. Mercury is environmentally pervasive as a byproduct of coal and fossil fuel combustion, paper processing [START_REF] Boening | Ecological effects, transport and fate of mercury: a general review[END_REF][START_REF] Van Velzen | Mercury in waste incineration[END_REF], and furthermore still used during illegal gold mining activities [START_REF] Goncalves | Evidence of reduced mercury loss and increased use of cyanidation at gold processing centers in southern Ecuador[END_REF]. Methylmercury (MeHg) is known for its irreversible toxic effects on humans and wildlife. This highly bioavailable organic Hg form acts as a neurotoxin, and damages DNA and RNA thereby causing permanent mobility damage [START_REF] Clarkson | The Toxicology of Mercury and its Chemical compounds[END_REF]. Methylmercury species bioaccumulate in marine food chains, with especially harmful levels in slow growing and predatory fish species [START_REF] Lavoie | Mercury transport and human exposure from global marine fisheries[END_REF]. Hence it is critical to assess levels of Hg and MeHg present in marine environments.

Mercury fulminate (HgFu) was originally formulated by alchemists in the seventeenth century, making it one of the oldest known explosive compounds. The first and second World Wars (WWI and WWII) stimulated extensive production and use of various types of munitions, including mines, anti-aircraft grenades and submarine torpedoes. In addition to the primary charge of e.g. trinitrotoluene (TNT), many WW anti-aircraft grenades contained HgFu as an initial ignition compound. The WWI production of HgFu in Germany increased from 104 t up to 357 t between 1910 and 1917 [START_REF] Matyáš | Primary explosives[END_REF]. Following a decline in manufacturing during the 1920s through 1930s (primarily due to its toxicity), HgFu output again increased in the 1940s with the inception of WWII [START_REF] Horowitz | Historical mercury releases from commercial products: Global environmental implications[END_REF]. As much as 15% of global Hg production was dedicated to the manufacturing of HgFu for munitions throughout WWI and WWII [START_REF] Horowitz | Historical mercury releases from commercial products: Global environmental implications[END_REF].

The Germany coastal city of Kiel has been an active naval base and manufacturing centre for submarines since 1864, and accordingly housed a notable reserve of munitions. Kiel was thus a prime target for bombing and destruction during WWII. Numerous air strike campaigns demolished Kiel between 1940 and1944, destroying ~80% of its buildings. About 20-30% of the munitions used during airstrikes and anti-aircraft defense activities did not explode [START_REF] Gichd | A Guide to Survey and Clearance of Underwater Explosive Ordnance[END_REF], resulting in unexploded ordnance (UXO) on land and in coastal waters. Following WWII, the Allies hastily disposed the large quantities of remaining munitions directly in the North Sea and Baltic Sea [START_REF] Carton | Historic Disposal of Munitions in U.S. and European Coastal Waters, How Historic Information Can be Used in Characterizing and Managing Risk[END_REF]. It is estimated that approximately 1.6 x10 6 t of munitions were disposed in German waters following WWI and II [START_REF] Böttcher | Munitionsbelastung der deutschen Meersgewässer -Bestandsaufnahme und Empfehlungen (stand 2011)[END_REF]. In effect between 360,000 -385,000 t of munitions were dumped in the Baltic Sea during the demilitarization campaign after 1945 [START_REF] Carton | Historic Disposal of Munitions in U.S. and European Coastal Waters, How Historic Information Can be Used in Characterizing and Managing Risk[END_REF]. This could have subsequently delivered an estimated 300 t of Hg just inside confined dump sites [START_REF] Beldowski | Sea-dumped ammunition as a possible source of mercury to the Baltic Sea Sediments[END_REF].

Munition dumps in coastal waters pose substantial dangers due to explosion risks to ship traffic, economic resource development and recreational as well as commercial fisheries [START_REF] Carton | Historic Disposal of Munitions in U.S. and European Coastal Waters, How Historic Information Can be Used in Characterizing and Managing Risk[END_REF][START_REF] Gichd | A Guide to Survey and Clearance of Underwater Explosive Ordnance[END_REF]. There is furthermore a raised awareness of threats that munition compounds and their degradation products that leach out of corroded ordnance pose to the marine ecosystems [START_REF] Carton | Historic Disposal of Munitions in U.S. and European Coastal Waters, How Historic Information Can be Used in Characterizing and Managing Risk[END_REF][START_REF] Beck | Spread, behavior and ecosystem consequences of conventional munitions compounds in coastal marine waters[END_REF][START_REF] Kampmeier | Exploration of the munition dumpsite Kolberger Heide in Kiel Bay, Germany: Example for a standardized hydroacoustic and optic monitoring approach[END_REF]. Kolberger Heide (KH) is a munition dumpsite located in Kiel Bay (KB) in the Baltic Sea, and proximate to active ship routes and densely populated areas. Geophysical observations have shown that KH contains over 1000 weapon objects, yielding an estimated 30,000 tons of munitions [START_REF] Kampmeier | Exploration of the munition dumpsite Kolberger Heide in Kiel Bay, Germany: Example for a standardized hydroacoustic and optic monitoring approach[END_REF]. The former anti-aircraft training site Dänisch-Nienhof (DN) is also located on the northern coast outside of Kiel, and was heavily bombed during WWII. This likely provided residual elemental Hg detonation fallout in addition to UXO remaining in coastal sediments. Munitions possibly impacted an area of approximately 10.5 km 2 around DN [START_REF] Helcom | Updated Fifth Baltic Sea pollution load compilation (PLC-5.5)[END_REF].

Recent findings suggest that sediments close to munitions have elevated Hg content [START_REF] Gębka | The impact of military activities on the concentration of mercury in soils of military training grounds and marine sediments[END_REF][START_REF] Beldowski | Sea-dumped ammunition as a possible source of mercury to the Baltic Sea Sediments[END_REF][START_REF] Siedlewicz | Presence of mercury and methylmercury in Baltic Sea sediments, collected in ammunition dumpsites[END_REF], however provide no assessment for how surrounding waters could be affected. The primary aim of this study was to evaluate if there is Hg contamination from legacy WW munitions in sediments around KH and DN which is further prevalent in the ambient water column of Kiel Bay and greater southwest Baltic Sea. Potential contributions were determined for dumped munitions and UXO in KH and DN towards the burden of various Hg compounds in the water column and sediments. Moreover, sediment water exchange fluxes were assessed for each site to evaluate if Hg and MeHg are discharged from sediments to overlying water. These measurements and experiments provide current regional background concentrations, as well as document if munition impacts represent a Hg source and danger to the local marine environment. The human population in the surrounding area of the Baltic Sea reaches nearly 90 million, therefore it is important to obtain a mechanistic and quantitative understanding of any toxic residuals release from historic munition including Hg and MeHg, and their distribution in the marine environment.

