Stephen G Kohler

Lars-Eric Heimbürger-Boavida

Philipp Assmy

Oliver Müller

Stefan Thiele

Maria G Digernes

Kuria Ndungu

Murat V Ardelan

Biotic transformation of methylmercury at the onset of the Arctic spring bloom

Keywords: Barents Sea, mercury, seasonality, Arctic Ocean, spring bloom

Despite the lack of local anthropogenic mercury sources, methylated mercury (MeHg) concentrations in Arctic biota are higher than in biota from lower latitudes. The main entry route occurs during the bioconcentration of seawater monomethylmercury (MMHg) into phytoplankton.

Despite the known seasonal changes in biological activity in the region, little is known about the seasonal cycling of total mercury (THg) and MeHg in the Arctic Ocean. Here, we report the concentrations of THg and MeHg in seawater sampled from the northwestern Barents Sea water column during late winter and spring. In the upper 500 m, the THg concentrations are significantly higher in spring (0.64 ± 0.09 pmol L -1 ) compared to late winter (0.53 ± 0.07 pmol L -1 ), driven by seasonal inputs to surface waters from atmospheric deposition and the dynamics of changing sea ice conditions. Contrastingly, the MeHg concentrations in spring were significantly lower (41 ± 39 fmol L -1 ) compared to late winter (85 ± 42 fmol L -1 ). We suggest that most MeHg is biotically demethylated by both phytoplankton and bacteria, with additional losses from photodemethylation and evasion. Our observations highlight the importance of demethylation during potential uptake of methylmercury coinciding with the Arctic spring bloom. Lastly, we use our new data together with previously published seasonal data in the region to construct a simplified seasonal mercury cycle in an Arctic marginal ice zone.

Introduction

The bioaccumulation of mercury (Hg) in the Arctic Ocean continues to remain an issue of concern for the marine ecosystem and indigenous peoples [START_REF] Basu | The impact of mercury contamination on human health in the Arctic: A state of the science review[END_REF]. Specifically, the bioconcentration of toxic monomethylmercury (MMHg) into phytoplankton and its subsequent bioaccumulation in the marine food web represents a risk to higher trophic level organisms [START_REF] Dietz | A risk assessment review of mercury exposure in Arctic marine and terrestrial mammals[END_REF]. The development and peak of primary productivity in the Arctic Ocean is restricted to the spring and summer (April to August), when light is available. In the spring, the phytoplankton bloom develops in the marginal ice zone (Strass and Nӧthig, 1996;[START_REF] Reigstad | Variations in hydrography, nutrients and chlorophyll a in the marginal ice-zone and the central Barents Sea[END_REF]. Several studies have highlighted phytoplankton blooms below fully consolidated sea ice as a result of the thinning and more dynamic icescape [START_REF] Assmy | Leads in Arctic pack ice enable early phytoplankton blooms below snow-covered sea ice[END_REF], Ardyna et al., 2020).

Concurrently, the Arctic Ocean receives seasonal inputs of mostly inorganic Hg (iHg) via atmospheric deposition, riverine input, erosion, snow, glacial and sea ice melt [START_REF] Dastoor | Arctic mercury cycling[END_REF]. However, seasonal cycling of Hg in the water column in the Arctic region is still poorly understood (Kohler et al., 2022a). As the Arctic spring bloom provides a crucial transfer of energy and potentially Hg into higher trophic levels, more knowledge on the amount of Hg available for bioconcentration prior and during the Arctic spring bloom is needed. Seasonal inputs from thawing permafrost, glacial melt, and erosion are expected to deliver greater amounts of iHg to the biologically productive Arctic Ocean shelf seas in the future [START_REF] Chételat | Climate change and mercury in the Arctic: abiotic interactions[END_REF] with most Hg expected to remain in the marine environment [START_REF] Araujo | Mercury isotope evidence for Arctic summertime reemission of mercury from the cryosphere[END_REF]. Hg can be scavenged to shelf sediments, advected with the halocline, cascaded into the deep basin [START_REF] Dastoor | Arctic mercury cycling[END_REF] or potentially transported into the Central Arctic Ocean via the transpolar drift [START_REF] Charette | The transpolar drift as a source of riverine and shelfderived trace elements to the central Arctic Ocean[END_REF].

Several major Hg inputs to the Arctic region have strong seasonal patterns. Atmospheric mercury depletion events (AMDEs) [START_REF] Schroeder | Arctic springtime depletion of mercury[END_REF] occur during polar sunrise in March and frequently occur throughout April, depositing iHg to snow, sea ice, and surface waters. Although a substantial fraction is photochemically reduced and reemitted back to the atmosphere as gaseous Hg 0 [START_REF] Durnford | The behavior of mercury in the cryosphere: A review of what we know from observations[END_REF]Dastoor 2011, Araujo et al., 2022), high concentrations of total Hg (THg) are seen in snow, sea ice and frost flowers [START_REF] Dommergue | Deposition of mercury species in the Ny-Alesund area (79 degrees N) and their transfer during snowmelt[END_REF], Chaulk et al., 2011). Sea ice, melting snow and glacial runoff can also provide additional Hg to the coastal and open ocean [START_REF] Schartup | Influence of the Arctic Sea-Ice regime shift on sea-ice methylated mercury trends[END_REF], Kim et al., 2020a, Dastoor et al., 2022). Rivers also deliver large amounts of iHg from the snowpack, surface soils, thawing permafrost, and erosion to the Arctic shelf seas during a freshet in springtime [START_REF] Sonke | Eurasian river spring flood observations support net Arctic Ocean mercury export to the atmosphere and Atlantic Ocean[END_REF][START_REF] Zolkos | Mercury export from Arctic great rivers[END_REF]. While new evidence suggests seasonal cycling of iHg in the upper water column during autumn and early winter (Kohler et al., 2022a) with suspected surface losses to deeper waters and sediments with sinking particles, water column measurements of Hg species from late winter are scarce [START_REF] Wang | Total and methylated mercury in the Beaufort Sea: the role of local and recent organic remineralization[END_REF].

Arctic surface waters are elevated in THg during the summer [START_REF] Heimbürger | Shallow methylmercury production in the marginal sea ice zone of the central Arctic Ocean[END_REF] from seasonal iHg inputs. Light reduces iHg to Hg 0 and photolytically degrades MeHg [START_REF] Sellers | Photodegradation of methylmercury in lakes[END_REF]. Gaseous Hg species can evade from the marginal ice zones [START_REF] Yue | The Marginal Ice Zone as a dominant source region of atmospheric mercury during central Arctic summertime[END_REF] while ionic Hg species can be transported downward with sinking particles [START_REF] Tesán Onrubia | Mercury export flux in the Arctic Ocean estimated from 234 Th: 238 U disequilibria[END_REF]. Ionic Hg is also often complexed to dissolved organic matter (DOM) and reduced sulfur compounds, which can affect Hg uptake and biogeochemical transformations between iHg and MeHg [START_REF] Ravichandran | Interactions between mercury and dissolved organic matter--a review[END_REF]). During and after the phytoplankton bloom, bacterial remineralization of sinking particulate organic matter fuels in situ MeHg production, leading to elevated subsurface concentrations of MeHg in the marginal ice zones and the permanently ice-covered central Arctic Ocean [START_REF] Heimbürger | Shallow methylmercury production in the marginal sea ice zone of the central Arctic Ocean[END_REF]. However, sea ice hinders both photodemethylation (Point et al., 2009) and the evasion of gaseous species (Hg 0 and dimethylmercury (DMHg)) (DiMento et al., 2019). Gaseous Hg species potentially build up under and within the ice throughout winter [START_REF] Andersson | Enhanced concentrations of dissolved gaseous mercury in the surface waters of the Arctic Ocean[END_REF], Schartup et al., 2020). Hg that is removed from surface waters via sinking particles and delivered to bottom sediments may ultimately be methylated to MeHg (Kim et al., 2020b). Resuspension of fine-grained sediment and diffusion of porewater can deliver both iHg and MeHg back to subsurface Arctic waters (Kim et al., 2020b). While in situ incubations indicate Arctic waters can be in net states of methylation or demethylation (Lehnherr et al., 2011), the advection of outflowing Arctic waters indicate that the Arctic Ocean is a net producer of MeHg [START_REF] Petrova | Mercury species export from the Arctic to the Atlantic Ocean[END_REF], Jonsson et al., 2022a). Consequently, understanding the timing and magnitude of both Hg methylation and demethylation is important. Due to a limited number of observations, the fate of marine MeHg throughout the Arctic winter and during the transition to the spring bloom is unknown.

We hypothesized the potential seasonality of pelagic Hg species in the northern Barents Sea in the late winter to spring transition due to seasonal Hg inputs and biological development. We measured unfiltered THg (the sum of all mercury species) and unfiltered MeHg (the sum of MMHg and DMHg) in seawater, under ice water, and sea ice on two seasonal cruises in late winter (March) and spring (end of April-May). We report Hg concentrations from seven stations along a southnorth transect and explore how hydrography, biological parameters, and seasonal processes affect our observations using literature and statistical relationships. We combine our Hg and environmental data with that of two other seasonal cruises in the region (Kohler et al., 2022a) to propose a simple seasonal model of the pelagic Hg cycle in surface waters of an Arctic marginal ice zone.

Oceanographic Conditions

The northern Barents Sea consists of water masses with both Atlantic Water (AW) and Polar Water (PW) origins [START_REF] Sundfjord | Suggested water mass defintions for the central and northern Barents Sea, and the adjacent Nansen Basin: Workshop Report[END_REF]. AW, characteristic of warmer water with higher salinity, flows into the northern Barents Sea from the south through the Barents Sea Opening and through troughs in the north [START_REF] Lundesgaard | Import of Atlantic Water and sea ice controls the ocean environment in the northern Barents Sea[END_REF]. This AW sinks below colder and fresher PW which arrives from the central Arctic Ocean and the Kara Sea in the east [START_REF] Loeng | Features of the physical oceanographic conditions of the Barents Sea[END_REF]. The boundary between these two water masses is often referred to as the polar front, and generally delineates the extent of sea ice in the region. Surface PW with colder temperatures entering the northern Barents Sea leads to sea ice formation and can advect additional sea ice into the region. In the summer, sea ice melt and surface heating form a stratified meltwater layer in the upper water column. By autumn, the beginnings of sea ice formation, cooling, and turbulence lead to water column mixing. This overturning of the water column continues throughout the polar night and into spring. Dense water formation on the Barents Sea shelf enters the Arctic Basin through deep water plumes along the continental shelf [START_REF] Rudels | Evolution of the Arctic Ocean boundary current north of the Siberian shelves[END_REF].

