# The open sea as the main source of methylmercury in the water column of the Gulf of Lions (Northwestern Mediterranean margin)

Daniel Cossa, Jörg Schäfer, Xavier Durrieu de Madron, Jean-Jacques Naudin

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Daniel Cossa, Jörg Schäfer, Xavier Durrieu de Madron, Jean-Jacques Naudin. The open sea as the main source of methylmercury in the water column of the Gulf of Lions (Northwestern Mediterranean margin). Geochimica et Cosmochimica Acta, 2017, 10.1016/j.gca.2016.11.037. hal-01485421

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# The open sea as the main source of methylmercury in the water column of the Gulf of Lions (Northwestern Mediterranean margin)

Cossa Daniel <fnref n="1" />,<fnref n="2" />,* , Durrieu De Madron Xavier <fnref n="3" />, Schäfer Jörg <fnref n="4" />, Lanceleur Laurent <fnref n="4" />, Guédron Stéphane <fnref n="2" />, Buscail Roselyne <fnref n="3" />, Thomas Bastien <fnref n="5" />, Castelle Sabine <fnref n="1" />, Naudin Jean-Jacques <fnref n="6" />

## Abstract :

Despite the ecologic and economical importance of coastal areas, the neurotoxic bioaccumulable monomethylmercury (MMHg) fluxes within the ocean margins and exchanges with the open sea remain unassessed. The aim of this paper is to address the questions of the abundance, distribution, production and exchanges of methylated mercury species (MeHgT), including MMHg and dimethylmercury (DMHg), in the waters, atmosphere and sediments of the Northwestern Mediterranean margin including the Rhône River delta, the continental shelf and its slope (Gulf of Lions) and the adjacent open sea (North Gyre).

Concentrations of MeHgT ranged from <0.02 to 0.48 pmol L⁻<fnref n="1" /> with highest values associated with the oxygen-deficient zone of the open sea. The methylated mercury to total mercury proportion (MeHgT/HgT) increased from 2% to 4% in the Rhône River to up to 30% (averaging 18%) in the North Gyre waters, whereas, within the shelf waters, MeHgT/HgT proportions were the lowest (1–3%). We calculate that the open sea is the major source of MeHgT for the shelf waters, with an annual flux estimated at 0.68 ± 0.12 kmol a⁻<fnref n="1" /> (i.e., equivalent to 12% of the HgT flux). This MeHgT influx is more than 80 times the direct atmospheric deposition or the in situ net production, more than 40 times the estimated “maximum potential” annual efflux from shelf sediment, and more than 7 times that of the continental sources. In the open sea, ratios of MMHg/DMHg in waters were always <1 and minimum in the oxygen deficient zones of the water column, where MeHg concentrations are maximum. This observation supports the idea that MMHg could be a degradation product of DMHg produced from inorganic divalent Hg.

**Keywords** : Mercury, Methylmercury, Ocean margin, Coastal area, Mediterranean

<footnote n="1">¹ Ifremer, Mediterranean Centre, BP 330, F-83507 La Seyne-sur-Mer, France</footnote>

<footnote n="2">² Université Grenoble Alpes, ISTerre, BP 53, F-38041 Grenoble, France</footnote>

<footnote n="3">³ Université de Perpignan, CNrs-Cefrem, 52, Ave. P. Alduy, F-66860 Perpignan Cedex, France</footnote>

<footnote n="4">⁴ Université de Bordeaux, Epoc, F-33615 Pessac, France</footnote>

<footnote n="5">⁵ Ifremer, Atlantic Centre, BP 21105, F-44311 Nantes Cedex 03, France</footnote>

<footnote n="6">⁶ Université P. et M. Curie, Lobb, F-66651 Banyuls-sur-Mer, France</footnote>

<footnote>* Corresponding author : Daniel Cossa, email address : dcossa@ifremer.fr</footnote>
Mercury (Hg) exists in the marine environment as elemental Hg (Hg⁰) and divalent Hg species, which include various inorganic (HgII) species (e.g., chlorocomplexes) and the two methylated species: monomethylmercury (MMHg) and dimethylmercury (DMHg).

Monomethylmercury is a neurotoxin that bioaccumulates in aquatic organisms and biomagnifies through trophic webs (e.g., Jensen and Jernelov, 1969; Clarkson and Magos, 2006). However, despite the ecological and economical importance of coastal areas, especially in terms of fish and shellfish production and capture, the distributions, sources and fluxes of methylated Hg species in the waters of the ocean margins are still poorly explored.

Methylated Hg sources for coastal waters include (i) inputs from upwellings, rivers, groundwaters, atmospheric deposition and waste water point sources, and (ii) *in situ* HgII
methylation in coastal waters and sediments (Cossa et al., 1996; Fitzgerald et al., 2007). The river-watershed contribution can be large due to direct inputs of MMHg to coastal waters (Coquery et al., 1997; Choe and Gill, 2003; Balcom et al., 2008 and 2015; Muresan et al., 2008; Guédron et al., 2012; Buck et al., 2015) and continental groundwaters *via* submarine estuaries (Ganguli et al., 2012). Contribution of the open ocean to the methylated Hg load of oceanic margin waters has also been evidenced: DMHg is conveyed, *via* upwellings, from ocean interior to surface coastal waters (Conaway et al., 2009). Atmosphere has been reported
as external methylated Hg sources, but available data are limited (e.g., Weiss-Penzias et al., 2012 and 2016). In situ Hg methylation has been observed in both coastal sediments (e.g., Gobeil and Cossa, 1993; Hammerschmidt et al., 2004; Hammerschmidt and Fitzgerald, 2006; Balcom et al., 2008; Hollweg et al., 2009 and 2010; Luengen and Flegal, 2009; Noh et al., 2013) and waters (Mason et al., 1993; Sunderland et al., 2010; Lehnherr et al., 2011; Wang et al., 2012; Lehnherr, 2014; Schartup et al., 2015). Experimental MMHg production from incubation of settling particles (“marine snow”) or water strongly suggests that water column methylation may be important worldwide (Monperrus et al., 2007; Ortiz et al., 2015). In summary, numerous internal and external sources of methylated Hg in coastal waters exist; however, their relative importance is not well established.

Pathways of Hg methylation in both oceanic and coastal waters are still poorly described, despite the oceanographically-consistent measurements of methylated Hg performed for three decades in the waters of the Atlantic Ocean (Mason et al., 1998; Mason and Sullivan, 1999; Bowman et al., 2015), the Pacific Ocean (Mason and Fitzgerald, 1990; Hammerschmidt and Bowman, 2012; Munson et al., 2015), and the Mediterranean Sea (Cossa et al., 1994, 1997 and 2009; Horvat et al., 2003). The highest concentrations of methylated Hg are consistently found in the oxygen deficient zones (ODZs). The generally significant correlation between methylated Hg and oxygen consumption (or organic carbon regeneration rates) have been interpreted as the result of net microbiological Hg<sup>II</sup><sub>i</sub> methylation at these depths (e.g., Mason and Fitzgerald, 1990; Cossa et al., 2009; Sunderland et al., 2009). The MMHg/DMHg molar ratios in the ODZs vary broadly (0.2 to >10) depending on location, even in the same oceanic region. Early North Atlantic data suggests that MMHg is a degradation product of DMHg in the water column (Fitzgerald and Mason, 1996; Mason et
al., 1998; Mason and Sullivan, 1999). However, production rate measurements, performed on Arctic waters, found contradictory results (Lehnheer et al., 2011). These authors found that DMHg production from Hg<sup>II</sup><sub>i</sub> was two orders of magnitude less than MMHg production, but faster than the rate of DMHg production from MMHg methylation. Consistent with these findings, recent results show that MMHg is the dominant form of methylated Hg in deep waters of the North Atlantic (Bowman et al., 2015). More recently, Jonsson et al. (2016) suggest that MMHg can be methylated on sulfide mineral surfaces, a pathway potentially responsible for much of the DMHg in oceanic waters. In summary, data on MMHg and DMHg distributions in marine waters are still scarce (especially in coastal areas), and, consequently, the methylation mechanism of Hg<sup>II</sup><sub>i</sub> remains uncertain.

In the present work, we address questions of abundance, distribution, production and exchanges of methylated Hg in the waters of the Northwestern Mediterranean margin including the Rhône River delta, the continental shelf and its slope (Gulf of Lions), the continental rise and the adjacent open sea (North Gyre) (Fig. 1). The objective is to assess the relative importance of internal and external sources of methylated Hg in the context of marine Hg cycle. For this, we have (i) monitored atmospheric deposition and riverine inputs of total methylated Hg (MeHgT = MMHg + DMHg), (ii) studied the MeHg distribution within the Rhône River plume, the freshwater-sea water mixing zone, the continental shelf-slope-rise system, and the adjacent open sea, and (iii) estimated the MeHg exchanges across various interfaces, including water/sediment and coastal/off-shore water interface. Additional data of total Hg (HgT = all the Hg species) were also collected in order to estimate the importance of MeHg fraction.

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## 2. Study area

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The Northwestern Mediterranean is characterized by the presence of a large continental shelf and the associated slope, both constituting the Gulf of Lions (Fig. 1). The water circulation in the Gulf of Lions is influenced in the South by the Northern Current, which is a part of a current system going from the Tyrrhenian Sea up to the Alboran Sea (Millot and Tapier-Letage, 2005). This Northern Current flows as a major vein along the upper part of the continental slope intruding onto the shelf (Fig. 1). The North Gyre and Gulf of Lions have contrasting hydrological and biological properties. The North Gyre is a typical oligotrophic open Mediterranean environment experiencing strong winter mixing of the surface and intermediate water masses, whereas the Gulf of Lions constitutes one of the few mesotrophic coastal regions within the Mediterranean Sea (Morel and André, 1991) largely influenced by the Rhône River freshwater inputs.

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The Gulf of Lions receives riverine inputs mainly from the Rhône River. The Rhône River is the major freshwater input to the western Mediterranean and its waters undergo three main processes before being diluted in the Gulf of Lions waters. First, freshwater is rapidly mixed with seawater within the few kilometers between Barcarin and the prodelta area (Fig. 1) (Elbaz-Poulichet et al., 1996). Secondly, the Rhône River plume is driven on the shelf by changeable continental winds (the northerly Mistral, southwesterly Tramontane and southeasterly-easterly Marin) and the cyclonic Northern Current (Fig. S1) (Naudin et al., 1997). The plume is periodically broken due to wind direction changes, producing “Low Salinity Water” lenses drifting on the shelf Naudin et al. (1997). Thirdly, below the Rhône River plume, the dense riverine particles settle abruptly, generating large sediment accumulation in the prodelta area up to several tens of centimeters per year in the proximal

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delta (~ 20 m) according to Charmasson et al. (1998), Radakovitch et al. (1999), Maillet et al. (2006) and Cathalot et al. (2010). Finer riverine material is exported farther on the Gulf of Lions shelf, undergoing a westward net transport through sedimentation/resuspension processes generated by infrequent easterly wind storm events (Durrieu de Madron et al., 2008; Ulses et al., 2008; Guizien, 2009; Marion et al., 2010; Bourrin et al., 2015).