Methods

Field Collection

Seawater and sediment for Hg analysis were collected aboard the RV Littorina in KB, southwest Baltic Sea (Figure 1). Seasonal field sampling (December, May and July) occurred during 2019 and 2020 in the vicinity of DN, the munition dump site KH, and control sites within KB (CR). The DN site was not sampled in December. No benthic biota were present within sediments, therefore no Hg measurements are available for any organisms. Seawater samples for coastal Hg concentrations were also collected on a cruise traverse in the south Baltic Sea between Kiel and Lübeck in September 2018 on RV Alkor (SI; Figure S1). All seawater samples were collected as depth profiles using a plastic-coated line with 8 L Go-Flo bottles (General Oceanics) and according to trace metal-clean GEOTRACES sampling techniques [START_REF] Cutter | Sampling and sample-handling protocols for GEOTRACES Cruises[END_REF]. Kiel Bay and especially KH are relatively shallow (<20 m), and profiles were collected between ~1-2 m below the surface and ~1-2 m above the bottom to minimize potential contamination risk from sediment disturbance or ship residue.

Water profiles were collected from between 3 to 7 stations at each site per season. Water were filtered through 0.2 μm filters (Nuclepore polycarbonate), then poured into acid-cleaned borosilicate glass bottles for the separate Hg species, preserved acidified with hydrochloric acid (0.2% v/v) upon returning to shore (<5 hours), then analyzed within ~5 months. Volumes of 500 mL were collected for MeHg measurements, and 250 mL for total Hg. Surface sediment samples were obtained by box corer in areas corresponding to water sampling. Sediment samples were selected to be visually uniform in their composition among stations and sites, primarily of a muddy mix with limited sand. Some rocky and sandy sediments areas in KB could not be obtained by box corer, so paired water and sediment samples were not available. Nevertheless, sediments were obtained from a wide region around munition and control sites (between 8-14 stations collected from each).

Intact sediment cores with their overlying water were additionally collected from each site with a multicorer during the July 2020 Littorina cruise (SI; Figure S2). These cores were used for sediment flux experiments to quantify sediment release and exchange of Hg species to the overlying water column. Duplicate cores were collected from 2 different adjacent DN and KH stations (e.g. FK1-1 and FK1-2 were one KH site), and one core was collected at two different CR sites (FC1 and FC2). The FC1 core contained some sand, while FC2 and the rest were composed of organics and muds. All cores were selected to be relatively consistent in sediment amounts and overlying water levels.

Water Analysis

Dissolved Hg was analyzed at GEOMAR via gas chromatography and cold-vapor atomic fluorescence spectrometry (CVAFS) following US EPA method 1631. Brominemonochloride (0.2% v/v; BrCl; Brooks Rand) was added <5 hours after sampling to preserve and completely extract complexed Hg. Hydroxylamine hydrochloride (0.1% v/v; Brooks Rand) was added to samples ~0.5 h prior to analysis to quench excess anions. Sample aliquots (100 mL) were poured into a sparging flask, and tin chloride reductant (20% w/v; 100 µL; Brooks Rand) was added before closing. The sample was purged with N 2 (150 ml/min; trace metal grade) onto a goldcoated bead trap for 10 min. The gold trap was then removed and placed on a TDM-II (Brooks Rand) instrument for desorption and CVAFS (Model-III; Brooks Rand). Triplicates were measured on select samples yielding a relative standard deviation (RSD) of <4.5% (LoD = 0.02 pM). Quantification and detection were verified with ERM-CA400 reference seawater, and was ~95% of accepted recovery. Dissolved MeHg species were analyzed following a direct ethylation method [START_REF] Munson | Determination of monomethylmercury from seawater with ascorbic acid-assisted direct ethylation[END_REF]US EPA method 1630). Briefly, acidified 50 mL sample aliquots were poured into the sparging flask with ascorbic acid (2.5% w/v; 200 µL; Fischer), neutralized using 8 M potassium hydroxide (KOH; 1.8 mL; Fischer), and adjusted to pH 4.9 with a 4 M ammonium acetate buffer (~860 µL). Sodium tetraethylborate (TEB; 100 µL; 1% w/v; Strem chemicals 98%) was added to each sparging flask, which was immediately closed tightly and allowed to react for 10 min.

Volatile ethyl derivates were then purged for 18 min onto a Tenax trap using N 2 (150 ml/min; trace-metal grade), and dried offline under N 2 for 5 min (~50 ml/min). The Tenex trap was removed and placed on the TDM-II for thermal desorption of ethylated Hg derivatives, then separated by isothermal gas chromatography (GC) and detected by CVAFS on the Model-III.

Select samples were analyzed multiple times, yielding an RSD <9.2% (LoD = 3.4 fM).

Sediment Analysis

Sediment samples were freeze dried and homogenized via milling prior to analysis. Total Hg in the sediments were analyzed using an Advanced Mercury Analyzer instrument (AMA254) at the Mediterranean Institute of Oceanography (MIO). The AMA254 works by thermal decomposition and atomic adsorption spectrometry. Quality assurance was validated with analysis of blanks and certified reference materials (CRM; MESS-4, within >81% accepted range) at the start and finish of each run, in addition to ~every 10 samples. Duplicates or triplicates were analyzed for each sample and RSD was <9.6%.

Sediment MeHg analysis was completed using a MERX-M (Brooks Rand) at MIO. First, a sample aliquot (between ~400-1200 mg) was carefully weighed into an acid cleaned glass tube.

Next, nitric acid (HNO 3 ; 5 mL) was added to the tube, which was then placed in an 80 o C oven to digest for 2 h. An aliquot of the digested sample was pipetted into fresh acid cleaned sample flasks containing deionized water (MQ, Millipore), then adjusted to pH 4.9 using acetate buffer (1.1 mL) and filled completely with MQ. The TEB (10 µL) was added to each sample and tightly sealed prior to analysis. The CRM IAEA-456 was digested and analyzed concurrently with the samples to verify MeHg recovery, and measured concentrations were within the accepted range [START_REF] Vassileva | Certification for trace elements and methyl mercury mass fractions in IAEA-456 marine sediment sample[END_REF]. Information about ancillary water and sediment methods (dissolved organic carbon (DOC), nutrients and organic matter) are available in SI.

