domain: chim sde

text:
Sylvain Bouchet
email: sylvain.bouchet@usys.ethz.ch

Anne L Soerensen

Erik Björn

Emmanuel Tessier

David Amouroux

Mercury Sources and Fate in a Large Brackish Ecosystem (the Baltic Sea) Depicted by Stable Isotopes

Keywords: Hg stable isotopes, Hg sources, MMHg photodegradation, dissolved organic matter

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INTRODUCTION

Humans are predominantly exposed to mercury (Hg) through the consumption of seafood products [START_REF] Sheehan | Global Methylmercury Exposure from Seafood Consumption and Risk of Developmental Neurotoxicity: A Systematic Review[END_REF] , in which Hg is mostly present as the neurotoxic compound monomethylmercury (MMHg). Concentrations of MMHg in seafood depend on the efficiency of MMHg bioconcentration from water to plankton and its bioamplification through food webs. [START_REF] Lavoie | Biomagnification of Mercury in Aquatic Food Webs: A Worldwide Meta-Analysis[END_REF][START_REF] Wu | The Importance of Bioconcentration into the Pelagic Food Web Base for Methylmercury Biomagnification: A Meta-Analysis[END_REF] The presence of inorganic Hg (the substrate for MMHg formation) in aquatic ecosystems can be ascribed to multiple sources: direct atmospheric deposition (dry and wet), watershed runoff and industrial discharges. It is critical to know the contribution from each source to fish MMHg in a given ecosystem to develop efficient policies of Hg emissions reduction. [START_REF] Blanchfield | Experimental Evidence for Recovery of Mercury-Contaminated Fish Populations[END_REF] The production of MMHg is essentially a biotic process controlled by the interplay between the activity of specific methylating microorganisms and the bioavailability of inorganic Hg. [START_REF] Regnell | Microbial Mercury Methylation in Aquatic Environments: A Critical Review of Published Field and Laboratory Studies[END_REF] On the contrary, the degradation of MMHg can proceed through several biotic and/or abiotic pathways with photodegradation considered a major sink in the photic zone. [START_REF] Barkay | Demethylation─The Other Side of the Mercury Methylation Coin: A Critical Review[END_REF][START_REF] Luo | Photochemical Behaviors of Mercury (Hg) Species in Aquatic Systems: A Systematic Review on Reaction Process, Mechanism, and Influencing Factor[END_REF] Over the past two decades, measurements of Hg stable isotopes has emerged as a major tool to advance our understanding of Hg cycling in the environment. Based on their mass-dependent and mass-independent isotope fractionation (Hg-MDF and -MIF, respectively), isotopes are useful for both differentiating Hg sources with contrasted isotopic signatures and informing on the extent, mechanisms and conditions of species transformations. [START_REF] Tsui | Review of Stable Mercury Isotopes in Ecology and Biogeochemistry[END_REF] In soils and sediments, binary or ternary mixing models have been used to discriminate between Hg originating from contaminated point sources (industrial and/or mining activities with slightly negative Hg-MDF and near zero Hg-MIF) [START_REF] Foucher | Tracing Mercury Contamination from the Idrija Mining Region (Slovenia) to the Gulf of Trieste Using Hg Isotope Ratio Measurements[END_REF][START_REF] Wiederhold | Mercury Isotope Signatures in Contaminated Sediments as Tracer for Local Industrial Pollution Sources[END_REF][START_REF] Gray | Mercury Isotope Fractionation during Ore Retorting in the Almadén Mining District, Spain[END_REF] and background Hg sources, i.e. terrestrial inputs with largely negative Hg-MDF and Hg-MIF versus the atmospheric ones. [START_REF] Lepak | Use of Stable Isotope Signatures to Determine Mercury Sources in the Great Lakes[END_REF][START_REF] Meng | An Integrated Model for Input and Migration of Mercury in Chinese Coastal Sediments[END_REF][START_REF] Jung | Elucidating Sources of Mercury in the West Coast of Korea and the Chinese Marginal Seas Using Mercury Stable Isotopes[END_REF] The contribution of atmospheric inputs is however especially difficult to estimate as both their Hg-MDF and -MIF can be highly variable in background atmosphere but especially when point-source emissions are involved. [START_REF] Kwon | Mercury Stable Isotopes for Monitoring the Effectiveness of the Minamata Convention on Mercury[END_REF] Lately the use of even Hg-MIF values (Δ 200 Hg and to a lesser extent Δ 204 Hg) has emerged to specifically discriminate gaseous Hg inputs, [START_REF] Lepak | Use of Stable Isotope Signatures to Determine Mercury Sources in the Great Lakes[END_REF][START_REF] Jiskra | Mercury Stable Isotopes Constrain Atmospheric Sources to the Ocean[END_REF] as atmospheric Hg 0 exhibits slightly negative Δ 200 Hg values whereas Hg(II) in wet and dry depositions is characterized by slight to large positive values. [START_REF] Jiskra | Mercury Stable Isotopes Constrain Atmospheric Sources to the Ocean[END_REF][START_REF] Chen | Unusual Fractionation of Both Odd and Even Mercury Isotopes in Precipitation from Peterborough, ON, Canada[END_REF] All Hg species undergo numerous reactions in surficial environments, e.g. in the atmosphere, soils, snow and water column, before being taken up by organisms at the base of food webs but only photochemical reactions specifically induce large and characteristic Hg-MIF of odd and even isotopes. [START_REF] Blum | Mercury Isotopes in Earth and Environmental Sciences[END_REF] The Δ 199 Hg/Δ 201 Hg slope has turned out to be a reliable indicator for the type of reactions involved (Hg(II) photoreduction vs MMHg photodegradation), [START_REF] Bergquist | Mass-Dependent and -Independent Fractionation of Hg Isotopes by Photoreduction in Aquatic Systems[END_REF] the mechanisms causing MIF (magnetic isotope effect vs nuclear volume effect), [START_REF] Zheng | Mercury Isotope Fractionation during Photoreduction in Natural Water Is Controlled by Its Hg/DOC Ratio[END_REF] and the Hg bonding environment including the nature of the Hg ligands and the Hg to ligands ratio. [START_REF] Zheng | Isotope Fractionation of Mercury during Its Photochemical Reduction by Low-Molecular-Weight Organic Compounds[END_REF][START_REF] Chandan | Mercury Isotope Fractionation during Aqueous Photo-Reduction of Monomethylmercury in the Presence of Dissolved Organic Matter[END_REF][START_REF] Bouchet | In Situ Photochemical Transformation of Hg Species and Associated Isotopic Fractionation in the Water Column of High-Altitude Lakes from the Bolivian Altiplano[END_REF] Similarly, the even Hg-MIF slopes are emerging as a new indicator of photochemical reactions occurring in the upper atmosphere. [START_REF] Sun | Mass-Dependent and -Independent Fractionation of Mercury Isotope during Gas-Phase Oxidation of Elemental Mercury Vapor by Atomic Cl and Br[END_REF][START_REF] Fu | Mass-Independent Fractionation of Even and Odd Mercury Isotopes during Atmospheric Mercury Redox Reactions[END_REF] Hg stable isotopes are also powerful tracers to examine the environmental controls of MMHg cycling, e.g. its production depth in aquatic environments or the influence of light transmission and attenuation for its photodegradation, [START_REF] Blum | Methylmercury Production below the Mixed Layer in the North Pacific Ocean[END_REF][START_REF] Sherman | Mercury Stable Isotopes in Sediments and Largemouth Bass from Florida Lakes, USA[END_REF][START_REF] Motta | Mercury Stable Isotopes in Flying Fish as a Monitor of Photochemical Degradation of Methylmercury in the Atlantic and Pacific Oceans[END_REF] by analyzing high-trophic level organisms that contain predominantly MMHg. They can also be used as ecological tracers to distinguish populations and provide unique information on animal movements and their foraging habits within ecosystems. [START_REF] Tsui | Review of Stable Mercury Isotopes in Ecology and Biogeochemistry[END_REF][START_REF] Cransveld | Mercury Stable Isotopes Discriminate Different Populations of European Seabass and Trace Potential Hg Sources around Europe[END_REF] One important assumption underlying these studies is the absence of Hg MIF in organisms after its accumulation. This is in contrast to Hg-MDF that has been shown to occur in higher organisms, due to MMHg degradation and excretion. [START_REF] Tsui | Review of Stable Mercury Isotopes in Ecology and Biogeochemistry[END_REF] Therefore, the MMHg isotopic signature in organisms is useful to infer the transformations it has undergone, especially its photodegradation that is mainly controlled by light radiation intensity and penetration as well as the chemical speciation of aqueous MMHg. [START_REF] Lehnherr | Importance of Ultraviolet Radiation in the Photodemethylation of Methylmercury in Freshwater Ecosystems[END_REF][START_REF] Hammerschmidt | Iron-Mediated Photochemical Decomposition of Methylmercury in an Arctic Alaskan Lake[END_REF] Direct photolysis of MMHg is not possible under environmental conditions in aquatic ecosystems and the presence of dissolved organic matter (DOM) is required to trigger MMHg photodegradation. [START_REF] Luo | Photochemical Behaviors of Mercury (Hg) Species in Aquatic Systems: A Systematic Review on Reaction Process, Mechanism, and Influencing Factor[END_REF] Both the concentration and composition of DOM influence the extent of photodegradation through their effect on light transmission, production/consumption of reactive oxygen species, and Hg complexation. [START_REF] Klapstein | Methylmercury Biogeochemistry in Freshwater Ecosystems: A Review Focusing on DOM and Photodemethylation[END_REF][START_REF] Yang | The Role of Fluorescent Dissolved Organic Matter on Mercury Photoreduction Rates: A Case Study of Three Temperate Lakes[END_REF] The rates of MMHg photodegradation increase with dissolved organic carbon (DOC) concentrations till a breaking point (5 -10 mg L - 1 ) after which the reaction is inhibited due to either shading or the scavenging of reactive oxygen species. [START_REF] Sherman | Mercury Stable Isotopes in Sediments and Largemouth Bass from Florida Lakes, USA[END_REF][START_REF] Girard | Photodemethylation of Methylmercury in Eastern Canadian Arctic Thaw Pond and Lake Ecosystems[END_REF][START_REF] Klapstein | Methylmercury Photodemethylation Is Inhibited in Lakes with High Dissolved Organic Matter[END_REF] The complexation of MMHg by thiol groups has been found to be key as it weakens the Hg-C bond [START_REF] Zhang | Photolytic Degradation of Methylmercury Enhanced by Binding to Natural Organic Ligands[END_REF][START_REF] Fernández-Gómez | Towards Universal Wavelength-Specific Photodegradation Rate Constants for Methyl Mercury in Humic Waters, Exemplified by a Boreal Lake-Wetland Gradient[END_REF][START_REF] Jeremiason | Photoreduction of Hg(II) and Photodemethylation of Methylmercury: The Key Role of Thiol Sites on Dissolved Organic Matter[END_REF] and photodegradation has been shown to be more efficient when aromatic DOM is involved. [START_REF] Qian | Why Dissolved Organic Matter Enhances Photodegradation of Methylmercury[END_REF][START_REF] Zhang | Probing the DOM-Mediated Photodegradation of Methylmercury by Using Organic Ligands with Different Molecular Structures as the DOM Model[END_REF] Accordingly, differences in MMHg photodegradation rates depending on the size or chemical fraction of DOM have been evidenced [START_REF] Kim | Effects of Natural Water Constituents on the Photo-Decomposition of Methylmercury and the Role of Hydroxyl Radical[END_REF][START_REF] Kim | Effects of Molecular Size Fraction of DOM on Photodegradation of Aqueous Methylmercury[END_REF] and in wetlands and rice fields, higher MMHg losses were related to optical signatures of less labile, humic DOM. [START_REF] Fleck | Concurrent Photolytic Degradation of Aqueous Methylmercury and Dissolved Organic Matter[END_REF] However, the effect of DOM quality on the extent of MMHg photodegradation has not been evaluated at large scale in aquatic ecosystems.

