Late-summer phytoplankton in western Lake Erie (Laurentian Great Lakes): bloom distributions, toxicity, and environmental influences David F. Millie Gary L. Fahnenstiel Julianne Dyble Bressie Ryan J. Pigg Richard R. Rediske David M. Klarer Patricia A. Tester R. Wayne Litaker Received: 7 May 2008 / Accepted: 25 January 2009 Ó Springer Science+Business Media B.V. 2009 Abstract Phytoplankton abundance and composi- tion and the cyanotoxin, microcystin, were examined relative to environmental parameters in western Lake Erie during late-summer (2003–2005). Spatially explicit distributions of phytoplankton occurred on an annual basis, with the greatest chlorophyll (Chl) a concentrations occurring in waters impacted by Maumee River inflows and in Sandusky Bay. Chlorophytes, bacillariophytes, and cyanobacteria contributed the majority of phylogenetic-group Chl a basin-wide in 2003, 2004, and 2005, respectively. Water clarity, pH, and specific conductance delin- eated patterns of group Chl a, signifying that water mass movements and mixing were primary determinants of phytoplankton accumulations and distributions. Water temperature, irradiance, and phosphorus availability delineated patterns of cyano- bacterial biovolumes, suggesting that biotic processes (most likely, resource-based competition) controlled cyanobacterial abundance and composition. Intracel- lular microcystin concentrations corresponded to Microcystis abundance and environmental parameters indicative of conditions coincident with biomass accumulations. It appears that environmental param- eters regulate microcystin indirectly, via control of cyanobacterial abundance and distribution. Keywords Algae  Chlorophytes  Cyanobacteria  Microcystin  Microcystis  Pandorina  Planktothrix R. R. Rediske Annis Water Resources Institute, Grand Valley State University, Muskegon, MI 49441, USA D. M. Klarer Division of Wildlife, Ohio Department of Natural Resources, Old Woman Creek National Estuarine Research Reserve, Huron, OH 44839, USA P. A. Tester  R. W. Litaker Center for Coastal Fisheries and Habitat Research, National Oceanic and Atmospheric Administration, Beaufort, NC 28516, USA R. J. Pigg Florida Wildlife Research Institute, Florida Fish and Wildlife Conservation Commission, Saint Petersburg, FL 33701, USA D. F. Millie (&) Florida Institute of Oceanography, University of South Florida, Saint Petersburg, FL 33701, USA e-mail: david.millie@myfwc.com G. L. Fahnenstiel Great Lakes Environmental Research Laboratory, Lake Michigan Field Station, National Oceanic and Atmospheric Administration, Muskegon, MI 49441, USA J. Dyble Bressie Great Lakes Environmental Research Laboratory, National Oceanic and Atmospheric Administration, Ann Arbor, MI 48105, USA R. J. Pigg College of Marine Science, University of South Florida, Saint Petersburg, FL 33701, USA 123 Aquat Ecol DOI 10.1007/s10452-009-9238-7