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The widespread and persistent contamination of freshwater environments by low concentrations of pesticides is a growing concern worldwide. In aquatic environments, herbicide pollution is of greatest concern for phytoplankton, due to their similarities to terrestrial plants. Through the use of Fast Repetition Rate Fluorometry (FRRF) during weeklong experiments on 10 phytoplankton cultures from 4 classes and 4 natural communities, we demonstrate that PSII-inhibiting herbicides, notably atrazine that is extensively used in North America, consistently have effects on freshwater phytoplankton photophysiology at concentrations far below concentrations affecting the most sensitive species in previous studies. The parameters specific to FRRF (Ρ, σ, τ 1 , τ 2 , τ 3 ) were those most sensitive to PSII inhibitors, compared to the standard fluorescence parameters derived from other fluorescence protocols such as Pulse Amplitude Modulation (PAM) fluorometry (F 0 , F m , F v /F m ) and extracted chlorophyll a concentrations. Based on these findings, existing national environmental guidelines and standards are insufficient to adequately prevent adverse effects of atrazine and other PSII inhibiting herbicides on algal physiology in aquatic ecosystems., Competing Interests: Declaration of competing interest The authors have no conflicts of interest to declare., (Copyright © 2019 Elsevier B.V. All rights reserved.)