In this study, we investigated Hg stable isotopic signature in sediment, seston and fish samples from the various basins of the Baltic Sea, a semi-enclosed sea with a large freshwater influence and spatial gradients in several key factors known to influence Hg species transformations and bioaccumulation including salinity, redox conditions, light, ice cover, presence of algae as well as OM concentration and quality. The salinity, temperature and nutrient loads are highest in the southern Baltic Sea leading to higher photosynthetic primary production rate and thus to the development of large oxygen depleted areas with a sustained production of MMHg in the anoxic part of the water column. [START_REF] Soerensen | Deciphering the Role of Water Column Redoxclines on Methylmercury Cycling Using Speciation Modeling and Observations From the Baltic Sea[END_REF] Because of a lower temperature and larger OM inputs by river discharge in the northern Baltic Sea, the primary production is lower and the fraction of allochthonous to autochthonous OM is higher, which can impact both MMHg production, bioavailability and bioaccumulation. [START_REF] Soerensen | Eutrophication Increases Phytoplankton Methylmercury Concentrations in a Coastal Sea-A Baltic Sea Case Study[END_REF][START_REF] Soerensen | Organic Matter Drives High Interannual Variability in Methylmercury Concentrations in a Subarctic Coastal Sea[END_REF] We combined concentrations and speciation data with the isotopic signatures observed in the various compartments, trophic levels and spatial origin to provide new insights into the sources, bioaccumulation and transformations of Hg species in this large brackish ecosystem. We especially discuss the factors controlling MMHg photodegradation in the various basins and highlight the impact of the terrestrial DOM gradient on the extent of this reaction.

EXPERIMENTAL SECTION Study site, sample collection and ancillary data

Sample types and stations from which sediment, plankton and fish samples were collected are presented in Figure 1 together with our division of the sampling locations into four Baltic Sea basins. Surface sediment samples (0 -2 cm, n = 29) were obtained from the Swedish Geological Survey archive and were selected from a variety of sites (coastal vs offshore and contaminated vs pristine) to provide a sufficient spatial coverage and representation of various sources of Hg. Samples were collected between 1992 and 2007 as part of Sweden's national environmental monitoring program that takes place every 6 th year and stored freeze-dried in closed containers. Seston samples (n = 24) were collected using a 90 μm mesh-size net during field campaigns in July-August 2015 and June-November 2016 that have been previously described. [START_REF] Soerensen | Deciphering the Role of Water Column Redoxclines on Methylmercury Cycling Using Speciation Modeling and Observations From the Baltic Sea[END_REF] In short, on cruises in the Baltic Proper and Kattegat, samples were collected from the upper 20 m of the water column but the net was closed before breaching the surface to avoid getting filamentous cyanobacteria, often present at the sea surface, in the net. On cruises in the Bothnian Bay and Sea, samples were collected from the entire water depth at each sampling station (8 -125 m).

Samples were always transferred from the net to 500 ml HDPE bottles and frozen on board.

