UV-B-induced acute toxicity of pyrene to the waterflea Daphnia magna in natural freshwaters

The effects of various water characteristics in natural freshwaters on the acute toxicity of one polycyclic aromatic hydrocarbon (PAH), pyrene, to a p elagic invertebrate Daphnia magna was studied under ultraviolet B (UV-B) ra diation and in the dark. Pyrene was photoactivated and was more toxic to D. magna in the presence of UV-B radiation. Dissolved organic material (DOM), measured as dissolved organic carbon (DOC), significantly reduced the phot oenhanced toxicity of pyrene. Under UV-B radiation the EC50 values were low er and in relation to the amount of DOM, ranging from 3.0 to 30.0 mu g/L py rene, whereas in the dark they were between 29.2 and 54.8 mu g/L and not re lated to the amount of DOM in the waters. Although the condition and mortal ity of the daphnids in the control groups were not affected by UV-B irradia tion, the increased toxicity was considered to be either an additive or a s ynergistic effect of both the photomodified pyrene and the stressing light conditions of UV-B. The measured binding of pyrene to DOM was low, although it was related to the amount of DOG. Despite the relatively high intensity of UV-B used, humic substances in the waters remained undegraded. It was t hus concluded that with their brownish-yellowish color, waters rich in humi c substances decreased the photomodification of the freely dissolved parent compound simply by diminishing the light penetration in these waters and, by implication, contact with the intact compound. These results suggest tha t DOM in surface waters plays an important role in protecting against the p hotoinduced toxicity of PAHs. (C) 1999 Academic Press.