On the western coast of Svalbard, the West Spitsbergen Current brings AW into the Eurasian Basin. This current continues parallel along the Barents Sea shelf break and past Franz Josef Land in the east. This AW shows a distinct seasonality, with warmer temperatures in autumn and colder temperatures in early spring [START_REF] Pérez-Hernández | Structure, transport, and seasonality of the Atlantic Water boundary current north of Svalbard: Results from a yearlong mooring array[END_REF]. As AW enters the Arctic Basin, it begins to cool and sinks below PW, forming an AW core at around 300 m depth in the water column north of Svalbard. As mixing occurs between AW and PW, continued temperature losses lead to the formation of modified Atlantic Water (mAW) below the pycnocline and AW core. As this water continues to cool below 0℃ and sinks, typically below 1000 m, it forms Eurasian Basin Deep Water (EBDW). In addition, extensions of the West Spitsbergen Current flowing onto the northern Barents Sea shelf [START_REF] Lundesgaard | Import of Atlantic Water and sea ice controls the ocean environment in the northern Barents Sea[END_REF] can form warm Polar Water (wPW). This wPW is not as salty as AW yet warmer than PW, and can be found in deeper depths of the northern Barents Sea shelf or at the surface during summer stratification. All samples were collected on a south-north transect in the northern Barents Sea and Nansen Basin (76 -83°N) on two cruises, Q1 from March 2-24, 2021 (hereafter referred to as late winter), and Q2 from April 27 -May 20, 2021 (hereafter referred to as spring), on R/V Kronprins Haakon as part of The Nansen Legacy project (Fig. 1). Seawater samples for Hg were taken at seven stations at 8 depths down to 500 m depth using Teflon-coated GOFLO bottles attached sequentially to a 600 m nonmetallic wire. At depths deeper than 500 m, Hg samples were collected with 10 L Niskin bottles mounted to a rosette. Upon retrieval, GOFLO bottles were transported immediately to a custom-made clean lab onboard. THg samples were collected unfiltered into pre-cleaned [START_REF] Cutter | Sampling and sample-handling protocols for GEOTRACES Cruises[END_REF] 40 mL borosilicate glass bottles with PTFE-lined caps with no headspace and kept unpreserved. MeHg samples were collected unfiltered into brand new 125 or 250 mL PET bottles and were acidified under a Class 100 clean air laminar flow hood to 0.4% v/v with double-distilled HCl. Upon acidification, dimethylmercury (DMHg) converts quantitatively to monomethylmercury (MMHg) [START_REF] Black | Stability of dimethyl mercury in seawater and its conversion to monomethyl mercury[END_REF]; as a result, the sum of MMHg and DMHg is measured as total methylated mercury (MeHg). All Hg samples were double-bagged and stored in the dark at 4°C until analysis. Field blanks for THg were collected by bringing laboratory MilliQ onboard and subsampling at the time of field sampling into precleaned borosilicate bottles and treated identically to seawater samples. However, as we could not guarantee trace metal clean Niskin bottles for samples collected below 500 m, these data are reported (SI Fig. 1) but not included in subsequent statistical processing when noted.

Field Sampling

Under ice water (UIW) was collected using a handheld 10 L Niskin bottle lowered into a hole cut into the sea ice at stations P4, P6, and P7 on both cruises. UIW was collected directly from the Niskin similar to the other seawater samples. Sea ice was collected at two stations, P4 and P7, using a 9 cm diameter ice corer (Kovacs Enterprises, Inc., USA), bagged whole and taken directly onboard into a -20°C cold lab. The bottom 10 cm of the ice core was cut using a handheld carbon steel saw. Approximately 3 mm of ice was shaved off from all sides of the ice section with a ceramic knife to remove any potential contamination before being double bagged and melted at room temperature. One frost flower sample was collected in late winter using plastic equipment and melted at 4°C. After melting, all samples were decanted into precleaned 40 mL borosilicate glass bottles and 125 or 250 mL PET bottles and stored in the dark at 4°C until analysis.

Mercury Analysis

The concentration of THg in the samples was measured by cold vapor atomic fluorescence spectroscopy (CVAFS) according to USEPA Method 1631 at the Mediterranean Institute of Oceanography (MIO) using a custom-made single gold trap setup described elsewhere [START_REF] Heimbürger | Shallow methylmercury production in the marginal sea ice zone of the central Arctic Ocean[END_REF], Kohler et al., 2022a), within one year after sampling. As samples were unfiltered to reduce contamination risk from filtration equipment, THg concentration includes both particulate and dissolved species of Hg. The THg concentration in all samples was calculated from a 6-point calibration curve traceable to NIST 3133 (National Institute of Standards and Technology) standard reference material in previously purged, Hg-free seawater. Standard bracketing of samples with a NIST 3133 matrix spike were used to correct for eventual instrument signal drift and ensure ongoing precision and recovery. Aliquots of certified reference materials ORMS-5 (Research Council of Canada) and ERM-CA400 (European Reference Materials) were spiked into previously purged Hg-free seawater matrix with recoveries of 93 ± 2% and 125 ± 4%, respectively. Selected seawater samples run in triplicate from both cruises had an average standard deviation of 0.05 pmol L -1 . Field blanks of MilliQ water were measured for THg for both late winter (0.31 ± 0.02 pmol L -1 , n = 5) and spring (0.23 ± 0.02 pmol L -1 , n = 3) cruises. As MilliQ water can potentially contain traces of Hg, we concluded that there was no significant contamination affecting our THg measurements.

The MeHg concentrations in seawater samples was measured using the isotope dilution (ID) technique via gas chromatography (GC) -sector field ICP-MS (GC-SF-ICP-MS) at MIO within one year after sampling [START_REF] Heimbürger | Shallow methylmercury production in the marginal sea ice zone of the central Arctic Ocean[END_REF]. Briefly, approximately 125 mL of seawater sample was spiked with enriched isotopes of i 199 Hg and MM 201 Hg (ISC Science) and equilibrated for 15 minutes. Samples were then adjusted to pH 3.9 using a sodium acetate buffer solution (ULTREX® II Ultrapure Reagent, J.T. Baker, USA) and NH3 (ULTREX® II Ultrapure Reagent, J.T. Baker, USA). A fresh aliquot of 1% sodium tetrapropylborate solution (Merseburger Spezialchemikalien) and 150 μL of isooctane (Sigma Aldrich) were added to derivatize and preconcentrate Hg species. Samples were sealed and shaken for 15 minutes before the organic extract was transferred to GC vials for analysis on a coupled GC (THERMO GC 1300 with GC220 transfer module) SF-ICP-MS (Thermo Element XR) system. Ultra-trace clean techniques and coupling to a high resolution ICP-MS allows a detection limit of 2 fmol L -1 and a limit of quantitation of 7 fmol L -1 (LOQ = 3.3*LOD). Selected seawater samples run in duplicate from both cruises had an average standard deviation of 7 fmol L -1 . The MM 201 Hg spike concentration used to calculate the sample MeHg concentrations had an accuracy of 94.4 ± 8.4%. The accuracy of our spike was determined using reverse isotope dilution using the Brooks Rand MMHgCl standard, traceable to NIST1641E.

Additional Data and Parameters

Water temperature and salinity were recorded on a Seabird SBE 9 CTD unit attached to a rosette. Quality controlled temperature, salinity, sea ice concentration, nutrients, chlorophyll a, particulate organic carbon, nutrients, and phytoplankton abundance data for all seasonal cruises were retrieved from the Norwegian Marine Data Centre and the Norwegian Polar Institute [START_REF] Gerland | CTD data from Nansen Legacy Cruise -Seasonal Cruise Q1[END_REF][START_REF] Ludvigsen | CTD data from Nansen Legacy Cruise -Seasonal Cruise Q2[END_REF][START_REF] Reigstad | CTD Data from Nansen Legacy Cruise-Seasonal Cruise Q3[END_REF][START_REF] Søreide | CTD Data from Nansen Legacy Cruise-Seasonal Cruise Q4[END_REF], Steer and Divine 2022, Marquardt 2022b, 2022c[START_REF] Kohler | Concentrations of total mercury, total methylated mercury, and selected trace elements in the northern Barents Sea as part of the Nansen Legacy project[END_REF], 2022e, Müller 2023a, 2023b, 2023c, 2023d, Chierici et al. 2021a, 2021b, Jones et al., 2022a, 2022b, Vader 2022, Assmy 2022a, 2022b). Part of the late winter MeHg data (station average, upper 400 m only) presented in this analysis was previously published in the bottom image in Fig. 6 in [START_REF] Dietz | A risk assessment review of mercury exposure in Arctic marine and terrestrial mammals[END_REF]. Flow cytometry, bacterial community analysis, and dissolved organic matter analysis details are presented in the Supplementary Information. Latitude and longitude coordinates for CTD stations differ slightly from GOFLO casts due to ship drift.

Conservative temperature (Θ), absolute salinity (SA), and potential density anomaly (σθ) were calculated in Ocean Data View. Water mass definitions were assigned based on the most recent report from the Nansen Legacy project for the northern Barents Sea [START_REF] Sundfjord | Suggested water mass defintions for the central and northern Barents Sea, and the adjacent Nansen Basin: Workshop Report[END_REF].

Statistical processing was performed for data above 500 m depth in Minitab software. Spearman rank correlation coefficients were calculated using a 95% CI with significant p-value < 0.05. Data selected for Student's t-test were log transformed for non-normal data distributions prior to evaluation using a 95% CI with significant p-value < 0.05. Log-log regression was performed on selected environmental variables with significance evaluated by F-test with a 95% CI and significant p-value < 0.05. Selected environmental parameters were chosen for Principal Component Analysis, where data is standardized prior to dimension reduction and plotted orthogonally as loadings plots for the first two principal components.

