¹Great Lakes Environmental Research Laboratory-Lake Michigan Field Station, National Oceanic and Atmospheric Administration, Muskegon, MI 49441 ²Florida Institute of Oceanography, University of South Florida and Fish and Wildlife Institute, Florida Fish and Wildlife Commission, St. Petersburg, FL 33701 ³Great Lakes Environmental Research Laboratory, National Oceanic and Atmospheric Administration, Ann Arbor, MI 48105 ⁴Center for Coastal Fisheries and Habitat Research, National Oceanic and Atmospheric Administration, Beaufort, NC 28516 ⁵Annis Water Resources Institute, Grand Valley State University, Muskegon, MI 49441 ⁶Old Woman Creek NERR, ODR Division of Wildlife, Huron, OH 44839

*Corresponding author: gary.fahnenstiel@noaa.gov

Abstract

   Distribution of the toxin microcystin was studied in Saginaw Bay, Lake Huron during two summer (August) cruises in 2003-2004. Intracellular microcystin concentrations averaged 1.1 µg L^-1 (range 0.01- 3.5 µg L^-1), and extracellular concentrations averaged only 0.09 µg L^-1 (range 0.01-0.18 µg L^-1). Highest microcystin concentrations were found in the nearshore regions of the bay, which were shallow and had high total phosphorus concentrations. Microcystin concentrations were strongly correlated with Microcystis aeruginosa abundance (r = 0.8). M. aeruginosa abundance was strongly correlated with total phosphorus, and growth rates were negatively correlated with C:P ratio. Particulate nutrient ratios suggested that Saginaw Bay algae were severely phosphorus deficient. Microcystin cell quotas for M. aeruginosa averaged 140 fg cell^-1, and were not correlated with any environmental factor or growth rates. In Saginaw Bay, phosphorus availability primarily influences microcystin concentrations through direct control of M. aeruginosa abundance and growth rates, and not through direct control of cellular microcystin synthesis.