In situ assessments of chemical toxicity in streams may be accomplished using natural periphytic communities when allowed to colonize an artificial substrate. The proliferation of Triclosan (TCS) in consumer products has resulted in its presence in wastewater influent, effluent, and subsequently in streams. In this study, the two objectives were: 1) assess the utility of a passive-diffusion periphytometer in toxicity tests, and 2) evaluate the growth-inhibiting effects of TCS on periphytic algae at the White River, Northwest Arkansas. The periphytometer was deployed for one week with seven replicates of nine treatments, including control (deionized H2O), methanol, low TCS (50 .g L-1), medium TCS (100 .g L-1), high TCS (500 .g L-1), nutrients (2000 .g PO4-P L-1 and 20000 .g NO3-N L-1), low TCS with nutrients, medium TCS with nutrients, and high TCS with nutrients. Relatively low stream nutrient concentrations were observed; maximum nitrate–nitrogen (NO3-N), total N, ammonium–nitrogen (NH4N-N), total organic carbon (TOC), and soluble reactive phosphorus (SRP) concentrations during the deployment were 230, 603
