Assessment of the toxicity of ash-loaded runoff from a recently burnt eucalypt plantation

Although wildfires are identified as an important source of polycyclic aromatic hydrocarbons (PAHs) and PAHs are well-known for their pernicious properties, the toxicity of runoff from recently burnt areas has received little research attention. This knowledge gap was addressed here through laboratory assays in which four aquatic species from distinct trophic levels were exposed to different dilutions of ash-loaded runoff. The runoff was collected in a recently burnt eucalypt stand in north-central Portugal on two occasions, immediately after the wildfire and about 1 year later. The total PAH load was about four times higher at the first than second sampling occasion (1194 vs. 352 ng l(-1)) but even the latter value was considerably higher than those reported by prior studies on burnt areas. In addition, the two runoff samples differed noticeably in PAH composition, with a clear predominance of naphthalene in the second sample. Both runoff samples produced significant inhibitory effects on the three species representing the lower trophic levels, that is, the bacteria Vibrio fischeri, the algae Pseudokirchneriella subcapitata and the macrophyte Lemna minor. The invertebrate Daphnia magna was not significantly affected but chronic tests are needed to discard the probable propagation of toxic effects from the lower trophic levels. Surprisingly, the runoff collected 1 year after the wildfire was the most toxic to V. fischeri, P. subcapitata and L. minor. Possibly, this was due to predominance of naphthalene in this sample. Surely, however, this demonstrated that detrimental off-site effects of wildfires are not necessarily limited to the immediate post-fire situation.