Mercury accumulation in laboratory-reared Chironomus riparius and in indegenous chironomids assemblages

Mercury is a priority pollutant characterized by a strong tendency to bioaccumulate in living organisms and biomagnify along trophic chains. These features follow from the transformation of inorganic mercury into the corresponding methylated form, mainly via bacterially-mediated mechanisms controlled by several environmental variables. From a risk-assessment point of view, mercury therefore requires an approach based on establishing its bioaccumulative potential in resident biota rather than the typical toxicity-based approach commonly used for other trace elements. In the case of small organisms (e.g., freshwater benthic invertebrates), collecting an adequate amount of biomass of resident biota for measuring Hg accumulation can become quite problematic. Resident organisms may also be absent due to e.g., strong pollution or cross-sites comparison hindered by the impossibility of collecting the same organisms at all monitoring points. Bioaccumulation experiments with laboratory-reared organisms therefore become attractive provided char they can accurately simulate Hg accumulation in indigenous biota. We have exposed reared larvae of Chirononzux riparius to sediments collected at various locations from an aquatic system impacted by Hg pollution. Assemblages of indigenous chironomids have also been collected and are currently being analyzed to validate laboratory results. In the laboratory experiments, adults of chironomids emerged after exposure to field-collected sediments have also been analyzed for Hg content. This type of measurement represents another useful tool of a combined laboratory-field approach to estimate the Hg load exported from contaminated aquatic systems to the surrounding terrestrial environments.