Mercury and methylmercury uptake and trophic transfer from marine diatoms to copepods and field collected zooplankton.

Mercury (Hg) and methylmercury (CH3Hg) are found at trace levels in most marine waters. These species, particularly CH3Hg, then ominously bioaccumulate through marine food chains eventually reaching potentially harmful levels in top oceanic wildlife. Accordingly, it is important to measure and evaluate uptake at environmentally relevant concentrations where trophic transfer initiates; during uptake in primary producers, and consumption by plankton grazers. Experiments using cultured copepods (Acartia tonsa) and field zooplankton assemblages were performed with two different sized diatom species labeled with stable isotopes of inorganic Hg (200Hg) and CH3Hg (CH3199Hg) at different concentrations. We observed size-specific effects on algal uptake and transfer to copepods, in addition to effects of Hg species concentration. Prey size effects were likewise observed on copepod assimilation efficiencies (AE). Average AE of 200Hg for copepods feeding on smaller diatoms was 50%, and 39% for larger diatoms. The AEs were much greater for CH3199Hg, yielding 71% for the smaller and 88% for the larger diatoms. These experiments add evidence demonstrating a significant relationship between Hg and CH3Hg exposure concentration and subsequent algal uptake and transfer to zooplankton. Furthermore, results imply that facilitated uptake of CH3Hg into algae occurs at low (~pM) concentrations, which has been suggested but not confirmed in previous research.