## 3 Results
### Bioaccumulation in Laboratory-Reared Organisms
This amount of food contained an absolute quantity of 6.12 ng Hg. Assuming that all the added food and associated Hg would have been ingested and assimilated by chironomids, one would expect an Hg concentration of about 0.402 mg kg\({}^{-1}\) in control organisms. Complete ingestion and assimilation are, however, unlikely and these simple calculations do not take into account the dynamic nature of contaminant accumulation in invertebrates [48; 49]. Overall, the addition of Tetramin(r) could explain the Hg accumulation observed in control chironomids, considering that _C. riparius_ has been shown to feed preferentially on added food during laboratory tests [50]. However, it remains an open question whether Hg levels in _C. riparius_ exposed to field sediments were determined solely by the addition of Tetramin(r). Organisms exposed to field-collected sediment likely had other carbon sources available besides Tetramin(r) (see the OC levels in Table 1) and larvae exposed to some field-collected sediments eventually accumulated less Hg than control larvae. Indeed, the addition of food can alter contaminant bioavailability in different ways depending on, for example, the contaminant getting adsorbed on the added food, which increases contaminant bioavailability, or not [50]. Except in control sediments, Hg concentration in Tetramin(r) was lower than Hg concentration in sediments (Table 2) implying that Hg bioaccumulation caused by sediment ingestion could have been partially diluted by the ingestion of cleaner Tetramin(r) particles. It is also important to remember that specimen of _L. variegatus_ exposed to lake sediments were not fed during the bioaccumulation tests. In this case, the accumulated Hg therefore reflect uptake from the element pool present in the sediment themselves. The differences in food addition between the two tests considered the provisions made by the corresponding norms. The OECD norms for _C. riparius_[33; 34] were developed for ecotoxicity testing of artificial spiked sediments in contrast to the USEPA, OECD and American Society for Testing and Materials (ASTM) norms for _L. variegatus_[35; 36; 37] that specifically focus on bioaccumulation. These differences may influence the comparison between the two organisms with regard to accumulated Hg in laboratory settings. On the other hand, these differences do not impact the main purpose of the present work, that consists in comparing Hg accumulation in indigenous vs. laboratory reared organisms. In theory, a preferential Hg accumulation from food in laboratory-reared _C. riparius_ should bring about a fixed extra-Hg accumulation compared with indigenous organisms. As explained in the previous paragraph, Hg accumulation from added food does not seem to have been the sole source of Hg during the tests with _C. riparius_. Further arguments in support of a moderate effect of added food on Hg accumulation are presented in Section 3.4. After 10 days of exposure, larvae of _C. riparius_ exhibited mortality >20% only for CARM-SL site. Similarly, larvae grown in CARM-SL sediments had the lowest average fresh weight (2.1 vs. 5.1 mg ind\({}^{-1}\) for control larvae). Average weight at the other sites ranged from 3.3 to 4.2 mg ind\({}^{-1}\). Apprecating sediment ecotoxicity goes beyond the scope of the present paper, but two aspects deserve a brief mention in this respect. The actual role of accumulated Hg in eliciting the observed biological effects may be assessed by comparing measured Hg concentrations with the corresponding critical body residue (CBR) for Hg or MMHg. [PERSON] et al. [51] determined average CBRs for MMHg of 10.5 mg kg\({}^{-1}\) wet tissue (w.w.) for lethality and 0.57 mg kg\({}^{-1}\) w.w. for reproductive effects in fish and invertebrates. [PERSON] et al. [52] estimated CBR of 0.046 mg kg\({}^{-1}\) w.w. as the no observed effect concentration (NOEC) for growth of hexagenid mayflies larvae. Assuming an average moisture content of 79% for chironomids [11], accumulation of total Hg in our pools of _C.