## 4 Discussions  
### DOC and Sulfides Drive MMHg Accumulation in Sediment PW and Effluxes Towards the SW  
The high MMHg concentrations found in PW are consistent with a previous study in Lake Uru Uru, which showed evidence that methylation was mediated mostly by SRB in the absence of sunlight [18]. Such nutrient-rich, alkaline waters with elevated levels in DOC and sulfates were also reported to favor the activities of anaerobic bacterial communities [4; 23; 24; 38]. During the wet season, the gradual rise in MMHg concentration in PW during the night illustrates the progressive installation of anoxia in surface sediment PW due to the rising respiration of the aquatic ecotopes, which is also reflected by the drop in dissolved oxygen in overlying waters (Figure 4a). This pattern is exacerbated during the dry season as a result of more eutrophic conditions, which enhance MMHg production and accumulation in the sediments [39]. During this latter season, both DOC and sulfide concentrations were positively correlated (R = 0.56, \(p<0.05\)), supporting the notion that active sulfate reduction in PW favors DOC release in PWs during the reduction of Fe-oxy(hydr)oxides [40]. Figure 3: **(a)** Filtered (F) THz (empty circles) and MMHg concentrations (dark cyan triangles up) and (**b**) percentage MMHg over the THz concentrations (dark cyan triangle down), dissolved organic carbon (DOC, semi-filled dark yellow circles), and hydrogen sulfide (H\({}_{2}\)S, grey circles) concentrations in sediment porewater during the wet (left panels, 20–21 May 2014) and the dry (right panel, 18 th to 19 th November 2014) seasons. Lines represent average concentrations for F (dotted line) and MMHg/THg (dashed line) concentrations and ratios. Interestingly, positive correlations were also found between sulfide and MMHg concentration (R = 0.73, \(p\) < 0.01, Figure S3) or percentages MMHg/THg (R = 0.66, \(p\) < 0.01), and between percentages MMHg/THg and DOC (R = 0.66, \(p\) < 0.01). This suggests that filtered MMHg was preferentially bound to sulfurized dissolved organic matter (DOM). In addition, the positive correlation found between sulfide and THg concentration (Figure S2) supports a limited precipitation of inorganic Hg with free sulfide or with FeS, which would have restricted the availability of Hg(II) for bio-methylation [41; 42; 43]. Hence, this rather supports the presence of Hg-SR complexes, which were reported to increase the bioavailability of Hg(II) for bio-methylation [14; 15]. In contrast, sulfide was negatively correlated with chalcophile redox-sensitive elements (i.e., Fe and V, \(p\) < 0.01, respectively, Figures S2 and S3), supporting the notion that the sulfate reduction in anoxic PWs favors their precipitation as authigenic sulfide minerals. The calculation of diffusive fluxes (Jsed, Figure 4b) for MMHg and HgII (=THg - MMHg) confirms both the seasonal and diel patterns observed in the PW and SW (Figures 2 and 3). Daily Jsed for HgII (16.2 \(\pm\) 7.3 ng m\({}^{-2}\) day\({}^{-1}\)) and MMHg (35.7 \(\pm\) 17.4 ng m\({}^{-2}\) day\({}^{-1}\)) were higher during the dry season compared to the wet one (13.3 \(\pm\) 9.1 ng m\({}^{-2}\) day\({}^{-1}\) and 5.4 \(\pm\) 7.8 ng m\({}^{-2}\) day\({}^{-1}\) for HgII and MMHg, respectively). These MMHg diffusive fluxes are much lower than those measured in the carbonate-rich sediments of Lake Titicaca (36 to 224 ng m\({}^{-2}\) day\({}^{-1}\)) but in the range of those reported for the organic-rich sediment of the same lake (13 to 60 ng m\({}^{-2}\) day\({}^{-1}\)[2]) and in coastal environments [3; 4; 6]. Figure 4: **(a)** Redox potential (ORP), dissolved oxygen saturation percentage (\(\%_{\rm sat}\)), and conductivity in overlying waters (OW) above the sediment–water interface (SWI); **(b)** filtered Hg(II) (=THg - MMHg) and MMHg diffusive fluxes (Jsed) at the SWI and (c) partitioning of THg and MMHg between filtered and particulate fractions in surface water, during the wet (left panels) and the dry (right panels) seasons.