The U.S. Geological Survey (USGS), in cooperation with the St. Croix River Research Station – Science Museum of Minnesota, updated a previously developed CE-QUAL-W2 hydrodynamic and water-quality model of Madison Lake, Minnesota (Smith and others, 2017). The previous version simulated phytoplankton into four general algal communities or groups: (1) Bacillariophyta (diatoms) and Chrysophyta (chrysophytes); (2) Chlorophyta (green algae); (3) Cyanophyta (blue-green algae); and, (4) Haptophyta and Cryptophyta (flagellates). For the updated model, the Cyanophyta group (originally referred to as blue-green algae) has been divided into two groups: a nitrogen-fixing Cyanophyta group, generally representative of Anabaena, Dolichospermum, and Cylindrospermopsis, and a non-fixing, buoyant Cyanophyta group, generally representative of Planktothrix, Microcystis, and Woronichinia.