## 4 Discussions
### Dial and Seasonal Changes in the Hg Partitioning between Dissolved and Particulate Phases in SW  
Previous studies reported limited net Hg methylation potentials in the water column, whereas elevated ones were found in floating bio-organic substrates and surface sediment of Lake Uru [18], supporting elevated MMHg production into fully anoxic environments or surface water microenvironments [12,48,49]. Although average MMHg concentrations were similar between the two seasons, the lower average percentages of filtered and unfiltered MMHg/THg with larger diel variations during the dry season compared to the wet one (i.e., 42 \(\pm\) 4% and 35 \(\pm\) 6%) can be explained by changes in water column biogeochemical conditions (i.e., dissolved oxygen, temperature and redox oscillations, and pelagic productivity or mobility), resulting in changes in the partitioning between the suspended solids and the solution. Alternatively, changes in the incident light radiation (see Section 4.3) between the two seasons may explain the seasonal differences. During the wet season, average percentages of filtered (<0.22 \(\upmu\)m) THg and MMHg in surface water were 58 \(\pm\) 21% and 71 \(\pm\) 23% of the UNF one, respectively, highlighting that both species were mainly present in the truly dissolved or colloidal phases (Figure 4c). Strikingly, sharp drops in the percentage of F/UNF THg and MMHg fractions (i.e., from \(\sim\)80 to \(\sim\)30%, Figure 4c) were found during the night, counterbalanced by the rise in both species onto particles synchronously with the drop in redox to negative values (Figure 2b). These percentages of F/UNF THg and MMHg were found to decrease with rising sulfides (R = \(-\)0.64 and \(-\)0.62, \(p\) < 0.05, for THg and MMHg, respectively) or DOC concentrations (R = \(-\)0.62 and \(-\)0.55, \(p\) < 0.05, for THg and MMHg, respectively) supporting the notion that DOC-bound MMHg or MMHg-SR complexes released from surface PW or produced in anoxic niches were sorbed onto suspended inorganic and organic particles. During the onset of reducing conditions, Mn concentrations (Figure S1) rose in the water column as a result of the dissolution of Mn oxides [50], whereas chalcophile redox-sensitive elements (i.e., Fe and V; Figure S1) decreased, likely due to their (co)precipitation as or onto antibigenic sulfide minerals [51]. Sulfate concentrations (5.4 \(\pm\) 5.1 g L\({}^{-1}\)) were at least an order of magnitude higher than those of Fe, and thus the production of sulfides drives the Fe levels in the SW. Hence, drops in THgF and MMHgF levels during reducing conditions found in the night-time period likely resulted from their adsorption or (co)precipitation onto/with colloidal and particulate FeS [2], and onto sulfurized OM [52]. They likely enriched the particulate pool through the flocculation of dissolved OM and the aggregation of colloids when SW became reducing [53,54]. During the dry season, Hg species in the reducing SW showed the opposite trend as for the wet season. THg and MMHg were mostly bound to particles (60 \(\pm\) 13% and 50 \(\pm\) 25% for particulate THgand MMHg, respectively), with higher partitioning onto particles during the day (Figure 4c). Such an opposite trend likely results from the more reducing, oxygen depleted, and sulfide-rich SWs, which favored both the MMHg production, upward efflux from the PW, and its preservation in the truly dissolved or colloidal form during the night in the dry season. In particular, the drop in oxygen was more accentuated during the night in the dry compared to the wet season, possibly resulting from a higher decomposition activity, triggered in turn by a higher productivity during the day. In addition, the higher content of algae and bacteria (both living or dead) in the SW during the dry season could have enhanced Hg and MMHg entrapment during the day as these organisms are known to accumulate MMHg in lake water columns [28; 45; 46; 55].