## 3 Results and Discussion
Such was also the case for samples from Blanco and San Martin rivers. The percentage of particulate organic carbon (POC) in SPM in all samples from the Itenez basin varied from 6 to 26% in June (Tables S3 and S4). Samples from Itenez lakes tended to contain higher POC percentages than those from the Itenez river. POC decreased significantly (\(p\) = 0.02) in September in Itenez River, but no significant variation was observed for its lakes. Lower percentages of organic carbon associated with suspended matter can be due to lower organic matter inputs to water bodies in dry months and carbon turnover. Although particulate organic carbon is less bioavailable than dissolved carbon, some carbon turnover takes place [20]. In June, the C:N ratio for particulate matter from the Itenez river, lakes, and tributaries showed values between 9.9 and 14.9. In September, the C:N ratio values were lower, fluctuating between 7.4 and 11.2 for all samples (Tables S3 and S4). This decrease was significant for both the Itenez river (\(p\) = 0.004) and its lakes (\(p\) = 0.009), Figure 3C, and it can be related to preferential sorption of nitrogen compounds, as observed by [PERSON] et al. [21]. For both June and September, and for both habitats (lakes and rivers), the obtained C:N values are characteristic of suspended matter originated from soil [17; 22]. This finding implies that the particulate organic carbon and the particulate mercury are mostly terrigenous in our study area. Terrigenous POC has a lower tendency to drive mercury methylation than autochthonous material [23], suggesting relatively low bioavailability of mercury. A significant negative correlation was observed between SPM and POC for June (\(\rho\) = \(-\)0.739, \(p\) \(<\) 0.001) and September ((\(\rho\) = \(-\)0.739, \(p\) \(<\) 0.001). Likewise, SPM and [Hg]w were negatively correlated in June (\(\rho\) = \(-\)0.640, \(p\) \(<\) 0.001) and September (\(\rho\) = \(-\)0.709, \(p\) \(<\) 0.001, see also Figure 4A,B). [PERSON] et al. [24] have also observed a negative correlation between SPM and [Hg]w for samples from the Negro, Solimoes, Amazon, and Madeira rivers. This \"dilution effect\" can be due to several processes. Higher loads of SPM generally imply that water also contains higher concentrations of colloids, which are not retained by filters but could absorb part of the available mercury [25]. [PERSON] et al. [6] indicate that there can be a \"growth dilution\" of particulate mercury caused by an increase in biomass attached to particles. On the other hand, for sampling points where strong resuspension of sediments was observed, POC's values and [Hg]w were much lower than in the other points. Sediments generally contain less organic carbon and mercury than suspended matter in the water column [3; 23]. This difference can be related to high heterotrophic activity in sediments at the benthic and hyperhoic zone [26]. High heterotrophic activity causes carbon turnover and can also cause mercury transformations, which leads to its separation from particles. A combination of the \"dilution effect\" and sediment resuspension could explain the behavior observed in Figure 4A,B. Figure 3: Characteristics of suspended particulate matter including the concentration of mercury by weight ([Hg]w) (**A**), organic carbon (**B**), and C/N ratio (**C**) for the Itenez River and associated lakes in June (early dry season) and September (late dry season). Particulate organic carbon was positively correlated with particulate mercury (see Figure 4C) both in June (\(\rho\) = 0.595, \(p\) = 0.004) and September (\(\rho\) = 0.595, \(p\) = 0.004). Such a positive relationship was also reported by other authors [23; 24], although in some other cases, no significant correlation was found [15; 17]. The nature of the samples of the particulate matter probably influences this correlation. [PERSON] et al. [17] found no correlation between [Hg]\({}_{\rm w}\) and the POC percentage in fine particulate matter.