Herring and cod samples (n = 45, female specimen of 2-5 and 2 years age, respectively) were obtained from the Swedish Environmental Specimen Bank. They were collected in fall 2013 as part of the Marine Monitoring Programme for Contaminants in Marine Biota and their length, age, weight as well as their muscle δ 15 N and δ [START_REF] Meng | An Integrated Model for Input and Migration of Mercury in Chinese Coastal Sediments[END_REF] C values were measured. [START_REF] Soerensen | The Swedish National Monitoring Programme for Contaminants in Marine Biota (until 2019 Year's Data) -Temporal Trends and Spatial Variations[END_REF] The latter were then corrected for lipids according to equation 1: [START_REF] Post | Getting to the Fat of the Matter: Models, Methods and Assumptions for Dealing with Lipids in Stable Isotope Analyses[END_REF] δ 13 Cnorm = δ 13 Cuntreat -3.32 + 0.99 × C:𝑁 eq 1

The Baltic herring (Clupea harengus) is found in the various basins throughout the Baltic Sea whereas the Atlantic cod (Gadus morhua) is not present in the northern part due to the low salinity. Moreover, there are two distinct stocks of cod, a western and an eastern one, that partially mix in the southern part of the Baltic Sea. [START_REF] Aro | A Review of Fish Migration Patterns in the Baltic[END_REF] The trophic levels (TL) of fishes were calculated with equation 2 by matching the δ [START_REF] Kwon | Mercury Stable Isotopes for Monitoring the Effectiveness of the Minamata Convention on Mercury[END_REF] N data from the fish with δ 15 N of mussels based on areas of collection in the Baltic Sea. TLfish = ( 15 Nfish - 15 Nmussels)/TLF + 2 eq 2 where TLF is the trophic level fractionation of  15 N and the trophic position of mussels are assumed to be two. We use a value of 2.4 ‰ for the trophic level fractionation as previously found for the Baltic Sea, [START_REF] Hansson | The Stable Nitrogen Isotope Ratio as a Marker of Food-Web Interactions and Fish Migration[END_REF] and δ [START_REF] Kwon | Mercury Stable Isotopes for Monitoring the Effectiveness of the Minamata Convention on Mercury[END_REF] N values for mussels from previous studies. [START_REF] Soerensen | The Swedish National Monitoring Programme for Contaminants in Marine Biota (until 2019 Year's Data) -Temporal Trends and Spatial Variations[END_REF][START_REF] Karlsson | Long-term changes in stable carbon and nitrogen isotopes in Blue mussels from Kvädöfjärden, Baltic Proper[END_REF] Cumulated PAR and UV irradiance data were obtained from the Swedish Meteorological and Hydrological Institute (SMHI) and averaged over the period 2012 -2013. Salinity, secchi depth and concentrations of humic substances were also obtained from SMHI and are from monthly cruises conducted across the Baltic Sea. When available, salinity and humic substances were taken from the same station as the fish and when not we used the nearest available station. Salinity and humic substances were averaged over the top 50 m depth. DOC concentrations were obtained from Seidel et al. 52

Measurements of Hg concentrations and isotopic composition.

HgT concentrations in sediments and some fish were available through the Swedish national environmental monitoring, [START_REF] Soerensen | The Swedish National Monitoring Programme for Contaminants in Marine Biota (until 2019 Year's Data) -Temporal Trends and Spatial Variations[END_REF][START_REF] Josefsson | Results from the National Environmental Monitoring Programme[END_REF] but seston samples as well as some fish samples for which HgT values were not available were analyzed with total Hg analyzers (either Leco AMA254 or Milestone DMA-80). MMHg concentrations in sediments, seston and fish were determined after acidic or basic extraction of freeze-dried samples using species-specific isotope dilution and (thermal desorption) gas chromatography inductively coupled plasma mass spectrometry according to established protocols. [START_REF] Monperrus | [END_REF][START_REF] Jonsson | Differentiated Availability of Geochemical Mercury Pools Controls Methylmercury Levels in Estuarine Sediment and Biota[END_REF] Extraction and/or quantification of Hg total and species concentrations were validated with various certified reference materials, i.e. IAEA 405, CNRC TORT-2&3, JCR ERM CE-464 and BCR-414, and NIST SRM-1947.

For stable isotopes analysis, sediments and seston samples (0.5 g) were first extracted with a 3:1 HNO3/HCl mixture for 12 h at 85 °C, then H2O2 was added and samples were further heated at 85 °C for 12 h. Fish samples (0.2 g) were extracted for 12 h at 85 °C with a 2:1 HNO3/ H2O2 mixture. Analyses were carried out with a multicollectorinductively coupled plasma-mass spectrometer (MC-ICP-MS, Nu Instruments) using either introduction with cold vapor generation (CVG) or dual amalgamation on gold (DA-Au) using a Tekran 2600 system for low-concentrated samples. [START_REF] Bérail | Determination of Total Hg Isotopic Composition at Ultra-Trace Levels by on Line Cold Vapor Generation and Dual Gold-Amalgamation Coupled to MC-ICP-MS[END_REF] In both cases, a desolvation nebulization system (DSN-100, Nu Instrument) was used to introduce a certified thallium solution (NIST-SRM-997) for instrumental mass-bias correction using the exponential fractionation law. We used a standard-sample bracketing system to calculate δ values (in ‰) relative to the reference Hg standard NIST SRM 3133. Reference materials NIST-8610 (UM-Almaden), -1944 and -1947, ERM BCR 414 and CE-464 were used as secondary standards (Table SI-1). Isotopic ratios were calculated by the Linear Regression Slope (LRS) method for the DA-Au introduction technique as described previously. [START_REF] Bérail | Determination of Total Hg Isotopic Composition at Ultra-Trace Levels by on Line Cold Vapor Generation and Dual Gold-Amalgamation Coupled to MC-ICP-MS[END_REF] The common  notation is used to express the Hg isotopic compositions:

where

RESULTS AND DISCUSSION

Sedimentary Hg reflects the mixing between terrestrial, industrial and atmospheric sources.