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## 3. Material and methods

To summarize the nomenclature used in the following text, $X_{UNF}$, $X_F$ and $X_P$ refer to unfiltered, dissolved (< 0.45 µm) and particulate, respectively (with X being a Hg species). Detailed protocols of the sampling and analytical techniques are given in SI 1. Sampling and water sample treatments were performed using ultraclean protocols, including acid cleaning of the plastic ware, and the use of plastic gloves and high purity grade reagents (e.g., SUPRAPUR HCl from Merck®, ULTREX $HNO_3$ from J.T. Baker®).

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### 3.1. *Sampling*

Sampling cruise identifications, locations and dates are summarized in Table S1.

*Atmospheric deposition.* Thirteen rain events were collected between April 2009 and January 2010 at a coastal site (La Seyne-sur-Mer, 43°.06.350′N; 5°53.117′E, Fig.1), located at the eastern end of the Gulf of Lions. The unfiltered samples were collected in Teflon (FEP) bottles and acidified (0.4 % v/v HCl) immediately after the water collection. The rain-collecting device (PP040, MTX®) was located 15 m above sea level. Details of the sampling are given by Castelle (2010). At the same site, 28 aerosol samples were collected on cellulose acetate membranes (0.22 µm) from July 2009 to March 2010. Each sample represents one

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147 week of pumping at a pumping rate of 14 L min⁻¹. Total gaseous Hg concentrations in the
148 atmosphere were monitored at the same time (Marusczak et al., 2015).

149 *Rhône River monitoring.* Rhône River Hg monitoring was performed over two monitoring
150 periods at Arles (Sta. SORA, Fig. 1): (i) a period of low river waters from March, 2009 to June,
151 2010, with only one rather weak flood event (~3000 m³ s⁻¹ on December the 2nd), and (ii)
152 during the flooding period of October-November 2008 (up to 3580 m³ s⁻¹). Surface freshwater
153 samples were collected twice a month at Station SORA in Arles (Fig. 1) by pumping through
154 polyethylene tubing using an all-Teflon (PFA) double-bellows pump (10-LPM, ASTI®).
155 Samples were collected in Teflon (FEP) bottles then filtered through hydrophilic Teflon
156 membranes (LCR, Millipore®) with a porosity of 0.45 µm. Membranes were stored at -18°C
157 in polycarbonate Petri dishes and the filtrates were acidified (0.4 % v/v HCl) and stored in
158 Teflon (FEP) bottles.

159 *Rhône delta mixing zone.* Brackish surface waters were sampled on 16 and 17 October 2008
160 from a rubber boat between Barcarin (Rhône River) and the sea (Fig.1); the locations of the
161 stations are indicated on Table S2a. The samples were collected directly into Teflon (FEP)
162 bottles. Samples were then filtered and stored as described in the previous section.

163 *BIOPROHFI cruise.* During the BIOPROHFI cruise (14th – 27th May 2006), two types of waters
164 were sampled: (i) the productive shelf waters and (ii) the mesotrophic slope waters. In the first
165 type, two Low Salinity Waters, originating from the Rhône River plume, were successively
166 tracked using a Lagrangian sampling protocol: lens “1” between May 14th and 18th,
167 corresponding to stations S1 to S68 and lens “2” between the 19th and 26th, corresponding to
168 stations S88 to S220 (Fig. S2). In the lenses, the 0-50 m layer was sampled by pumping with
169 the all-Teflon pneumatic pump (10-2PM, ASTI®) and through polyethylene tubing, directly
into a class 100 on-board clean laboratory; the water depth varied between 60 and 100 m during the drifting. Two deep casts (0-900 m) were performed on May the 26th at the slope foot (Stas. S221 and S230, Fig. S2); their coordinates are given in Table S2a.

**CASCADE cruise.** During the CASCADE cruise (1st – 23rd March 2011), water was collected (i) on the inner shelf from the Rhône prodelta to the south-western end (Stas. A to D), (ii) at the head of Cap de Creus canyon (Sta. E), and (iii) thirteen deep casts on the shelf edge (Stas. L-01 and M-12), the slope foot (Stas. L-03 and M-10) and within the North Gyre (Stas. Antarès, S2400, L-05-08-10-12 and M-03-05-08) (Fig. 1). Two to twelve water depths were sampled for each cast depending on the height of the water column (Fig. 1 and Table S2). Sediment cores were collected along the Gulf of Lions shelf, at the head of the Cap de Creus canyon and in the abyssal plain in the North Gyre area during the CASCADE cruise (Table S2b). A multicore sampler (Oktopus GmbH Multiple corer with 8 tubes of 100 mm diameter) allowing the sampling of the undisturbed benthic interface (Barnett et al., 1984) was used.

## 3.2. Sample treatment

**Water and particles.** The samples from the 0-50 m layer of the shelf waters during the BIOPRHOFI cruise were filtered through polycarbonate membranes (0.45 µm, Nuclepore®). Filtrate was collected in Teflon (FEP) bottles and acidified with HCl (0.4 %, v/v). The collection of particles from the Low Salinity Waters, for subsequent MMHgP, particulate carbon and phosphorus, and pigments determinations, was performed using in-line filtration through pre-heated glass fiber filters (GF/G, Whatman®). Deep-water samples (> 50 m) were collected during the BIOPRHOFI and CASCADE cruises by rosette-mounted 5L bottles (1010X-Niskin, General Oceanics®) equipped with a CTD probe. These samples were not filtered and were analyzed only for HgTUNF and MeHgTUNF. Total Hg was determined on board, whereas
193 MMHg and MeHgT analyses were performed in the laboratory within 2 months after the
194 cruise on the acidified samples stored in the dark at +4°C in a double wrapping of polyethylene
195 bags.

196 *Sediment pore water*: The sediment pore water was extracted, from below the sediment
197 surface (-1 cm), using Milli-Q (Millipore®) water-rinsed microporous polymer tube samplers
198 (Rhizon SMS, Rhizosphere Research Products®) attached to an acid washed all-polypropylene
199 syringe (Guédron et al., 2012).

200 **3.3. Chemical analyses**

201 *Total mercury*. Total Hg, in filtered and unfiltered samples, was measured on board within a
202 few minutes of sampling. The release of Hg from its ligands was achieved by addition of a
203 BrCl solution. This technique is known as the Environmental Protection Agency from the
204 United States of America standard method N° 1631

205 (www.epa.gov/sites/production/files/2015-08/documents/method_1631e_2002.pdf). The
206 detection limit (DL = 3 times the standard deviation of the blank) was 0.1 pmol L⁻¹ and the
207 reproducibility varied according to the concentration level between 5 and 15 % (Cossa et al.
208 2003). The accuracy of HgT measurements was tested using the ORMS-3 water sample,
209 which is a certified reference material (CRM) from the National Research Council of Canada.

210 Our measurements were always within the confidence limits given for the CRM (12.6 ± 1.1
211 pg mL⁻¹; http://inms-ienm.nrc-cnrc.gc.ca/calserv/crm_files_f/ORMS-3_f.pdf).

212 *Methylated Hg species*. Total methylated Hg and MMHg were measured on filtered and
213 unfiltered samples and DMHg was calculated as the difference between MeHgT<sub>UNF</sub> and
214 MMHg<sub>UNF</sub>. Since DMHg is converted into MMHg at low pH (Mason, 1991; Black et al.,
215 2009a) MeHgT was determined on acidified samples. Monomethyl Hg was determined after
216 bubbling samples for 40 min with argon in order to remove DMHg before acidification. Total
217 methylated Hg and MMHg were determined as volatile MMHg hydride by purge and cryo-
218 trapping gas chromatography and detected as elemental Hg vapor by atomic fluorescence
219 spectrometry according to Stoichev et al. (2004). The mercury hydrides (from MMHg and
220 Hg<sup>II</sup><sub>i</sub>) were formed with NaBH<sub>4</sub>, sparged from the sample with helium, concentrated and then
221 separated by cryogenic chromatography before detection. During this set of analyses, the low
222 blank (< 0.01 pmol) and its reproducibility (2 %) allowed and limits of quantification (LQ) as
223 low as 0.005 and 0.015 to 0.075 pmol L<sup>-1</sup>, respectively. The analytical reproducibility varied
224 with time between 6 and 15 %.

225 MeHgT<sub>P</sub> was determined by atomic fluorescence spectrometry after acid extraction,
226 ethylation and chromatographic separation according to Liang et al. (1994). The analytical
227 performances were established by analyzing a Certified Reference Material from the
228 International Atomic Energy Agency (IAEA-142, Horvat et al., 1997). The reproducibility
229 was 10 % and the DL 20 pmol g<sup>-1</sup>; recovery varied from 80 to 100 %.

230 *Isotopic incubations*. Incubation experiments were performed onboard immediately after
231 sampling using a <sup>199</sup>Hg<sup>II</sup> stable isotope spike to study Hg methylation potentials in
232 Mediterranean bottom water samples as described for tropical lake water incubations in
233 Huguet et al. (2010). These experiments aimed at simulating the temporal resuspension of
234 reducing sediments in oxic bottom water and evaluating their impact on gross methylation
235 compared to that in bottom water without sediment resuspension. For this, we determined
236 gross methylation in (a) unfiltered bottom water and (b) slurries of surface sediment
237 resuspended in unfiltered bottom water as follows:
238 - (a) Aliquots of unfiltered seawater from the bottom of the shelf water column
239 were transferred into acid-cleaned 125 mL Teflon (FEP) bottles, spiked with ¹⁹⁹HgII (Oak
240 Ridge, 92 % purity) to a nominal concentration of 1000 ng ¹⁹⁹HgII L⁻¹ and incubated at
241 constant temperature (12 °C) in the dark for 48, 96 and 192 hours (n = 2 for each time;
242 manual shaking twice per day). Aliquots (n = 2) of unfiltered bottom water were spiked
243 and methylation was immediately stopped for control. After each incubation time,
244 methylation was stopped by adding 1 mL of 12 M HCl (INSTRA analyzed, J.T. Baker®)
245 and the bottles were stored in double sealed polypropylene bags at 4°C and in the dark
246 until analysis for isotopic composition of the different Hg species.

247 - (b) Surface sediments from the shelf at Stations A to D were added to 100 ml of
248 unfiltered bottom water from the respective site to obtain a nominal suspended particle
249 concentration of 100 mg L⁻¹; three aliquots per site) in acid-cleaned 125 mL Teflon (FEP)
250 bottles, to simulate resuspension events.

251 The slurries were then spiked with ¹⁹⁹HgII (Oak Ridge, 92 % purity) to a nominal
252 concentration of 25 ng of Hg per gram of sediment slurry, including water and sediment,
253 and incubated at constant temperature (12 °C) in the dark for 192 hours. Incubation was
254 stopped by freezing (-20°C). The samples were stored frozen, freeze-dried and stored
255 cool in the dark until extraction with HNO₃ (6M, ULTREX, J.T. Baker®) and analysis. The
256 applied protocol including derivation (propylation with NaBPr₄), extraction and the
257 analytical setup, i.e. gas chromatography (Focus GC, Thermo Fischer Scientific®)
258 coupled to ICP-MS (X7, Thermo Fischer Scientific®) using a thermostatic interface is
259 based on the methods described in detail elsewhere (Monperrus, 2004). The difference
260 between the MM¹⁹⁹Hg measured and the MM¹⁹⁹Hg naturally present (calculated from the
261 amount of MM<sup>200</sup>Hg measured) represents the MM<sup>199</sup>Hg produced during the incubation
262 of the <sup>199</sup>Hg<sup>II</sup> added. Methylation potential was estimated by dividing the amount of
263 MM<sup>199</sup>Hg formed by the amount of <sup>199</sup>Hg<sup>II</sup> recovered after the respective incubation
264 periods, as described by Monperrus et al. (2007).

