This dataset contains algal identification and enumeration data for phytoplankton samples collected by the U.S. Geological Survey (USGS) between July 2019 and November 2019 at seven reservoirs across the United States. Reservoirs sampled included Lake Koshkonong, Wisconsin, Pelican Lake, Minnesota, Lake Ida, Minnesota, Pomme de Terre, Minnesota, Lake Emily, Minnesota, Milford Lake, Kansas, and Jordan Lake, North Carolina. The samples were analyzed at the Caribbean - Florida Water Science Center (CFWSC) Phycology Laboratory using morphology-based microscopy methods. This data is part of a larger multi-agency project between the U.S. Environmental Protection Agency, the National Aeronautics and Space Administration, the National Oceanic and Atmospheric Administration, and USGS called the Cyanobacteria Assessment Network (CyAN). The goal of the CyAN project is to develop a satellite-based, early warning system to detect harmful algal blooms (HABs) in freshwater systems.