Effects from log-yard stormwater runoff on the microalgae Scenedesmus subspicatus: Intra-storm magnitude and variability

This paper describes the effects posed by stormwater runoff from an industrial log-yard on the microalgae Scenedesmus subspicatus. The effects of stormwater runoff sampled during two rain events were determined by exposing S. subspicatus cells to different concentrations (% v:v) of each sample. The effects were measured as the percentage change in growth rates in relation to a control culture after exposure times of 24, 48, 72 and 96 h. The runoff from the first rain event had no negative effects to S. subspicatus, posing in most cases growth stimulation, whereas the runoff from the second rain event inhibited algae growth. Differences in runoff physico-chemical characteristics combined with the hydrological factors of each rain event explained these opposite effects. The hypothesis of toxic first flush phenomenon was confirmed in the second rain event on the basis of normalized inhibitory effects and runoff volume. It was found that 42, 51 and 50% of the inhibitory effects during exposures of 24, 48 and 72 h were associated with the initial 4% of the total discharged volume. The fact that negative effects were observed in the two runoff events analyzed, raises concern about the potential environmental threats posed by runoff originated from wood-based industrial areas during the entire hydrological year.