## 1 Introduction  
Mercury is a persistent environmental contaminant that can severely threaten human and environmental health. Because of its peculiar environmental chemistry, characterized by the presence of a gaseous species (Hg(0)) and of organic complexes (mainly monomethylmercury--MMHg), mercury is ubiquitous and tends to bioaccumulate and biomagnify strongly in food webs [1]. Although Hg is a neurotoxin and one of the most (eco)toxic trace elements, its chronic toxicity is linked essentially to dietary uptake [2]. Protection of top predators from secondary poisoning via ingestion of contaminated preys must, therefore, be the primary focus of Hg risk management. These specific management needs are reflected, _inter alia_, in the current European regulation that establishes an Environmental Quality Standard for Hg based on concentration in prey fish [3]. However, as much as 65% of the diet of non-piscivorous fish can come, directly or indirectly, from benthic sources with sediment-dwelling organisms representing a crucial link between primary producers, detrital depositsand top predators [4; 5]. Indeed, mercury concentrations at the base of the food web, rather than variation in biomagnification rates, seem to be the main factor controlling Hg transfer in food webs [6; 7]. Considering that sediments represent a major repository for many persistent chemicals introduced into aquatic systems, it is important to understand to what extent sediment-bound Hg is potentially available for remobilization via benthic organisms [8]. Indeed, from a risk-assessment point of view, mercury 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 [9; 10]. Data from low trophic levels (i.e., plankton and benthic invertebrates) are necessary to estimate the trophic magnification factors (TMF) for a given food web to fulfil e.g., European regulatory requirements for bioaccumulative chemicals [11]. In the case of small organisms such as freshwater benthic invertebrates, collecting an adequate amount of biomass of resident biota for measuring Hg accumulation can be quite a time- and resource-intensive exercise. Resident organisms may be absent due to e.g., seasonal reproductive cycles or in certain situations of strong contamination, while cross-sites comparisons may be hindered by the impossibility of collecting the same organisms at all monitoring points. Bioaccumulation experiments with laboratory-reared organisms, therefore, become attractive to assess the bioaccumulative potential of different chemicals, especially in medium- to large-sized monitoring programs. However, contaminant bioaccumulation even in taxonomically close species can vary because of different ecological traits [12; 13; 14] and simple sediment sampling operations can increase the availability of several trace elements in laboratory settings [15]. The use of in situ bioaccumulation tests [16; 17] would prevent problems related to sampling and sample handling, but performing in situ exposures in deep waters is subject to the availability of divers and becomes unfeasible for depths above 20 m. Following all these considerations, it is clearly necessary to verify that laboratory tests using individual model species can predict Hg accumulation by indigenous biota with an acceptable degree of accuracy. In the present study, laboratory-reared specimens of the model benthic organisms _Chironomus riparius_ (first-instar larvae) and specimens of _Lumbriculus variegatus_ were exposed to sediments collected at various sites in Lake Maggiore (Northwestern Italy), a holo-oligomic lake still showing signs of legacy Hg pollution from a chlor-alkali plant eventually dismissed in 1998. Assemblages of indigenous chironomids and oligochaetes were also collected and taxonomically identified to gather information about their spatial distribution, and to compare their different feeding strategies. Furthermore, adults of _C.