As sediment samples were selected to represent both coastal and offshore as well as contaminated and pristine environments, the range of HgT concentrations is large, from 64 to 10 690 ng g -1 with a median value of 140 ng g -1 (interquartile range (IQR) = 167) (Table SI-2). MMHg concentrations range from 0.1 to 13.6 ng g -1 and contribute on average 0.7 ± 0.7 % to HgT, similar to other coastal environments and great lakes. [START_REF] Guédron | Mercury Contamination Level and Speciation Inventory in Lakes Titicaca & Uru-Uru (Bolivia): Current Status and Future Trends[END_REF][START_REF] Fitzgerald | Marine Biogeochemical Cycling of Mercury[END_REF] Our HgT values are overall in agreement with two recent studies that demonstrated a strong anthropogenic pollution during the second half of the past century that overall increased Hg concentrations in the Baltic Sea sediments over its pre-industrial background (20 -50 ng g -1 ) [START_REF] Leipe | Mercury in Baltic Sea Sediments-Natural Background and Anthropogenic Impact[END_REF] and also led to high Hg concentrations in some local hotspots (industrial and war waste sites). [START_REF] Leipe | Mercury in Baltic Sea Sediments-Natural Background and Anthropogenic Impact[END_REF][START_REF] Kwasigroch | Distribution and Bioavailability of Mercury in the Surface Sediments of the Baltic Sea[END_REF]  202 Hg in sediments ranges from -2.3 to -0.1 ‰ (Table SI-2) and with the exception of 4 outliers, it is linearly and negatively correlated to 1/HgT (Figure SI-1, r² = 0.5, p-value = 10 -4 ). This trend indicates mixing between a lighter Hg end-member with an average δ 202 Hg value of -2.18 ± 0.45 ‰ (calculated for an average background HgT concentration of 35 ng g -1 ) and a heavier Hg end-member with an average δ 202 Hg value of -0.55 ± 0.11 ‰ (yintercept in Figure SI-1), the latter being very similar to the value previously found by Gehrke. [START_REF] Gehrke | Mercury Cycling in the Marine Environment: Insights from Hg Stable Isotopes[END_REF] Although the data are relatively scattered, no specific trend is apparent between basins suggesting that both end-members are relatively homogeneous across the sites investigated. The heavy end-member value is typical of industrial sources and matches the δ 202 Hg value reported for cinnabar (HgS) 63 as well as the values previously found in sediments from polluted sites across Sweden. [START_REF] Wiederhold | Mercury Isotope Signatures in Contaminated Sediments as Tracer for Local Industrial Pollution Sources[END_REF] With respect to their concentrations, the 4 outliers exhibit a much lighter isotopic composition with  202 Hg values of about -2.3 ‰ and slightly positive odd-MIF values ( 199 Hg from 0.05 to 0.10 ‰). Their isotopic composition is very similar to those reported for a coastal site polluted by elemental Hg (exhibiting specifically low  202 Hg values) located in the Bothnian Sea south of these 4 samples. [START_REF] Wiederhold | Mercury Isotope Signatures in Contaminated Sediments as Tracer for Local Industrial Pollution Sources[END_REF] The light endmember is close to the negative  202 Hg values previously reported for boreal forest soils (-2.6 to -2.9 ‰), which results from the large MDF that occur during uptake and oxidation of atmospheric gaseous Hg by foliage. [START_REF] Jiskra | Mercury Deposition and Re-Emission Pathways in Boreal Forest Soils Investigated with Hg Isotope Signatures[END_REF][START_REF] Demers | Mercury Isotopes in a Forested Ecosystem: Implications for Air-Surface Exchange Dynamics and the Global Mercury Cycle: MERCURY ISOTOPES IN A FORESTED ECOSYSTEM[END_REF] This MDF is conserved when Hg is transferred to soils and adjacent aquatic ecosystems [START_REF] Jiskra | Source Tracing of Natural Organic Matter Bound Mercury in Boreal Forest Runoff with Mercury Stable Isotopes[END_REF] and the higher  202 Hg values in sediments compared to soils can reflect both the photoreduction of this terrestrially derived Hg(II) in the water column [START_REF] Rose | Effects of Ultraviolet Radiation on Mercury Isotope Fractionation during Photo-Reduction for Inorganic and Organic Mercury Species[END_REF] and its mixing with heavier Hg inputs, i.e. atmospheric and/or industrial Hg brought in the various basins by direct deposition, river discharges, [START_REF] Soerensen | Eutrophication Increases Phytoplankton Methylmercury Concentrations in a Coastal Sea-A Baltic Sea Case Study[END_REF] and/or sediment redistribution.

The range of Hg odd-MIF values in sediments is narrow (Table SI-1,  199 Hg = -0.16 -0.10 ‰) as commonly observed, [START_REF] Blum | Mercury Isotopes in Earth and Environmental Sciences[END_REF] There is a positive trend between  199 Hg and 1/HgT in sediments but it is not significant (Figure SI-1, p-value = 0.35). However, sediments with low Hg content exhibit significantly higher  199 Hg than values previously observed in boreal forest soils (-0.5 to -0.25 ‰), suggesting again mixing with atmospheric Hg (wet and dry) and/or photoreduction of Hg(II) before its deposition to sediments. The  199 Hg/ 201 Hg slope in sediments is indeed 1.1 ± 0.3 (not shown, r² = 0.59, p-value = 3.10 -4 ) close to the one imprinted by Hg(II) photoreduction that can occur in the water column, [START_REF] Bergquist | Mass-Dependent and -Independent Fractionation of Hg Isotopes by Photoreduction in Aquatic Systems[END_REF][START_REF] Rose | Effects of Ultraviolet Radiation on Mercury Isotope Fractionation during Photo-Reduction for Inorganic and Organic Mercury Species[END_REF] but also in the atmosphere. [START_REF] Fu | Mass-Independent Fractionation of Even and Odd Mercury Isotopes during Atmospheric Mercury Redox Reactions[END_REF] Regarding Hg even-MIF,  200 Hg values ranged from -0.04 to 0.08 ‰ and  204 Hg from -0.22 to 0.14 ‰ (Table SI-2, Figure 3), which is also similar to previous observations in freshwater and coastal sediments. [START_REF] Lepak | Use of Stable Isotope Signatures to Determine Mercury Sources in the Great Lakes[END_REF][START_REF] Sonke | Sedimentary Mercury Stable Isotope Records of Atmospheric and Riverine Pollution from Two Major European Heavy Metal Refineries[END_REF][START_REF] Meng | Mercury Isotope Variations within the Marine Food Web of Chinese Bohai Sea: Implications for Mercury Sources and Biogeochemical Cycling[END_REF] No significant relationships were found between Hg even-MIF and concentrations and the data scattering render their interpretations difficult. The discrimination of Hg sources to sediments is difficult as their Hg isotopic signature results from the mixture of multiple sources and processes inducing isotopic fractionation. Isotopic signatures of soils and especially atmospheric Hg, for which both Hg-MDF and -MIF values can be highly variable, need to be characterized at a regional level before the contribution of each source of Hg to the Baltic Sea sediments can be better deciphered.

Biota Hg concentrations and MDF are primarily controlled by trophic level.

HgT in seston range from 1.5 to 60 ng g -1 (d.w., average 18 ± 16 ng g -1 , n = 23) with the highest values found at the northernmost, least saline station Harufjarden (31 ± 16 ng g -1 , Figure SI-2). The percentage of MMHg in seston vary over the full range, from 0.5 to 98 % and averages 27 ± 22 % of HgT. In fish muscle, HgT concentrations range from 34 to 530 ng g -1 (d.w.) and average 137 ± 102 ng g -1 to which MMHg contribution is constant at 93 ± 3 %. The lowest concentrations are found in herrings from the Bothnian Sea offshore station (48 ± 12 ng g -1 , Table SI-3) while the highest are found in cod from the Fladen station (312 ± 135 ng g -1 ) which is explained by the higher trophic level of cod compared to herring. Overall, the trophic magnification slope determined from our samples is 0.14 ± 0.08 (Figure SI-3a, r 2 = 0.78), which is similar to the value previously reported for a pelagic food chain from the Baltic Sea, [START_REF] Nfon | Trophodynamics of Mercury and Other Trace Elements in a Pelagic Food Chain from the Baltic Sea[END_REF] and also in line with values compiled by Lavoie et al. [START_REF] Lavoie | Biomagnification of Mercury in Aquatic Food Webs: A Worldwide Meta-Analysis[END_REF] for a variety of ecosystems (average slope for HgT = 0.16 ± 0.11).

