Mixture and single-compound toxicity using Daphnia magna : comparisons with estimates of concentration addition and independent action

Sammanfattning: Aquatic organisms are usually not exposed to single substances but rather to mixtures of toxicants in streams located in agricultural areas. The transport of pesticides used in the agricultural area in Sweden is monitored with continuous environmental supervision every year. During 2002-2008 an average of 10 pesticides were found in each sample and 40% of the samples contained substances with concentrations higher than the Water Quality Standards. Chemical risk management is normally based on single-test evaluations. A problem when assessing mixture toxicities is that the constituents and concentrations of pollutants in the environment vary greatly. Therefore experimental testing of all possible combinations of constituents in a mixture in the environment is not possible. Models such as concentration addition and independent action have been developed that allow an estimation of the toxicity of mixtures on the basis of the toxicity of the single compounds. In most cases, these models give quite accurate estimations of the toxicity of mixtures. This study was aimed at testing three insecticides (pirimicarb, fenitrothion and esfenvalerate) neurotoxic to Daphnia magna in order to evaluate if synergistic, antagonistic or strictly additive effects occur when added together in a mixture. The study also aimed at investigating if the mixed exposure toxicity can be predicted with any of the concentration addition and independent action models. The selected insecticides have been used frequently in Swedish agriculture and have been found above the Water Quality Standards in Swedish surface waters for several years during environmental monitoring. The highest concentration found in surface waters for esfenvalerate exceeded the NOEC and therefore negative effects on Daphnias can be expected. The highest concentration found in surface waters for all three insecticides were higher than the Water Quality Standards values. Therefore all three insecticides have the potential to be toxic to aquatic life with the concentrations found in agricultural streams during surveys. EC50 values obtained from the mixed exposure tests were lower than the EC50 values obtained from the single exposure tests for all three insecticides. Esfenvalerate showed the highest increase in toxicity, 80% in EC50 value, pirimicarb 50% and fenitrothion 45%. Independent action predicted the toxicity accurately at EC50 but the concentration addition model is the preferable model to work with as it generally predicts a higher toxicity than independent action, and therefore gives a “worse case scenario”. There is a need for further studies in order to see how these three insecticides interact with each other to see if the combination shows synergism, antagonism or additivity. Studies have found that concentrations that cause biological changes in Daphnia and other cladocerans are significantly lower than lethal concentrations. Therefore there is a risk that concentrations found in the environment can lead to changes in the entire ecosystem.