The true toxicity of combined pollutants in freshwater


Courtesy of European Commission, Environment DG

Aquatic organisms are exposed to many stressors, including a variety of pollutants from human activities. New research suggests that commonly used models in toxicological studies can fail to adequately predict the range of effects of complex mixtures of chemicals in aquatic environments. Stress caused by combinations of man-made pollutants found in water systems can compound the effects of natural stressors, such as low oxygen levels, on freshwater organisms. Currently, assessing the effects of multiple contaminants is based on the known actions of the individual compounds.

Two different concepts, concentration addition (CA) and independent action (IA), are used in aquatic toxicology to model the expected mixture toxicity of individual chemicals. CA assumes that the toxins act in the same manner on an organism and can interact with each other and IA assumes that each of the toxins in the mixture act differently on the organism and have no interaction with each other.

Researchers used experimental data to test the reliability of these models when assessing the impact of cadmium and carbendazim contaminants on water fleas under varying conditions of dissolved oxygen (DO) levels. Water fleas (Daphnia magna) are organisms found in freshwater systems around the world. They are commonly used to evaluate the impact of toxins in aquatic environmental studies.

Cadmium (Cd) is found in industrial waste and run-off from mines, although low levels of cadmium compounds also occur naturally. The fungicide, Carbendazim (CBZ), is used in agriculture and is known to disrupt hormone functions. Both contaminants are harmful to aquatic life.

Observations on the survival and feeding of water fleas were used to evaluate the lethal and sublethal exposures to the single and joint effects of Cd and CBZ. Experiments with lethal end-points are indicative of the effects of high dose, short-term (acute) exposure, while sublethal studies are used to evaluate long-term (chronic) exposure to lower doses. Chronic low dose exposure is found in many aquatic systems contaminated with manmade pollutants.

Overall, results comparing experimental data with the models showed that at acute exposures, the IA model adequately predicted joint toxicity of Cd and CBZ at low DO levels when synergism of the compounds was observed. However, both CA and IA models underestimated the joint toxic effects at sublethal levels of exposure to Cd and CBZ. Further research is needed to develop toxicological methods that better estimate the effects of combinations of pollutants.

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