## 3 Results
### Bioaccumulation in Laboratory-Reared Organisms  
In tests with _C. riparius_, exposure temperatures, pH values and oxygen and ammonia concentrations remained within acceptable values and within 10% of the initial values for all exposure conditions. A similar situation was observed for tests with _L. variegatus_ with the exception of N-NH\({}_{4}\) which tended to decrease over time (data not shown). As specified in Section 2.4, overlying water in the tests with _L. variegatus_ was renewed as needed when N-NH\({}_{4}\) levels became higher that 1 mg L\({}^{-1}\). The observed differences in Hg bioaccumulation should therefore reflect variations in Hg bioavailability from sediments collected at the various locations independently of uncontrolled deviations in master hydrochemical parameters in certain exposure beakers. After 10 day of exposure, chironomids survival in control organism was 100% and above 80% in all sediment except CARM-SL (66%). Emergence at day 28 was 100% for control organisms and above 70% for all sediments. The absence of mortality in controls guarantees good test conditions and the survival of organisms exposed to field sediments was high enough to recover sufficient material for Hg analyses. Direct comparison of accumulated Hg concentrations in _C. riparius_ and _L. variegatus_ was possible for sites PAL-SL, PAL-P, BAV-SL, BAV-P and CARM-SL. Median Hg levels were similar for the two organisms (0.249 vs. 0.220 mg kg\({}^{-1}\), n = 5). On the other hand, no correlation existed between Hg accumulation in _C. riparius_ versus _L. variegatus_ (Spearman rho = 0.68; \(p>\) 0.5, n = 5) considering only sediments that were tested with both organisms) or with the corresponding Hg concentrations measured in test sediments (\(p\) = 0.058 for tests with _C. riparius_; \(p\) = 0.5 for tests with _L. variegatus_). In laboratory settings, sediments were homogenized before use and organisms' ecology is expected to be less important in determining bioaccumulation differences than under real field conditions. The relative standard deviation among replicate Hg bioaccumulation analyses in the experiments with _L. variegatus_ (n = 2-9; see Table S1) were within 15% except for PAL-SL (30%) and BAV-SL (80%). For both samples, only two replicate measurements could be performed due to availability of sediments and the variability among measurements was calculated using the formula:  
[Absolute value (Measurement 1 - Measurement 2)]/square root of 2  
The formation of two distinct sediment layers, a light brown upper layer about 0.5 cm thick covering a thicker layer of black sediment, was observed in the aquaria used for _L. variegatus_ exposures. Analyses of Hg concentration in the two layers of selected sediment samples usually agreed to within 10% (Table S3). Mercury content in control organisms was between 0.2 and 0.28 mg kg\({}^{-1}\); comparable to levels measured in specimen exposed to lake sediments excluding the most contaminated sediment samples (i.e., PAL-SL and, for _C. riparius_ only, PAL-01). The use of filtered Lake Maggiore water (tests with _C. riparius_) vs. synthetic water (tests with _L. variegatus_) does not seem to have introduced any systematic bias in Hg accumulation during laboratory tests. An important contribution of the aqueous phase to Hg accumulation should have resulted in a systematically higher Hg burden of _C. riparius_ compared with _L. variegatus_; which was not observed (Table 2). Although the Hg content in filtered Lake Maggiorewaters was not determined, this observation suggests that most of the Hg burden in test organisms was linked to dietary uptake. The Tetramin(r) food used to feed _C. riparius_ and control _L. variegatus_ during the tests contained \(0.051\pm 0.001\) mg kg\({}^{-1}\) d.w. Hg (n = 3). In the case of chironomids, 0.4 g Tetramin(r) per larva per day were administered over the 10-day test [33; 34]. The pooled control sample that was analysed for Hg consisted of 30 specimens which, taken collectively, were fed 120 mg Tetramin(r).