Mercury Sediment-water Exchange Experiment

Cores for sediment water exchange (flux) experiments were sealed using fitted caps for protection against contamination and to prevent gas exchange. Caps provided continuous overlying water mixing, though were briefly opened during sampling. Cores were held to ~19-20 o C and protected from most light during the experiment. Samples for DOC and total dissolved Hg and MeHg were siphoned from overlying core water with an acid cleaned syringe, then filtered, preserved and analyzed as noted above and in SI. Each core was sampled 5 times over a period of 42 h. Oxygen saturation and temperature were measured and tracked at each sampling point, and remained relatively constant. Surface sediment (0-2 cm) from each core was sectioned off immediately after draining the core at the end of the experiment for Hg, MeHg and organic characteristics, and analyzed as stated.

Following overlying water sampling at each time point, this amount (~350 mL) was replaced with water collected from depth at the corresponding site in KB in order to keep the volume consistent. Discharge of Hg and MeHg release were calculated from the averaged linear regression slope of the Hg species concentration change over time (see SI for equations), then averaged to obtain a general flux for each core. Any difference in Hg and MeHg concentrations for replacement water was considered for the calculations. Calculations were normalized to the approximate core water volume (3740 cm 3 ).

Results & Discussion

Sediment Hg Species

Sediment Hg and MeHg concentrations displayed a wide spread among sites (Figure 2). Control stations (CR) acting as background, had the lowest Hg concentrations, ranging from ~2.4 to 59.3 ng/g Hg (average 14 ± 18 ng/g), compared to DN which ranged between ~2.7 -137.5 ng/g (average 47 ± 46 ng/g). At the DN location sediments were collected both inside and outside the reported historic training area to allow comparisons with other nearshore measurements, and showed a range consistent with overall sediment characteristics in KB (SI; Table S1). Sediments from KH had the highest Hg concentrations, revealing a greater average of 124.5 ± 76 ng/g Hg, and larger range of ~12 to 237.8 ng/g Hg. Sediment MeHg concentrations showed less variability between sites, with average values of 6.1 ± 4 pg/g for CR, 12 ± 5 pg/g for DN, and 18.8 ± 7 pg/g for KH. The MeHg concentrations at KH were highest and ranged between 9 to 30 pg/g (Figure 2; bottom). Notably, sediment %MeHg (MeHg/Hg) levels were all <0.6% (SI; Table S1), with lowest %MeHg found at KH (0.03 ± 0.02%). All sites were significantly different for Hg (p=<0.0001, n=61, ANOVA) and strongly different for MeHg (p=0.0001, n=33, ANOVA). Overall KH sediments were predominantly higher in Hg and MeHg (Figure 2), and sediments between sites displayed notable heterogeneity compared to the consistency of water column observations (see section 3.3). Organic parameters (nitrogen content by percent weight: N %wt, sulfur content: S %wt, organic carbon: C-org, molar ratio: [C/N], total carbon: TC %wt) facilitated us to resolve some associations among sites. There were clear correlations between Hg concentrations for sites and all analyzed organic matter parameters (Table 1). Organic content was highest and most variable for KH, followed by intermediate levels at DN and lowest amounts found in CR sites (SI; Table S1). The TC levels were 3.47 ± 2.2% at KH and 2.49 ± 1.7% for DN, with lowest TC of 0.71 ± 0.9% in CR sites. Overall average sediment S content was 0.4 ± 0.4%, with highest levels found at KH (0.61 ± 0.4%), compared to DN (0.34 ± 0.2%) and CR (0.12 ± 0.2%). Significant relationships were observed between nearly all organic matter parameters and MeHg for KH stations (Table 1). There were also significant relationships between MeHg and organic elements for the overall dataset as well as the CR sites (excluding [C/N]). Conversely, no significant relationships were present for DN stations between any organic matter parameters and Hg or MeHg. But there were large deviations in organic matter levels, and ranges exhibited much overlap for DN and KH (SI; Table S1). Sediments are a long-term repository for adsorbed contaminants, and may form a legacy pollution source which can release Hg species once disturbed. Mercury and MeHg transfer from sediment into the water column may occur through passive diffusion, advection and/or resuspension [START_REF] Bloom | Speciation and Cycling of Mercury in Lavaca Bay, Texas, Sediments[END_REF]. Reported preindustrial Hg concentrations in Baltic Sea surface sediments range between <20 to 40 ng/g [START_REF] Leipe | Mercury in Baltic Sea Sediments-Natural background and anthropogenic impact, Earth's chemistry[END_REF]. Whereas modern sediment Hg concentrations typically extend between <20 to 350 ng/g (Uscinowicz et al., 2011;[START_REF] Siedlewicz | Presence of mercury and methylmercury in Baltic Sea sediments, collected in ammunition dumpsites[END_REF][START_REF] Kwasigroch | Distribution and bioavailability of mercury in the surface sediments of the Baltic Sea[END_REF], with higher levels associated to pollution sources, thereby including munitions.

There is a large variety of anthropogenic Hg sources which contributes to watersheds and coastal sediments, incorporating direct inputs from local industries including coal combustion and waste incineration [START_REF] Van Velzen | Mercury in waste incineration[END_REF][START_REF] Beldowska | Factors influencing variability of mercury input to the southern Baltic Sea[END_REF]. Dating of Baltic Sea sediments by [START_REF] Leipe | Mercury in Baltic Sea Sediments-Natural background and anthropogenic impact, Earth's chemistry[END_REF] indicated that Hg values greatly increased around the mid-1970s.

A specific Hg point source derived from isotopic ratios analysis was not noticeable. However, given that there was a coal fired power plant operating on the outskirts of Kiel (switched to natural gas in 2020), it is expected that some Hg within the Bay results from energy production.

Yet it is unlikely that atmospheric Hg deposition from fossil fuel combustion would occur specifically in sediments only around KH and less so at DN or CR sites. Both DN and KH are located at the Kiel fjord outlet, and subjected to nearshore land runoff or other coastal anthropogenic Hg sources. Visually the DN site exhibited a similar sediment consistency to KH.

The CR and DN sites have muddy as well as mixed sand sediments, while KH was muddy and more organic with several samples containing sand in the mix. Hence, sediment basis and organic matter content are important, but not the only explanation for differences in Hg and MeHg concentration between KH and CR or DN. [START_REF] Orani | Temporal variation of trace elements, rare earth elements and Pb isotope ratios in sediment core from Kiel Bay, western Baltic Sea[END_REF] documented sedimentary Hg enrichments occurring during the period of 1917-1970 in KB. Notably, this time period also corresponded with both WWI and WWII, during which munitions would have been predominantly deployed, exploded and eventually corroded.