265 *Particulate organic carbon, phosphorus and pigments.* Particulate organic carbon
266 concentrations were measured according to Hedges and Stem (1984) with a Shimadzu®
267 analyzer (TOC-5000 Series). Soluble reactive phosphorus (SRP) was determined in
268 seawater with an auto-analyzer using the standard molybdate blue method (Murphy and
269 Riley, 1962) as detailed in the protocol by Aminot and Kérouel (2007). Particulate
270 phosphorus (P<sub>P</sub>) determination used the same colorimetric method but after magnesium
271 nitrate oxidation (Ormaza-Gonzalez and Statham, 1996). Chlorophyll pigments were
272 determined following the standard method by Strickland and Parsons (1972).

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## 4. RESULTS

275 **4.1. Atmospheric deposition**

276 Total Hg concentrations in unfiltered rain water varied from 10 to 80 pmol L<sup>-1</sup>, with a mean of
277 31 ± 22 pmol L<sup>-1</sup> (n = 13), whereas MeHgT concentrations varied from 0.10 to 1.25 pmol L<sup>-1</sup>,
278 with a mean of 0.59 ± 0.33 pmol L<sup>-1</sup> (n = 13), i.e., ~2% of the HgT. These ranges are similar
279 to those published for other coastal areas of the Northern Hemisphere (e.g., Hammerschmidt
280 et al., 2007; Marusczak et al., 2011; Weiss-Penzias et al., 2012). Methylated Hg
281 concentrations in the 28 aerosol samples were undetectable due to the too-small amount of
282 sample collected on the membrane.
283 **4.2. Rhône River and its plume**

284 During the low flow period MeHg~F~ and MeHg~P~ concentrations varied in the picomolar range
285 (Table 1), MeHg being predominantly (90%) associated with suspended particles. The mean
286 methylated Hg fractions (MeHg/HgT) were 4 and 2 % as dissolved and particulate,
287 respectively.

288 The variations of dissolved MeHg concentrations (MeHgT~F~) during the mixing of
289 Rhône freshwater with Gulf of Lions saltwater, between Barcarin ferry and the prodelta area,
290 are illustrated in Figure 2a. Concentrations of MeHgT~F~ varied from 0.02 to 0.14 pmol L⁻¹ and
291 the distribution follows a conservating mixing line (R² = 0.75, *p* < 0.01) with higher
292 concentrations in the freshwater and lower concentrations in the seawater end-member (Fig.
293 2a), similarly to observations of Noh et al. (2013) in the Mekong River delta.

294 At the bottom of the Low Salinity Waters lenses (~50m), salinity ranged from 38.2 to
295 38.4, which are typical values for waters of the Northern Current. The lowest salinity in the
296 Low Salinity Waters (32.5) at surface indicates a ~20 % dilution of marine waters with the
297 Rhône River waters. The MeHgT~F~ concentrations were in the femtomolar range, with 35 %
298 of values lower than the LQ (i.e., <0.015 pmol L⁻¹) and maximum concentration of 0.069
299 pmol L⁻¹ (Table 2). MeHgT~F~ fraction varies from <0.3 to 5.5 %, averaging 1.5 ± 1.0 % of the
300 HgT~F~. The time series of MeHgT~F~ concentrations in the Low Salinity Waters varied little with
301 depth (Fig. 3). Interesting to note is that high MeHgT~F~ concentrations occurred at the highest
302 salinities, suggesting a marine MeHg source (Fig. 2b).

303 **4.3. Vertical profiles along shelf and slope**
On the shelf, average MeHgT<sub>UNF</sub> concentrations were 0.026 ± 0.024 pmol L<sup>-1</sup> (n = 40) (Table 3 and Fig. 4a). The MeHgT<sub>UNF</sub> / HgT<sub>UNF</sub> ratios averaged roughly 2 %, ranging from 0.1 to 6.3 % (Table 3).

Tables 2 and 3 summarize the results obtained for the “slope foot” (900-1800m bottom depth) water column in May 2006 (Sta. S-230, BIOPRHOFI cruise) and in March 2011 (Sta. L-03 and M-10, CASCADE cruise). Concentrations of MeHgT<sub>UNF</sub> ranged from 0.02 to 0.38 pmol L<sup>-1</sup>, averaging 18 % of the HgT<sub>UNF</sub> (Tables 2 and 3). MeHgT<sub>UNF</sub> were significantly higher below than above 100 m (*p*<0.05), suggesting removal and/or photodegradation for methylated Hg in upper layer and regeneration and/or production below (Fig. 4b, Table 2).

## 4.4. Deep profiles in the North Gyre

Summary statistics for Hg species concentrations are given in table 3. Methylated Hg concentrations in unfiltered samples ranged from 0.020 pmol L<sup>-1</sup> at surface at Sta. S2400 to 0.478 pmol L<sup>-1</sup> at 500 m at Sta. L-10 (Fig. 5a). Expressed as a fraction of the HgT, MeHgT varied between 16 to 30 % (Table 3). The MeHgT<sub>UNF</sub> profiles are typical of methylated Hg vertical distribution in open ocean (e.g., Fitzgerald et al., 2007; Cossa et al., 2011; Mason et al., 2012), i.e., very low concentrations at surface increasing with depth (Fig. 5a). At five stations, MMHg was determined in addition to MeHgT, allowing calculation of DMHg concentrations by difference. Results indicate MMHg/DMHg ratios always lower < 1, varying from 0.04 to 0.81 (Fig. 5b).

## 4.5. Sediment pore waters

Concentrations of MeHgT<sub>F</sub> in pore waters extracted from surface sediments (0-2 cm) were clearly higher (4-20 times) than in the overlaying waters and in the corresponding lower-water
column (Tables 3 and 4). Moreover, concentrations in sediment pore waters of North Gyre were one order of magnitude higher than the shelf pore waters, ranging from 2.05 to 2.18 pmol L⁻¹ and from 0.07 to 0.68 pmol L⁻¹, respectively (Table 4).

## 4.6. Ex situ methylation experiments

Concentrations of inorganic Hg in sediments ranged from 0.432 ± 0.030 nmol g⁻¹ (Sta. B) to 0.598 ± 0.122 nmol g⁻¹ (Sta. A), with a mean of 0.519 ± 0.084 nmol g⁻¹. The results of the sediment incubation experiments showed clearly modified MM¹⁹⁹Hg/MM²⁰⁰Hg ratios compared to both natural values (0.72 ± 0.005; n = 3) and isotope ratios observed in control samples where incubation was stopped immediately after the spike (0.76 ± 0.023; n = 3; Fig. 6a). The three replicate incubations performed for each site generally provided reproducible results, revealing clear differences between the stations. The sediment samples from Sta. A (MM¹⁹⁹Hg/MM²⁰⁰Hg = 1.36 ± 0.06; n = 3) showed the highest methylation, whereas isotope ratios in the samples from Sta. B (MM¹⁹⁹Hg/MM²⁰⁰Hg = 0.85 ± 0.02; n = 3) were relatively close to natural values (¹⁹⁹Hg/²⁰⁰Hg = 0.73). Stations C (MM¹⁹⁹Hg/MM²⁰⁰Hg = 1.05 ± 0.11; n = 3) and D (MM¹⁹⁹Hg/MM²⁰⁰Hg = 1.11 ± 0.10; n = 3) showed intermediate and similar MM¹⁹⁹Hg/MM²⁰⁰Hg, yet clearly different from control samples (Fig. 6a). From these isotope ratios, we estimated gross methylation rates between 0.009 and 0.083 % d⁻¹ with means of 0.010 ± 0.001 % d⁻¹ (Sta. B), 0.020 ± 0.005 % d⁻¹ (Sta. C), 0.029 ± 0.010 % d⁻¹ (Sta. D), and 0.059 ± 0.021 % d⁻¹ (Sta. A). Incubations of unfiltered bottom water showed changing MM¹⁹⁹Hg/MM²⁰⁰Hg over time, reaching values of up to 8.7 after 192 h of incubation (n = 2, data not shown). The estimated methylation potentials for unfiltered bottom water ranged from 0.00019 % d⁻¹ (after 48 h) to 0.00043 % d⁻¹ (after 96 and 192 hours; Fig 6b).
350 **5.1. External sources of MeHg**

351 **5.1.1. Advection from open sea**

352 In the North Gyre, the MeHgT<sub>UNF</sub> concentrations varied from values below DL to 0.48 pmol L<sup>-1</sup> (Table 3), which is similar in magnitude to those reported in the last ten years for the open 353 Western Mediterranean waters (Cossa and Coquery, 2005; Kotnik et al., 2007; Cossa et al., 354 2009 and Heimbürger et al., 2010). The vertical MeHgT distribution patterns (Fig. 5) are 355 consistent with the now classical oceanic MeHg behavioral model characterized by (i) a 356 microbial net methylation within the oxycline, and (ii) a photodemethylation in surface 357 waters. Photochemical demethylation is well documented in experimental and natural 358 conditions (Suda et al., 1993; Black et al., 2012; Kim et al., 2016), whereas microbial 359 methylation is supported by the occurrence of a peak of MeHg in the ODZs (e.g., Mason and 360 Fitzgerald, 1990; Cossa et al., 2009; Sunderland et al., 2009). This model has recently been 361 supported by isotopic Hg compositions observed in the waters of the Pacific Ocean (Blum et 362 al., 2013). At stations L-10 and L-12, where water stratification was well established (Fig. 363 S3), O<sub>2</sub>-deficient Leventine Intermediate Water and maximum MeHgT concentrations were 364 found between 200 and 400 m (Fig. 5a). For these two stations, and additionally Sta. M-10, 365 statistically significant (*p*<0.01) relationships between MeHgT (pmol L<sup>-1</sup>) and dissolved O<sub>2</sub> 366 (µmol L<sup>-1</sup>) exist (Fig. 7a), with similar regression coefficients, averaging -0.0063 ± 0.0001. 367 For other stations, which comprise coastal sites (Stas. L-01, L-03 and M-12) and/or well 368 mixed water column (Sta. S2400), the correlations were weak or inexistent. The 369 corresponding average regression coefficient obtained for the relationships between MeHgT 370 and apparent oxygen utilization (AOU) is +0.0059 ± 0.0007, which is close to the value 371
calculated for the whole Western Mediterranean (+0.0039, R² = 0.77, n = 40, according to Cossa et al., 2009). In various parts of the World Ocean analogous distribution patterns have been observed, showing undetectable methylated Hg concentrations in surface waters and peaks deeper in the water column, where O₂ reaches a minimum due to organic carbon remineralization (e.g., Mason and Fitzgerald, 1990; Cossa et al., 2009; Sunderland et al., 2009; Cossa et al., 2011). According to the model, settling particulate organic carbon is both a source of Hg<sup>II</sup><sub>i</sub> to microbiologically-active waters and a source of organic matter to sustain bacterial activity. A specific gene cluster is linked to Hg methylation in a variety of microorganisms, including sulfate and iron-reducing bacteria and others (Park et al., 2013; Gilmour et al., 2013).