Hydrography & sea ice

In general, all stations in the northern Barents Sea showed both similarities and differences in water mass distributions (SI Figs. 2 & 3) in both late winter and spring, with changes influenced by sea ice coverage or temperature shifts [START_REF] Jones | Inorganic carbon and nutrient dynamics in the marginal ice zone of the Barents Sea: seasonality and implications for ocean acidification[END_REF], Van Engeland et al., 2023). In the late winter and spring sampling periods, station P1 south of the polar front had a mAW signature, with SA greater than 35.06 g kg -1 and Θ > 0℃. Stations P2, P3, and P4, all north of the polar front, had PW (Θ < 0℃, σθ < 27.97 kg m -3 ) in surface waters and wPW (Θ > 0℃, SA < 35.06 g kg -1 ) in deeper waters extending to the bottom. A deepening of PW in spring at stations P2-P5 could suggest rapidly changing sea ice conditions (SI Fig. 4), with small increases in salinity indicating refreezing and brine rejection. Station P5 had low temperatures, with PW throughout the entire water column in both seasons. Stations P6 and P7 intersect the West Spitsbergen Current, with PW and wPW in surface waters extending until approximately 200 m depth, where both mAW and AW (Θ > 2℃, SA > 35.06 g kg -1 ) extends down to about 500 m. In waters below 500 m, mAW persists until about 800 m depth and EBDW (Θ < 0℃, SA > 35.06 g kg -1 ) fills the Nansen Basin.

At the time of sampling in late winter, first-year sea ice covered all stations except for P1 (Marquardt et al., 2023a). Some sea ice changes were observed during sampling (SI Fig. 4), but in general most stations were covered by close drift ice. Minor changes in ice coverage occurred before sampling for shelf stations, while P6 and P7 showed significant variations in ice coverage.

However, by the end of the late winter campaign, P6 and P7 were also covered by close drift ice.

Shelf stations P2-P5 showed dynamic ice conditions in between the late winter and spring cruises, while P6 and P7 were covered by slightly older sea ice in spring (Marquardt et al., 2023a).

THg and MeHg distributions in the water column during late winter and spring

THg concentrations in late winter (Fig. 2, top left) in the northern Barents Sea and Nansen Basin were on average 0.56 ± 0.17 pmol L -1 (Table 1). In spring (Fig. 2, top right), THg concentrations in the northern Barents Sea and Nansen Basin were on average 0.69 ± 0.24 pmol L -1 (n = 67, ± 1SD, range: 0.46 -2.37 pmol L -1 ). In the upper 500 m, our average THg concentrations on the spring cruise (0.64 ± 0.09 pmol L -1 , n = 56) were significantly higher (log transformed, Student's t-test, p < 0.05) compared to late winter (0.53 ± 0.07 pmol L -1 , n = 51). On the shelf, our THg concentrations in both late winter and spring (Table 1) are similar to THg concentrations in the southern Barents Sea Opening (0.43 ± 0.14 pmol L -1 ) [START_REF] Petrova | Mercury species export from the Arctic to the Atlantic Ocean[END_REF] and comparable to previous expeditions in this area (0.63 ± 0.19 pmol L -1 in summer, and 0.46 ± 0.07 pmol L -1 in early winter) (Kohler et al., 2022a). In late winter, shelf stations P1 -P5 exhibited relatively constant THg concentration profiles with depth, similar to early winter 2019 (Kohler et al., 2022a) while spring vertical THg profiles are similar to observed THg profiles in summer 2019 (Kohler et al., 2022a), with enrichments in surface waters < 50 m. Depths where MeHg concentration < LOQ (7 fmol L -1 ) have been removed from the visualization. The x-axis displays latitude and the nonlinear y-axis is in units of depth (m). Transect created with Ocean Data View (Schlitzer 2021). 1, Fig. 2 middle left) along the entire transect during late winter were 112 ± 76 fmol L -1 (n = 58, ± 1SD). At station P6 and P7, MeHg concentrations were lowest near the surface and directly below the ice (UIW = 59 ± 11 fmol L -1 , n = 2). At P7, MeHg concentrations greater than 250 fmol L -1 began at 300 m depth in the AW core of the West Spitsbergen Current.

MeHg concentrations (Table

By the spring cruise, MeHg concentrations along the entire transect were 69 ± 75 fmol L -1 (n = 60, ± 1SD) (Fig. 2, middle right) and covered a relatively large range (Table 1). In the upper 500 m, our average MeHg concentrations on the spring cruise (41 ± 39 fmol L -1 , n = 49) were significantly lower (log transformed, Student's t-test, p < 0.05) compared to late winter (85 ± 42 fmol L -1 , n = 51). Shelf stations in spring exhibited low MeHg concentrations in general (Table 1), with the lowest MeHg concentrations observed at P1 in mAW and some measurements below the LOQ of 7 fmol L -1 . Previous seasonal expeditions on the northern Barents Sea shelf showed similarly constant MeHg distributions with depth, albeit with slightly higher average MeHg concentrations (Kohler et al., 2022a). Our spring MeHg concentrations on the shelf however, are also lower than both the Barents Sea Opening (~ 60 ± 30 fmol L -1 ) [START_REF] Petrova | Mercury species export from the Arctic to the Atlantic Ocean[END_REF] and among the lowest reported for the Arctic region (Jonsson et al., 2022a). Percent MeHg (%MeHg) in both seasons follows a similar distribution pattern as MeHg concentrations (Figure 2, bottom), with higher %MeHg observed in subsurface waters. In late winter, %MeHg is 20 ± 13% (n = 58) while spring %MeHg is 10 ± 10% (n = 60). Similar to MeHg, there is a significant difference in %MeHg in the upper 500 m (log transformed, Student's t-test, p < 0.05) between late winter (16 ± 9%, n = 51) and spring cruises (7 ± 8%, n = 49).

The highest THg concentrations for both late winter and spring were observed in deep waters of P6 and P7 between 500 -1500 m depth and both maxima are observed 10 m from the bottom of the Nansen Basin (SI Fig. 1). Our unfiltered THg observations in basin waters in both late winter and spring are within the ranges reported in the central Arctic Ocean [START_REF] Heimbürger | Shallow methylmercury production in the marginal sea ice zone of the central Arctic Ocean[END_REF][START_REF] Wang | Subsurface seawater methylmercury maximum explains biotic mercury concentrations in the Canadian Arctic[END_REF], Agather et al., 2019, Jonsson et al., 2022b) and the Fram Strait [START_REF] Petrova | Mercury species export from the Arctic to the Atlantic Ocean[END_REF]. Similarly, relatively high MeHg concentrations were observed in basin waters > 500 m (SI Fig. 1). These findings in late winter are similar to deep water MeHg concentrations observed in the Fram Strait [START_REF] Petrova | Mercury species export from the Arctic to the Atlantic Ocean[END_REF] and the central Arctic Ocean (Jonsson et al., 2022b).

However, as we could not guarantee trace metal clean Niskin bottles for our deep water sampling (> 500 m), some caution is needed when interpreting these results. S1.

Seasonal inputs of THg to surface waters

Elevated THg in surface waters in spring could suggest inputs from AMDEs or sea ice processes, where changing sea ice conditions facilitate atmospheric deposition to surface waters. Although much of the AMDE-derived Hg is photochemically re-emitted back to the atmosphere from the cryosphere during the summer [START_REF] Dastoor | Arctic mercury cycling[END_REF], a temporary enrichment could exist during spring. Under ice water (UIW) taken at stations P4, P6 and P7, in late winter was 0.58 ± 0.02 pmol L -1 (n = 3), similar to the remaining THg levels in the water column. However, the bottom 10 cm of ice cores taken at P4 and P7 had low THg concentrations, with values of only 0.34 pmol L -1 and 0.20 pmol L -1 , respectively. As sea ice appeared to both melt and freeze in between our two sampling periods (SI Fig. 4), it is plausible that dynamic melt and freeze processes facilitated additional THg to surface waters. However, changes in integrated THg in seawater (upper 100 m)

do not show a correlation with the average fraction of open water (SI Fig. 5). We suspect that the heterogeneity in sea ice and snow may deliver variable amounts of Hg to surface waters, as the sea ice covering our stations during the two seasonal cruises have different origins and drift patterns (Marquardt et al., 2023a). Previous studies in the Arctic found higher concentrations and a wide range of THg (range: 0.089 -60.8 pmol L -1 ) in both first year ice and multiyear ice compared to seawater (Chaulk et al., 2011[START_REF] Beattie | Total and methylated mercury in Arctic multiyear sea ice[END_REF], Schartup et al., 2020, Jonsson et al., 2022b).

Although we measured low THg concentrations in the bottom sea ice in late winter, THg can vary by up to an order of magnitude throughout the core, with highest concentrations near the sea iceatmosphere interface [START_REF] Beattie | Total and methylated mercury in Arctic multiyear sea ice[END_REF]. As sea ice drifts and snow and ice melts between cruise samplings (SI Fig. 4), these reservoirs could supply additional Hg to surface seawater [START_REF] Huang | Modeling the mercury cycle in the sea ice environment: a buffer between the polar atmosphere and ocean[END_REF]. Furthermore, any leads in the ice pack during these changing ice conditions could also facilitate direct atmospheric deposition of AMDEs to surface seawater or deposit additional Hg to the cryosphere [START_REF] Moore | Convective forcing of mercury and ozone in the Arctic boundary layer induced by leads in sea ice[END_REF].

Refreezing also results in the rejection of brine between sampling periods (SI Fig. 4), with brine THg concentrations potentially up to 355 pmol L -1 (Chaulk et al., 2011). Although we did not measure brine directly, a frost flower formed from brine expulsion in late winter had a concentration of 24 pmol L -1 (n = 1). By spring, UIW concentrations were 1.00 ± 0.30 pmol L -1 (n = 3), and higher than the average concentration for the remaining water column. Likewise, in sea ice, the THg concentration in the bottom 10 cm of ice cores taken at P4 and P7 were higher in spring, with values of 1.55 pmol L -1 and 2.12 pmol L -1 , respectively. Spring THg concentrations in the bottom of ice cores are higher compared to our late winter observations, which contrasts with a seasonal study in Antarctica [START_REF] Nerentorp Mastromonaco | Seasonal study of mercury species in the Antarctic sea ice environment[END_REF]. Local melt and freeze processes, downward brine movement, and varying sea ice advection are most likely responsible for the differences observed in the two studies. However, seasonal increases in THg in the Antarctic water column from winter to spring were suggested to be a result of both atmospheric and sea ice processes [START_REF] Nerentorp Mastromonaco | Speciation of mercury in the waters of the Weddell, Amundsen and Ross Seas (Southern Ocean)[END_REF]. Consequently, we conclude that our observed seasonal THg water column concentrations most likely reflect similar seasonal inputs and processes.

