## 5 Conclusions  
Our results confirm previous observations made in Lake Uru Uru showing that diel changes in MMHg concentration overwhelm seasonal ones, with higher MMHg accumulation in surface water during the night-time and the dry season. We here bring new insights into the key role of the sediment compartment as a major source of MMHg for the SW, with a larger contribution in MMHg efflux towards the water column during the dry season when the lake is relatively shallow, more eutrophicated, and with the redoxcline likely moving above the SWI. Our results suggest that MMHg accumulation in the PWs fluctuates depending on diel redox oscillations around the SWI, driving both the bacterial sulfate reduction and bio-methylation. We here confirm the previous hypothesis from enriched isotope incubation that MMHg is preferentially produced in the PW and supports its release toward the SW associated with dissolved/colloidal OM or organic thiols. In the water column, both the pelagic light-dependent organisms and the flocculation/aggregation of dissolved OM with colloids drive the MMHg partitioning between dissolved and particulate phases during diel redox oscillations. Finally, although elevated MMHg diffusive fluxes demonstrate that MMHg loading from the porewater to the water column is large, both the photochemical and biological methylmercury degradation control the net MMHg accumulation in the water column. The following are available online at [[http://www.mdpi.com/2076-3417/10/21/7936/s1](http://www.mdpi.com/2076-3417/10/21/7936/s1)]([http://www.mdpi.com/2076-3417/10/21/7936/s1](http://www.mdpi.com/2076-3417/10/21/7936/s1)), Figure S1: Seasonal and diel cycles for vanadium (V), manganese (Mn), calcium (Ca); potassium (K), magnesium (Mg), sodium (Na), total sulfur (S), and silicon (Si) in surface water. Figure S2: Scatter plot with linear regression for porewater concentrations of (a) manganese (Mn) vs. hydrogen sulfide (H\({}_{2}\)S) and dissolved oxygen (O2--measured in epibentic water above the SWI with a multiparameter probe); (b) iron (Fe) and vanadium (V) vs. H\({}_{2}\)S; (c) Filtered total mercury THz vs. H\({}_{2}\)S and V. Linear regression was performed for all data (both seasons). Figure S3: Scatter plot for (upper panels) PW MMHg concentrations vs. hydrogen sulfide (H\({}_{2}\)S) and vanadium (V), and (bottom panels) MMHg diffusive fluxes (Jsed MMHg) vs. (H\({}_{2}\)S) and redox potential (measured in epibentic water above the SWI with a multiparameter probe)[PERSON] Figure S4: Average precipitation and wind velocity at Oruro during the period 2010 to 2016 [68][PERSON]  
Conceptualization, D.A. ([PERSON]), S.G., D.P., D.A. ([PERSON]), and S.B. ; methodology, S.G., D.A. ([PERSON]), D.P., S.B., and D.A. ([PERSON]); validation, S.G., D.A. ([PERSON]), D.P., S.B., and D.A. ([PERSON]); formal analysis, S.G., D.A. ([PERSON]), S.B., D.P., E.T., S.R.-L., C.H., P.-S., M.F., and S.B. ; Field investigation, S.G., D.A. ([PERSON]), D.A. ([PERSON]), S.B., D.P., E.T., S.R.-L., C.H., P.-F.-S., M.F., I.Q.-L., and S.B. ; data curation, S.G.; writing--original draft preparation, S.G.; writing--review and editing, S.G., D.A. ([PERSON]), S.B., and D.A. ([PERSON]); supervision, S.G., D.A. ([PERSON]), S.B., and D.A. ([PERSON]); project administration, D.A. ([PERSON]) and S.G.; funding acquisition, S.G., D.A. ([PERSON]), S.B., and D.A. ([PERSON]). All authors have read and agreed the published version of the manuscript. This research was funded by Agence National pour la Recherche, as a contribution to the LA PACHAMMA project (ANR CESA program, No ANR-13-CESA-0015-01, PI: D. Amouroux: [EMAIL_ADDRESS]), COMIBOL project (INSU CNRS/IRD EC2 CO Program, PI: D. [PERSON]: [EMAIL_ADDRESS]), EUTTITICAA project (founded by the Impuestos Directos a los Hidrocarburos IDHadministated by the Universidad Mayor de San Andres, PI: D. [PERSON]: [PERSON]DRESS]) and TRACISOMER supported by a grant from Labex OSUG@2020 (PI: [PERSON]: [EMAIL_ADDRESS]).