With the exception of one outlier ( 202 Hg = -2.47 ‰), the  202 Hg values in seston range from -1.75 to -0.26 ‰ and averaged -1.1 ± 0.5 ‰ (1 SD, Figure 2a and Table SI-4). These are very similar to sediments in range and average value (-1.2 ± 0.6 ‰ , 1 SD), which stems from the seston samples being dominated by Hg(II). There is only a weak relationship between  202 Hg in seston and 1/HgT (Figure SI-4a), reflecting both the presence of MMHg in different proportions and the fact that seston and sediments do not accumulate Hg(II) with the exact same signature (see below). As expected, the  202 Hg values in fish are higher (-0.29 to 0.73 ‰, Figure 2b) and the range between the lowest and highest value is smaller (1.0 ‰) than for seston (1.5 ‰) and sediments (2.2 ‰). This reflects a homogenization (decrease in variability) of the Hg isotopic composition higher up in the food webs with higher organisms displaying a more integrative MMHg isotopic signature. The average  202 Hg from our fish samples is 0.27 ± 0.23 ‰ (1 SD) with the lowest values found in herring from Angskarsklubb (-0.08 ± 0.15 ‰, 1 SD) whereas the highest values are found in herring from Fladen and Bothnian Sea offshore (0.55 ± 0.12 and 0.51 ± 0.09 ‰, respectively, 1 SD). Overall,  202 Hg values in biota increase on average by 0.5 ± 0.1 ‰ per trophic level (Figure SI-3b), consistent with values found in the Chinese Bohai Sea. [START_REF] Meng | Mercury Isotope Variations within the Marine Food Web of Chinese Bohai Sea: Implications for Mercury Sources and Biogeochemical Cycling[END_REF] Higher trophic level biota thus accumulates a higher share of heavier Hg isotopes in MMHg (Figure SI-3c) and variations in  202 Hg among biota are mostly explained by their trophic level. However, many seston samples from the Bothnian Bay and cod from Fladen are below this trend revealing the accumulation of isotopically lighter Hg species. For seston from the Bothnian Bay, we prescribe this to the terrestrial inputs, most important to this basin, carrying lighter Hg isotopes. For cod from Fladen, they have been shown to have a more pronounced benthic feeding strategy as they live in shallow waters, [START_REF] Funk | The Forgotten Feeding Ground: Patterns in Seasonal and Depth-specific Food Intake of Adult Cod GADUS MORHUA in the Western Baltic Sea[END_REF] and they are therefore likely to accumulate more MMHg originating from sediments, that is less (photo)fractionated, compared to the pelagic herring and cod from SO Gotland that live in deeper waters.

Odd Hg-MIF in seston reflects in situ photochemical reactions and even Hg-MIF atmospheric Hg sources.

 199 Hg values in seston range from -0.04 to 2.18 ‰ (Table SI-4) and seston samples are found in intermediate positions, depending on their trophic level and Hg speciation, between the background sediment and the fish signatures on the  199 Hg - 202 Hg plot (Figure 2a). Overall, the  199 Hg/ 201 Hg slope in seston is 0.92 ± 0.11 (Figure 2c), close to the slope produced by the photoreduction of Hg(II). [START_REF] Bergquist | Mass-Dependent and -Independent Fractionation of Hg Isotopes by Photoreduction in Aquatic Systems[END_REF] This results from the predominance of Hg(II) over MMHg (73 vs 27 % average, respectively) and indicates that most of the Hg present in seston has undergone some photochemical processing, either Hg(II) photoreduction or MMHg photodegradation. However, the  199 Hg signal in seston is also positively albeit weakly correlated to the % MMHg (Figure SI-4b, R² = 0.50) and negatively to the % Hg(II) (not shown), suggesting that the MIF signal is dominated by the contribution of MMHg in seston of higher trophic level. The weak correlation between MIF values and % MMHg is likely due to the spatial and seasonal variability of photochemical processes affecting both MMHg and Hg(II) in the water column before their uptake. Photochemical reactions are affected by a number of parameters, among which light radiation intensity and wavelength as well as DOM concentrations are the most important, [START_REF] Lehnherr | Importance of Ultraviolet Radiation in the Photodemethylation of Methylmercury in Freshwater Ecosystems[END_REF][START_REF] Klapstein | Methylmercury Biogeochemistry in Freshwater Ecosystems: A Review Focusing on DOM and Photodemethylation[END_REF][START_REF] Amyot | Effect of Solar Radiation on the Formation of Dissolved Gaseous Mercury in Temperate Lakes[END_REF] and all of them vary both spatially and temporally across the Baltic Sea (see below).

Even Hg-MIF values in sediment, seston and fish are presented in Figure 3. Median  200 Hg values in seston and fish are higher than in sediments (0.12, 0.03 and 0.01 ‰, respectively), which is consistent with previous studies on aquatic invertebrates and fish, [START_REF] Blum | Methylmercury Production below the Mixed Layer in the North Pacific Ocean[END_REF][START_REF] Sherman | Mercury Stable Isotopes in Sediments and Largemouth Bass from Florida Lakes, USA[END_REF][START_REF] Lepak | Factors Affecting Mercury Stable Isotopic Distribution in Piscivorous Fish of the Laurentian Great Lakes[END_REF][START_REF] Sackett | Nitrogen, and Mercury Isotope Evidence for the Biogeochemical History of Mercury in Hawaiian Marine Bottomfish[END_REF][START_REF] Motta | Mercury Cycling in the North Pacific Subtropical Gyre as Revealed by Mercury Stable Isotope Ratios[END_REF] but it is particularly noteworthy that several values are above 0.2 ‰ in seston. Hg isotopic data in phyto-or zooplankton are particularly scarce, [START_REF] Motta | Mercury Cycling in the North Pacific Subtropical Gyre as Revealed by Mercury Stable Isotope Ratios[END_REF][START_REF] Perrot | Higher Mass-Independent Isotope Fractionation of Methylmercury in the Pelagic Food Web of Lake Baikal (Russia)[END_REF] but the maximum value of 0.58 ‰ observed in our study is to the best of our knowledge the highest reported in such samples. These values are actually similar to values reported for seawater in the Canadian Arctic [START_REF] Štrok | Development of Pre-Concentration Procedure for the Determination of Hg Isotope Ratios in Seawater Samples[END_REF][START_REF] Štrok | The Mercury Isotope Composition of Arctic Coastal Seawater[END_REF] and higher than many values reported for precipitations collected at low latitudes in the temperate zone, [START_REF] Demers | Mercury Isotopes in a Forested Ecosystem: Implications for Air-Surface Exchange Dynamics and the Global Mercury Cycle: MERCURY ISOTOPES IN A FORESTED ECOSYSTEM[END_REF][START_REF] Sherman | Investigation of Local Mercury Deposition from a Coal-Fired Power Plant Using Mercury Isotopes[END_REF][START_REF] Enrico | Atmospheric Mercury Transfer to Peat Bogs Dominated by Gaseous Elemental Mercury Dry Deposition[END_REF][START_REF] Gratz | Isotopic Composition and Fractionation of Mercury in Great Lakes Precipitation and Ambient Air[END_REF][START_REF] Wang | Mass-Dependent and Mass-Independent Fractionation of Mercury Isotopes in Precipitation from Guiyang, SW China[END_REF][START_REF] Rolison | Isotopic Composition of Species-Specific Atmospheric Hg in a Coastal Environment[END_REF] with the exception of samples collected in Peterborough (44 °N) where  200 Hg reached up 1.2 ‰ in both rain and snow. [START_REF] Chen | Unusual Fractionation of Both Odd and Even Mercury Isotopes in Precipitation from Peterborough, ON, Canada[END_REF] The median  200 Hg value is higher for seston than fish but this could result from a sampling bias as there are fewer seston samples that were not collected in the same year and their distribution is skewed towards northern stations compared to fish. In seston, the  200 Hg values are higher than the  204 Hg ones and even though not significant, the Δ 200 Hg/Δ 204 Hg slope (-0.28, r² = 0.12, p-value > 0.05, Figure SI-4c) is very similar to the slopes reported for precipitations and marine biota. [START_REF] Kwon | Mercury Stable Isotopes for Monitoring the Effectiveness of the Minamata Convention on Mercury[END_REF][START_REF] Jiskra | Mercury Stable Isotopes Constrain Atmospheric Sources to the Ocean[END_REF] Our data therefore strengthen previous studies in suggesting that pelagic plankton and invertebrates preferentially accumulate Hg(II) originating from wet depositions that are characterized by positive Δ 200 Hg and negative Δ 204 Hg values, [START_REF] Demers | Mercury Isotopes in a Forested Ecosystem: Implications for Air-Surface Exchange Dynamics and the Global Mercury Cycle: MERCURY ISOTOPES IN A FORESTED ECOSYSTEM[END_REF][START_REF] Enrico | Atmospheric Mercury Transfer to Peat Bogs Dominated by Gaseous Elemental Mercury Dry Deposition[END_REF][START_REF] Fu | Domestic and Transboundary Sources of Atmospheric Particulate Bound Mercury in Remote Areas of China: Evidence from Mercury Isotopes[END_REF] or MMHg formed from this Hg(II) pool.