Potential uptake of THg by the spring bloom

Increased THg concentrations in surface waters and sea ice in spring could also be driven by the onset of primary productivity (SI Fig. 6). Development of ice algae during the spring bloom can take up THg from brine and seawater [START_REF] Burt | Mercury uptake within an ice algal community during the spring bloom in first-year Arctic sea ice[END_REF], explaining our increased THg concentrations in the bottom of sea ice. In the water column, Spearman's rank correlation analysis in spring indicated significant positive relationships between THg, particulate organic carbon (POC) and chlorophyll a (Table S3). Additionally, elevated chlorophyll a, pico-, nano-, and microphytoplankton in surface waters were observed in spring, indicating recent development of the phytoplankton spring bloom (SI Figs. 6 & 9). Log-log regression models between THg, POC, and chlorophyll a concentrations in the upper 500 m for all stations in spring (Fig. 4A,4C) showed a positive and statistically significant relationship (F-test, p < 0.05). Compared to late winter, this analysis suggests that THg is more associated with the particulate phase. If we assume POC as a proxy for total particulate matter and a constant partition coefficient Kd to calculate dissolved Hg (dHg) and particulate Hg (pHg) (SI Equations 3 and 4), this also indicates a greater percentage of pHg in spring when compared to late winter. A strong positive Spearman rank correlation also exists between POC and chlorophyll a (Table S3, p <0.05), suggesting that the positive correlation of THg with both of these biological indicators may suggest adsorption and/or uptake of Hg species to primary producers during the spring bloom (Burt et al., 2014, Luengen andFlegal 2009). However, it is important to note that higher THg concentrations in spring suggest that seasonal inputs increase both pHg and dHg concentrations, with a significant fraction of THg in the dissolved phase. While our THg relationships to biological parameters during the spring bloom suggests that THg is either taken up into primary producers by diffusion or adsorbed to cell surfaces, the remaining dHg in the water column can also be complexed by extracellular algal exudates [START_REF] Cossart | Role of phytoplankton in aquatic mercury speciation and transformations[END_REF]. In the dissolved phase, Hg shows strong affinity to DOM and especially to reduced sulfur ligands [START_REF] Ravichandran | Interactions between mercury and dissolved organic matter--a review[END_REF]). Indeed, we saw increased relative intensities of dissolved organic sulfur compounds (CHOS) in surface waters in both sampling periods compared to deeper waters (SI Fig. 7), most likely released by phytoplankton. These biologically driven processes, such as uptake by phytoplankton and organic matter complexation, could keep THg sourced from the atmosphere or sea ice in surface waters as well as facilitate transformations of Hg species.

MeHg dynamics in late winter and spring

Significant differences in MeHg concentrations from late winter to spring can be driven by several biogeochemical processes. As our spring MeHg concentrations showed a significant decline compared to late winter, the collective sinks of MeHg in the water column outweigh MeHg sources. Potential MeHg sinks to consider are photodemethylation, particle scavenging of MMHg, atmospheric evasion of DMHg, and dark demethylation processes. To systematically evaluate the contribution of each of these potential sinks, we used both literature and field data to calculate the percentage contribution to the observed difference in the upper 100 m. It is important to note that our observed differences in MeHg are net values, which can lead to high uncertainty when estimating the contribution from various sources and sinks.

We integrated the water column for MeHg for each station to calculate the rate of change per day, called ΔMeHg (Kohler et al., 2022a) for each station using the following equation:

𝛥𝛥𝛥𝛥𝛥𝛥𝛥𝛥𝛥𝛥 = (� 𝑧𝑧 0 [𝛥𝛥𝛥𝛥𝛥𝛥𝛥𝛥] 𝑆𝑆𝑆𝑆 𝑑𝑑𝑑𝑑 -� 𝑧𝑧 0 [𝛥𝛥𝛥𝛥𝛥𝛥𝛥𝛥] 𝐿𝐿𝐿𝐿 𝑑𝑑𝑑𝑑 ) / 𝑑𝑑𝑑𝑑
Here, we used the trapezoidal integration method to calculate from 0 to 100 m depth, where z = depth, SP = spring, and LW = late winter, and t = time in days between sampling of each station.

The total integrated pools are summarized in Table S4. All stations show a net decline, or loss, in the integrated pool from late winter to spring with an average of -81 ± 36 pmol m -2 d -1 (n = 7), demonstrating that sinks outweigh sources for observed change in MeHg concentrations.

Our measured declines of MeHg pools by spring could be due to increased photodemethylation during the transition to polar day. However, both lower water temperatures and the presence and thickness of snow and sea ice hinder MeHg photodemethylation (Point et al., 2009). Previous estimates in the Arctic Ocean suggest photodemethylation rates of 0.04 d -1 (Lehnherr et al., 2011) and 0.0082 d -1 [START_REF] Zhang | A global model for methylmercury formation and uptake at the base of marine food webs[END_REF] at the sea surface. Assuming first-order kinetics and using ΔMeHg, our calculated rate constant for our seven stations in the upper 100 m between late winter to spring is an average MeHg loss of 0.021 ± 0.013 d -1 (n = 7). This loss, although a net loss and not directly comparable, agrees well with the estimated photodemethylation rate constants; however, the photodemethylation rate would be attenuated with depth and dependent on available light. To estimate the percentage of the MeHg pool potentially degraded due to photodemethylation at each station, we used photosynthetically active radiation (PAR) approximated to 15 E m -2 d -1 between the months of March and May 2021 (SI Fig. 8) and rate constant from Arctic waters of 0.001 m 2 E -1 when expressed to solar irradiance (Lehnherr et al., 2011). After accounting for the fraction of open water (with a simple assumption of zero transmittance through snow-covered sea ice) and attenuation of PAR with depth (Table S6), our calculations suggest that photodemethylation can only account for 5 ± 4% of our observed ΔMeHg in the upper 100 m (Fig. 5). The highest percentage of photodemethylation ( 11%) is observed at station P1 due to no sea ice coverage. This calculation carries a high uncertainty due to additional variables we could not account for, such as the partial transmittance of light through sea ice, enhanced or inhibited photodemethylation in the presence of DOM, and scattering and absorption of light by both particles and DOM. Nevertheless, potential photodemethylation plays a relatively small role compared to our observed changes in MeHg pools. Particulate scavenging could also potentially serve to transport particle-bound MMHg (pMMHg) to deeper waters. As the total amount of sinking particles, including POC, are low in late winter waters due to the lack of biological productivity (Dybwad et al., 2022), we could assume that scavenging of pMMHg is negligible. Furthermore, a previous study has estimated that only 4% of pHg is pMMHg, which might be overestimated due to several samples below the detection limit [START_REF] Agather | Distribution of mercury species in the Western Arctic Ocean (US GEOTRACES GN01)[END_REF]. Additionally, observed POC fluxes across our transect in late winter were low at 27 ± 9 mg C m -2 d -1 (Bodur et al., 2023) with no flux attenuation, suggesting minimal vertical pHg transport. For comparison, POC fluxes in spring were 178 ± 199 mg C m -2 d -1 (Bodur et al., 2023). While we only measured MeHg and do not know exactly when primary production began to develop, we made several assumptions to attempt to determine the contribution of pMMHg scavenging to ΔMeHg (Table S6), with an estimate of 1 ± 1% of ΔMeHg (Fig. 5).

In surface waters, shifts in ice coverage could lead to evasion of DMHg trapped below the ice and help explain lower MeHg in surface waters during spring. In UIW, MeHg concentrations were 59 ± 11 fmol L -1 (n = 3) in late winter and 36 ± 16 fmol L -1 (n = 3) in spring. As our average UIW concentrations of MeHg in spring were lower than in late winter, it is possible DMHg has evaded to the atmosphere during melting and freezing, or DMHg has built up in the overlying sea ice [START_REF] Schartup | Influence of the Arctic Sea-Ice regime shift on sea-ice methylated mercury trends[END_REF]. Previous studies have emphasized the importance of the evasion of DMHg from Arctic waters (Baya et al., 2015[START_REF] Soerensen | A mass budget for mercury and methylmercury in the Arctic Ocean[END_REF] but recent studies of DMHg in sea ice and surface seawater [START_REF] Agather | Distribution of mercury species in the Western Arctic Ocean (US GEOTRACES GN01)[END_REF], Jonsson et al., 2022) suggest a relatively modest evasion rate. Although we did not measure DMHg, we expect a large proportion of our surface MeHg concentrations to consist of MMHg, similar to other Arctic Ocean studies (Baya et al., 2015, Agather et al., 2019, Jonsson et al., 2022b). We attempted to quantify the amount of DMHg evasion during our transition to spring with additional limitations and assumptions (Table S6), with atmospheric evasion making up only 3 ± 3% of ΔMeHg (Fig. 5). As this estimate is a gross value and does not account for atmospheric re-deposition of MMHg, it may be overestimated.

Water mass distribution along our transect did not change drastically from late winter to spring (Van Engeland et al., 2023), suggesting relatively minimal importance of advection of lower concentration MeHg waters in the upper 100 m. However, station P6 and P7 do intercept the West Spitsbergen Current, with slight shifts from wPW to PW in spring (Van Engeland et al., 2023).

MeHg concentrations in PW are lower than wPW in late winter (One-way ANOVA with Tukey's test, p < 0.05), and could contribute to the observed ΔMeHg. Nevertheless, PW is significantly different between the two cruises, so for the sake of simplicity we have assumed advection plays a negligible role in explaining our observed ΔMeHg.

The importance of dark demethylation processes in unfiltered waters has been highlighted in previous studies using spiked stable Hg isotopes to measure demethylation rates (Lehnherr et al., 2011[START_REF] Monperrus | Mercury methylation, demethylation and reduction rates in coastal and marine surface waters of the Mediterranean Sea[END_REF]. In addition, the recent MeHg annual budget for the East Siberian

Sea attributed approximately 50% of potential MeHg sinks to gross dark demethylation (Kim et al., 2020b). Consequently, we attribute the remaining ΔMeHg observed (90 ± 8%, Fig. 5) as dark demethylation processes and hypothesize that they are mainly driven by biological activity.

