## 3 Results
### Predicting Hg Bioaccumulation in Indigenous Organisms from Laboratory Bioaccumulation Experiments
Sediment freezing to kill the indigenous fauna before laboratory tests may have modified Hg bioavailability before exposure of laboratory organisms and should be verified as a possible confounding factor. Furthermore, changes in the availability of sediment-bound elements between field and laboratory conditions have been documented [15] and may be valid in the case of mercury. These factors probably contributed also to the absence of correlation in Hg bioaccumulation by indigenous oligochaetes and _L. variegatus_. Calculation of BSAF takes into account changes in Hg sedimentary content and suggests that, except for site BAV-SL, the bioavailability of sediment-bound Hg to chironomids was higher in laboratory conditions than in the field, while the situation was more variable for oligochaetes (Table 2). In the case of accumulation by _L. variegatus_, some replicate measurements were highly variable (Table S1) and could affect BSAF estimation based on the average concentrations (Table 2). The actual species composition of indigenous assemblages (Table S5) is another factor probably contributing to differences in Hg bioaccumulation between indigenous and laboratory organisms. In general, oligochaetes represented from 52% to 94% of the annual total absolute abundance at all the surveyed stations. Diptera chironomids were the second most abundant (1%-43% of the entire assemblage) and diverse taxon, followed by bivalves (1%-5%). The remaining taxa groups represented an insignificant part of the benthic community (0.2%-1.2%) and can be considered rare (Table S5). In detail, PAL-SL has the highest biodiversity in term of number of taxa found (35 taxa compared to  
Figure 3: Scatterplot of measured vs. estimated Hg concentration values for ‘Indigenous chironomids’ and ‘Field sediments’. Estimated values were obtained using regressions 1 and 2 (see text) for chironomids and sediments, respectively. The dashed lines indicate the ideal 1:1 correspondence. the other stations ranging between 14 and 29 taxa) and also the highest number of dettritivorous taxa (18 taxa compared to other stations ranging between 7 and 16 taxa). However, BAV-SL showed the highest density for dettritivorous chironoids (and the second highest for oligochaetes--Figure 4), while BAV-P had the highest density of dettritivorous oligochaetes (Figure 4). These high densities suggest the presence of nutrient enrichment at the two stations. The feeding habits of indigenous oligochaetes and chironomids as a whole (Figure 4) are not necessarily those of _C. triparius_ and _L. variegatus_, which could result in different Hg accumulation patterns for indigenous and laboratory organisms. While indigenous dettritivorous oligochaetes were always the predominant group at all the surveyed stations, dettritivorous chironomids (specifically, _C. anthracrius_ gr.) were the most abundant feeding group only at BAV-SL (Figure 4). _Chironomus anthracinus_ ingests particulate matter freshly deposited at the sediment surface [55]. Mercury bioaccumulation in this species likely reflects Hg content at the sediment-water interface with a degree of vertical selectivity that cannot be captured in laboratory tests with homogenised sediment aliquots. Conversely, the scraper _Prodiamesa olivacea_ and the gatherer _Pagastiella orophila_ were the most abundant species at PAL-SL (Table S5). The latter species has been shown to feed selectively on pennate diatoms [56], which could be an additional source of Hg not available during laboratory experiments. Furthermore, individuals of _C. triparius_ analysed for Hg at the end of laboratory exposure were all in the fourth larval instar, while indigenous chironomids from the various taxa most likely were at different stage of development. The importance of feeding habits in interpreting differences in Hg bioaccumulation between indigenous oligochaetes and _L. variegatus_ is less clear considering that dettritivorous oligochaetes predominated at all sites (Figure 4 and Table S5).