Excess Hg in some KH sediment can likely be attributed to the degradation and dissolution of WW munitions containing HgFu. It has been reported that HgFu rapidly dissolves into surrounding seawater upon contact [START_REF] Gosnell | Effects of salinity and temperature on seawater dissolution rate of initial detonation agent mercury fulminate[END_REF]. Mercury readily adheres to sediment and resuspended particles and thus would deposit in nearby sediments [START_REF] Bloom | Speciation and Cycling of Mercury in Lavaca Bay, Texas, Sediments[END_REF]Ulrich et al., 2001). Thus, it is feasible that any release and deposition in sediments of Hg derived from HgFu would remain close to the munition source under typical conditions. [START_REF] Beldowski | Sea-dumped ammunition as a possible source of mercury to the Baltic Sea Sediments[END_REF] proposed that dumped conventional munitions from WWII represent a distinctive source of Hg for the Baltic Sea. In their study the highest Hg concentration observed in the vicinity of ammunition was 322 ng/g, in agreement with the upper range of reported Baltic observations of 300 ng/g (HELCOM, 2010). Comparatively, sediment Hg concentrations from around the Bornholm Deep chemical munition dump site in the Baltic Sea ranged between 34 to 91.7 ng/g Hg [START_REF] Beldowski | Sea-dumped ammunition as a possible source of mercury to the Baltic Sea Sediments[END_REF], whereas 500 ng/g Hg has been reported for the Gotland Deep munition dumpsite (Uscinowicz et al., 2011). Enhanced sediment Hg concentrations in this study (>125 ng/g) were found only at the KH munition dump site.

Multibeam measurements in the area of our study allowed quantification and mapping of munitions [START_REF] Kampmeier | Exploration of the munition dumpsite Kolberger Heide in Kiel Bay, Germany: Example for a standardized hydroacoustic and optic monitoring approach[END_REF]. Geophysical observations continue to reveal anomalies which are indicative of munitions (M. Kampmeier, personal communication). Unfortunately, due to safety concerns, we were unable to sample directly below munitions or report coordinates where anomalies reside in the KB area. Three sediment stations which were sampled close to a geophysical abnormality had comparatively higher Hg concentrations: K6 (138 ng/g) and K7

(137 ng/g), whereas the third (K15) yielded 160 ng/g. Two of the 3 stations also had higher MeHg concentrations: K15 (26.2 pg/g), and K6 (25.5 pg/g), though K7 was lower at 12.5 pg/g, which was close to the KB average (14.5 ± 8 pg/g).

Methylmercury in sediments was spatially variable, but within the range of reported concentrations [START_REF] Beldowski | Sea-dumped ammunition as a possible source of mercury to the Baltic Sea Sediments[END_REF][START_REF] Siedlewicz | Presence of mercury and methylmercury in Baltic Sea sediments, collected in ammunition dumpsites[END_REF]. However, Beldowski et al.

(2019) reported higher MeHg concentrations in some sediments near KH than were observed in our study (e.g. <650 pg/g), yet did not directly link elevated MeHg to munitions. The methylation of Hg within sediments depends on a range of factors, including sediment composition (i.e. grain size), as well as ecological, physical and chemical properties, typically governed by organic matter, oxygen and S concentrations [START_REF] Bloom | Speciation and Cycling of Mercury in Lavaca Bay, Texas, Sediments[END_REF][START_REF] Boening | Ecological effects, transport and fate of mercury: a general review[END_REF][START_REF] Bravo | Biotic formation of methylmercury: A bio-physio-chemical conundrum[END_REF]. [START_REF] Beldowski | Total, methyl and organic mercury in sediments of the Southern Baltic Sea[END_REF] noted that gradients in sediment properties influenced discrepancies in MeHg in sediment for the Gulf of Gdansk, located off Poland in the southern Baltic.

It appears that some of the significantly elevated concentrations of Hg observed in KH sediment are resulting from the release of WW munition derived HgFu. Enhanced organic material concentrations are favorable to consolidating Hg within sediments and facilitate to keep Hg proximate to where it was released, likely resulting in overall higher Hg around KH. It is possible that sediments around DN harbors some WW residues considering the higher Hg compared to CR sites. However, organic matter concentrations in DN sediments were lower and less favorable to storage of Hg compared to KH. Furthermore, degrading munitions at the KH dumpsite continues to supply Hg, whereas DN levels are more related to historic fallout, background or alternative sources.

Mercury Fluxes from Sediments

There were overall no pronounced differences between sediment to water exchange fluxes of Hg for the CR and DN sites (Figure 3; Table 2). Most cores show a low Hg release over time (<0.5 nmol m -2 d -1 ). In contrast, both FK2 cores from KH had considerably higher overall Hg fluxes (>2 nmol m -2 d -1 ), matching enhanced concentrations of Hg found in sediment and overlying waters (Table 2). Both FD1 cores from DH also showed a positive Hg flux, and overlying water concentrations were higher at the incubation experiments end, however sediment Hg concentrations were not markedly higher, and their variability (±SD) likely negated any positive output. The FK1 cores displayed high sediment concentrations yet did not have remarkably high Hg fluxes. Several cores exhibited negative flux rates, indicating Hg incursion instead of release from sediment.

Results for MeHg fluxes derived from core incubations were different to those for Hg.

The FK1 cores held high sediment MeHg concentrations, yet had a lower MeHg flux with greater spread than FK2 cores (Table 2). The FD1-2 core exhibited both high MeHg and Hg fluxes from sediment, although the associated FD1-1 core had a negative MeHg flux (and higher ±SD). Yet the large standard deviations for both FD1 cores encompass both outputs. The FK2 cores provided substantial MeHg release into the overlying waters. Both FK2 cores yielded MeHg flux rates that were strikingly higher than for all other cores and sites (>400 pmol m -2 d -1 ; Table 2), even considering the high variation. There were also quite enhanced MeHg concentrations found in overlying core waters, with levels up to 1000-fold higher than others (>2000 fM; Figure 3

top).

Table 2. Observed sediment concentrations for Hg and MeHg, and %MeHg, total carbon (TC) and sulfur (S) in flux core samples for CR (FC1, FC2), and duplicate cores for KH (FK1, FK2) and DN (FD1, FD2) stations (see SI; Figure S2). Average calculated (flux) sediment-water exchange rates (± SD) for the experiment duration are reported for Hg and MeHg for each core. Methylating bacteria typically increase their activity and MeHg release during warmer summer months [START_REF] Ullrich | Mercury in the aquatic environment: a review of factors affecting methylation[END_REF][START_REF] Gosnell | Seasonal Cycling and Transport of Mercury and Methylmercury in the Turbidity Maximum of the Delaware Estuary[END_REF][START_REF] Bravo | Biotic formation of methylmercury: A bio-physio-chemical conundrum[END_REF], and flux cores were collected in July when Hg methylation theoretically is higher. It is feasible that the first time point is not accurate, as waters measured then had been sitting in the dark cores during transport and setup in the lab (>14 h), and thus likely contained an unrealistic baseline for MeHg.

