Refining and testing a trace metal biomonitor (Chaoborus) in highly acidic lakes

Biomonitor organisms can be used to estimate bioavailable contaminant concentrations provided that a reliable model exists between organism and environmental contaminant concentrations. We evaluate a recently proposed model designed to relate Cd concentrations in a proposed biomonitor, the insect Chaoborus, to free cadmium ion concentrations, [Cd2+], in lakes. In highly acidic metal-contaminated lakes, Cd concentrations in the biomonitor tend to be depressed despite the often high Cd2+ concentrations in these lakes. We test the hypothesis than one or several of the cations H+, Cu2+, Ni2+, and Zn2+, which also tend to be present in high concentrations in such lakes, compete with Cd2+ for biological uptake sites and thereby reduce Cd accumulation in the biomonitor. We show that to effectively relate Cd concentrations in the biomonitor to those in its environment, only competition from H+ needs to be taken into account, suggesting that H+ is the major competitor with Cd2+ at biological uptake sites and that the competitive effects of the other metal ions can be ignored. We also show that while it is best to treat the various species of Chaoborus separately, species can be pooled with only moderate loss of predictive power. In contrast to Cd, Chaoborus larvae could not be used to monitor the concentrations of either copper or zinc because the concentrations of these essential metals appear to be regulated by the larvae.