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 comm unities is one of the problems encountered when carrying out risk asse ssment 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 w hich a laboratory strain of Daphnia magna was added to a natural, fres h water plankton community and exposed to three levels of a metal mixt ure. The mixtures, composed of arsenic, cadmium, chromium, copper, mer cury, lead, nickel and zinc, were characterized by the sum of toxic un its of dissolved metal concentrations (Sigma TUdis), on the basis of E C(50) values obtained in a laboratory study (Enserink et al., 1991). O nly a small part of the metals (about one quarter) was present in diss olved form, the remainder was bound to particulate matter. The Sigma T Udis remained constant throughout the experiment. On the basis of the population development (integrated density), an EC(50) of 0.83 Sigma T Udis was calculated for D. magna. All other dominant cladoceran specie s were more sensitive than D. magna, with EC(50) values of 0.53 Sigma TUdis for Ceriodaphnia spp. and 0.60 Sigma TUdis for D. cucullata resp ectively. Copepods were less sensitive (0.98 Sigma TUdis) and rotifers were about as sensitive as cladocerans. Significant bioconcentration factors could be calculated for arsenic, nickel, lead and mercury. Tox ic effects on zooplankton resulted in elevated phytoplankton biomass d ue to reduced grazing. Only rotifer species were able to benefit from extra food availability, but could not prevent phytoplankton blooms fr om forming. It is concluded that, although the relative importance of the individual metals and the physico-chemical conditions in the natur al plankton community differs from laboratory test conditions, the tox ic effects on D. magna are about comparable, and that some dominant na turally present cladoceran species are more sensitive than D. magna. C opyright (C) 1996 Published by Elsevier Science Ltd.