Stream mesocosm experiments were conducted with the anionic surfactants alkyl sulfate (AS) and alkyl ethoxylate sulfate (AES) to determine the ecotoxicological effect thresholds on stream communities. Autotrophic and heterotrophic periphyton, protozoan, and invertebrate population and community effect assessments were conducted over an 8-week exposure period in the summer to fall seasons following a 2- to 3-month colonization period. Concentrations spanned environmentally low to unrealistically high concentrations to challenge the sensitivity of the test system and to understand the potential array of effects. In both experiments mayfly taxa and clam populations were significantly impaired at high concentrations (582-1585 mu g surfactant/l). Indirect effects, associated with increased heterotrophic periphyton biomass in surfactant-treated streams at 224-1586 mu g surfactant/l, supported increased densities of oligochaetes and gastropods. Mayfly habitat may have become sub-optimal during the AS experiment as a result of the heterotrophic biofilm at concentrations above 224 mu g/l. These experiments resulted in ecotoxicological no-observed-effect-concentrations (NOECs) of 224 mu g AS/I and 251 mu g AES/I. Structure-activity relationships would have predicted AES as 10-fold more toxic than AS. Differences between the prediction and mesocosm observations likely reflected confounding effects of heterotrophic periphyton in the AS experiment. The invertebrate community NOECs assure the existence of a margin of environmental safety for both alkyl sulfate and alkyl ethoxylate sulfate.
