Seasonal temperature declines do not decrease periphytic surfactant biodegradation or increase algal species sensitivity

The effects of seasonally decreasing river water temperature on surfactant biodegradation and algal sensitivity are reviewed from four stream mesocosm studies conducted over a 5-year period. Seasonal temperatures ranged from 28 to 0°C over all studies and temperature declines were approximately 9 to 14°C over the course of each individual study. Mesocosm periphyton were naturally colonized on tile substrata with in-flowing river water for a period of 3 to 8 weeks prior to the initiation of sampling. Streams were dosed for 8 to 11 weeks with μg/L (ppb) quantities of the surfactants C12-alkyl sulfate (C12-AS), C45E2.17S-alkyl ethoxysulfate (AES), C25E6-alkyl ethoxylate (AE) or 0 to 13% final effluent during the sampling period. Mineralization of C12-AS and AE by periphyton in the dosed streams generally increased over the dosing period while mineralization remained approximately constant in the control streams. The results from the AE study occurred with an increase in periphyton heterotrophic respiration. Mineralization of AES increased over the dosing period in streams receiving the highest dose of AES and remained constant in streams receiving lower doses. All studies involving surfactant exposure demonstrated a positive correlation between surfactant concentration and mineralization during periods of seasonal temperature decline. Mineralization of AE by periphyton dosed with final effluent increased slightly over the testing period. Periphytic algal taxonomy and biovolume were evaluated during the AES study. Overall, these tests showed no increases in species sensitivity over the testing period. Taken collectively, these results indicate that there is no correlation between naturally decreasing seasonal temperatures and lower rates of surfactant mineralization or increased species sensitivity by naturally acclimated periphyton.