Biotic demethylation of MeHg

As POC fluxes were low in late winter (Bodur et al., 2023), it is likely that there is no substantial in situ biotic Hg methylation occurring throughout the water column. Additionally, despite a higher concentration of THg by spring, both MeHg concentrations and % MeHg have decreased (Fig. 2), suggesting methylation processes were dominated by demethylation processes. However, the increase in phytoplankton abundance by orders of magnitude in surface waters along the transect in spring could support biotic transformations of Hg species (SI Fig. 6). Upon development of the phytoplankton bloom, we observed that diatoms, particularly species of the genus Thalassiosira, had the highest relative abundance in the upper 90 m at five out of seven stations along our transect (SI Fig. 9). Our regression analysis (Fig. 4) supports the hypothesis that THg is indeed potentially taken up by primary producers. Unlike THg, however, MeHg has significant negative relationships with both POC and chlorophyll a in spring (Fig. 4B, 4D, F-test, p < 0.05), supporting declining MeHg concentrations in association with biological particles. As diatoms and other marine phytoplankton passively take up MMHg (Mason et al., 1996, Lee and[START_REF] Soerensen | A mass budget for mercury and methylmercury in the Arctic Ocean[END_REF], we hypothesize that our observed MeHg losses in surface waters could be due to phytoplankton uptake and subsequent demethylation. This hypothesis is further supported by relatively low contributions of the other MeHg sinks in surface waters (Fig. 5) and possible biological demethylation mechanisms identified in literature. Previous research has observed reduction of inorganic Hg II to Hg 0 in the diatom Thalassiosira weissflogii [START_REF] Morelli | Changes in the non-protein thiol pool and production of dissolved gaseous mercury in the marine diatom Thalassiosira weissflogii under mercury exposure[END_REF] while the green alga

Chlamydomonas reinhardii was demonstrated to demethylate MMHg [START_REF] Bravo | Speciesspecific isotope tracers to study the accumulation and biotransformation of mixtures of inorganic and methyl mercury by the microalga Chlamydomonas reinhardtii[END_REF].

Another study in a freshwater lake has shown that demethylation of intracellular MMHg correlated positively with diatom abundance [START_REF] Cossart | Species-specific isotope tracking of mercury uptake and transformations by pico-nanoplankton in an eutrophic lake[END_REF]. Recently, a study suggested that phytoplankton can intracellularly demethylate MeHg through the production of reactive oxygen species [START_REF] Liang | Lightindependent phytoplankton degradation and detoxification of methylmercury in water[END_REF].

This MMHg uptake during the onset of the Arctic spring bloom could potentially facilitate the seasonal bioaccumulation of MMHg by higher trophic level organisms throughout the summer. A previous study in an Arctic lake observed decreasing MeHg levels in water from winter to spring, yet saw a seasonal increase in MeHg concentrations in freshwater zooplankton [START_REF] Varty | Methylmercury transport and fate shows strong seasonal and spatial variability along a high Arctic freshwater hydrologic continuum[END_REF].

Furthermore, another winter to spring increase of MeHg concentrations in zooplankton was observed in waters of the Canadian Arctic Archipelago (Pućko et al., 2014). While other mechanisms like zooplankton grazing activity or direct methylation in the gut could explain their observed seasonal Hg biota concentrations (Pućko et al., 2014), we speculate that our seasonal changes of water column MeHg partly reflect a MMHg biomagnification pathway for higher trophic level organisms in the Arctic's productive spring and summer seasons.

Further studies on Hg methylation activity in specific marine phytoplankton species demonstrated that MeHg demethylation positively correlated with thiol concentrations in algal exudates [START_REF] Li | The potential of mercury methylation and demethylation by 15 species of marine microalgae[END_REF]. We observed more elevated relative intensities of CHOS compounds in surface waters compared to deeper waters (SI Fig. 7). Dissolved CHOS compounds could facilitate the transformation of MMHg to DMHg via localized reduced thiol groups [START_REF] Jonsson | Dimethylmercury formation mediated by inorganic and organic reduced sulfur surfaces[END_REF] demonstrated that the presence of the merB gene, found in certain bacteria and responsible for the demethylation of MeHg to Hg II , is present in deep waters of the Arctic Ocean [START_REF] Bowman | Distribution of mercury-cycling genes in the Arctic and equatorial Pacific Oceans and their relationship to mercury speciation[END_REF]. In bacteria, the mer operon, encoding for merA and merB genes, exists primarily as a detoxifying mechanism for high concentrations of Hg, in a process called reductive demethylation.

Lineages of a particular bacterial strain genome containing merA and merB, with high resistance and strong degradation capacity of MeHg, are present in polar regions (Sanz-Sáez et al., 2022).

On our transect, potential reductive demethylators (Alteromonas, Marinobacter, and Oleibacter) are present, but in low relative abundance during both sampling periods compared to the entire bacterial and archaeal community (SI Text). However, bottom waters at shelf stations had higher total abundances of bacteria and higher relative abundances of potential demethylators than surface waters in late winter and could support MeHg degradation during the winter-spring transition (SI Text). This includes methanotrophic bacteria and sulfate reducing bacteria, which can also participate in oxidative demethylation of MeHg under anoxic conditions [START_REF] Barkay | Demethylation─the other side of the mercury methylation coin: a critical review[END_REF].

While dark abiotic demethylation could influence our losses of MeHg in deep waters, more research is needed to uncover these mechanisms. Dissolved CHOS compounds, seen in both surface and deeper waters in both late winter and spring (SI Fig. 6), could facilitate transformation and demethylation of MMHg as previously discussed. Additionally, resuspension of sediments may supply manganese oxide to bottom waters, resulting in both the demethylation and scavenging of MMHg and iHg (Vlassopolous et al., 2018[START_REF] Murphy | Demethylation or Sorption? The fate of methylmercury in the presence of manganese dioxide[END_REF], Kohler et al., 2022a). However, due to the uncertainty of these potential abiotic processes, we hypothesize much of our MeHg in deeper waters is also biotically demethylated from late winter into spring.

Hg in basin deep waters ( > 500 m)

Bottom waters below 500 m at stations P6 and P7 are elevated in THg compared to the shelf stations (SI Fig. 1). These elevated concentrations in EBDW could contain additional Hg from resuspended sediment or sinking particles, as suggested in the Canadian Basin [START_REF] Agather | Distribution of mercury species in the Western Arctic Ocean (US GEOTRACES GN01)[END_REF]. As these basin stations are close to the shelf break, turbidity along the continental slope caused by the West Spitsbergen Current may resuspend and deliver Hg-rich particles into the basin.

Throughout the Arctic winter (December to March), overturning on the Barents Sea shelf generates a nepheloid layer of suspended sediment which can sink as dense water plumes into the basin [START_REF] Rudels | Atlantic sources of the Arctic Ocean surface and halocline waters[END_REF][START_REF] Gardner | Distribution, sources, and dynamics of particulate matter along Trans-Arctic sections[END_REF][START_REF] Rogge | Carbon dioxide sink in the Arctic Ocean from cross-shelf transport of dense Barents Sea water[END_REF]. High concentrations of Hg and other trace elements in surface sediments in the Eurasian Basin have suggested hydrodynamic transport and scavenging of particles sourced from the shelf (Kohler et al., 2022b). Beam transmittance data (SI Fig. 10), however does not support significant resuspension of sediments in basin waters. For MeHg, stations P6 and P7 show similar patterns in late winter and spring, with the highest MeHg concentrations observed at 500 m and below. In EBDW, MeHg concentrations remain elevated above 170 fmol L -1 at P7 until 2500 m depth. Compared to the shelf, EBDW and deeper waters remain enriched in MeHg. These observations are in line with previous measurements in the Fram Strait with maximum MeHg between 400 -1000 m [START_REF] Petrova | Mercury species export from the Arctic to the Atlantic Ocean[END_REF]. Diffusion of porewaterderived MeHg could help explain elevated MeHg concentrations along the continental slope by station P6 (Kim et al., 2020b). Although both MeHg and THg concentrations in the deep basin agree with previous literature [START_REF] Wang | Total and methylated mercury in the Beaufort Sea: the role of local and recent organic remineralization[END_REF][START_REF] Heimbürger | Shallow methylmercury production in the marginal sea ice zone of the central Arctic Ocean[END_REF], our sampling with standard Niskin bottles lends a degree of caution.

Seasonal cycle of Hg in the northern Barents Sea

Our observations for both THg and MeHg suggest a distinct seasonality of Hg in the northern Barents Sea marginal ice zone from late winter to spring. Based on our analysis, we conclude that the main drivers of both THg and MeHg changes in surface waters are ultimately driven by seasonal Hg inputs and the biotically driven processes of the phytoplankton spring bloom. In deeper waters, biotic demethylation is likely to play a more important role than abiotic demethylation mechanisms. A previous study suggested iHg seasonality in this region during the transition from summer to winter, with no significant change in MeHg concentrations (Kohler et al., 2022a). By assuming Hg follows a distinct seasonal cycle in the Arctic Ocean every year, we combined our observations from late winter and spring 2021 with that of summer and early winter 2019 (Kohler et al., 2022c,d) to illustrate a simple seasonal cycle of Hg over the course of a year in the northern Barents Sea surface waters (Fig. 6). To best illustrate the seasonal dynamics of THg and MeHg, we integrated our results from the surface to 100 m depth for each station (Tables S4 &S5), where most biological productivity and Hg input occurs. from seasonal cruises (this study) and a previous study (Kohler et al., 2022a) in the northern Barents Sea. Graphs are superimposed on a simplified Arctic seasonal cycle in the marginal ice zone with polar day occurring between April and August and polar night beginning November until February. Green particles represent the development of the ice algal and phytoplankton spring blooms. Integrated pools for THg and MeHg are found in Tables S4 andS5. The x-axis is in months. Note the difference in scales on the y-axis.

Seasonal patterns of both THg and MeHg emerge and appear to be driven by the development of biological activity. The THg pools are higher during the polar day, while MeHg pools are at the minimum at the onset of the phytoplankton bloom and maximum in the second half of the year.

As hypothesized, seasonal inputs and biological activity increase THg concentrations in surface waters. To better assess how relevant environmental drivers impact seasonal THg and MeHg concentrations, a PCA was conducted for each seasonal cruise (Fig. 7), with the first two principal components explaining > 60% of the variance for each cruise (Table S7). Clustering of THg with biological parameters POC and chlorophyll a in spring and summer suggest seasonal changes in THg concentrations are driven by biological activity and its partitioning to the particulate phase.

Notably, this includes potential uptake of Hg species, transformation to MeHg during remineralization of POC, and downward transport with sinking particles. Sinking POC supports the production of subsurface in situ MeHg throughout polar day [START_REF] Heimbürger | Shallow methylmercury production in the marginal sea ice zone of the central Arctic Ocean[END_REF], reflected in increasing MeHg pools progressing into summer (Fig. 6) and the general trend of positive MeHg loadings on the first component with depth, nutrients, and salinity for the spring and summer cruises. Late summer POC fluxes (Bodur et al., 2023), and increased proportions of manganese oxides leading into the polar night (Xiang and Lam 2020) lower THg pools by early winter (Kohler et al., 2022a).

