Can flooded organic matter from sediments predict mercury concentrations in zooplankton of a perturbed lake?

Mercury (Hg) and the composition of organic matter (OM) in sediments and large zooplankton (>190 μm) have been analysed in a perturbed lake (Lake 154) in northern Québec, Canada. We investigated whether methylmercury concentrations ([MeHg]) in flooded sediments can predict [MeHg] in zooplankton. [MeHg] at the uppermost sediment layers located immediately downstream of an inundated system (LA-40) are 10 times higher than at other sediment stations. Zooplankton organisms sampled downstream of LA-40 contain higher [MeHg] (183±50 ng g−1 dry wt.) than those of neighbouring natural lakes (66±37 ng g−1 dry wt.). OM in sediment and zooplankton samples has been characterised applying atomic carbon/nitrogen ratios (C/N) and stable carbon isotopic ratios (δ13C). While C/N ratios in zooplankton from Lake 154 (5.6±0.4) do not differ from samples of natural lakes (5.7±0.1), δ13C signatures are more depleted at stations close to the discharge of LA-40 (−33±0.4‰) with respect to zooplankton from unperturbed lakes (−30.6±0.7‰). Results of this tracer study suggest that the biochemical composition of OM in flooded sediments did not modify the biochemical composition of zooplankton and can therefore not predict [MeHg] in zooplankton.