Evaluation of laboratory derived toxic effect concentrations of a mixture of metals by testing fresh water plankton communities in enclosures

The extrapolation of single-species laboratory studies to natural communities is one of the problems encountered when carrying out risk assessment of chemicals in the environment. This is especially applicable to extrapolation problems where mixtures of chemicals are involved. To investigate whether the laboratory-derived mixture toxicity approach is valid under field conditions, a mesocosm experiment was set up in which a laboratory strain of Daphnia magna was added to a natural, fresh water plankton community and exposed to three levels of a metal mixture. The mixtures, composed of arsenic, cadmium, chromium, copper, mercury, lead, nickel and zinc, were characterized by the sum of toxic units of dissolved metal concentrations (ΣTUdis), on the basis of EC50 values obtained in a laboratory study (Enserink et al., 1991). Only a small part of the metals (about one quarter) was present in dissolved form, the remainder was bound to particulate matter. The ΣTUdis remained constant throughout the experiment. On the basis of the population development (integrated density), an EC50 of 0.83 ΣTUdis was calculated for D. magna. All other dominant cladoceran species were more sensitive than D. magna, with EC50 values of 0.53 ΣTUdis for Ceriodaphnia spp. and 0.60 ΣTUdis for D. cucullata respectively. Copepods were less sensitive (0.98 ΣTUdis) and rotifers were about as sensitive as cladocerans. Significant bioconcentration factors could be calculated for arsenic, nickel, lead and mercury. Toxic effects on zooplankton resulted in elevated phytoplankton biomass due to reduced grazing. Only rotifer species were able to benefit from extra food availability, but could not prevent phytoplankton blooms from forming. It is concluded that, although the relative importance of the individual metals and the physico-chemical conditions in the natural plankton community differs from laboratory test conditions, the toxic effects on D. magna are about comparable, and that some dominant naturally present cladoceran species are more sensitive than D. magna.