It has been proposed that even Hg-MIF values in precipitation increases with latitude, implying an upper atmosphere origin of even-MIF. [START_REF] Wang | Mass-Dependent and Mass-Independent Fractionation of Mercury Isotopes in Precipitation from Guiyang, SW China[END_REF][START_REF] Cai | Mass-Independent Fractionation of Even Mercury Isotopes[END_REF] Our data support this (i) as the Baltic Sea is located north of Peterborough (roughly from 55 to 65 °N) and (ii) because there is an apparent gradient in  200 Hg average values along the Baltic Sea, from 0.06 ± 0.20 ‰ in the south, western basin (Kattegat) to 0.22 ± 0.20 ‰ in the northern basin (Bothnian Bay). The highest even-MIF values are found in the northern basin which can be explained by the fact that (i) deep stratospheric intrusions are likely more common towards the north as the troposphere height decreases towards the poles, [START_REF] Cai | Mass-Independent Fractionation of Even Mercury Isotopes[END_REF] and (ii) snow precipitation is more important in this basin (> 40 % of total precipitations vs 20 % in the south) [START_REF] Arheimer | Regulation of Snow-Fed Rivers Affects Flow Regimes More than Climate Change[END_REF] and the highest even-MIF values have been reported in snow until now. [START_REF] Chen | Unusual Fractionation of Both Odd and Even Mercury Isotopes in Precipitation from Peterborough, ON, Canada[END_REF] Similarly to odd-MIF, the  200 Hg signal in seston is also overall positively (albeit weakly) correlated to the % MMHg (Figure SI-4d, R² = 0.37, p-value < 0.005), which indicates that at higher tropic levels in the food web the even-MIF signal is eventually dominated by the contribution of MMHg to total Hg. However, some elevated values are also found in samples with MMHg < 30 %, demonstrating an important contribution of Hg(II) in some cases. Our data suggests that the analysis of seston is promising to discriminate the contribution of Hg wet deposition to the water column. It seems to be able to reflect this atmospheric signature at a higher temporal and spatial resolution compared to fish that accumulates a broader temporal and spatial average MMHg signal or sediments, where it is largely diluted by the terrestrial and anthropogenic Hg inputs.

Odd Hg-MIF in fish reveals basin-specific MMHg photodegradation patterns.

Odd Hg-MIF is found in all fish samples, whatever the fish species or sub-basins, with  199 Hg ranging from 0.33 to 3.04 ‰ and  201 Hg from 0.21 to 2.26 ‰ (Figure 2 and Table SI-3). The  199 Hg/ 201 Hg slope is 1.31 ± 0.02 (Figure 2c, linear fit, no weighting, pvalue < 0.005, n = 51) when considering all fish samples, which is typical for freshwater fish as it reflects the slope imprinted during MMHg photodegradation in freshwater with excess DOM compared to MMHg. [START_REF] Blum | Mercury Isotopes in Earth and Environmental Sciences[END_REF][START_REF] Bergquist | Mass-Dependent and -Independent Fractionation of Hg Isotopes by Photoreduction in Aquatic Systems[END_REF][START_REF] Bouchet | In Situ Photochemical Transformation of Hg Species and Associated Isotopic Fractionation in the Water Column of High-Altitude Lakes from the Bolivian Altiplano[END_REF][START_REF] Sherman | Mercury Stable Isotopes in Sediments and Largemouth Bass from Florida Lakes, USA[END_REF][START_REF] Lepak | Factors Affecting Mercury Stable Isotopic Distribution in Piscivorous Fish of the Laurentian Great Lakes[END_REF][START_REF] Perrot | Higher Mass-Independent Isotope Fractionation of Methylmercury in the Pelagic Food Web of Lake Baikal (Russia)[END_REF][START_REF] Tsui | Variation in Terrestrial and Aquatic Sources of Methylmercury in Stream Predators as Revealed by Stable Mercury Isotopes[END_REF] However, cod from Fladen present a lower  199 Hg/ 201 Hg ratio of 1.08 ± 0.22 (n = 5) compared to the other fish even if their MMHg content is similar (90.9 -98.6 %). This likely results from both their more pronounced benthic feeding that lead to the accumulation of both pelagic and benthic MMHg that is less or not photodegraded, and their migration strategy. It has been evidenced that most individuals from the population in Kattegat (Fladen) undertake spawning migrations towards the North Sea, [START_REF] Hüssy | Where Do You Come from, Where Do You Go: Early Life Stage Drift and Migrations of Cod Inferred from Otolith Microchemistry and Genetic Population Assignment[END_REF] and it is well established that a lower slope is imprinted during MMHg photodegradation in low DOM marine waters. [START_REF] Blum | Mercury Isotopes in Earth and Environmental Sciences[END_REF][START_REF] Motta | Mercury Stable Isotopes in Flying Fish as a Monitor of Photochemical Degradation of Methylmercury in the Atlantic and Pacific Oceans[END_REF][START_REF] Cransveld | Mercury Stable Isotopes Discriminate Different Populations of European Seabass and Trace Potential Hg Sources around Europe[END_REF][START_REF] Meng | Mercury Isotope Variations within the Marine Food Web of Chinese Bohai Sea: Implications for Mercury Sources and Biogeochemical Cycling[END_REF][START_REF] Masbou | Hg-Stable Isotope Variations in Marine Top Predators of the Western Arctic Ocean[END_REF] There is a good separation of the various fish populations in the  202 Hg - 199 Hg space (Figure 2b), suggesting that they could be discriminated based on their Hg isotopic signatures as previously demonstrated for juvenile Seabass at the European scale. [START_REF] Cransveld | Mercury Stable Isotopes Discriminate Different Populations of European Seabass and Trace Potential Hg Sources around Europe[END_REF] This separation stems partly from the multiple Hg sources entering the food web and the food web structure as discussed above but also from a different degree of MMHg photodegradation between basins controlled by a combination of environmental factors. There is a clear upward shift from an average  199 Hg of 0.53 ± 0.29 ‰ in herrings from Ostergarnsholm (Baltic Proper) to 2.58 ± 0.48 ‰ in Harufjarden (Bothnian Bay). Contrary to seston,  200 Hg values in herring are similar in the northern and central stations (from 0.03 ± 0.05 ‰ for the Baltic Proper offshore to 0.08 ± 0.06 ‰ for Holmoarna, vs -0.01 ± 0.04 ‰ for Fladen, Figure 4a). This result demonstrates that MMHg is formed from an homogenous pool of Hg with respect to atmospheric sources and therefore that the odd Hg-MIF trends observed are due to differences in MMHg photodegradation rates.

