## 3 Results
### Predicting Hg Bioaccumulation in Indigenous Organisms from Laboratory Bioaccumulation Experiments  
Mercury concentrations in indigenous and laboratory organisms showed a good correlation in the case of chironomids (Spearman rho = 0.88, \(p\) = 0.033, n = 6), but no correlation in the case of oligochaetes (Spearman rho = 0.43, \(p\) = 0.35, n = 7). The resulting robust linear regression to predict Hg levels in indigenous chironomids based on results from standardized tests with _C. riparius_ was:  
\[[\text{Hg}]_{\text{ChInd}}=(0.566\pm 0.047)\cdot[\text{Hg}]_{\text{ChLab}}+(0.045 \pm 0.049) \tag{1}\]  
where \([\text{Hg}]_{\text{ChInd}}\) is the Hg concentrations estimated in indigenous chironomids using regression (1), \([\text{Hg}]_{\text{ChLab}}\) is the accumulated mercury concentrations measured in _C. riparius_ after 10-days exposure to field sediments in the laboratory, and all values are in mg kg\({}^{-1}\) d.w. According to the regression's slope, laboratory chironomids seemed to accumulate about twice as much Hg as indigenous organisms. It is noteworthy that Hg levels in 'Lab\({}_{\text{Ch}}\) sediments' (see Table 2) were also correlated with those in 'Field' sediments (Spearman rho = 0.94, \(p\) = 0.017, n = 6) and that the slope of the corresponding linear regression was:  
\[[\text{Hg}]_{\text{SegmentField}}=(0.493\pm 0.002)\cdot[\text{Hg}]_{\text{SegmentChLab} }+(0.062\pm 0.007) \tag{2}\]  
where \([\text{Hg}]_{\text{SegmentField}}\) is the mercury concentration estimated in field sediments from where indigenous organisms were recovered for Hg analysis using regression (2), \([\text{Hg}]_{\text{SegmentChLab}}\) is the Hg concentration measured in sediments used to expose _C. riparius_ in the laboratory (Table 2), and all values are in mg kg\({}^{-1}\) d.w. Regression (2) shows that Hg concentrations in sediments used for laboratory tests were, on average, twice those measured in sediment aliquots used to isolate indigenous organisms. With two exceptions (see below), differences between measured and predicted Hg concentrations were less than 40% for indigenous chironomids and less than 10% for the 'Field' sediments (Figure 3). Regressions (1) and (2), therefore, suggest that the doubling of Hg accumulation in laboratory sediments used during the bioaccumulation tests with _C. riparius_ was responsible for the doubling of Hg accumulation in laboratory organisms compared with indigenous chironomid assemblages. Under such hypothesis, food addition during the laboratory tests with _C. riparius_ would be of minor importance in determining the measured Hg body burdens. The possible factors causing the observed differences in Hg concentrations among sediment aliquots have been discussed in Sections 3.1 and 3.2. On the other hand, regression (1) overestimated Hg levels in indigenous chironomids from site PAL-SL by about 3-fold and regression (2) underestimated Hg concentration in 'Field' sediments from site PAL-01 by 2.5-fold. With respect to these outliers, a limitation of the present study is that the variability of Hg accumulation in indigenous organisms was not determined because only one composite sample per site could be obtained to reach useful quantities of organisms to carry out the analyses despite the significant sampling effort. Similar considerations apply to laboratory bioaccumulation tests with _C. riparius_, in which organisms from replicate exposures had to be pooled before analysis due to their limited mass. On the other hand, laboratory experiments with _L. variegatus_ allowed for replicated analysis (Table S1) and showed a low inter-replicate variability (less than 30%) in most albeit not all cases. Assuming a 30% variability for replicate bioaccumulation measurements, the differences in Hg accumulation between indigenous chironomids and laboratory-exposed _C. riparius_ for site PAL-SL (Figure 3) would not be solely controlled by variable Hg levels in different sediment aliquots.