Furthermore, we observed here more significant regression coefficient with a steeper slope for the inverse relationship between DMHg and dissolved O₂ compared to that for MMHg and dissolved O₂ (Fig. 7b), suggesting that the organic matter regeneration is more directly connected to DMHg formation than MMHg formation. This fact is consistent with the hypothesis proposed by Fitzgerald and Mason (1996) that MMHg in marine waters is not formed directly, but is a degradation product of DMHg. The probability of this pathway is reinforced by the observation that the highest MMHg/DMHg ratios occur during the convection period (Fig. S3), when the O₂ depletion is limited and consequently net Hg methylation is depressed (Fig. 5b). However, recent results (e.g., Jonsson et al., 2016; Sorensen et al. 2016) leave the question of the Hg methylation pathway in the ocean open to debate.

The Northern Current is the main source of waters for the Gulf of Lions. The horizontal flux across the shelf-open sea boundary has been estimated to vary at different periods of the
year between 0.07 and $0.35 \times 10^6 m^3 s^{-1}$ (Durrieu de Madron et al., 2003). The same authors also estimate a shelf-slope exchange as ~10% of the along-slope transport, namely $0.2 \times 10^6 m^3 s^{-1}$. The chemical characteristics of waters entering the shelf are well represented by 0-100 m water layer found at Sta. Antarès. Using a $MeHgT_{UNF}$ mean concentration of $0.11 \pm 0.02$ pmol L$^{-1}$ (n = 3), the MeHgT entering the shelf waters from the open sea *via* the Northern Current can be estimated as $680 \pm 120$ mol a$^{-1}$, representing 12% of the HgT flux.

### 5.1.2. Atmospheric deposition

The MeHgT contribution from the atmospheric bulk wet deposition on the Gulf of Lions, based on results from section 4.1 and a surface area of the Gulf of $12 \times 10^3 km^2$, can be calculated as $2.5 \pm 1.4$ mol a$^{-1}$. In the absence of quantifiable analytical determinations, we assume a MeHgT dry deposition similar to the wet (as it is for the HgT in the Mediterranean environment according to Cossa and Coquery, 2005). Thus, the total MeHgT deposition into the waters of the Gulf of Lions can be estimated at ~5 mol a$^{-1}$, which is 2% of the total HgT deposition.

### 5.1.3. Riverine and groundwater inputs

The amount of MeHgT transported annually by the Rhône River into the Gulf of Lions waters is estimated to be 5 mol a$^{-1}$ for the dissolved phase and 51 mol a$^{-1}$ for the particulate phase (see S1 S2 for detailed calculation). Extrapolating these figures to transports from all the Gulf of Lions rivers gives a continental surface runoff contribution of $5.8 \pm 2.0$ and $59.7 \pm 28.1$ mol a$^{-1}$, for the dissolved and particulate MeHgT, respectively. The conservative mixing of dissolved MeHg between fresh and marine waters (Fig. 2a) allows an estimation of the annual net MeHgT efflux to the Gulf of Lions as equal to the gross efflux calculated above. Based on previous results, and owing to the fact that the Rhône River provides 85% and 83% all
riverine dissolved and solid discharges respectively (Gairoard et al., 2012), the total riverine
MeHg flux (dissolved + particulate) to the Gulf ranges within 53.3 - 77.7 mol a⁻¹, with a 99 %
probability. At the scale of the Gulf of Lions, this mean flux is equivalent to ~ 10 % of the
MeHgT flux from marine source. To this surface runoff, the potential contribution of
groundwater should be added (Black et al. 2009b; Ganguli et al., 2012). In the absence of
direct measurement, a hypothetical estimate can be performed. Assuming values of
groundwater discharges to the Gulf of Lions of 2-30 % of the riverine discharge (Ollivier et
al., 2008), and MeHgT concentrations in groundwater similar to the only figures available to
date – i.e., those measured along the Southern Californian coasts (0.2-1.0 pmol L⁻¹, according
to Ganguli et al., 2012) –, we can approximate the MeHgT contribution of submarine
freshwater discharges to the Gulf of Lions waters to be in the range 0.2-17.4 mol a⁻¹. Given
the large uncertainties in this estimate, the MeHgT contribution from groundwater sources
remains to be assessed based on direct measurements. However, we can conclude that the
continental water contribution (gross efflux) is < 95 mol a⁻¹ and small compared to open sea
(680 ± 120 mol a⁻¹) as a source of MeHgT for the Gulf of Lions.

### 5.13. *Efflux from the shelf sediments*

The shelf sediments can be a source of MeHgT *via* the diffusion of soluble phase from
pore water across the sediment-water interface, *via* biopumping, and *via* the resuspension of
the surficial sediments during storm events. All these processes have not been determined,
however, in order to compare their magnitude with other MeHg sources, a “maximum
potential efflux” of MeHg from sediment has been estimated.

The pore-waters of the shelf surface sediments are MeHgT-enriched compared to the
overlying waters (Table 4). Using a simplified diffusion model (SI 3), we can calculate
potential diffusion effluxes from sediments varying from 0.2 to 2.6 pmol m⁻² d⁻¹ depending on the shelf stations and averaging 1.1 ± 0.8 pmol m⁻² d⁻¹ (n = 6). For the surface area of the Gulf of Lions shelf, which is 12 x 10³ km², we estimate an annual potential diffusive flux of MeHgT from the sediment of 4.7 ± 3.5 mol, a figure similar to the dissolved riverine input (5.8 ± 2.0 mol a⁻¹). However, such a flux is not supported by MeHgT distribution in the water column of the shelf (Stas. A-E), since no vertical trend evoking a gradient near the bottom has been observed (Fig. 4a). In addition, a diffusive Hg flux out of the sediment may be counteracted by the presence of the oxic layer near the sediment surface, where upward diffusing Hg species may be trapped (Muresan et al. 2007). On the continental shelf of the Gulf of Lions, oxygen penetration depth in the sediments increases with water depth and distance from shore (Pruski et al, 2015). Oxidised conditions can be restricted from some millimeters to some centimeters in the proximal Rhône (20-30 m), extended down to 2-5 cm at 60 m depth (Table 4) and to 5-10 cm offshore at 100 m depth. In the sediment cores collected on the Gulf of Lions shelf during the CASCADE cruise, an oxic layer was always present at the benthic interface (Table 4). This layer should act as an efficient barrier for upward diffusion of the pore water MeHgT<sub>F</sub>, by adsorption onto oxyhydroxides (logK<sub>dMeHg</sub> equal to 6.4 and 7.7 for FeOₓ and MnOₓ, respectively, according to Muresan et al., 2007) or onto organic matter associated with oxides according to the model by Feyte et al. (2010).

If diffusion of MeHgT from sediments is unlikely, MeHg advection from sediment *via* bioirrigation and biopumping is more probable. Assuming this advective flux to be 3 times the diffusion flux, as calculated by Hammerschmidt and Fitzgerald (2008) for the Northeastern Atlantic coastal sediments, the biomediated MeHg efflux from sediments would be ~14.1 mol a⁻¹.
Release of porewater-borne MeHgT during resuspension of surface sediment is an alternative methylated Hg source for shelf waters. Assuming a 2-cm thick layer (with a mean porosity of 0.82, according to Cathalot et al., 2010) of MeHgT-enriched pore water resuspended ($0.3 \pm 0.2$ pmol L$^{-1}$ according to Table 4) and $12 \times 10^3$ km$^2$ for the Gulf of Lions shelf surface area, the quantity of MeHgT mobilized at each storm event would be $0.07 \pm 0.05$ mol. According to Guizien (2009) the magnitude of the period with waves higher than 1.5 m on the Gulf of Lions shelf is around 10 % of the year. This means that a maximum of 37 days of resuspension (i.e., ~ fifteen 2.5 days-long storm events, Bourrin et al., 2012; Dumas et al., 2014) can be estimated leading to the annual injection of $2.7 \pm 1.9$ mol of MeHgT in the water column. Thus, the “maximum potential efflux” of MeHg from the shelf sediment of the Gulf of Lions would be ~$16.8$ mol a$^{-1}$. Of course, factors such as demethylation at the sediment-water interface as well as MeHg readsorption on resuspended particles may reduce this figure. It is noteworthy that this “maximum potential efflux” is equivalent to 2.5 % of the methylated Hg entering the Gulf of Lions from the sea and 28 % of the riverine particulate MeHgT flux, from which it partially derives.

## 5.2. Internal MeHg production

### 5.2.1. Shelf and slope waters

The average of MeHgT<sub>F</sub> concentrations in the Low Salinity Waters was low (< 0.05 pmol L<sup>-1</sup>) and not different from the rest of the Gulf of Lions shelf waters (t-test, *p* > 0.01, Tables 2 and 3). The average MeHgT<sub>F</sub> / HgT<sub>F</sub> ratio was 1.5 %. These low levels are consistent with active photodemethylation in the surface waters (e.g., Black et al., 2012). The highest MeHgT<sub>F</sub> concentrations occurred where highest salinities were present at the bottom of the Low Salinity Waters lenses (Fig. 2b, Fig. 3), suggesting two possible origins for methylated Hg: its
advection from the external sources quantified above (Northern Current waters and shelf

sediment) and/or its production in the shelf water column. Methylated Hg production

associated with planktonic production/degradation in the mixed layer has been shown already

in the Mediterranean Sea (Monperrus et al., 2007; Heimbürger et al., 2010) and elsewhere

(e.g., Lehnherr et al., 2011).

In the surface waters of the Low Salinity Waters, significant relationships exist between

MeHgT<sub>P</sub> and particulate phosphorus (MeHgT<sub>Ppmol L<sup>-1</sup></sub> = 0.0723P<sub>µmol L<sup>-1</sup></sub> - 0.0048, R<sup>2</sup> = 0.44,

0.01<*p*<0.05, Fig. S4a), and between MeHgT<sub>P</sub> and total pigments (MeHgT<sub>Ppmol L<sup>-1</sup></sub> =

0.0044Pig<sub>µg L<sup>-1</sup></sub> - 0.0078, R<sup>2</sup> = 0.43, 0.01<*p*<0.05, Fig. S4b). This relationship suggests

methylated Hg accumulation in the phytoplankton of the Low Salinity Waters; however, its

origin is unidentified. Interestingly, MeHgT<sub>P</sub> is significantly inversely correlated with the

MeHgT<sub>F</sub> in Low Salinity Water surface waters (MeHgT<sub>Fpmol L<sup>-1</sup></sub> = - 0.600MeHgT<sub>Ppmol L<sup>-1</sup></sub> +

0.030, R<sup>2</sup> = 0.46, *p*<0.05, Fig. S5). With a regression coefficient not different from 1 (-0.600

± 0.415; *p* = 0.05), this relationship suggests that the two phases were exchanging the MeHgT

standing stock present in the water.

Along the slope (Sta. S230), MeHgT<sub>UNF</sub> distribution is related to soluble reactive

phosphorus (SRP) with the following relationship: MeHgT<sub>UNFpmol L<sup>-1</sup></sub> = 0.92SRP<sub>µmol L<sup>-1</sup></sub> + 0.04

(R<sup>2</sup> = 0.95, *p*<0.01). The regression coefficient is in the range of values reported for open

Mediterranean waters, namely 0.89 to 1.24 (see Table 2 in Cossa et al., 2009). These values

calculated for unfiltered samples are more than 10 times higher than the regression coefficient

for the relationship between MeHg<sub>P</sub> and P<sub>P</sub> mentioned above (0.0723). If the mineralization of

settling particules were responsible for the MeHg<sub>F</sub> versus SRP relationship, similar regression

coefficients should be found for the particulate and the dissolved phases. On the other hand,
510 the discrepancy in the regression coefficients between MeHg<sub>F</sub> vs SRP and MeHg<sub>P</sub> vs P<sub>P</sub>

511 suggests that MeHg-enriched water is advected with Northern Current and/or that net Hg

512 methylation occurs in the organic matter regeneration zone of the slope water column. The

513 significant increase in MeHgT concentrations in the slope waters when dissolved oxygen

514 decreases (R<sup>2</sup> = 0.90; n = 21, p<0.01) supports this latter interpretation (Fig. S6).