The maximum  199 Hg values found in the Baltic Sea are relatively low compared to values observed in pelagic fish from other fresh or marine waters (e.g. Lake Baikal, high altitudes Tibetan lakes, North American great Lakes or the Pacific ocean) [START_REF] Perrot | Higher Mass-Independent Isotope Fractionation of Methylmercury in the Pelagic Food Web of Lake Baikal (Russia)[END_REF][START_REF] Xu | Linking Mercury, Carbon, and Nitrogen Stable Isotopes in Tibetan Biota: Implications for Using Mercury Stable Isotopes as Source Tracers[END_REF][START_REF] Lepak | Factors Affecting Mercury Stable Isotopic Distribution in Piscivorous Fish of the Laurentian Great Lakes[END_REF][START_REF] Motta | Mercury Stable Isotopes in Flying Fish as a Monitor of Photochemical Degradation of Methylmercury in the Atlantic and Pacific Oceans[END_REF] that often reach 6 -7 ‰, sometimes up to 9 ‰, indicating that the extent of MMHg photodegradation in the Baltic Sea is relatively limited. Based on a MIF enrichment factor of 14.3 ‰, [START_REF] Chandan | Mercury Isotope Fractionation during Aqueous Photo-Reduction of Monomethylmercury in the Presence of Dissolved Organic Matter[END_REF][START_REF] Bouchet | In Situ Photochemical Transformation of Hg Species and Associated Isotopic Fractionation in the Water Column of High-Altitude Lakes from the Bolivian Altiplano[END_REF] we calculated that the extent of MMHg photodegradation before incorporation into the food web averages 4 -5 % in the central Baltic Sea (Baltic Proper), 6 % in the Kattegat and up to 16 % in the northern, less saline basins (Bothnian Sea and Bay). This limited MMHg photodegradation can be explained by a combination of factors, i.e. the amount of incident light, its penetration in the water column and the residence time of MMHg in the photic zone. Solar radiation is about 2-5 times lower in boreal regions compared to temperate or tropical ones and the light penetration in ice-free water of the Baltic Sea is comparably low. The secchi depth averages 11 m in the south Kattegat strait to 3 -5 m in the Bothnian Bay (Figure SI-5a) compared to e.g. about 15 m in the Laurentian Great Lakes [START_REF] Lepak | Factors Affecting Mercury Stable Isotopic Distribution in Piscivorous Fish of the Laurentian Great Lakes[END_REF] and 45 m in the Atlantic and Pacific oceans. [START_REF] Motta | Mercury Stable Isotopes in Flying Fish as a Monitor of Photochemical Degradation of Methylmercury in the Atlantic and Pacific Oceans[END_REF] The light penetration and residence time of Hg (about one year) [START_REF] Soerensen | Eutrophication Increases Phytoplankton Methylmercury Concentrations in a Coastal Sea-A Baltic Sea Case Study[END_REF] are limited by relatively high concentrations of DOM and/or POM throughout the entire Baltic Sea compared to marine waters and recurrent "massive" algae blooms that occur especially in the central part of the Baltic Sea, starting with diatoms and dinoflagellates in spring which are then replaced by filamentous cyanobacteria over the summer. [START_REF] Hjerne | Climate Driven Changes in Timing, Composition and Magnitude of the Baltic Sea Phytoplankton Spring Bloom[END_REF] It is noteworthy that the lowest  199 Hg values are indeed observed at the Ostergarnsholm and Baltic proper offshore stations where algae blooms form a thick cover that severely decreases light penetration in the water column but also likely protects Hg species from light exposure following their internalization. Moreover, light penetration is also limited by ice cover in the Baltic Sea and this certainly lower the extent of MMHg photodegradation overall, especially in the northern basins (Bothnian Sea and Bay), as already demonstrated in other northern ecosystems. [START_REF] Point | Methylmercury Photodegradation Influenced by Sea-Ice Cover in Arctic Marine Ecosystems[END_REF] The  199 Hg values in fish presented here are thus likely lower than expected as the maximum ice extents were above average from 2011 to 2013 (177 000 to 309 000 km 2 compared to an average value of 140 000 km 2 over the past 30 years). On the contrary, values in seston might be higher than expected as ice extents over 2015 -2016 were lower than average (51 000 to 110 000 km 2 ). 93 Finally, production of MMHg in the anoxic parts of the water column has been previously established in the Baltic Proper basin [START_REF] Soerensen | Deciphering the Role of Water Column Redoxclines on Methylmercury Cycling Using Speciation Modeling and Observations From the Baltic Sea[END_REF] and deep waters could thus also contribute a non or less fractionated MMHg to biota. Further studies are required to clarify this point as it was previously found that turbulent diffusion across the hypoxic-anoxic interface was a likely source of MMHg to the hypoxic zone but not to normoxic waters where fish spend most of their time. [START_REF] Soerensen | Deciphering the Role of Water Column Redoxclines on Methylmercury Cycling Using Speciation Modeling and Observations From the Baltic Sea[END_REF]

MMHg photodegradation is driven by the terrestrial fraction of DOM.

There is a negative relationship between  199 Hg in herring and incident light irradiance over the inner Baltic Sea (Kattegat excluded), both with respect to UV (Figure 4a) and PAR (Figure SI-5a). This trend cannot be explained by light penetration as the secchi depth overall decreases from south to north and therefore shows no correlation with  199 Hg (Figure SI-5b&c). [START_REF] Stock | Satellite Mapping of Baltic Sea Secchi Depth with Multiple Regression Models[END_REF] The  199 Hg value in Kattegat is higher than expected from the trend and this is likely the result of the larger irradiance and deeper light penetration in this basin, about twice that of the others. The negative relationship between  199 Hg and light penetration in the inner Baltic Sea (Baltic Proper, Bothnian Sea and Bothnian Bay) is a surprising finding as previous studies have found a positive correlation between the extent of MMHg photodegradation and light penetration in both fresh and marine waters. [START_REF] Sherman | Mercury Stable Isotopes in Sediments and Largemouth Bass from Florida Lakes, USA[END_REF][START_REF] Motta | Mercury Stable Isotopes in Flying Fish as a Monitor of Photochemical Degradation of Methylmercury in the Atlantic and Pacific Oceans[END_REF][START_REF] Lepak | Factors Affecting Mercury Stable Isotopic Distribution in Piscivorous Fish of the Laurentian Great Lakes[END_REF] There is also a clear negative correlation of  199 Hg values with salinity over the inner Baltic Sea (Figure SI-6a, y = -0.5 + 3.7, r² = 0.9, Kattegat excluded) that demonstrates the importance of freshwater inputs to enhance photodegradation of MMHg along the mixing zone. This increase in  199 Hg towards the northern, less saline waters, is well explained by the concentration of humic substances (Figure SI-7a, slope = 0.18, r 2 = 0.8), that also increases from an average of 8.3 ± 0.2 µg L -1 in Ostergarnsholm to 17.5 ± 1.5 µg L -1 in Harufjarden (while DOC concentrations only increases from 3.5 to 3.9 mg L -1 , Figure SI-7b).

