## 2 Materials and Methods  
Water samples were taken in June (early dry season) and September (late dry season) 2007 from a longitudinal transect in the Itenez River, which is located at the frontier between Bolivia and Brasil and from the tributaries Blanco and San Martin. The sampled transect in Itenez river is 660 km long, starting at the \"Piso Firme\" locality (S 13\({}^{\circ}\)31\({}^{\prime}\)36.9\({}^{\prime\prime}\) W 61\({}^{\circ}\)49\({}^{\prime}\)40.7\({}^{\prime\prime}\)) and finishing 100 km upstream of the confluence with Mamore River (S 12\({}^{\circ}\)11\({}^{\prime}\)43.0\({}^{\prime\prime}\) W 64\({}^{\circ}\)40\({}^{\prime}\)00.5\({}^{\prime\prime}\)). Floodplain lakes related to the Itenez, San Martin, and Blanco Rivers were also sampled. Additional samples were obtained from three streams that cross Bolivian territory influenced by the \"San Simon\" goldmine and from three tributaries of Itenez that cross highly deforested Brazilian territory. The location of the sampling points can be observed in Figure 1. A more detailed description of the study area can be found in [11]. Five-liter plastic containers were used for sampling. These were conditioned by following a slight modification of the protocol proposed by [PERSON] et al. [14]. The containers were first washed with tap water and metal-free detergent and rinsed with MilliQ water. After this, they soaked overnight with a 7% NaOH solution. They were then rinsed with MilliQ water and soaked with 10% HCl for an additional 12 h. Finally, they were rinsed three times with MilliQ water. Once clean, the containerswere kept in hermetic plastic bags until their use. For the sampling of SPM, GF/F (0.7 \(\upmu\)m) filters were used. Before sampling, the filters were placed in packages wrapped with aluminum foil and heated up to 500 \({}^{\circ}\)C for 8 h. Upon cooling, the filters were weighted and individually placed in Petri dishes. Water was collected from the mainstream of rivers and the center of lakes by submerging the containers around 1 m deep. Following [13], the person in charge of sampling wore clean gloves throughout the procedure. The containers were rinsed three times with the river or lake water before being filled. Additionally, in each sampling point, temperature (\({}^{\circ}\)C), electric conductivity, and pH were measured with CONSORT C353 multimeter (Belgium). The depth of water was also registered. SPM was obtained in the field by vacuum filtration of the water sample. The volume of water that caused saturation of the filter was annotated. Each filter was then placed in a Petri dish and stored at \(-17\)\({}^{\circ}\)C until analysis. Mercury content of SPM was determined at the Laboratorio de Calidad Ambiental of the University of San Andres (La Paz, Bolivia). The filters that contained SPM were oven-dried at 40 \({}^{\circ}\)C for 24 h and weighted. Each filter was then introduced in a thermoresistant screw-capped 50 mL glass tube. 1 mL of nitric acid and 100 \(\upmu\)L of hydrochloride acid were added to the tubes, and these were sealed with Teflon and capped. All tubes were heated at 100 \({}^{\circ}\)C for two hours. After cooling, 200 \(\upmu\)L of hydrogen peroxide was added, and the tubes were again heated at 100 \({}^{\circ}\)C for two hours. 5 mL of ultrapure water was finally added. Mercury of the digested samples was quantified with Cold Vapor Atomic Fluorescence Spectrometer (CVAFS, Brooks Rand Model III, Seattle, WA, USA) using the EPA 1631 Revision E: (EPA 2002) protocol. Reference samples were obtained from the National Research Council of Canada: TORT-2 lobster hepatopancreas and MESS-3 Marine Sediment Reference Materials for Trace Metals (NRC-CNRC 2016). Total mercury concentration in SPM was expressed by water volume ([Hg]\({}_{v}\)) and SPM weight ([Hg]\({}_{w}\)). SPM from filters was additionally analyzed for particulate organic carbon (POC) and nitrogen in the Laboratorio de Medios Analiticos Cayenne- IRD in French Guyana with the CHNS THERMOQUEST AS2100 analyzer.