515 In summary, net Hg methylation seems to occur along the slope water column, whereas

516 we find little evidence for it to occur in the Low Salinity Water lenses drifting on the Gulf of

517 Lions shelf. This observation probably indicates that particulate organic matter degradation,

518 which governs the Hg<sup>II</sup><sub>i</sub> methylation, is favored when the water column is deep enough to

519 sustain a sufficiently long residence time of settling particles. In contrast, on the shelf, where

520 the water column depth is limited, organic matter degradation mostly occurs in the surface

521 sediments. Incubation experiments allowed estimating the Hg methylation potentials within

522 the Gulf of Lions water and sediment.

523 **5.2.2. In situ Hg methylation potential**

524 Week-long (192 h) incubation experiments suggest gross Hg methylation t rates of 0.009 to

525 0.083 % d<sup>-1</sup> during sediment resuspension. In addition, incubations of bottom seawater

526 sampled on the shelf during winter downwelling gave Hg methylation potentials (over 192 h)

527 ranging from 0.0002 to 0.0004 % d<sup>-1</sup>. Interestingly, the methylation rates increased during the

528 first 96 h of incubation then remained constant at the maximum level after 96 to 192 h (Fig.

529 6b). Applying the above methylation potentials to the flux of inorganic particulate Hg

530 calculated from the mean concentration of inorganic particulate Hg measured in March 2011

531 in the Gulf of Lions (0.519 nmol g<sup>-1</sup>) and the downwelling mass flux of 0.4 x 10<sup>12</sup> g per storm

532 event (2-3 days-long; Bourrin et al., 2012; Dumas et al., 2014), the gross methylation
observed in sediment slurries may account at best for 0.02 to 0.17 mol·d⁻¹ MeHgT (i.e. 0.05-0.42 mol MeHgT per storm event), that is 0.7 to 6.3 mol per year for 15 storms a year. These considerations suggest that the contribution of MeHgT produced in sediment slurries resulting from sediment resuspension in bottom water and short-term settling/resuspension cycles may be similar in magnitude to MeHgT release from porewater (2.7 mol·a⁻¹). If we apply the gross methylation rates obtained in bottom water (up to 0.0004 % d⁻¹) to the whole water column (with a HgIIᵢ of 0.85 pmol L⁻¹), the maximum methylation rate in the entire Gulf of Lions waters (~1.8 x 10³ km³) should not exceed 2.2 mol·a⁻¹.

541

542

## 6. SUMMARY AND CONCLUSIONS

Reviewing the oceanic Hg biogeochemical cycle, Mason et al. (2012) sum up that, while atmospheric deposition is the main source of inorganic divalent Hg in open ocean systems, most of the MMHg accumulating in ocean fish should derive from *in situ* production in the upper water column. Is this model proposed for the open seas also valid for the coastal environment? Here, we have addressed the questions of the distribution and sources of methylated Hg in the waters of the Northwestern Mediterranean margin including the continental shelf (Gulf of Lions) and the adjacent open sea (North Gyre). In summary, it appears that the proportion of methylated Hg to HgT increased seaward, from the freshwaters (Rhône River waters) to the shelf waters (Gulf of Lions waters) and the open ocean (North Gyre waters). Highest MeHgT concentrations are associated with the ODZs of the North Gyre. Despite the observed *in situ* methylation of inorganic Hg in sediment and waters within the system (<6.3 and ~2.2 mol·a⁻¹, respectively), external methylated Hg sources are the largest for the Gulf of Lions. The adjacent open ocean is the dominant source, with 680 ± 120
556 mol of MeHg per year. Continental sources (river and groundwater) account for < 95 mol a⁻¹,
557 with MeHg being mainly associated with continental particulate matter, a phase which is not
558 directly available for pelagic biota. Contributions from atmospheric deposition and sediment
559 resuspension are estimated to be ~5 and <16.8 mol a⁻¹, respectively. We conclude that (i) the
560 methylation of inorganic Hg in the ODZs of the open sea is the main source of methylated Hg
561 in the Northwestern Mediterranean margin waters, and that (ii) sedimentary sources have a
562 lower influence on the distribution of MeHgT in the water column, even though high
563 methylated Hg concentrations in the sediments may cause the exposure of organisms feeding
564 on food webs linked to the benthic environment. Our findings underline the ecological
565 importance of the idea that, even in coastal contaminated environments, methylated Hg
566 transfer into food webs is driven by the efficiency of processes that determine MeHgT inputs
567 to the water column (Sunderland et al., 2010; Chen et al., 2014). We are in favor of an
568 approach where, in coastal ecosystems, the origin of MeHg accumulated through pelagic and
569 benthic food webs are differentiated using stable Hg isotopes (Mason et al., 2012).

570

571 **Acknowledgments:** Thanks are due to Bernard Averty, Jean-François Chiffoleau, Nicole
572 Garcia, Christine Sotin and Roger Kerouel who participated to sampling or analytical works
573 and to captains and crews of R/V SUROIT (BIOPRHOFI cruise) and R/V ATALANTE (CASCADE
574 cruise). Special thank to François Dufois for his helpful comments and to Mac Vautour for
575 editing the English. The manuscript was greatly improved by numerous comments of one
576 referee and the associated editor. This research received funding from the French National
577 Research Agency (EXTREMA project, N° ANR-06-VULN-005), and the European
578 Commission's Seventh Program (HERMIONE project, N° 226354), the Région Aquitaine
579 (FEDER Aquitaine-1999-Z0061) and the MERMEX / MISTRALS project. It is a contribution

580 to the international LOICZ program.

581
582

583

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Figure Captions

859 **Figure 1.** Study site in the Northwestern Mediterranean, with main circulation patterns.

860 Stations location during the CASCADE cruise (March 2011). Station coordinates are given in

861 Table S2.

862 **Figure 2.** Dissolved MeHg (MeHgT<sub>F</sub>) *versus* salinity. (a) Estuarine mixing zone, (b) Low Salinity Water lenses. The solid line represents the linear relationship between MeHgT<sub>F</sub> and salinity (MeHgT<sub>F</sub> = - 0.0025*Sal + 0.1258, R<sup>2</sup> = 0.76), which suggests conservative mixing.

865 **Figure 3.** Time series of dissolved methylmercury (MeHgT<sub>F</sub>) and salinity in the Low Salinity Water lenses (LSWs) drifting on the Gulf of Lions' shelf and slope. Note that the LSW lenses occur mainly in the top 0-20 m depth.

868 **Figure 4.** Methylmercury (MeHgT) vertical profiles in the water of the shelf (a) and the continental slope (b) of the Gulf of Lions. UNF subscribes refer to unfiltered samples. Error bars correspond to standard deviations of 2 to 4 samples collected during temporal monitorings.

872 **Figure 5.** Vertical profiles of methylated mercury (MeHgT) in the North Gyre water column.

873 (a) Unfiltered (MeHgT<sub>UNF</sub>) water samples and (b) MMHg/DMHg ratios.

874 **Figure 6.** (a) Isotopic composition of MeHgT after incubation (192 h) of slurries of surface sediments in unfiltered bottom water and (b) gross methylation rates in incubations of unfiltered seawater from the bottom of the shelf water column, showing increasing methylation rates over time during the first 96h of incubation followed by the highest and constant methylation rates after 96h. Error bars represent standard deviation for multiple analyses (n=3) of the same sample. For the methylation rates, error bars estimated from multiple injections of the same sample (n=3) were smaller than symbol size.

881 **Figure 7.** Relationships between methylated mercury (MeHgT) and dissolved oxygen (Dissolved O<sub>2</sub>) in unfiltered samples from the North Gyre stations. (a) Methylated mercury (MeHgT) and (b) monomethylmercury (MMHg) and dimethylmercury (DMHg). The probability threshold (*p*<0.01) is reached for R<sup>2</sup> ≥ 0.55. No dissolved O<sub>2</sub> measurements were available for station "Antarès".

885

886
Tables

<table><caption>**Table 1.** Rhône River. A statistical summary of the concentrations in dissolved ($X_F$ < 0.45µm) and particulate ($X_P$ > 0.45µm) HgT and MeHg, measured in the waters at Arles (Sta. SORA). HgT values measured in 1994-1995 are from Cossa and Coquery (2005). SD: standard deviation; n: number of samples.</caption><thead><tr><th></th><th>HgT<sub>F</sub><br>(pmol L<sup>-1</sup>)</th><th>HgT<sub>P</sub><br>(nmol g<sup>-1</sup>)</th><th>MeHg<sub>F</sub><br>(pmol L<sup>-1</sup>)</th><th>MeHg<sub>P</sub><br>(nmol g<sup>-1</sup>)</th></tr></thead><tbody><tr><td>Average ± SD (n)</td><td>2.45 ± 2.05 (24)</td><td>0.85 ± 0.45 (27)</td><td>0.100 ± 0.035 (23)</td><td>0.017 ± 0.008 (26)</td></tr><tr><td>Min. – Max.</td><td>0.40 – 9.25</td><td>0.20 – 2.15</td><td>0.035 – 0.185</td><td>0.004 – 0.032</td></tr></tbody></table>

