Solar radiation‐induced toxicity of anthracene to Daphnia pulex

Photoinduced anthracene toxicity to Daphnia pulex was investigated. Organisms were exposed to three nominal anthracene concentrations (3.0, 9.6 and 30.0 μg L −1 ) in static bioassays on clear, partly cloudy and cloudy days. A “shell coating” technique was used to achieve concentrations within the aqueous solubility range of anthracene and to obviate the need for a carrier solvent. Photoinduced anthracene toxicity was not observed under laboratory lighting conditions; it occurred only in the presence of solar radiation. A dose‐response relationship existed for both anthracene concentration and solar radiation intensity. Anthracene was only slightly less toxic to organisms transferred into water containing no anthracene before exposure to solar radiation. This indicates that toxicity resulted from activation by solar radiation of material present on or within the animals and not in the water. Activation appeared to be of anthracene molecules and not anthracene degradation products, since similar concentrations of anthraquinone, the primary and most stable degradation product of anthracene, were not toxic at similar solar radiation intensities. Additionally, a series of filters was used to selectively remove UV wavelengths from solar radiation to determine the photoactive wavelengths. Mylar film absorbs in the UV‐B region (285 to 315 nm) of solar radiation and Corning 0‐52 glass absorbs essentially the entire spectrum of UV wavelengths (285 to 380 nm). Placement of Mylar film over bioassay beakers diminished photoinduced anthracene toxicity only slightly, whereas Corning 0‐52 glass reduced toxicity proportionate to the reduction in UV intensity. Thus, wavelengths in the UV‐A region (315 to 380) are primarily responsible for photoinduced anthracene toxicity.