Site core ID

Hg

Subsequently, MeHg is unstable, and will rapidly demethylate naturally and via photoreduction processes [START_REF] Costa | Photoreduction and evolution of mercury from seawater[END_REF][START_REF] Ullrich | Mercury in the aquatic environment: a review of factors affecting methylation[END_REF][START_REF] Luo | Photochemical behaviors of mercury (Hg) species in aquatic systems: A systematic review on reaction processes, mechanisms, and influencing factor[END_REF]. This would explain the comparably steep drop in MeHg levels after the first time point in the flux core experiment.

Accordingly, MeHg sediment water exchange flux rates are only reported for the times after 6 h (Table 2), as exchange had stabilized. Sediment methylation rates are usually calculated for 2 h as demethylation typically overrides methylation over longer periods (Hintermann et al., 2000).

Hence MeHg demethylation in ambient overlying core water was greater or consistent with what could be resupplied via sediment efflux. This is evident by the relatively consistent MeHg concentrations (considering SD) measured in core waters over the experiment duration (Figure 3). Several cores had negative fluxes, indicating MeHg demethylation or particle adsorption and deposition (i.e. onto sediments). Yet enhanced MeHg production and higher Hg species fluxes were observed at site FK2. Given the variability in sediment water exchange rates for most of the cores it is evident that only select KH sediments transferred Hg and MeHg into overlying waters.

The methylation process can be influenced by organic matter elemental composition, and increased microbial activity might likewise be driven by organic matter abundance and lability [START_REF] Bravo | Biotic formation of methylmercury: A bio-physio-chemical conundrum[END_REF][START_REF] Capo | Oxygen-deficient water zones in the Baltic Sea promote uncharacterized Hg methylating microorganisms in underlying sediments[END_REF]. A minor association with C-org was observed for the MeHg flux rates (r 2 =0.449, p=0.033, n=10), however not for Hg flux rates (r 2 =0.311, p=0.093, n=10) for the sediment cores. The Hg flux rate was furthermore not related to S content (r 2 =0. 255, p=0.136, n=10). But there was some significance between S content and MeHg flux rate (r 2 =0.406, p=0.047, n=10). This suggests that the degree of MeHg formation and output for flux cores could have been in part controlled by organic matter and S content of sediment, while Hg release was independent of those factors. Thus, if there is a sufficient pool of Hg, then it is possible for local sulfur reducing bacteria (SRB) and other sediment microbes to transform and then facilitate transfer of MeHg into overlying waters.

Methylmercury in flux core sediments was spatially variable, though within the range of site values as well as other reported concentrations [START_REF] Beldowski | Sea-dumped ammunition as a possible source of mercury to the Baltic Sea Sediments[END_REF][START_REF] Siedlewicz | Presence of mercury and methylmercury in Baltic Sea sediments, collected in ammunition dumpsites[END_REF]. It was surprising that excessive MeHg sediment concentrations were not measured in FK2 cores considering the higher MeHg and Hg fluxes. Moreover, the MeHg as well as %MeHg was higher in FK1 core sediments. Accordingly, the high MeHg discharge recorded in FK2 cores could explain the low levels of sediment MeHg and %MeHg observed. Specifically, if MeHg is produced and released at high rates, then the MeHg would be low in sediment due to its transfer instead to overlying waters. There is evidence of this occurring in estuarine systems [START_REF] Gosnell | Seasonal Cycling and Transport of Mercury and Methylmercury in the Turbidity Maximum of the Delaware Estuary[END_REF]. Once released into the shallow water column and subjected to UV light MeHg can be rapidly demethylated back to Hg [START_REF] Costa | Photoreduction and evolution of mercury from seawater[END_REF][START_REF] Luo | Photochemical behaviors of mercury (Hg) species in aquatic systems: A systematic review on reaction processes, mechanisms, and influencing factor[END_REF]. Likewise, even if

MeHg is supplied into the water column via sediment release, then it would not necessarily be measurable at high levels for long.

Dissolved organic matter (DOM) and DOC can influence Hg speciation, methylation and photoreduction within marine and fresh waters and sediments [START_REF] Ullrich | Mercury in the aquatic environment: a review of factors affecting methylation[END_REF][START_REF] Mason | An examination of the factors influencing the flux of mercury, methylmercury and other constituents from estuarine sediment[END_REF][START_REF] Bravo | The interplay between total mercury, methylmercury and dissolved organic matter in fluvial systems: A latitudinal study across Europe[END_REF]Lou et al., 2020). The DOC concentrations were higher in covering waters from core experiments compared to those measured in the water column. The FK2 core waters contained the highest DOC at ~933 µM, whereas FD1 had 809 µM and FD2 had 883 µM.

DOC concentrations for control site (FC1 and FC2) and FK1 cores were ~698 µM (± 186). Thus, higher Hg and MeHg levels for the FK2 core waters were associated with higher DOC in covering waters. However, DOC concentrations were not disproportionate relative to other cores as to form a dominat factor towards enhanced Hg species levels in overlying waters of the FK2 core. Furthermore, DOC concentrations did not selectively change during the experiment.

Under low oxygen conditions in sediments Hg can be transformed into MeHg by SRB, and can furthermore bind to sulfide which is produced from sulfate [START_REF] King | Sulfate-Reducing Bacteria Methylate Mercury at Variable Rates in Pure Culture and in Marine Sediments[END_REF][START_REF] Ma | Mercury methylation by anaerobic microorganisms: A review[END_REF][START_REF] Bravo | Biotic formation of methylmercury: A bio-physio-chemical conundrum[END_REF][START_REF] Capo | Oxygen-deficient water zones in the Baltic Sea promote uncharacterized Hg methylating microorganisms in underlying sediments[END_REF]. Oxygen saturation had decreased by ~10% in all overlying core waters by the end of the experiment, yet did not reach hypoxic or anoxic levels.

Therefore, it is not likely that Hg speciation within individual core waters was influenced by oxygen state during experiments, especially as the experiment was not under completely closed conditions. Yet even if dissolved Hg or MeHg is released by sediments, the natural oxidative state within the shallow and well mixed water column and reactivity of ambient particles ensues that most MeHg would quickly demethylate and the Hg will redeposit back into sediments.