<table><caption>**Table 2.** Low Salinity Water lenses (LSW) and slope foot waters (SFW, Sta. S230). A statistical summary of HgT and MeHgT (MeHgT=MMHg+DMHg) measurements during the BIOPRHOFI cruise. Average ± standard deviation (number of samples) and range (Min. – Max.). (*) mean and standard deviation were calculated with the 87 concentration values, which included 30 measurements lower than the detection limit, which have been put equal to the half of the detection limit.</caption><thead><tr><th></th><th>HgT<br>(pmol L<sup>-1</sup>)</th><th>MeHgT<br>(pmol L<sup>-1</sup>)</th><th>MeHgT/HgT<br>(%)</th></tr></thead><tbody><tr><td>LSW (1-50 m layer;<br>bottom ≤120 m) Filtered<br>samples (&lt;0.4µm)</td><td>1.57 ± 0.74 (84)<br>0.61 – 3.50</td><td>0.021* ± 0.012* (87)<br>&lt;0.015 – 0.069</td><td>1.5 ± 1.0 (84)<br>&lt;0.3 – 5.5</td></tr><tr><td>SFW (&lt; 100 m layer;<br>bottom at 1386 m)<br>Unfiltered samples</td><td>1.22 ± 0.13 (3)<br>0.98 – 1.35</td><td>0.13 ± 0.10 (7)<br>0.026 – 0.241</td><td>10 ± 7 (3)<br>2 – 18</td></tr><tr><td>SFW (100-900 m layer;<br>bottom at 1386 m)<br>Unfiltered samples</td><td>1.41 ± 0.06 (4)<br>1.33 – 1.48</td><td>0.32 ± 0.13 (11)<br>0.23 – 0.38</td><td>18 ± 1 (4)<br>16 – 20</td></tr></tbody></table>
<table><caption>**Table 3.** Gulf of Lions shelf, slope and North Gyre waters (CASCADE cruise). Summary statistics for HgT<sub>UNF</sub> and MeHgT<sub>UNF</sub> concentrations. Average ± Standard deviation (number of samples) and range (Min. – Max.). Station locations are indicated on figure 1 and coordinates are given on table S2. (*) Values calculated with concentrations lower than the DL put equal to the half of the DL.</caption><thead><tr><th>Station<br>(water layer sampled)</th><th>HgT<sub>UNF</sub><br>(pmol L<sup>-1</sup>)</th><th>MeHgT<sub>UNF</sub><br>(pmol L<sup>-1</sup>)</th><th>MeHgT<sub>UNF</sub>/HgT<sub>UNF</sub><br>(%)</th></tr></thead><tbody><tr><td colspan="4" style="text-align: center;">Inner shelf (bottom &lt;100 m)</td></tr><tr><td>A (1-90 m)</td><td>2.04 ± 1.11 (7)<br>1.27 – 4.47</td><td>0.041 ± 0.034 (9)*<br>&lt;0.015 – 0.119</td><td>1.5 ± 0.9 (7)<br>0.6 – 3.4</td></tr><tr><td>B (1-90 m)</td><td>1.17 ± 0.29 (10)<br>0.78 – 1.61</td><td>0.034 ± 0.021 (10)*<br>&lt;0.015 – 0.066</td><td>3.1 ± 2.2 (10)<br>0.5 – 6.3</td></tr><tr><td>C (1-90 m)</td><td>1.40 ± 1.08 (10)<br>0.58 – 4.20</td><td>0.007 ± 0.009 (11)*<br>&lt;0.015 – 0.035</td><td>0.9 ± 1.8 (10)<br>0.1 – 6.0</td></tr><tr><td>D (1-90 m)</td><td>1.61 ± 1.23 (9)<br>0.92 – 3.80</td><td>0.024 ± 0.014 (10)*<br>&lt;0.015 – 0.048</td><td>2.2 ± 1.7 (9)<br>0.3 – 5.1</td></tr><tr><td>A/B/C/D (1-90m)</td><td>1.52 ± 1.00 (36)<br>0.58 – 4.47</td><td>0.026 ± 0.024 (40)*<br>&lt;0.015 – 0.119</td><td>1.9 ± 1.9 (36)<br>0.1 – 6.3</td></tr><tr><td colspan="4" style="text-align: center;">Slope edge and head of the Cap de Creus canyon (bottom at 100-300 m)</td></tr><tr><td>E (2-290 m)</td><td>0.99 ± 0.52 (20)<br>0.58 – 2.94</td><td>0.026 ± 0.048 (8)*<br>&lt;0.015 – 0.141</td><td>1.5 ± 2.0 (8)<br>0.3 – 4.7</td></tr><tr><td>L-01 (5-250 m)</td><td>1.07 ± 0.10 (6)<br>1.00 – 1.26</td><td>0.033 ± 0.023 (6)<br>0.020 – 0.076</td><td>3.0 ± 1.8 (6)<br>1.8 – 6.1</td></tr><tr><td>M-12 (10-130 m)</td><td>1.02 ± 0.12 (4)<br>0.93 – 1.14</td><td>0.049 ± 0.006 (4)<br>0.042 – 0.056</td><td>4.8 ± 0.8 (4)<br>4.1 – 6.0</td></tr><tr><td colspan="4" style="text-align: center;">Slope foot (bottom at 900-1800 m)</td></tr><tr><td>M-10 (10-1100 m)</td><td>1.01 ± 0.15 (10)<br>0.66 – 1.13</td><td>0.271 ± 0.088 (10)<br>0.092 – 0.355</td><td>26.2 ± 6.2 (10)<br>13.9 – 33.4</td></tr><tr><td>L-03 (10-1860 m)</td><td>1.22 ± 0.12 (10)<br>1.13 – 1.53</td><td>0.177 ± 0.077 (8)<br>0.020 – 0.266</td><td>14.9 ± 6.5 (8)<br>1.3 – 22.0</td></tr></tbody></table>
North Gyre (bottom at > 2000 m)

Antarès (10-2500m) 1.05 ± 0.12 (9) 0.235 ± 0.107 (9) 21.8 ± 8.8 (9)

0.84 – 1.23 0.090 – 0.375 11.3 – 33.8

L-05 (10-2200 m) 1.04 ± 0.04 (10) 0.252 ± 0.040 (9) 24.3 ± 3.9 (9)

0.98 – 1.11 0.199 – 0.309 18.8 – 30.1

L-08 (10-2150 m) 1.07 ± 0.04 (10) 0.307 ± 0.045 (10) 28.7 ± 4.3 (10)

1.01 – 1.13 0.263 – 0.394 24.2 – 38.6

L-10 (10-2360 m) 1.02 ± 0.12 (10) 0.314 ± 0.113 (10) 30.2 ± 8.4 (10)

0.82 – 1.11 0.166 – 0.478 20.3 – 43.2

L-12 (10-2500m) 1.13 ± 0.18 (10) 0.237 ± 0.079 (10) 21.3 ± 5.1

0.64 – 1.23 0.136 – 0.398 13.4 – 32.4

S2400 (10-2400m) 1.11 ± 0.01 (10) 0.175 ± 0.123 (10) 15.8 ± 11.1 (10)

1.08 – 1.12 0.020 – 0.326 1.8 – 25.4

M-08 (10-2010m) 1.15 ± 0.12 (10) 0.305 ± 0.052 (10) 26.8 ± 5.9 (10)

1.03 – 1.45 0.245 – 0.367 17.7 – 35.7

M-05 (10-2490 m) 1.14 ± 0.04 (10) 0.216 ± 0.062 (10) 18.8 ± 5.5 (10)

1.05 – 1.19 0.106 – 0.302 10.0 – 26.9

M-03 (10-2580 m) 1.24 ± 0.10 (10) 0.276 ± 0.056 (10) 22.6 ± 5.5 (10)

1.09 – 1.41 0.137 – 0.320 9.7 – 27.9

M-01 (10-2500 m) 1.06 ± 0.24 (10) 0.278 ± 0.132 (10) 24.7 ± 8.7 (10)

0.53 – 1.24 0.066 – 0.374 8.9 – 34.6

909

910

911

912

913

914

915

916

917

918

919

920
**Table 4.** Gulf of Lions shelf and North Gyre (CASCADE cruise). Summary statistics for MeHgT<sub>F</sub> (<0.4µm) in the first 2 cm of the sediment pore waters and bottom water (10 cm above the water sediment interface). The thickness of the oxidized layer (cm) is based on the redox potential value. In italic and brackets are the MeHgT concentrations (pmol g<sup>-1</sup>) in the solid phase.

<table><thead><tr><th rowspan="2">Station</th><th>Oxidized layer thickness</th><th>MeHgT<sub>F</sub> (pmol L<sup>-1</sup>)</th><th>MeHgT<sub>F</sub> (pmol L<sup>-1</sup>)</th></tr><tr><th>(cm)</th><th>Pore water</th><th>Bottom water</th></tr></thead><tbody><tr><td colspan="4" style="text-align: center;">Inner shelf (bottom &lt;100 m)</td></tr><tr><td>A-04</td><td>2</td><td>0.071 (4.0)</td><td>0.022</td></tr><tr><td>B-05</td><td>3</td><td>0.268 (3.5)</td><td>0.008</td></tr><tr><td>C-05</td><td>3</td><td>0.684 (3.0)</td><td>0.040</td></tr><tr><td>D-06</td><td>4.5</td><td>0.254 (3.5)</td><td>0.040</td></tr><tr><td>E-03</td><td>-</td><td>0.259</td><td>0.040</td></tr><tr><td>E-08</td><td>4.5</td><td>0.258</td><td>0.040</td></tr></tbody></table>

926

927
<figure><img src="image_2.png" /><figcaption>Fig. 1</figcaption></figure>

928

929

<figure><img src="image_3.png" /><figcaption>Fig. 2</figcaption></figure>

930

931

932

933

934

935
<figure><img src="image_4.png" /><figcaption>Fig. 3</figcaption></figure>

<figure><img src="image_5.png" /><figcaption>Fig. 4</figcaption></figure>
<figure><img src="image_6.png" /><figcaption>Fig. 5</figcaption></figure>

946

947

<figure><img src="image_7.png" /></figure>

<figure><img src="image_8.png" /><figcaption>Fig. 6</figcaption></figure>

948

949

950

951

952

953
<figure><img src="image_9.png" /><figcaption>Fig. 7</figcaption></figure>
957 Supplementary material including Suppl. Figures (S1-S6), Suppl. Tables (S1 and S2),

958 Suppl. Information (SI1, SI2 and SI3) and references

959

960 Supplementary Figure Captions

961 **Figure S1.** Schematic representation of the Gulf of Lions water circulation patterns.

962 **Figure S2.** Trajectories of the desalted water lenses (LSWs) drifting on the Gulf of Lions' shelf and

963 slope.

964 **Figure S3.** Potential temperature vertical profiles along the “L” transect (Fig. 1). Stratified stations are

965 characterized by high temperature gradients (red zones), whereas homogenized water column are

966 illustrated by a bleu monicolor.

967 **Figure S4.** Particulate methylmercury (MeHg~P~) *versus* (a) particulate phosphorus, and (b) pigments at

968 stations 221 and 230. See figure S2 for station locations.

969 **Figure S5.** Dissolved methylmercury versus particulate methylmercury in desalted water lenses (LSW)

970 drifting on the Gulf of Lions' shelf and slope. See figure S2 for station locations.

971 **Figure S6.** Dissolved methylmercury (MeHg~F~) *versus* dissolved oxygen within desalted water lenses

972 (LSW) drifting on the Gulf of Lions' shelf and slope. See figure S2 for station locations.

973
<figure><img src="image_10.png" /><figcaption>Fig. S1</figcaption></figure>

974

<figure><img src="image_11.png" /><figcaption>Fig. S2</figcaption></figure>

975
<figure><img src="image_12.png" /><figcaption>Fig. S3</figcaption></figure>

976

<figure><img src="image_13.png" /><figcaption>Fig. S4</figcaption></figure>

977
<figure><img src="image_14.png" /><figcaption>Fig. S5</figcaption></figure>

978

<figure><img src="image_15.png" /><figcaption>Fig. S6</figcaption></figure>

979

980
Supplementary Tables

**Table S1.** Sampling cruises. (1) http://www.com.univ-mrs.fr/MOOSE/spip.php?article46; (2) http://www.smtdr.fr/page5.html; (3) http://flotte.ifremer.fr/fleet/Presentation-of-the-fleet/Vessels/Deep-sea-vessels/Le-Suroit/Virtual-tour-of-N-O-Le-Suroit; (4) http://flotte.ifremer.fr/fleet/Presentation-of-the-fleet/Vessels/Deep-sea-vessels/L-Atalante. See also the map on figure 1.