The contribution of terrestrial DOC to the total DOC in the Baltic Sea has previously been estimated to increase from about 40 % in the Baltic Proper to 50 % in the Bothnian Sea and up to 80 % in the Bothnian Bay. [START_REF] Deutsch | Tracing Inputs of Terrestrial High Molecular Weight Dissolved Organic Matter within the Baltic Sea Ecosystem[END_REF] In this study from Deutsch et al., [START_REF] Deutsch | Tracing Inputs of Terrestrial High Molecular Weight Dissolved Organic Matter within the Baltic Sea Ecosystem[END_REF]  13 C values in high molecular weight DOM (> 1 kDa) ranged from about -27 to -25 ‰ across the Baltic Proper, Bothnian Sea and Bay basins and were found to be linearly correlated with salinity values ranging from 2 to 7 psu. Here we find that across the inner Baltic basins,  199 Hg values in herring are negatively correlated to the  13 C values measured in the fish themselves, ranging from about -26 to -20 ‰ (Figure 4b, y = -0.38 -7.36, r² = 0.81, p-value < 0.001, regression performed on individual points). The  13 C values also point to two "sub-populations" of herring in the northern Bothnian Bay (Harufjarden station) with distinct  199 Hg values (average 1.9 ± 0.1 and 2.9 ± 0.1 ‰, Figure 4b). This suggests that the gradient in humic substances concentrations To the best of our knowledge, this is the first study to demonstrate at the ecosystem scale that MMHg photodegradation is enhanced by increasing low concentrations of terrestrial DOM. The importance of low molecular weight, aromatic DOM for the MMHg photodegradation reaction was previously demonstrated in several lab or field experiments with addition of model compounds or purified DOM extracts. For example, additions of fulvic acid up to several ten's of mg L -1 in natural waters were found to promote the reaction. [START_REF] Jeremiason | Photoreduction of Hg(II) and Photodemethylation of Methylmercury: The Key Role of Thiol Sites on Dissolved Organic Matter[END_REF][START_REF] Qian | Why Dissolved Organic Matter Enhances Photodegradation of Methylmercury[END_REF][START_REF] Kim | Effects of Natural Water Constituents on the Photo-Decomposition of Methylmercury and the Role of Hydroxyl Radical[END_REF] On the other hand, the negative effect of high molecular weight humic acid has been demonstrated with purified extracts, [START_REF] Kim | Effects of Natural Water Constituents on the Photo-Decomposition of Methylmercury and the Role of Hydroxyl Radical[END_REF] but also by field studies focused on lakes with high DOC levels. [START_REF] Sherman | Mercury Stable Isotopes in Sediments and Largemouth Bass from Florida Lakes, USA[END_REF][START_REF] Klapstein | Methylmercury Photodemethylation Is Inhibited in Lakes with High Dissolved Organic Matter[END_REF] These latter studies have overall evidenced a decrease of the photodegradation extents with increasing DOC concentrations but the tipping point between enhancement and inhibition remains overall poorly defined. However, it has been suggested that 6 -7 mg L -1 of DOC seem to be optimal conditions for MMHg photodegradation in natural waters, [START_REF] Bouchet | In Situ Photochemical Transformation of Hg Species and Associated Isotopic Fractionation in the Water Column of High-Altitude Lakes from the Bolivian Altiplano[END_REF][START_REF] Girard | Photodemethylation of Methylmercury in Eastern Canadian Arctic Thaw Pond and Lake Ecosystems[END_REF] and here we found it to be at its highest where the more terrestrial and highest concentrations of DOC are found in the Baltic Sea (i.e. averages of 16 -18 µg L -1 for humic substances and 3.9 mg L -1 for DOC at Holmoarna and Harufjarden). It gives a new perspective to the negative relationship previously observed between MMHg and humic OM in the northern Baltic Sea, which was hypothesized to be caused by a reduced availability of Hg(II) for microbial MMHg formation. [START_REF] Soerensen | Organic Matter Drives High Interannual Variability in Methylmercury Concentrations in a Subarctic Coastal Sea[END_REF] Our findings now strongly suggests that MMHg photodegradation also plays an important role for this negative relationship. It remains to be tested if terrestrial OM driven photodegradation is an important MMHg sink also in other types of coastal seas with higher light irradiance and penetration (e.g. in temperate regions). Based on our results it can be expected that the magnitude of this MMHg sink will increase with the large increased delivery of terrestrial OM to lakes, rivers and coastal ecosystems expected especially in the world's northern regions with climate and other global changes. Further studies are thus required to quantitatively assess the impacts of the various OM fractions on MMHg photodegradation, especially the opposing effects of low and high molecular weight compounds, in order to better predict future MMHg concentrations in water and biota.

Supporting Information.

Hg isotopic composition of the various reference materials; Hg concentrations and isotopic composition in

  from coastal to open-sea conditions generally observed in the Baltic Sea has a strong impact on the extent of MMHg photodegradation. Particularly high humic substances concentrations up to 70 µg L -1 are observed along the Bothnian Bay coast (Figure SI-6b). Altogether, the relationships between Hg odd-MIF and salinity, concentrations of humic substances and  13 C strongly point to the terrestrial fraction of DOC as the key factor controlling the photodegradation of MMHg along the salinity gradient of this large estuarine ecosystem.

  sediments from the Baltic Sea;  202 Hg versus 1/HgT and  199 Hg versus  202 Hg in sediments from the various basins of the Baltic Sea; Hg concentrations and isotopic composition in fish from the Baltic Sea; Hg concentrations and isotopic composition in seston from the Baltic Sea; HgT concentration in seston and fishes versus latitude and salinity; log10HgT concentrations versus 15N,  202 Hg versus trophic level and % MMHg versus trophic level for all biota samples (seston and fishes);  202 Hg versus 1/HgT;  199 Hg versus % MMHg;  200 Hg versus  204 Hg and  200 Hg versus % MMHg in seston samples from the Baltic Sea;  199 Hg in herring from the various basins versus incident PAR irradiance, Secchi depth in the various basins versus incident PAR irradiance and  199 Hg in herring from the various basins versus Secchi depth.;  199 Hg in herring from the various basins as a function of concentrations of humic substances and DOC.

Figure 1 :

 1 Figure 1: Map of the Baltic Sea with the various types of samples analyzed and their location. Specific sampling stations are indicated for seston and fishes while several sediment samples were collected at each sediment area indicated. There are several latitudinal gradients across the various basins with increases in salinity and light irradiation, and decreases in concentration and terrestrial fraction of DOM, from the Bothnian Bay to the Kattegatt.

Figure 2 :

 2 Figure 2: (a)  199 Hg versus  202 Hg in seston (contours of sediments and fish are given for indication); (b)  199 Hg versus  202 Hg in fish samples according to their basins, stations and fish species; (c)  199 Hg versus  201 Hg in fish and seston samples.

Figure 3 :Figure 4 :

 34 Figure 3: Distribution of  204 Hg (grey boxes) and  200 Hg (red boxes) in sediment, seston and fish samples.

  xxx can be either 204, 202, 201, 200 or 199 and ( xxx Hg/ 198 Hg)NIST 3133 is the averaged isotopic ratio of the two bracketing standards. The  notation is used to express the mass independent fractionation (MIF), calculated as Δ xxx Hg = δ xxx Hg-βkin×δ202 Hg where βkin = ln(m198/mxxx)/ln(m198/m202).[START_REF] Blum | Recent Developments in Mercury Stable Isotope Analysis[END_REF]

		𝐻 𝑥𝑥𝑥 𝑔
	𝛿 𝑥𝑥𝑥 𝐻𝑔 ‰ =	𝐻 198 𝑔 𝑠𝑎𝑚𝑝𝑙𝑒 𝐻 𝑥𝑥𝑥 𝑔	-1 × 1000
		𝐻 198 𝑔 𝑁𝐼𝑆𝑇3133

Acknowledgments.

This work was funded by the Swedish research council FORMAS (grants 2014-1088 and 2021-00942).

We warmly acknowledge Aleksandra Skrobonja for the Hg total and speciation analyses of the seston samples. We thank the Swedish Environmental Specimen Bank, part of the Swedish Museum of Natural History, for providing fish samples and the Swedish Geological Survey for providing sediment samples for the study.