Less clear is the behavior of both THg and MeHg throughout the polar night into late winter. It was proposed that potential scavenging by manganese oxides could be a driver of THg during Arctic winter (Kohler et al., 2022a), although a negative trend in THg pools from early winter to late winter is not observed (Fig. 6). We speculate that changes in Hg speciation, such as increased proportions of gaseous Hg 0 and DMHg could protect against scavenging losses. A recent experimental study argued that gross dark reduction rates of Hg II to Hg 0 are significant in both photic and aphotic zones, with dark reduction most likely dominant in the Arctic except during polar day [START_REF] Lamborg | Dark reduction drives evasion of mercury from the ocean[END_REF]. Reduction could be driven by bacteria with the merA gene, and potentially include bacteria with both merA and merB, facilitating declining MeHg pools throughout the polar night (Fig. 6). The bacterial community in this region shifts from carbon cycling in summer to nitrogen cycling in winter [START_REF] Thiele | Seasonality of the bacterial and archaeal community composition of the Northern Barents Sea[END_REF], suggesting a possible link between MeHg and the nitrogen cycle [START_REF] Despins | Linked mercury methylation and nitrification across oxic subpolar regions[END_REF] during the polar night. In our PCA analysis, we observed that MeHg and nitrite (NO2) exhibited opposite loadings on PC2 for all cruises (Fig. 7). This observation could link nitrite-oxidizing Nitrospina, known to harbor and express hgcAB-like proteins for Hg methylation [START_REF] Gionfriddo | Microbial mercury methylation in Antarctic sea ice[END_REF][START_REF] Villar | Widespread microbial mercury methylation genes in the global ocean[END_REF], as a driver for in situ seasonal MeHg concentrations in the Arctic. Our in situ concentration data however, show a net loss of the MeHg pool from winter to late winter (Fig. 6). As our studies comprise two different sampling years (2019 vs. 2021), more seasonal and interannual studies are needed to fully understand the drivers of MeHg concentrations in the Arctic water column. To the best of our knowledge, only one model has attempted to represent seasonal cycling of Hg in the Arctic Ocean [START_REF] Fisher | Riverine source of Arctic Ocean mercury inferred from atmospheric observations[END_REF]. In this model, sources and sinks of Hg are categorized by month, with subsurface waters resupplying Hg to surface waters through vertical mixing.

However, the latest Arctic budget highlights that export of Hg to deeper waters is most likely underestimated [START_REF] Dastoor | Arctic mercury cycling[END_REF], further supporting our hypothesis that seasonal changes in Hg may be driven by spring Hg inputs and changes in the particulate phase. Our new seasonal data help fill in this knowledge gap but continue to remain a "seasonal snapshot" from both a seasonal and interannual perspective. Recent studies on seasonality in this region highlight large seasonal and interannual variability in the northern Barents Sea regarding water masses and sea ice dynamics [START_REF] Jones | Inorganic carbon and nutrient dynamics in the marginal ice zone of the Barents Sea: seasonality and implications for ocean acidification[END_REF][START_REF] Kohlbach | Earlier sea-ice melt extends the oligotrophic summer period in the Barents Sea with low algal biomass and associated low vertical flux[END_REF], Van Engeland et al., 2023). Specifically, interannual variability may exert a greater impact on perceived seasonality, as our four cruises were sampled in two different years (2019 vs. 2021). As a result, our interpretations between the spring to summer transition and the early winter to late winter transition are subject to both seasonal and interannual variability, and thus more speculation. Nevertheless, we propose that our seasonal cycle of Hg in the northern Barents Sea is supported by our observational data and is reflected in current literature on Arctic Hg cycling.

Conclusion

We show that both THg and MeHg concentrations in the water column of the northern Barents Sea exhibit dynamic changes throughout the year. Variations in THg and MeHg are driven by seasonal processes during the early spring, such as Hg input to surface waters and biological activity. Notably, we observed a significant decline in MeHg concentrations during the development of the phytoplankton spring bloom. We suggest biotic demethylation mediated by phytoplankton and bacteria as the main mechanism underlying our observations. This demethylation by phytoplankton and bacteria during the spring bloom may mitigate the bioaccumulation of Hg up the food web. Although it is difficult to completely disentangle these losses and transformations of MeHg, our data suggests that potential seasonal uptake of MeHg during the spring bloom represents an important entry point of Hg into the Arctic marine ecosystem.

Using our current data, we constructed a simplified seasonal Hg cycle in the northern Barents Sea marginal ice zone and conducted additional analysis to try and identify major environmental parameters driving both THg and MeHg concentrations. THg was clustered with chlorophyll a and POC in spring and summer, suggesting seasonal changes of THg are modulated by changes in primary productivity, particles, and food web processes. MeHg concentrations also exhibited a seasonal pattern, with potential uptake and demethylation during spring, followed by hypothesized in situ production throughout the summer. Our simple seasonal cycle also encompasses potential interannual variability, which may underestimate additional drivers of Hg concentrations such as sea ice coverage and changing water masses. In addition, the northern Barents Sea has limited riverine inputs, highly variable sea ice coverage and a unique water mass distribution compared to the other Arctic shelf seas. Thus, the applicability of our proposed seasonal cycle may have limited use in other Arctic regions, like the river-influenced Siberian shelf seas or less biologically productive Central Arctic Ocean. Nevertheless, our study represents an important first step in identifying Hg seasonal processes and drivers in Arctic waters, with additional seasonal environmental data from the Nansen Legacy project available for use in budgets and models. We suggest future Arctic Hg budgets and coupled ocean-atmosphere/earth system models include an element of seasonality.

The projected climate driven inputs of increasing amounts of Hg from melting glaciers and thawing permafrost to Arctic shelf seas during the onset of the productive season may facilitate greater uptake of MMHg in the marine food web [START_REF] Chételat | Climate change and mercury in the Arctic: abiotic interactions[END_REF]. As ongoing climate change continues to transform the Arctic region, more research is needed to elucidate how region-specific seasonal processes affect Hg distributions in the Arctic Ocean and the surrounding shelf seas.

Specifically, we suggest more studies examining the connection between demethylation and biological uptake in the marginal ice zones as well as seasonal measurements of gaseous Hg species. We suspect that these transformation processes and rates that control MeHg distributions in the water column, and subsequently uptake in biota, follow a seasonal pattern.

Integration of THg and MeHg

We used trapezoidal integration for the upper 100 m in the water column for both THg and MeHg concentrations (Kohler et al., 2022). We assumed that the first available sampling depth (10 m or UIW depending on station) represented the sea ice-ocean interface (0 m) or the atmosphere-ocean interface (0 m). For 100 -200 m integration, stations P2 and P5 were integrated to bottom sampling depths less than 200 m. Stations were linearly interpolated to 100 m using the closest available sampling depths. The exact sampling dates and days between for each station were used to calculate the rate constants and ΔMeHg values. Where MeHg samples were less than the LOD or LOQ, the LOD (2 fmol L -1 ) and LOQ (7 fmol L -1 ) were used as the sample depth's concentration for integration, respectively. As most samples are run in singles, uncertainty in Δ (±1SD) for late winter and spring ΔMeHg and ΔTHg was calculated using the absolute THg reproducibility reported (± 0.05 pmol L -1 ) and absolute MeHg reproducibility reported (± 7 fmol L -1 ) for all depths and propagated using the sum of squares method for integration, and reported as ± 1 SD.

Depth matching for environmental parameters

THg and MeHg sampling geographic coordinates and depths differ slightly from environmental parameter sampling geographic coordinates due to ship drift during multiple samplings. Station samplings were assumed to be identical despite multiple sampling CTD casts and deviance from exact coordinates. For Spearman rank analysis, regression analysis, and PCA, environmental data were linearly interpolated to sampling depths of THg and MeHg for each cruise using the closest sampling depths when necessary. Otherwise, matching depths were assumed to be from the same water parcel for data analysis.

Percentage of ΔMeHg calculations

Photodemethylation

Delta photodemethylation (Table S6, ΔMeHgpd, pmol m -2 d -1 ) was estimated using a similar equation ( 1 (2)

Scavenging of MMHg

Delta MMHg scavenging (Table S5, ΔMeHgscav, pmol m -2 d -1 ) was estimated using several assumptions. First we calculated pHg in the upper 100 m using our THg concentrations at each discrete depth in late winter. To do this, we used in situ POC concentrations (Marquardt 2022a(Marquardt , 2022b) ) at each sampling depth as a proxy for total particulate matter and Kd for Hg in the Arctic shelf (1.38*10 6 L kg -1 , Tesán Onrubia et al., 2020) to calculate dHg (3) and pHg (4). We also assumed that Kd does not change seasonally nor with depth. Then, we assumed that pMMHg makes up approximately 4% of pHg [START_REF] Agather | Distribution of mercury species in the Western Arctic Ocean (US GEOTRACES GN01)[END_REF]. We used our estimated pMMHg and POC concentrations to estimate a pMMHg/TPM ratio in the upper 100 m, by assuming that POC in late winter makes up approximately 25% of TPM (Dybwad et al., 2022). We used the average late winter POC fluxes reported in Bodur et al., 2023 using a short term sediment trap method (27 mg C m -2 d -1 ) and assumed it represented 25% TPM flux of 108 mg m -2 d -1 , which is in agreement with previous TPM fluxes in the area determined by long term moorings (Dybwad et al., 2022).

As POC fluxes were not attenuated with depth in late winter (Bodur et al., 2023), we multiplied our average pMMHg/TPM ratio by our estimated TPM flux to calculate a pMMHg flux at 100 m depth (5), for an average of 0.76 ± 0.06 pmol m -2 d -1 for ΔMeHgscav.

𝑑𝑑𝛥𝛥𝛥𝛥 = 𝑇𝑇𝑀𝑀𝑀𝑀 ((𝐾𝐾 𝑑𝑑 * 𝑆𝑆𝑂𝑂𝑃𝑃)+1) (3) 𝑝𝑝𝛥𝛥𝛥𝛥 = 𝑇𝑇𝛥𝛥𝛥𝛥 -𝑑𝑑𝛥𝛥𝛥𝛥 (4)
𝛥𝛥 𝛥𝛥𝛥𝛥𝛥𝛥𝛥𝛥 𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠 = 𝑝𝑝𝛥𝛥𝛥𝛥𝛥𝛥𝛥𝛥 𝑇𝑇𝑆𝑆𝑀𝑀 * 𝑇𝑇𝑃𝑃𝛥𝛥 𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓 (5)

Evasion of DMHg

We followed a similar approach to Kim et al., 2020 (and references therein) for calculating delta Baya et al., 2015). We multiplied our estimated DMHg evasion flux by the average fraction of open water (fOW) in the previous 60 days at each station from sampling in May, assuming sea ice completely restricts atmospheric evasion (DiMento et al., 2019).