Mercury in the Water Column

Stations in Kiel Bay feature shallow and well mixed waters, resulting in no pronounced temperature and salinity variations in depth profiles. There was a slight increase for salinity and decrease in temperature and oxygen saturation with depth for CR stations in May, as is consistent with offshore observations (SI; Figure S3). Overall vertical nutrient profiles did not show major deviations with depth or amongst sites for a given month or season. (See SI for more detail on measured nutrients.)

The Hg concentrations were not significantly higher at KH and DN compared to the outer CR sites, with most observations below 2 pM (Figure 4; SI, Figure S4). These values also reside within the reported range of the waters collected directly in Kiel Fjord as well as our measurements further offshore along the German Baltic coast (0.8 -2.1 pM; SI, Figure S1 & Table S2), and those reported for nearby coastal and Baltic waters [START_REF] Soerensen | Eutrophication increases phytoplankton methylmercury concentrations in a coastal sea -A Baltic Sea case study[END_REF][START_REF] Kuss | The impact of the Major Baltic Inflow of December 2014 on the Mercury Species Distribution in the Baltic Sea[END_REF].

The MeHg concentrations were relatively uniform with depth for stations throughout KB (SI; Figure S4 & Table S2), although some shallow stations at DN had slightly higher concentrations (Figure 4). There was not strong evidence of elevated MeHg concentrations in the water column (<6 m) following release from shallow sediments, as MeHg concentrations in overlying waters of flux core experiments were much higher than measured at all sites (Figure 3).

Average water column MeHg concentrations in May were 24 ± 9 fM (CR), 28.7 ± 13 fM (DN) and 24.5 ± 12 fM (KH), hence low and typical for Baltic and coastal waters. [START_REF] Soerensen | Eutrophication increases phytoplankton methylmercury concentrations in a coastal sea -A Baltic Sea case study[END_REF] reported MeHg concentrations of 1500 fM for deep waters in Gotland basin off eastern Sweden, however noted the rest of the south Baltic did not exceed ~100 fM. Overall, MeHg was not enhanced at KH or DN or overall in waters of KB compared to other coastal regions, and was consistent with MeHg measurements in the south Baltic [START_REF] Soerensen | Eutrophication increases phytoplankton methylmercury concentrations in a coastal sea -A Baltic Sea case study[END_REF][START_REF] Kuss | The impact of the Major Baltic Inflow of December 2014 on the Mercury Species Distribution in the Baltic Sea[END_REF]. Dissolved %MeHg did not exceed 6% at any station, and overall KB levels were 2.7 ± 0.6% (December), 2.5 ± 1.5% (May) and 0.8 ± 0.4% (July).

Water profiles exhibited no or minimal distinctive features indicating well-mixed water column at the various stations. There was no significant difference between KH and DN compared to CR sites for Hg in May (p=0.11,n=35,ANOVA). On the other hand, there were significant differences in Hg concentrations between seasons (December, May and July) for KH (p=0.001, n=32, ANOVA) and CR sites (p=0.05, n=27, ANOVA). There were also no differences in MeHg concentrations in water samples between sites in May (p=0.223, n=53, ANOVA), but strong seasonal differences were observed for both CR (p=0.001, n=27, ANOVA)

as well as KH sites (p=0.024, n=30, ANOVA). Considering seasonal differences, concentrations were highest in July (Hg) or May (MeHg) for all sites. However average %MeHg values for all sites were lowest in July (<1%; SI, Table S2). Thus, extensive sunlight radiation likely facilitated photoreduction of MeHg in shallow waters during summertime [START_REF] Costa | Photoreduction and evolution of mercury from seawater[END_REF][START_REF] Luo | Photochemical behaviors of mercury (Hg) species in aquatic systems: A systematic review on reaction processes, mechanisms, and influencing factor[END_REF].

Some Baltic Sea regions exhibit enhanced concentrations of Hg (i.e. >4 pM) and MeHg within its brackish waters [START_REF] Pempkowiak | Mercury in water and sediments of the southern Baltic Sea[END_REF][START_REF] Elmgren | Understanding Human Impact on the Baltic Ecosystem: Changing Views in Recent Decades[END_REF][START_REF] Beldowski | Total, methyl and organic mercury in sediments of the Southern Baltic Sea[END_REF][START_REF] Gubelit | Nutrient and metal pollution of the eastern Gulf of Finland coastline: Sediments, macroalgae, microbiota[END_REF][START_REF] Soerensen | Eutrophication increases phytoplankton methylmercury concentrations in a coastal sea -A Baltic Sea case study[END_REF][START_REF] Kuss | The impact of the Major Baltic Inflow of December 2014 on the Mercury Species Distribution in the Baltic Sea[END_REF], but concentrations within KB were not higher compared to other coastal waters. Kiel Bay supports an active industrial shipyard and much merchant and military traffic, and thus has several prospective sources of anthropogenic Hg [START_REF] Kwasigroch | Distribution and bioavailability of mercury in the surface sediments of the Baltic Sea[END_REF]. However, there were no observations of waters reaching what would be considered contaminated levels for dissolved Hg and MeHg. Even the highest concentrations at DN and KH did not exceed 55 fM MeHg or 2 pM Hg (Figure 4). Furthermore, data from KB indicates a decreasing trend in Hg within biological tissues over the last 20 years [START_REF] Helcom | Updated Fifth Baltic Sea pollution load compilation (PLC-5.5)[END_REF]. Consistent water column concentrations indicate no site-specific contamination of Hg and MeHg emitting from WWII munitions, in contrast to the sediments. Likewise, higher or point source specific fish contamination has not been associated with the WWII munition dump sites in KB [START_REF] Kammann | Deep impact? Is mercury in dab (Limanda limanda) a merker for dumped munitions? Results from munition dump site Kolberger Heide (Baltic Sea)[END_REF].

