Toxicity of atrazine was determined for natural phytoplankton communities in lake water samples from Missisquoi Bay (Lake Champlain, CA, US) at monthly intervals from May to August 2005 in order to assess how this herbicide could affect phytoplankton dynamics. Atrazine was added at 0, 5, 20, and 50 μ g L− 1 in 14 L treatments and incubated under simulated ambient light conditions (43 μ mol m− 2 s− 1). Phytoplankton community composition changes in the treatments were followed using a phytoplankton-specific fluorometer (FluoroProbe) and size fractionated filtration and analysis of extracted chlorophyll-a. Phycocyanin-rich Cyanobacteria decreased with increasing atrazine concentration in June and July and increased with atrazine concentration in August. Further experimentation utilizing a fast repetition rate fluorometer revealed that atrazine impacts on phytoplankton autofluorescence can impair FluoroProbe measurements over the short term (1 h to < 7 days).

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