DMHg evasion (

∆𝛥𝛥𝛥𝛥𝛥𝛥𝛥𝛥 𝑀𝑀𝑠𝑠 = 𝐾𝐾 𝑤𝑤 * [𝐶𝐶 𝐷𝐷𝑀𝑀𝑀𝑀𝑀𝑀 𝑤𝑤𝑤𝑤𝑤𝑤𝑤𝑤𝑤𝑤 -� 𝑃𝑃 𝐷𝐷𝐷𝐷𝐷𝐷𝐷𝐷 𝑤𝑤𝑎𝑎𝑤𝑤 𝑀𝑀 �] * 𝑓𝑓 𝑂𝑂𝐿𝐿 (6)

Dark demethylation

We calculated percentage dark methylation (ΔMeHgdark) as the proportion of ΔMeHg unaccounted for from the sum of ΔMeHgpd, ΔMeHgscav and ΔMeHgev for each station.

Flow cytometry (FCM) Analysis

To measure the abundance of microorganisms (prokaryotes [bacteria and archaea], pico-and nanophytoplankton), four replicate water samples (1.8 mL) were taken, fixed with glutaraldehyde (0.5 % final concentration) at 4 °C for a minimum of 2 h, flash-frozen in liquid nitrogen and stored at -80 °C until analysis. Samples for the analysis of prokaryote abundance were first diluted 10 times with 0.2-μm-filtered TE buffer (Tris 10 mM and EDTA 1 mM, pH 8), then stained with a green fluorescent nucleic acid dye (SYBR Green I; Molecular Probes, Eugene, OR, USA) and incubated for 10 min at 80 °C and measured on a FACS Calibur (Becton Dickinson, Oxford, UK) flow cytometer [START_REF] Marie | Enumeration of marine viruses in culture and natural samples by flow cytometry[END_REF]. Samples were counted at a low flow rate of around 60 µL min-1 and groups discriminated on a biparametric plot of green fluorescence (BL1) vs. side scatter (SSC). Pico-and nanosized phytoplankton abundance was measured using an Attune® Acoustic Focusing Flow cytometer (Applied Biosystems by Thermo Fisher Scientific, Waltham, MA, USA)

and the different phytoplankton groups were discriminated based on their red fluorescence (BL3) vs. orange fluorescence (BL2) [START_REF] Paulsen | Synechococcus in the Atlantic gateway to the Arctic Ocean[END_REF]. The data for all cruises can be found at the Norwegian Marine Data Centre (Mȕller et al., 2023a(Mȕller et al., , 2023b(Mȕller et al., , 2023c(Mȕller et al., , 2023d)).

Bacterial Community Analysis

We investigated the bacterial and archaeal community with respect to potential demethylators (Christakis et al., 2018), methanotrophs, and sulfate reducing bacteria (SRB) using 16S rRNA gene amplicon sequences collected on our transect to help explain the MeHg concentrations at different depths. Using PICRUSt2 with standard parameters in bioconda, the metabolic potential of the different amplicon sequence variants was inferred based on their location in a phylogenetic tree of fully sequenced organisms and the genomic assets of the closest relative in this tree [START_REF] Grüning | Bioconda: sustainable and comprehensive software distribution for the life sciences[END_REF][START_REF] Douglas | PICRUSt2 for prediction of metagenome functions[END_REF]. Using KEGG-numbers, the marker genes merA and merB (reductive demethylation), dsrA (sulfate reduction), and mmoX (methanotrophy) were extracted and used to identify SRB, methanogens, and methanotrophs. For merA and merB, only Oleibacter was identified from the samples and was complemented with manual searches for Alteromonas and Marinobacter (Sanz-Sáez et al., 2022). Similarly, no mmoX genes were predicted and known methanotrophs were extracted based on recent literature [START_REF] Guerrero-Cruz | Methanotrophs: discoveries, environmental relevance, and a perspective on current and future applications[END_REF]. SRB were well covered by the PICRUSt2 results. In the water column, the relative abundance of potential reductive demethylators and SRB was highest in the deepest sample of station P7 with 1.9% and 0.7% (potential reductive demethylators) and 4.8% and 3.9% (SRB) in late winter and spring.

Potential reductive demethylators were otherwise low in relative abundance, rarely exceeding 0.1% of the total bacterial and archaeal community. Higher abundance was found in most samples at 10 m and below 200 m depth as compared to the samples in between, regardless of bottom depth for both seasons, although on average the abundance was higher in late winter (~0.1%) than in spring (~0.06%). SRB were similarly abundant in both seasons with 0.39% in late winter and 0.37% in spring but were lower in surface samples (10 and 20 m) as compared to deeper samples.

Methanotrophs were also low in abundance with ~0.15% both in late winter and spring. The highest abundance was found in both seasons at P1 (near bottom depth) with 1.3% in spring as compared to 0.7% in late winter. Generally, the abundance was higher in samples > 20 m.

Dissolved Organic Matter (DOM) Analysis

DOM seawater and under ice water samples were collected at stations P1, P4, P6, and P7 on both late winter and spring sampling expeditions. Water samples were immediately extracted after collection using solid phase extraction [START_REF] Dittmar | A simple and efficient method for the solid-phase extraction of dissolved organic matter (SPE-DOM) from seawater[END_REF]) and stored at -20℃ until analysis. on the spring cruise, with depth, total mercury (THg) methylated mercury (MeHg) particulate organic carbon (POC), chlorophyll a (Chla), nitrite (NO2), nitrate (NO3), phosphate (PO4), silicate (SiOH4), conservative temperature (Θ), absolute salinity (SA), and bacteria.

A significant p-value (p < 0.05) marked in bold.

Depth THg

Figure 1 :

 1 Figure 1: Map of sampling locations (stations P1-P7) in the northern Barents Sea region. A solid black line indicates the approximate location of the Polar Front, while the dashed colored arrows show a simplified water mass scheme and current direction, with Atlantic Water (AW) in red and Polar Water (PW) in blue. The color bar indicates water depth. Map created with Ocean Data View (Schlitzer 2021).

Figure 2 :

 2 Figure 2: THg concentrations (top, pmol L -1 ), MeHg concentrations (middle, fmol L -1 ), and % MeHg as THg (bottom) along the shelf-basin transect in the northern Barents Sea during late winter (left) and spring (right) in 2021. The maximum plotted THg concentration range is 1.25 pmol L -1 .

Figure 4 :

 4 Figure 4: Log scale scatterplots with log-log regressions of THg (left) and MeHg (right) concentrations against POC (top) and chlorophyll a (bottom) concentrations in the upper 500 m for both late winter (blue) and spring (orange) in the northern Barents Sea in 2021.

Figure 5 :

 5 Photodemethylation MMHg Scavenging DMHg Evasion Dark Demethylation

  , with a proposed disproportionation reaction mechanism of two MMHg molecules to form DMHg and iHg. Inorganic Hg could be reduced to Hg 0 or be scavenged, while DMHg could evade to the atmosphere as previously highlighted. Complexation of MMHg to dissolved thiols in DOM could also reduce uptake of MMHg by phytoplankton (Lee and Fisher 2017) reducing MMHg bioavailability. MMHg-DOM complexes can also undergo enhanced photodemethylation (Qian et al., 2014) in surface waters, although the specific mechanisms remain debated (Barkay and Gu 2021). Additional Arctic seasonal studies are needed to characterize the types of DOM and its role in driving MeHg biogeochemical processes in the water column. Collectively though, our data and literature support biotically mediated MeHg demethylation during the development of the spring bloom. Interestingly, deeper waters (100 to 500 m) also show a net loss of MeHg concentrations, with an average loss rate of 0.015 ± 0.011 d -1 (n = 7) in the 100 -200 m depth interval. While potential phytoplankton demethylation could explain MeHg losses in surface waters, we expect different biotic processes to affect MeHg losses in subsurface and bottom waters. Recent research has

Figure 6 :

 6 Figure 6: Integrated water column (0 to 100 m) observations of THg (top) and MeHg (bottom)

Figure 7 :

 7 Figure 7: Principal component analysis (PCA) loadings plot for each seasonal cruise in the northern Barents Sea from this study and previous cruises where Hg observations are available (<

  ) asKim et al., 2020. MeHg concentrations (CMeHg, pmol L -1 ) at each station were linearly interpolated for each depth in the surface 100 m in late winter using our observed concentrations at discrete depths. The photodemethylation rate was estimated based on satellite PAR (I0, 15 E m -2 d -1 ) in the region in April 2021 multiplied by the photodemethylation rate constant (kpd, 0.001 m 2 E -1 )(Lehnherr et al., 2011). The kpd was assumed constant over 100 m, while PAR (I) was attenuated by depth (z) using equation (2) found in Kim et al., 2020, with original reference Paulson and Simpson 1977, type I (clear water). We accounted for the sea ice coverage by multiplying by the average fraction of open water (fOW) at each station in the previous 60 days from sampling in May, assuming zero transmittance through snow-covered sea ice. 𝛥𝛥𝛥𝛥𝛥𝛥𝛥𝛥𝛥𝛥 𝑝𝑝𝑝𝑝 = ∫ 100 0 𝐶𝐶 𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑀 * (𝑓𝑓 𝑂𝑂𝐿𝐿 * 𝑃𝑃𝑃𝑃𝑃𝑃 * 𝑘𝑘 𝑝𝑝𝑝𝑝 )

  Extracts were analyzed by LTQ-Velos-Pro Orbitrap mass spectrometer (Thermo Scientific, Germany) with an electrospray ionization source (ESI) in negative mode. Each sample spectrum was calibrated internally in mass range 200-800 m/z with data resolution of 100 000. Additional detailed instrument parameters are noted in Hawkes et al., 2016. Dissolved organic sulfur compounds (CHOS) formulas are determined based on the following elemental criteria: C4-50 H4-100O 2-40N 0-2S 0-1, hydrogen to carbon ratio (H/C) = 0.3 -2.2, oxygen to carbon ratio (O/C) = 0-1, double bond equivalent-oxygen (DBE-O) = -10 -10, valence electron as even number, mass to charge ratio (m/z) = 150-800 and mass defect (mass after nominal mass) = -0.1 -0.3.

