Nickel dynamics in the lakewater metal biomonitor Chaoborus

Abstract

Nickel (Ni) is a widespread contaminant present at toxic concentrations in aquatic systems in the vicinity of some mining and smelting operations. However, its accumulation by aquatic animals has been little studied and there are few biomonitors for this metal. Recently, larvae of the aquatic insect Chaoborus were shown to be effective as biomonitors for Ni concentrations in lakewater. Since animals are more effective as biomonitors when we understand how they take up their contaminants (from water or from food) and the rate at which they exchange contaminants with their surroundings, we set out to measure these parameters for Chaoborus. To achieve these goals, we exposed the components of a laboratory food chain (green alga, cladoceran, Chaoborus) to realistic Ni concentrations. We found that the majority (≈65%) of the Ni taken up by Chaoborus flavicans comes from lakewater, with the remainder coming from its planktonic prey ( Daphnia magna). This result is consistent with the low mean efficiency (14%) with which C. flavicans assimilated Ni from its prey. To explain the low efficiency of Ni uptake from food we measured the subcellular distribution of Ni in prey, which predicted that the majority of the Ni in prey (≈55%) was available for assimilation by the predator. This potential Ni uptake efficiency was only reached in animals that ingested few prey, likely because their gut passage time was longer than those ingesting many prey. We also measured Ni uptake and loss by C. flavicans exposed to Ni in water then used these data to parameterize a mechanistic bioaccumulation model that allowed us to describe Ni exchange between this insect and water. Lastly, we used these model constants, along with field measurements of Ni in 10 Canadian lakes, to predict Ni concentrations in field populations of Chaoborus. Model predictions overestimated Ni concentrations in field populations by a factor of 4. We suggest that uncertainties in the rate constant for Ni uptake from water and a lack of measured Ni concentrations in the prey eaten by Chaoborus larvae in the field could explain this difference.