## 2 Materials and Methods
### Test with Chironomus riparius  
_Chironomus riparius_ Meigen 1804 (Diptera, Chironomidae) adults were initially obtained from a culture held at the laboratory ECT Oekotoxidologie GmbH., Florsheim Main (Germany). Chironomids were reared in 40 L aquaria containing a 4 cm layer of sand (<250 \(\upmu\)m) and 8-10 cm of aerated tap water. The culture was kept at 20 \(\pm\) 2 \({}^{\circ}\)C under a daily photoperiod (16:8 h light:dark; 800 lux). The overlying water was renewed every other day and organisms were fed weekly with Tetramin(r) fish food (Tetrawerke, Melle, Germany) at 1 mg per organism. Bioaccumulation tests were carried out following the main indications of the Organisation for Economic Co-operation and Development (OECD) guidelines 218 and 219 [33; 34] and lasted 10 days.  
Sediments samples were frozen at \(-\)20 \({}^{\circ}\)C to eliminate indigenous macrofauna and transferred at 4 \({}^{\circ}\)C for one week before the start of the bioaccumulation experiment. Three days before the addition of the animals, 250 mL glass beakers were filled with 50 mL of wet weight sediment (combed through 2 mm sieve to eliminate debris and coarse material that could have perturbed the well-being of test organisms) and 200 mL of filtered (1 \(\upmu\)m), aerated Lake Maggiore water gently added to the sediment. About 15 L of Lake Maggiore water were collected in the middle of the basin in the pelagic area of the lake on a single day during the sampling period, filtered directly on site and stored at 4 \({}^{\circ}\)C in the dark until use. The typical composition of Lake Maggiore water in the topmost 25 m of the water column in March 2010 (regular monitoring of CNR-IRSA, Verbania-Pallanza offices) was as follows: Ca = 22.6 mg L\({}^{-1}\), Mg = 3.68 mg L\({}^{-1}\), Na = 3.06 mg L\({}^{-1}\), K = 1.51 mg L\({}^{-1}\), Cl = 2.96 mg L\({}^{-1}\), SO\({}_{4}\) = 29.2 mg L\({}^{-1}\), N-NO\({}_{3}\) = 855 \(\upmu\)g L\({}^{-1}\), total organic carbon = 0.7 mg L\({}^{-1}\), hardness = 72 mg L\({}^{-1}\) as CaCO\({}_{3}\), conductivity = 150 \(\upmu\)s cm\({}^{-1}\), alkalinity = 0.829 meg L\({}^{-1}\), pH = 7.53. To allow sediments and water to equilibrate, the beakers were covered with a plastic Petri dish and kept in the dark at 20 \(\pm\) 1 \({}^{\circ}\)C and without aeration for three days (Figure S1).  
Five replicate beakers were prepared for each site along with a laboratory control consisting of artificial clean sediment prepared according to OECD 218/219 [33; 34]. At the start of the test, the overlying water of each beaker was gently aerated for 2 h and 15 first instar larvae chosen at random were transferred to all replicate and control beakers. Tests were performed under artificial light (800 lux) with a 16:8 h light:dark photoperiod. The animals were fed three times a week with a Tetramin(r) suspension at 0.4 mg per organisms for the first 10 days of exposure and at 0.7 mg per organisms for the rest of the test. The water lost through evaporation was replaced with spring water. Temperature, pH, conductivity and dissolved oxygen were measured in all the beakers at the beginning and at the end of the tests. Organisms in two replicate beakers were manually recovered after 10 days, weighted, kept in lake water without feeding for 8 h to purge their guts, pooled and freeze-dried for Hg analysis. In the other three replicates, covered with nets, exposure continued until 25-28 days to collect emerging adults for Hg analysis. Each replicate was gently aerated with a flow-rate of 1-2 bubbles per second.