Table 1 :

 1 Average (± one standard deviation) and range of THg, MeHg, concentrations and % MeHg on both late winter and spring cruises for all water column samples, shelf stations (P1 -P5), 310 basin stations (P6, P7), under ice water (UIW), ice cores, and frost flowers (FF). at stations P6 and P7. No statistically significant differences were observed for THg nor MeHg concentrations in wPW, which comprised only subsurface and bottom waters at our stations(Van Engeland et al., 2023). As PW comprises most of our shelf stations and surface waters at P6 and P7, our observed changes in THg concentrations are best explained by external inputs to surface waters such as atmospheric deposition or ice melt instead of changes in water mass. In addition, our general transect (Fig.2) and insignificant differences in wPW (Table2) do not suggest major inputs of benthic THg nor MeHg during this time period. We also cannot rule out potential concentration differences in individual water parcels between sampling periods, but we infer that our observed THg profiles and differences in THg concentrations in spring reflect seasonal THg inputs to surface waters. Similarly, we hypothesize our lower MeHg concentrations in spring are driven by internal biogeochemical processes rather than external inputs and/or water masses. THg (top) and MeHg (bottom) concentrations averaged for water masses in both late winter (LW) and spring (SP) in the northern Barents Sea in 2021. Water masses are Atlantic Water (AW), modified Atlantic Water (mAW), Polar Water (PW), and warm Polar Water (wPW). Box plots display median (± IQR) and individual data points for each water mass. For averages and sample sizes, see Table

	311
			Total Hg (pmol L -1 )				Methylated Hg (fmol L -1 )
		Late winter	Spring			Late winter			Spring
		Average	Range	Average	Range		Average	Range	%	Average	Range	%
		± 1SD		± 1SD			± 1SD		MeHg as	± 1SD		MeHg as
									THg			THg
	All water	0.56 ±	0.40 -	0.69 ±	0.46 -	All water	112 ± 76	20 -340	20 ± 13	69 ± 75	< 7 -288	10 ±
	column	0.17	1.61	0.24	2.37	column	(n =58)			(n = 60)
	data	(n = 60)			(n = 67)	data
	Upper	0.53 ±	0.40 -	0.64 ±	0.46 -	Upper	85 ± 42	20 -254	16 ± 9	41 ± 39	<7 -204	7 ±
	500 m	0.07	0.77	0.09	0.94	500 m	(n = 51)			(n = 49)
	only	(n = 51)			(n = 56)	only
	Shelf	0.54 ±	0.47 -	0.64 ±	0.46 -	Shelf	71 ± 32	20 -161	13 ± 5	23 ± 13	< 7 -60	4 ±
	(P1-P5)	0.07	0.77	0.08	0.84	(P1-P5)	(n = 34)			(n = 33)
	all	(n = 34)			(n = 40)	all
	depths					depths
	Basin	0.59 ±	0.40 -	0.76 ±	0.47 -	Basin	171 ± 82	72 -340	30 ± 15	127 ± 79	21 -288	18 ±
	(P6, P7)	0.24	1.61	0.36	2.37	(P6, P7)	(n = 24)			(n = 27)
	all	(n = 26)			(n = 27)	all
	depths					depths
	UIW	0.58 ±	0.56 -	1.00 ±	0.73 -	UIW	59 ± 11	50 -67	10 ± 2	36 ± 17	26 -55	4 ±
		0.02	0.60	0.30	1.32		(n =2)			(n = 3)
		(n = 3)			(n = 3)
	Ice core	0.27 ±	0.20 -	1.83 ±	1.55 -	Ice core	n/a			n/a
		0.10	0.34	0.40	2.12
		(n = 2)			(n = 2)
	FF	24.03		n/a		FF	n/a			n/a
		(n = 1)
	312	To understand how changes in physical oceanography may affect our distributions of both THg
	313	and MeHg species, we averaged Hg concentrations by water mass in the upper 500 m, visualized
	314	in Fig. 3 and summarized in Table S1. Significant differences (log transformed, Student's t-test, p
		< 0.05) for both THg and MeHg concentrations in late winter and spring were observed for PW.

observed in waters at station P1. No difference was observed for MeHg in mAW, although we 317 assumed this is mainly due to local MeHg concentration differences in surface mAW at station P1 and subsurface mAW

Table S6

 S6

	, ΔMeHgev) (6). This included using estimated windspeeds from
	Longyearbyen airport in March and April 2021 (~ 8 m s -1 ), surface water temperatures (~ 10 m
	depth), at each station in late winter, temperature dependent Schmidt numbers for CO2 and DMHg
	to calculate the DMHg transfer coefficient (Kw), and temperature dependent Henry's law constant
	for DMHg (H). We estimated the DMHg (CDMHg water) concentration in surface waters using the %
	DMHg as MeHg as reported by Jonsson et al., 2022 (~ 6% in primary mixed layer) at shallowest
	sample depth for MeHg in late winter, and a literature value of 0.023 pmol m -3 for DMHg in air
	(CDMHg air,

603 -0.414 0.378 -0.311 -0.315 -0.081 0.802 0.674 0.823 S A 0.649 -0.329 0.319 -

  0.155 -0.255 -0.057 0

			MeHg POC Chla NO2 NO3 PO4 SiOH4	Θ	SA
	THg	-0.609
	MeHg	0.394 -0.42
	POC	-0.622 0.714 -0.582
	Chla	-0.707 0.708 -0.467 0.895
	NO 2	-0.05 0.045 -0.308 0.212 0.251
	NO 3	0.66 -0.417	0.51 -0.489 -0.534 -0.22
	PO 4	0.724 -0.484 0.542 -0.59 -0.64 -0.235 0.937
	SiOH 4	0.668 -0.324 0.438 -0.389 -0.472 -0.162 0.938 0.897
	Θ	0.

.723 0.621 0.724 0.856

	Bacteria	-0.249 0

.469 -0.389 0.468 0.438

  -0.061 0.107 0.021 0.188 0.166 0.249

Table S4 :

 S4 Integrated THg pools (pmol m -2 ) from 0 -100 m from all seasonal cruises completed in the area. We used data fromKohler et al. 2022a for summer and early winter.

			Integrated Total Hg pool (pmol m -2 )
	0 -100 m	Late winter	Spring	Summer	Early winter
		(March 2021)	(May 2021)	(August 2019)	(December 2019)
	P1	51647 ± 1696	68280 ± 1620	64033 ± 869	37050 ± 1736
	P2	51197 ± 1620	67652 ± 1541	63050 ± 865	40050 ± 1800
	P3	55570 ± 1620	63074 ± 1620	64483 ± 922	40267 ± 1829
	P4	49786 ± 1601	67616 ± 1601	73635 ± 838	51400 ± 1866
	P5	53409 ± 1677	63737 ± 1541	78773 ± 886	50136 ± 1753
	P6	52700 ± 1994	72807 ± 1743	63300 ± 1115	43955 ± 2049
	P7	52244 ± 2016	63251 ± 1989	55300 ± 1089	58200 ± 2200

Table S5 :

 S5 Integrated MeHg pools (pmol m -2 ) from 0 -100 m from all seasonal cruises completed in the area. We used data fromKohler et al. 2022 for summer and early winter.

	Integrated

Methylated Hg pool (pmol m -2 ) 0 -100 m Late winter (March 2021) Spring (May 2021) Summer (August 2019) Early winter (December 2019)

	P1	5486 ± 237	710 ± 293	10133 ± 583	9350 ± 648
	P2	3999 ± 227	2187 ± 216	10000 ± 604	9513 ± 731
	P3	6912 ± 227	1341 ± 260	10317 ± 583	9450 ± 555
	P4	7339 ± 224	1378 ± 245	9444 ± 548	9433 ± 583
	P5	6611 ± 235	2640 ± 216	9550 ± 547	11000 ± 547
	P6	8297 ± 279	5878 ± 244	13000 ± 679	10290 ± 624
	P7	13381 ± 282	5485 ± 278	12850 ± 679	14350 ± 679

Table S6 :

 S6 ΔMeHg (pmol m -2 d -1 ) for 0-100 m, calculated from 0 -100 m integrated pools for late winter and spring. A negative value assigned in front of each ΔMeHg indicates in a negative change in the MeHg pool from late winter to spring, or a loss of MeHg.

	Station	Days between	ΔMeHg	ΔMeHgpd	ΔMeHgscav	ΔMeHgev
		sampling	pmol m -2 d -1	pmol m -2 d -1	pmol m -2 d -1	pmol m -2 d -1
	P1	56	-85 ± 7	-10	-1	-7
	P2	56	-32 ± 6	-3	-1	-2
	P3	57	-98 ± 6	-3	-1	-2
	P4	57	-105 ± 6	-1	-1	-1
	P5	56	-71 ± 6	-1	-1	0
	P6	58	-42 ± 6	-3	-1	-2
	P7	58	-136 ± 7	-3	-1	-2

Table S7 :

 S7 Percent of variance explained for the first two plotted principal components for each cruise.

		% of Variance
	Cruise	PC 1 PC 2
	Late Winter 2021 (LW) 43.3	20.2
	Spring 2021 (SP)	52.8	17.0
	Summer 2019 (SU)	56.5	12.1
	Winter 2019 (W)	46.8	16.2

R., & Heimbürger-Boavida, L-E. Mercury export flux in the Arctic Ocean estimated from 234 Th:

Acknowledgements

Funding for this study was provided by The Nansen Legacy project (RCN# 276730) and the Anders Jahres fond til vitenskapens fremme. The authors would like to thank the captain, crew members, cruise leaders, and fellow scientists onboard R/V Kronprins Haakon for cruises Q1 and Q2. Specifically, the authors would like to thank L. Kull and M. Adams for assistance in collecting mercury samples onboard and N. Torres-Rodriguez and A. Dufour for assistance in analysis.

Additional thanks are to the sea ice coring teams and under ice water sampling teams for help in collecting ice samples. We also thank N. Sanchez and M. Petrova for their insightful comments on this manuscript. Lastly, we thank F. A. Cnossen for her artistic design of the graphical abstract.

CRediT authorship contribution statement

Stephen G. Kohler: Conceptualization, methodology, validation, formal analysis, investigation, writing -original draft, visualization, funding acquisition. Lars-Eric Heimbürger-Boavida: conceptualization, methodology, validation, investigation, writing -review and editing, visualization, supervision. Philipp Assmy: investigation, formal analysis, writing -review and editing, visualization. Oliver Müller: investigation, formal analysis, writing -review and editing, visualization. Stefan Thiele: investigation, formal analysis, writing -review and editing. Maria G. Digernes: Investigation, formal analysis, visualization. Kuria Ndungu: writing -review and editing, supervision. Murat V. Ardelan: Conceptualization, methodology, writing -review and editing, funding acquisition, supervision.

Supplementary Material

Biotic transformation of methylmercury at the onset of the Arctic spring bloom

Stephen G.