The deep,ultraoligotrophic piedmont lakes of Northern Patagonia(Argentina)are located in pristine and barely impacted areas,along a wide latitudinal range.Several studies have reported moderate to high total mercury(Hg)concentrations and contrasting methylmercury(CH3Hg+)production in different lake compartments.Sources of Hg for western Patagonian terrestrial and aquatic ecosystems are still not clear;while point sources can be ruled out and atmospheric deposition is a plausible source,along with contribution from active volcanic areas of the Andes.In this investigation,we reported a noticeably seasonal,spatial(between lake branches),and vertical(between water column strata)heterogeneity in the total Hg concentrations found in the pelagic zone of Lake Moreno.Sterile water samples taken in a depth profile of the lake showed moderate to high concentrations of CH3Hg+ in autumn with a decreasing trend with depth.Our results indicated that Hg is largely allocated in the plankton fraction between 10-53 μm;which dominated within the euphoric(epilimnetic)zone of the lake due to the high densities attained by two species of the dinoflagellate Gynmodinium.The 53-200 μm planktonic size fraction(comprising rotifers,ciliates and immature stages of crustaceans)and the > 200 μm fraction(calanoid copepods and cladocerans)were found to bear strikingly lower total Hg concentrations,suggesting that the magnification of Hg at the planktonic consumer level is negligible.