Grazer-mediated regeneration of methylmercury, inorganic mercury, and other metals in freshwater.

Whereas it is well established that zooplankton can transfer various pollutants such as mercury (Hg) from primary producers to higher trophic levels, less is known on the effects of their activities on the recycling of Hg in aquatic ecosystems. Here, the impact of zooplankton grazing efficiency and excretion/egestion processes on metal concentrations in freshwater was investigated. Isotopically labeled algae (HgCl, MeHgCl) was used as a food source and the transfer of the selected isotopes to the culture medium during grazing was measured. In parallel, the potential for the recycling of major ions and metals, including selected essential and non-essential metals, was investigated for this could help to track the effects of sloppy feeding. To highlight the role of feeding behavior, a large filter-feeder (Daphnia magna) was compared to a smaller selective feeder zooplankton taxon (calanoid copepods), with the latter being expected to cause more sloppy feeding than the former. The experiments demonstrated that zooplankton grazing of both taxa significantly influenced the concentrations of the particulate portion of both inorganic Hg (IHg) and monomethylmercury (MeHg) in water. In contrast, only Daphnia significantly increased the concentration of dissolved IHg, whereas the concentration of dissolved MeHg was not affected by either grazer. The results also suggested that both taxa affected the concentrations of dissolved Fe, Zn, SO and rare earth elements via sloppy feeding, whereas only Daphnia significantly increased the concentration of dissolved Cu via this mechanism. The effects of excretion/egestion were negligible except for dissolved IHg and Cu in Daphnia treatment. These results highlight a neglected pathway of IHg and MeHg recycling in the water column in freshwater ecosystems with potentially important consequences for trophic transfer.