Water Development in the southwest Baltic Sea

The Baltic Sea is a unique and important ecosystem incorporating an overall shallow basin with a mean depth of 55 m and no permanent current structure. Water renewal from the North Sea is relatively poor for the Baltic, as it enters only via scattered inlets flowing through the southwest corner, yielding an overall low and fluctuating salinity. Marine waters moving into KB originate from the northern Kattegat and North Sea [START_REF] Stedmon | Tracing water mass mixing in the Baltic-North Sea transition zone using the optical properties of colored dissolved organic matter[END_REF], delivering one of the smallest measured semi-diurnal tidal ranges (typically ~5-10 cm). Atmospheric and riverine sources dominate aquatic Hg inputs to the Baltic Sea. Recent estimates indicate that rivers introduce approximately 70% of Hg to the Baltic [START_REF] Lilja | Screening study on occurrence of hazardous substances in the eastern Baltic Sea[END_REF][START_REF] Beldowska | Factors influencing variability of mercury input to the southern Baltic Sea[END_REF][START_REF] Helcom | Updated Fifth Baltic Sea pollution load compilation (PLC-5.5)[END_REF], whereas the remaining 30% enters via erosion and atmospheric deposition (HELCOM 2010; [START_REF] Leipe | Mercury in Baltic Sea Sediments-Natural background and anthropogenic impact, Earth's chemistry[END_REF][START_REF] Beldowski | Total, methyl and organic mercury in sediments of the Southern Baltic Sea[END_REF]. Inputs are typically high considering the historical and extensive use of coal as an energy source throughout Germany and surrounding countries, which considerably contribute to atmospheric and depositional Hg levels.

River catchments with farmlands can also deliver high Hg inputs owing to a historical use of fungicides associated with Hg [START_REF] Beldowska | Factors influencing variability of mercury input to the southern Baltic Sea[END_REF]. Accordingly, it is clear that the dominant atmospheric and riverine Hg sources provide a homogeneous effect for Baltic Sea waters. This would in turn conceal lesser point sources of Hg such as directly seeping out of munitions at KH.

Consequently, it is likely that Hg species in the KB water column are more influenced by seasonal factors (i.e. temperature or hypoxia) than proximity to underlying relic WW munitions.

To further investigate seasonal variations, water samples were collected from the nearby station Boknis Eck, located outside Eckernförde Bay within the study area (Figure 1). Boknis Eck is one of the oldest continuous monitoring stations for marine parameters worldwide, with approximate monthly sampling since April 1957 [START_REF] Lennartz | Long-term trends at the Boknis Eck time series station (Baltic Sea), 1957-2013: does climate change counteract the decline in eutrophication?[END_REF]. Boknis Eck consistently experiences seasonal anoxia during late summer/early fall, partially due to decreasing oxygen solubility with increasing temperature, and enhanced respiration rates from reduced atmospheric oxygen supply by ventilation due to water column stratification [START_REF] Lennartz | Long-term trends at the Boknis Eck time series station (Baltic Sea), 1957-2013: does climate change counteract the decline in eutrophication?[END_REF]. Dissolved Hg and MeHg samples were collected periodically at Boknis Eck during 2020-2021 in order to characterize any seasonal impacts on Hg species and concentrations (Figure 5). Through spring the waters were well mixed with no distinct profile features noted, and exhibited low Hg and MeHg as observed for KB waters. However, Hg and MeHg concentrations increased in summer and autumn. The MeHg was highest in June and Hg peaked in September at depth and coincided with recorded anoxia and organic matter remineralization. The %MeHg at Boknis Eck was between 1-2% for April and June, but <1% for other months. In winter, the Hg and MeHg decreased to low values and the observed profile features dispersed. Hence, it is likely that waters in summer through to September featured characteristics that facilitated methylation, such as eutrophication and hypoxia, compared to spring or winter when enhanced mixing and oxygen renewal occur. Baltic waters typically exhibit enhanced primary production and high concentrations of freshwater derived marine and terrestrial DOM [START_REF] Lennartz | Long-term trends at the Boknis Eck time series station (Baltic Sea), 1957-2013: does climate change counteract the decline in eutrophication?[END_REF][START_REF] Seidel | Composition and Transformation of Dissolved Organic Matter in the Baltic Sea[END_REF] [START_REF] King | Sulfate-Reducing Bacteria Methylate Mercury at Variable Rates in Pure Culture and in Marine Sediments[END_REF][START_REF] Gosnell | Seasonal Cycling and Transport of Mercury and Methylmercury in the Turbidity Maximum of the Delaware Estuary[END_REF][START_REF] Ma | Mercury methylation by anaerobic microorganisms: A review[END_REF][START_REF] Bravo | Biotic formation of methylmercury: A bio-physio-chemical conundrum[END_REF][START_REF] Capo | Oxygen-deficient water zones in the Baltic Sea promote uncharacterized Hg methylating microorganisms in underlying sediments[END_REF]. The KB water column was not anoxic, but it is likely that the anoxic horizon in its sediments was shallower and caused an elevated flux of MeHg to overlying waters. This was evident at the munition dump KH considering the active FK2 flux cores had >4000 fM MeHg within the overlying water.

However overall the well-mixed and oxygenated water column dissipates and removes most effluxes. Hence, though sediments harbor Hg from munition dumps and other anthropogenic sources, it currently does not transmit enhanced fluxes into overlaying waters or provide a distinctive higher water column concentration of Hg for KB residents.

Conclusion

This study observed high Hg concentrations in sediments of Kiel Bay (KB) and most prominently around the munition dump site Kolberger Heide (KH). There were some elevated

Figure 1 .

 1 Figure 1. Map of stations for water and sediment sample collection in Kiel Bay in the southwest Baltic Sea (north Germany). Sites are abbreviated as background control (CR), Dänisch Nienhof (DN), and the munition dump site Kolberger Heide (KH). Section distances (km) for each site are indicated. Water profiles (5 total) were collected at the Boknis Eck monitoring station (yellow target) during 2020-2021 to provide additional water inventory.

Figure 2 .

 2 Figure 2. Mercury (Hg [ng/g] dry weight; top) and methylmercury (MeHg [pg/g] dry weight; bottom) concentrations in sediments collected within Dänisch Nienhof (DN), munition dump site Kolberger Heide (KH) and background control sites (CR).

Figure 5 .

 5 Figure 5. Dissolved Hg (pM) and MeHg (fM) concentrations in several monthly profiles collected at the monitoring station Boknis Eke during 2020-2021.