<table><thead><tr><th>Name of the cruise</th><th>Location of the sampling</th><th>Mode of collection</th><th>Date</th><th>Type of samples collected</th></tr></thead><tbody><tr><td rowspan="2">Rhône monitoring project</td><td rowspan="2">Rhône River at Arles</td><td rowspan="2">SORA pumping station<sup>1</sup></td><td>April to June and October to November 2008</td><td>Particles</td></tr><tr><td>March 2009 to June 2010</td><td>Filtered waters and particles</td></tr><tr><td>Rhône delta study</td><td>Rhône fresh-seawater mixing zone</td><td>Barcarin ferry<sup>2</sup> and Zodiac</td><td>October 2008</td><td>Filtered waters and particles</td></tr><tr><td>BIOPRHOFI cruise</td><td>Shelf and slope</td><td>R/V Suroit<sup>3</sup></td><td>May 2006</td><td>Filtered and unfiltered waters. and particles</td></tr><tr><td>CASCADE cruise</td><td>Shelf, slope, canyon and rise</td><td>R/V Atalante<sup>4</sup></td><td>March 2011</td><td>Filtered and unfiltered waters and sediment cores (solid and pore waters)</td></tr><tr><td>Atmospheric monitoring</td><td>Coastal site (La Seyne-sur-Mer)</td><td>Rain collector</td><td>April 2009 to January 2010</td><td>Wet and dry deposition</td></tr></tbody></table>

988

989

990

991

992

993
<table><caption>**Table S2a.** Water sampling stations: Cruise, coordinates and bottom depth.</caption><thead><tr><th>Cruise</th><th>Station</th><th>Latitude (N)</th><th>Longitude (E)</th><th>Bottom depth (m)</th></tr></thead><tbody><tr><td>Rhône River monitoring</td><td>SORA (Arles)</td><td>43°40.722'</td><td>4°37.278'</td><td>4</td></tr><tr><td>Rhône mixing zone</td><td>I-II</td><td>43°15.360'</td><td>4°26.460'</td><td>6</td></tr><tr><td>Rhône mixing zone</td><td>III-IV</td><td>43°15.012'</td><td>4°26.892'</td><td>8</td></tr><tr><td>Rhône mixing zone</td><td>V-VII</td><td>43°15.018'</td><td>4°26.862'</td><td>13</td></tr><tr><td>Rhône mixing zone</td><td>VIII-XIV</td><td>43°13.758'</td><td>4°28.968'</td><td>10</td></tr><tr><td>BIOPRHOFI</td><td>S12</td><td>43°05.555'</td><td>4°47.848'</td><td>104</td></tr><tr><td>BIOPRHOFI</td><td>S16/18</td><td>43°02.395'</td><td>4°54.328'</td><td>123</td></tr><tr><td>BIOPRHOFI</td><td>S26</td><td>43°03.156'</td><td>4°55.379'</td><td>120</td></tr><tr><td>BIOPRHOFI</td><td>S33</td><td>43°02.857'</td><td>4°56.328'</td><td>123</td></tr><tr><td>BIOPRHOFI</td><td>S47</td><td>43°02.066'</td><td>4°57.294'</td><td>123</td></tr><tr><td>BIOPRHOFI</td><td>S54</td><td>43°02.008'</td><td>4°55.098'</td><td>123</td></tr><tr><td>BIOPRHOFI</td><td>S68</td><td>43°00.897'</td><td>4°54.132'</td><td>122</td></tr><tr><td>BIOPRHOFI</td><td>S88</td><td>43°03.014'</td><td>4°30.747'</td><td>103</td></tr><tr><td>BIOPRHOFI</td><td>S95</td><td>43°00.035'</td><td>4°28.624'</td><td>267</td></tr><tr><td>BIOPRHOFI</td><td>S108</td><td>42°58.267'</td><td>4°29.126'</td><td>358</td></tr><tr><td>BIOPRHOFI</td><td>S116</td><td>42°58.095'</td><td>4°31.054'</td><td>132</td></tr><tr><td>BIOPRHOFI</td><td>S129</td><td>42°56.984'</td><td>4°32.381'</td><td>131</td></tr><tr><td>BIOPRHOFI</td><td>S138</td><td>42°56.986'</td><td>4°34.895'</td><td>131</td></tr><tr><td>BIOPRHOFI</td><td>S152</td><td>42°57.621'</td><td>4°36.471'</td><td>127</td></tr><tr><td>BIOPRHOFI</td><td>S159</td><td>42°57.746'</td><td>4°38.387'</td><td>125</td></tr><tr><td>BIOPRHOFI</td><td>S171</td><td>42°55.144'</td><td>4°37.313'</td><td>269</td></tr><tr><td>BIOPRHOFI</td><td>S178</td><td>42°53.911'</td><td>4°34.218'</td><td>495</td></tr><tr><td>BIOPRHOFI</td><td>S220</td><td>42°42.374'</td><td>4°36.422'</td><td>971</td></tr><tr><td>BIOPRHOFI</td><td>S221</td><td>42°41.884'</td><td>4°32.451'</td><td>1122</td></tr><tr><td>BIOPRHOFI</td><td>S230</td><td>42°34.071'</td><td>4°27.957'</td><td>1386</td></tr><tr><td>CASCADE</td><td>Antarès</td><td>42°48.150'</td><td>6°07.466'</td><td>2497</td></tr><tr><td>CASCADE</td><td>L01</td><td>42°01.950'</td><td>3°29.284'</td><td>239</td></tr><tr><td>CASCADE</td><td>L03</td><td>42°01.967'</td><td>3°53.250'</td><td>1887</td></tr><tr><td>CASCADE</td><td>L05</td><td>42°01.998'</td><td>4°17.400'</td><td>2199</td></tr><tr><td>CASCADE</td><td>L08</td><td>42°02.017'</td><td>5°06.067'</td><td>2166</td></tr></tbody></table>
<table><caption>**Table S2b.** Sediment sampling stations: Cruise, coordinates and bottom depth.</caption><tr><td>CASCADE</td><td>L10</td><td>42°01.998'</td><td>5°30.150'</td><td>2398</td></tr><tr><td>CASCADE</td><td>L12</td><td>42°55.9845</td><td>5°41.816'</td><td>2536</td></tr><tr><td>CASCADE</td><td>S2400</td><td>42°02.000'</td><td>4°41.533'</td><td>2391</td></tr><tr><td>CASCADE</td><td>M01</td><td>41°08.033'</td><td>4°41.750'</td><td>2691</td></tr><tr><td>CASCADE</td><td>M03</td><td>41°25.967'</td><td>4°41.783'</td><td>2614</td></tr><tr><td>CASCADE</td><td>M05</td><td>41°44.033'</td><td>4°41.466'</td><td>2522</td></tr><tr><td>CASCADE</td><td>M08</td><td>41°19.984'</td><td>4°41.816'</td><td>1979</td></tr><tr><td>CASCADE</td><td>M10</td><td>42°19.988'</td><td>4°41.850'</td><td>1096</td></tr><tr><td>CASCADE</td><td>M12</td><td>42°55.984'</td><td>4°41.816'</td><td>126</td></tr><tr><td>CASCADE</td><td>A-08</td><td>43°12.984'</td><td>04°50.371'</td><td>99</td></tr><tr><td>CASCADE</td><td>B-08</td><td>43°12.570'</td><td>04°07.670'</td><td>91</td></tr><tr><td>CASCADE</td><td>C-08</td><td>42°56.054'</td><td>03°29.580'</td><td>93</td></tr><tr><td>CASCADE</td><td>D-09</td><td>42°38.515'</td><td>03°18.408'</td><td>90</td></tr><tr><td>CASCADE</td><td>E-06</td><td>42°20.725'</td><td>03°21.414'</td><td>280</td></tr><tr><td>CASCADE</td><td>X-1 to X-33</td><td>42°20.867'</td><td>3°21.484'</td><td>290</td></tr></table>

<table><thead><tr><th>Cruise</th><th>Station</th><th>Latitude (N)</th><th>Longitude (E)</th><th>Bottom depth (m)</th></tr></thead><tbody><tr><td>CASCADE</td><td>A-04</td><td>43°18.092'</td><td>04°50.635'</td><td>57</td></tr><tr><td>CASCADE</td><td>B-05</td><td>43°20.375'</td><td>04°08.127'</td><td>64</td></tr><tr><td>CASCADE</td><td>C-05</td><td>43°03.382'</td><td>03°22.305'</td><td>60</td></tr><tr><td>CASCADE</td><td>D-06</td><td>42°40.594'</td><td>03°10.800'</td><td>61</td></tr><tr><td>CASCADE</td><td>E-03</td><td>42°19.951'</td><td>03°20.943'</td><td>120</td></tr><tr><td>CASCADE</td><td>E-08</td><td>42°21.106'</td><td>03°22.029'</td><td>442</td></tr><tr><td>CASCADE</td><td>S2400</td><td>42°01.985'</td><td>04°41.807'</td><td>2333</td></tr><tr><td>CASCADE</td><td>M01</td><td>41°08.005'</td><td>04°41.798'</td><td>2625</td></tr><tr><td>CASCADE</td><td>M08</td><td>42°19.989'</td><td>04°41.832'</td><td>1942</td></tr></tbody></table>
# Supplementary Information

1004

## SI1. Methods

### Sample treatment

Water and particles. The samples from the 0-50 m layer of the shelf waters during the BIOPRHOFI cruise were collected by pumping with an all-Teflon pneumatic pump (10-2PM, ASTI®) and through polyethylene tubing, directly into a class 100 on-board container, with the consequence that the seawater was never in contact with the atmosphere of the ship. All the plastic wares were previously acid-cleaned according to ultraclean sample handling protocols (e.g., Cossa et al. 2003). Discrete water samples were collected in 2L Teflon (FEP) bottles inside the container, where filtrations were then performed on sub-samples using acid washed polycarbonate membranes (0.45 µm, Nuclepore®). Filtrate (250 mL) was collected in Teflon (FEP) bottles and acidified with HCl (0.4 %, v/v, Suprapur, Merck®) for subsequent analyses of “dissolved” fraction of total and methylmercury (HgT<sub>F</sub> and MeHgT<sub>F</sub>). The collection of particles from the Low Salinity Waters, for subsequent MMHg<sub>P</sub>, CHNP and pigments determinations, was performed using in-line filtration through glass fiber filters (GF/G, Whatman®) previously cleaned by heating at 450°C for 24h and mounted in a Teflon (PTFE)/stainless steel filter holder (Ø142 mm, Schleicher & Schuell®) within the container. Up to 60 liters of water were filtered this way in order to collect enough material for the various particulate analyses. Samples for dissolved gaseous Hg (DGM) analyses were collected in a 1L Teflon bottle (FEP) according to the traditional method used for dissolved oxygen determination in order to avoid gas evasion during the collection. Deep-water samples (> 50 m) were collected during the BIOPRHOFI and CASCADE cruises by rosette-mounted 5L bottles (1010X-Niskin, General Oceanics®) equipped with a CTD probe. These samples were not filtered and analyzed only for HgT<sub>UNF</sub> and MeHgT<sub>UNF</sub>. Dissolved gaseous Hg and HgT were determined on board, while MMHg and MeHgT analyses were performed in the laboratory within 2 months after the cruise on the acidified samples stored in the dark at +4°C in a double wrapping of polyethylene bags.

