## 2 Materials and Methods
### Sampling Location, Strategy, and Elemental Analysis  
Surface water and sediment porewater samples were collected during two campaigns performed in May (end of the wet season) and November (end of the dry season) 2014. Surface water was sampled with a Teflon-coated Go-Flo trace metals sampler. Samples were then stored unfiltered or filtered (0.22 \(\upmu\)m PVDF) in acid-washed FEP Teflon containers and acidified (except for dissolved gaseous mercury samples) with HCl (0.5%, _v/v_, Ultrex grade--Baker). Surface sediment porewaters (0-5 cm) were sampled with 5 cm long microporous polymer tube samplers (RhizonCSS(r), 0.2 \(\upmu\)m porosity; Rhizosphere Research Products, the Netherlands) over \(\sim\)4 h, following published protocols [3; 23].  
Dissolved gaseous mercury (DGM) was purged from unfiltered water samples on-site within \(\sim\)0.5 h after sampling and collected on gold-coated sand traps [24]. DGM was analyzed by thermodesorption (600 \({}^{\circ}\)C) and cold vapor atomic fluorescence spectrometry (CV-AFS) [24]. Filtered (F) and unfiltered (unf) total mercury concentrations (THg) in surface and pore waters were determined by cold vapor atomic fluorescence spectrometry (CV-AFS) after conversion of all mercury species into Hg0 followed by detection using a Tekran(r) (Model 2500, Canada) [25; 26]. Filtered monomethylmercury (MMHgF) concentrations were analyzed using an ethylation purge and trap-gas chromatograph-AFS analyzer (MERX System, Brooks Rand(r), Seattle, WA, USA) following published protocols [27; 28]. All samples were run in duplicate and quantified using the standard addition technique. Unfiltered monomethylmercury (MMHgunf) concentrations were analyzed by propylation ID-GC-ICPMS (S.I.2.2) following published protocols [4; 29; 30].  
Dissolved organic carbon concentrations (DOC) were determined using a Non-Dispersive Infra-Red (NDIR) CO\({}_{2}\) Shimadzu(r) (Model VCSN, Japan) spectrometer after humid oxidation in a sodium persulfate solution at 100 \({}^{\circ}\)C. Major elements (Ca, Si, K, Na, Mg, and S) in filtered water (SW and PW) were measured by ICP-AES (Varian model 720 ES) within the analytical chemistry platform of ISTerre (OSUG-France) and trace elements (Fe, Al, Mn, V, Sb) by ICP-MS (Nexion, Perkin Elmer) within the analytical chemistry platform of Univ. de Pau et des Pays de l'Adour (UPPA). Anions were measured by ionic chromatography using a 332 Metrohm apparatus and its accuracy was evaluated with a Carl Roth 2668.1 standard. Hydrogen sulfide (H\({}_{2}\)S) was determined using a  
Figure 1: (**a**) Study site location: location of Lake Uru Uru in the Bolivian Altiplano and (**b**) sampling site (UU\({}_{12}\)) in the southern part of the lake.  
modification of the diamine method described elsewhere [31; 32] and analyzed with HPLC (Agilent ) after calibration with a Radiello(r) solution (Code 171, Italy) for H\({}_{2}\)S [28].  
A submersible multiparameter probe (HI-9828, Hanna Instruments, Woonsocket, RI, USA) was used to characterize the water physico-chemistry (pH, conductivity, dissolved oxygen concentration and saturation, and temperature). Data collection was conducted every three seconds for two minutes at the surface (0-10 cm) and above the sediment-water interface (SWI) at each time of sampling [28].