  Hg concentrations in sediments near Dänisch-Nienhof (DN), where munition deployment may have delivered Hg during combat operations and training. But Hg levels in DN sediments are not as high as those at KH, possibly reflecting background, or a historical burden by artillery sources containing Hg rather than any new HgFu supplied from recent munition corrosion. Munitions are only a notable source of Hg contamination when in direct vicinity of WWII ordnance. Enhanced organic matter concentrations furthermore facilitated better storage of Hg species in KH sediments compared to DN and CR. All the same, there was no clear evidence of sediment MeHg which could be associated with munitions, and overall sediment MeHg and %MeHg were low.Sediment-water exchange core experiments suggest that sediment around KH could provide elevated Hg and MeHg fluxes into overlying waters. However, the variations in Hg and MeHg concentration concentrations over time and between cores indicated that higher levels were not established in the open and well mixed oxygenated water column of KB. The mechanism of release for possible munition derived Hg species from sediment was not assessed in this study, thus this requires further exploration. But it is likely that sediments containing a higher content of organic matter, such as those surrounding KH, could accumulate and release more Hg and MeHg than sediments which are sandy or barren.Our results indicate that munitions are not a noticeable source of Hg contamination to waters of KB. These findings align with other observations indicting that fish found in KB were not impacted by Hg sourced from the munition dump KH. Overall water column Hg and MeHg values were relatively uniform throughout the region, and not higher within waters in the vicinity of the munition dump site KH. Thus, whilst enhanced fluxes into overlying waters were observed from FK2 cores, general removal and mixing processes in the water column resulted in an absence of clear sediment source signatures at KH or in KB. There were minor seasonal concentration variations for Hg and MeHg values, such as increased levels during the summer season, and hence possible subsidiary changes associated with environmental parameters (e.g. sunlight, oxygen or DOC). For example, water column %MeHg was highest in December and lowest in July, which may be associated with longer days and enhanced sunlight duration, and thus stronger MeHg photodegradation during summertime. However, above all for Kiel and Baltic citizens, Hg and MeHg do not presently reach concerning or endangering concentrations in waters overlying the resting grounds of historic WWs munitions.

Table 1 .

 1 Regression coefficients (r 2 , n=29) between Hg and MeHg and organic matter parameters (nitrogen content by weight: N %wt; total carbon: TC %wt; sulfur content: S %wt; organic carbon: C-org; and molar ratio: [C/N]). Associations are noted for the overall sediments, control sites (CR; n=8), Dänisch Nienhof (DN; n=8) and Kolberger Heide (KH; n=13).

		Organic
		parameter overall	CR	DN	KH
	Hg	N %wt	0.490	0.987	0.446	0.623
			p=<0.001	p=<0.001	p=0.101	p=0.001
	Hg	TC %wt	0.502	0.988	0.529	0.648
			p=<0.001	p=<0.001	p=0.063	p=0.001
	Hg	S %wt	0.538	0.983	0.374	0.689
			p=<0.001	p=<0.001	p=0.144	p=<0.001
	Hg	C-org	0.504	0.988	0.429	0.647
			p=<0.001	p=<0.001	p=0.109	p=<0.001
	Hg	[C/N]	0.383	0.779	0.023	0.361
			p=0.0001	p=0.003	p=0.743	p=0.029
	MeHg	N %wt	0.489	0.566	0.266	0.426
			p=<0.001	p=0.059	p=0.067	p=0.001
	MeHg	TC %wt	0.492	0.553	0.258	0.429
			p=<0.001	p=0.037	p=0.058	p=0.001
	MeHg	S %wt	0.486	0.543	0.303	0.386
			p=<0.001	p=0.025	p=0.060	p=0.001
	MeHg	C-org	0.487	0.561	0.243	0.410
			p=<0.001	p=0.028	p=0.064	p=0.001
	MeHg	[C/N]	0.452	0.656	0.300	0.210
			p=0.038	p=0.153	p=0.120	p=0.624

[ng/g] MeHg [pg/g] %MeHg TC [% wt.] S [% wt.] Hg flux [nmol m -2 d -1 ] MeHg flux [pmol m -2 d -1 ] FC1

		2.45	1.72	0.08	0.13	0.02	-0.428 ± 2.9	4.523 ± 19.9
	FC2	36.99	15.17	0.04	1.85	0.35	0.393 ± 2	14.369 ± 17.7
	FD1-1	38.91	23.19	0.06	2.07	0.31	-0.013 ± 1.5 -14.597 ± 34.7
	FD1-2	37.91	13.55	0.04	1.76	0.22	1.962 ± 1.5	26.942 ± 22.4
	FD2-1	83.32	14.94	0.02	4.46	0.89	1.209 ± 4.2	-5.618 ± 17.6
	FD2-2	81.49	8.28	0.01	4.46	0.87	-0.095 ± 3.5	-7.675 ± 29.1
	FK1-1	105.09	46.77	0.04	4.26	0.79	0.131 ± 2.4	6.604 ± 76.6
	FK1-2	179.27	66.61	0.04	2.53	0.46	0.154 ± 0.6	16.633 ± 35.4
	FK2-1	212.57	42.97	0.02	5.90	1.11	2.979 ± 1.5 934.18 ± 580.4
	FK2-2	253.54	28.73	0.01	5.79	1.05	2.828 ± 0.4 440.71 ± 669.1

  . The DOC concentrations did not differ between KH and CR or DN, measuring around 330 µM (± 77 µM) for May and July. These values agree with the seasonal changes in DOC between ~200 -380 µM reported by[START_REF] Lodeiro | Seasonal variations in proton binding characteristics of dissolved organic matter isolated from the Southwest Baltic Sea[END_REF] for Boknis Eck. Thus, DOC concentrations are not a prominent factor for different Hg levels for Boknis Eck, and overall relatively consistent within KB.Approximately 10% of the Baltic Sea has anoxic or hypoxic waters(Cartensen et al., 2014), which can potentially lead to elevated MeHg concentrations.[START_REF] Soerensen | Eutrophication increases phytoplankton methylmercury concentrations in a coastal sea -A Baltic Sea case study[END_REF] proposed that eutrophication and the ensuing anoxic bottom waters resulted in an up to 80-fold increase in MeHg concentrations for Gotland Basin. This likely occurred at KB to a smaller extent as demonstrated at Boknis Eck, where MeHg concentrations increased in waters coinciding with the seasonal anoxia of late summer and autumn. As noted, SRB that methylate Hg are more active, and hence methylate more Hg with warmer temperatures

Acknowledgements

Much appreciation goes to the Captain and crew of the FS Littorina for their assistance during the careful logistics and sample collection around the munition sites and Kiel Bay. We would also like to thank André Mutzberg, Florian Evers, Bettina Domeyer and Aurèlie Dufour for their knowledge and help with sample collection and analysis. We would further like to thank Christian Schlosser, as well as Jens Greinert, Mareike Kampmeier and Marc Seidel for help with geophysical observations. Travel to France was possible with a PROCOPE-Mobility grant.

Funding for this research was supported by a grant from Wirtschaftsförderung und Technologietransfer Schleswig-Holstein GmbH (WT.SH: MUNISEE Nr. 123 18 011).