Sediment cores: The cores were collected along the Gulf of Lions shelf, at the head of the Cap de Creus canyon and in the abyssal plain in the North Gyre area during the CASCADE cruise. A multicore sampler (Oktopus GmbH Multiple corer with 8 tubes of 100 mm diameter) allowing the sampling of the undisturbed benthic interface (Barnett et al., 1984) was used. The pore water was drained using Milli-Q (Millipore®) water-rinsed microporous polymer tube samplers (Rhizon SMS, Rhizosphere Research Products®) fixed on an acid washed all-polypropylene syringe (Guédron et al., 2012). Collected pore water (the two first cm below the water-sediment interface) was filtered through a
hydrophilic Teflon membrane (0.45 µm, Millex-LCR, Millipore®), then acidified with high purity HCl (0.4 % v/v, Suprapur, Merck®) and stored in the dark until MMHg analysis. According to Guédron et al. (2012), Rhizon samplers preferentially recover water from the sediment macropores, containing the readily exchangeable chemical species.

## Chemical analyses

**Total mercury.** Total Hg in filtered and unfiltered samples were measured on board within a few minutes of sampling. In order to access all the mercury chemical species, present in the sample, the release of Hg from its ligands was achieved by a BrCl solution (0.1 mL of a 0.2 M solution is added to a 100 mL sample), and then the HgII was reduced to Hg0 with an acidic SnCl2 solution (0.2 mL of a 1 M solution is added to a 100 mL sample). This technique derives from the original Bloom and Crecelius (1983) method and has been described in detail by several authors (e.g., Gill and Fitzgerald, 1988; Horvat et al., 1991; Mason and Fitzgerald, 1993) and is now known as the US-EPA standard method N° 1631. The Hg0 vapor generated by the reduction is amalgamated on a gold (Au) trap then released by heating into an Atomic Fluorescence Spectrometer (2500, Tekran®). For both measurements, the detection limit (DL) was 0.1 pmol L-1 and the reproducibility varied according to the concentration level between 5 and 15 % (Cossa et al. 2003). The accuracy of HgT measurements was tested using the ORMS-3 certified reference material (CRM) from the National Research Council of Canada. Our measurements were always within the confidence limits given for the CRM (12.6 ± 1.1 pg mL-1; http://inms-ienm.nrc-cnrc.gc.ca/calserv/crm_files_f/ORMS-3_f.pdf).

**Methylated Hg species.** Total methylated Hg and MMHg were measured on filtered and unfiltered samples, whereas DMHg was not measured, but calculated as the difference between MeHgT and MMHg. Total methylated Hg was determined on acidified samples, which means that both MMHg and DMHg were determined in the same time, since DMHg is converted into MMHg at low pH (Mason, 1991; Black et al., 2009a). Monomethyl Hg was determined with the same technique as MeHgT, but after bubbling 350 mL samples for 40 min with argon (Ar) at a flow rate of 250 mL min-1 in order to remove DMHg before acidification. Total methylated Hg and MMHg were determined as volatile MMHg hydride by purge and cryo-trapping gas chromatography and detected as elemental Hg vapor by atomic fluorescence spectrometry (AFS). The mercury hydrides (from MMHg and HgII) were formed with NaBH4, sparged from the sample with helium (He) (250 mL min-1), concentrated and then separated (50 mL min-1) by cryogenic chromatography before being converted in Hg0 in a furnace (800°C) and detected by the AFS detector. The hydride generation technique was initially proposed by Filippelli et al. (1992), modified by Tseng et al. (1998), and then improved by Stoichev et al. (2004) and Cossa et al. (2009). Last authors optimized the method in order to detect sub-picomolar
1069 levels in seawater by lowering the reagent amount (addition of only 0.6-4.0 mL of a NaBH₄ solution of
1070 0.5 % (w/v) to a 30-200 mL water sample) and using a very stable and sensitive detector with an
1071 absolute DL of ~1 femtomol of Hg (AFS detector 2500 model equipped with a mirror-coated quartz
cuvette, Tekran®). The hydrides are formed within a silanized borosilicate glass reactor (5 % DMDCS
1072 in toluene), then concentrated at low temperature (in liquid nitrogen) and separated by heating (up to
1073 90°C) in a silanized borosilicate glass tube of 4 mm interior diameter and filled with Chromosorb
1074 WAW-DMCS (60/80 mesh impregnated with 15 % OV-3). The vector gas was Hg-free He, purified
1075 by passing through charcoal and gold filters. During this set of analyses, the blank (< 0.01 pmol) and
1076 its reproducibility (2 %) allowed DLs (calculated as 3 times the standard deviation of the blank)
1077 ranging from 0.005 to 0.025 pmol L⁻¹ depending on the volume of the sample analyzed, and limits of
1078 quantification (calculated as 10 times the standard deviation of the blank) ranging from 0.015 to 0.075
1079 pmol L⁻¹. The analytical reproducibility varied with time between 6 and 15 %. The accuracy was not
1080 directly estimated because no certified reference seawater for MMHg was available. The calibration
1081 was performed using the dilutions of a 1 g L⁻¹ stock MMHg solution in isopropanol. The dilutions of
1082 the stock solution were performed in HCl (0.4 %, w/v, Suprapur, Merck®) water solution. Three times
1083 a day, 2 µL of saturated Hg⁰ vapor was injected into the chromatographic system through a septum in
1084 order to check the response of the instrument and verify the hydride yields. Details of the analytical
1085 system are given in a technical paper (Cossa et al. 2003).
1086

1087 MeHgT<sub>P</sub> was determined by atomic fluorescence spectrometry after HNO<sub>3</sub> (4M, Suprapur, Merck®) extraction, ethylation of the MMHg and Hg<sup>II</sup>, followed by chromatographic separation of the volatile ethylated compounds according to Liang et al. (1994). The accuracy, reproducibility and DL, established analyzing a Certified Reference Material from the International Atomic Energy Agency (IAEA-142, Horvat et al., 1997), were 80-120 % (recovery), 10 % (coefficient of variation) and 4 ng g<sup>-1</sup> (3 times the standard deviation of the blank).

1093 *Dissolved gaseous Hg*. For analysis of DGM, 300 mL of sample was purged for 30 min with ultra-high
1094 purity nitrogen stripped of Hg⁰ by passage through Au traps, at a flow rate of 300 L min⁻¹,
1095 corresponding to a calculated extraction rate of 78 % (results were corrected for this yield). Volatilized
1096 Hg species were trapped and concentrated on an Au trap (Braman and Johnson, 1974), subsequently
desorbed by pyrolysis and quantified by gas-phase AFS. Dissolved gaseous Hg net production was
1097 evaluated by incubating unfiltered water samples in an incubator located on the deck of the vessel
1098 exposed to sunlight radiations. Temperature was controlled by continuously pumping sea-surface
1099 water through the incubator using a through-flow system. These *ex-situ* incubations were performed in
1100 batch experiments during 2 to 12 h periods under the following conditions: (i) absence of light (FEP
1101
1102 Teflon bottles, wrapped in Al foil); (ii) presence of light (FEP Teflon bottles). Transparent FEP Teflon bottles absorbed 2.5 % of total incident radiation according to Amyot et al. (1997). Net DGM production was estimated without taking into account the possible re-oxidation of Hg⁰.

1105

## SI2. Riverine flux calculations

1107 In the absence of any significant relationship between “dissolved” HgT and RR water discharge the HgT<sub>F</sub> flux was calculated as the product of weighted average of Hg concentrations and average discharge (Meybeck and Ragu, 1996; Gairoard et al. 2012). On the contrary, in the case of particulate Hg, we took into account the existing HgT<sub>P</sub> dependencies upon hydrological changes (Fig. 3). Rhône river discharge and the suspended particulate matter concentrations were obtained from the Compagnie Nationale du Rhône (http://www.cnr.tn.fr/). In these conditions, the HgT discharging from the RR between June 2009 and June 2010 has been calculated to be 85 and 800 mol as “dissolved” and particulate, respectively.

1114 Considering that during the studied period the Rhône discharge was atypically low, and using the water and particulate mean discharges calculated on the basis of the last 36 years (Gairoard et al., 2012) the annual HgT efflux from the Rhône River to the GoL would be close to 2.7 and 0.13 kmol for the particulate and “dissolved” phases respectively. Assuming average ratios of MeHg/HgT of 1.9 and 4.1 % (Table 1 of the main manuscript), the best estimates of the MeHg annual Rhône effluxes is calculated to be 51 and 5 mol for particulate and the “dissolved” phases, respectively.

1120

## SI3. Modeling the diffusive fluxes

1122 MeHg<sub>F</sub> diffusive fluxes were estimated at the stations A to E benthic boundary layer (BBL) using Fick's first law (Eq. 1):

$$J = - (\phi D_w / \theta^2) (\delta C / \delta x)_{\text{BBL}} \quad [1]$$

1125 Where, *J* is the flux of the solute with concentration *C* at depth *x*, *φ* is the sediment porosity (Eq. 2), *θ* is the tortuosity (Eq. 3), and *D*<sub>w</sub> is the molecular diffusion coefficient of the solute in seawater. Measuring porosity, the tortuosity was approached using Boudreau's formulation:

$$\phi = \text{pore water volume} / (\text{solid volume} + \text{pore water volume}) \quad [2]$$

$$\theta^2 = 1 - \ln(\phi^2) \quad [3]$$
1130 The *Dw* for MeHg<sub>F</sub> as MeHgCl were determined coupling the linear regressions of the infinite dilution diffusion for cations and anions against temperature (Boudreau, 1996) with the infinite-dilution diffusion for ion pairs (Applin and Lasaga, 1984). The expression was calculated for temperature salinity and pressure from an empirical equation developed by Kukulka et al. (1987). The adjustment for pore water viscosity of normal seawater was small at no more than 7 % (Li and Gregory, 1974). The respective approximations for MeHg<sub>F</sub> at T = 18 °C, S = 35 and P = 2 bar were 1.84 x 10<sup>-5</sup> and 8.65 x 10<sup>-6</sup> cm<sup>2</sup> s<sup>-1</sup>.

1136
1137 **Table:** MeHgT<sub>F</sub> diffusive fluxes from shelf sediments in the Gulf of Lions (Northwestern Mediterranean).
1138 The MeHg gradient at the sediment-water interface is estimated using the difference between MeHg
1139 concentrations in surface sediment pore waters (2 cm below the interface) and bottom waters (10 cm
1140 above the water sediment interface).

<table>
  <thead>
    <tr>
      <th rowspan="2">Station</th>
      <th>MeHgT<sub>F</sub> (pmol m<sup>2</sup> day<sup>-1</sup>)</th>
      <th>MeHgT<sub>F</sub> (pmol L<sup>-1</sup>)</th>
    </tr>
    <tr>
      <th>Flux</th>
      <th>Diff<sub>Pore water and Bottom water</sub></th>
    </tr>
  </thead>
  <tbody>
    <tr>
      <td>A-04</td>
      <td>0.20</td>
      <td>0.049</td>
    </tr>
    <tr>
      <td>B-05</td>
      <td>1.04</td>
      <td>0.26</td>
    </tr>
    <tr>
      <td>C-05</td>
      <td>2.58</td>
      <td>0.644</td>
    </tr>
    <tr>
      <td>D-06</td>
      <td>0.86</td>
      <td>0.214</td>
    </tr>
    <tr>
      <td>E-03</td>
      <td>0.88</td>
      <td>0.219</td>
    </tr>
    <tr>
      <td>E-08</td>
      <td>0.87</td>
      <td>0.218</td>
    </tr>
  </tbody>
</table>

1141
1142
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1194

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