¹1Department of Biology and Clarkson Center for the Environment, Clarkson University, 8 Clarkson Avenue, Potsdam, New York, 13699-5805, USA
²Department of Earth and Environmental Science, Plattsburgh State University, 101 Broad Street, Plattsburgh, New York, 12901, USA

*Corresponding author: mtwiss@clarkson.edu

Abstract

   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 µg L^-1 in 14 L treatments and incubated under simulated ambient light conditions (43 µmol 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 hr to < 7 days).