Effects of zooplankton grazing and daily temperature changes on mercury cycling in freshwater ecosystems

Fan Qin, Environmental sciences, University of Quebec in Trois-Rivieres, Trois-Rivieres, QC, Canada, Marc Amyot, Biological sciences, University of Montreal, Montreal, QC, Canada and Andrea Bertolo, Environmental sciences, University of Quebec in Trois-Rivieres, Trois Rivieres, QC, Canada

Background/Question/Methods:
Zooplankton can transfer various pollutants such as mercury (Hg) from primary producers to higher trophic levels. However, the efficiency of this trophic transfer could be reduced due to inefficient feeding (i.e. sloppy feeding) and/or inefficient assimilation (i.e. excretion/egestion). Unfortunately, the effects of these mechanisms on the release of pollutants are little known. Here, we experimentally tested the impact of zooplankton grazing on metal concentrations in freshwater. We focused on inorganic Hg and methylmercury (MeHg) concentrations in water by using isotopically labeled algae with stable Hg isotopes (200HgCl2, Me198HgCl) in a controlled experiment. We also explored the potential effects on other elements by analyzing main ions and rare-earth elements (REEs). To highlight the role of feeding behavior, we compared a filter-feeder (Daphnia manga) to a grasper zooplankton taxon (calanoid copepods).
Results/Conclusions:
Our results show that the presence of both grazers increased the concentrations of the particulate phase of both inorganic Hg and MeHg. In contrast, only Daphnia significantly increased the concentration of dissolved inorganic Hg. The concentration of dissolved MeHg was not affected by the grazer. Short-term (i.e. 30 min) effects of both zooplankton taxa have been observed for Fe, Cu, Zn, SO42- and REEs in the first sampling. However, these effects were transient except for Cu and Ce (a REEs), with higher concentrations in D. magna treatment than the control but decreased significantly in both zooplankton treatments, respectively, during the complete experiments.
Our study demonstrated the recycling of Hg between primary producer and consumer and highlighted a new source of inorganic Hg and MeHg in the water column for trophic transfer in freshwater ecosystems. Under the current global warming event, vertical migrate zooplankton may enhance their grazing activity in the water column, and consequently affect the distribution and speciation of Hg and MeHg along with other elements in aquatic ecosystems.