This U.S. Geological Survey (USGS) data release provides phytoplankton data collected from Seneca Lake, New York between October 9-10, 2019. All data are reported as raw calculated values and are not rounded to USGS significant figures. This data release was produced in compliance with the open data requirements as a way to make scientific products associated with USGS research efforts and publications available to the public. This dataset includes all routine and quality assurance/quality control samples collected at twelve sampling locations on Seneca Lake. Phytoplankton were identified to the lowest possible taxonomic level, and abundance (density reported as both natural units and cells) and biovolume are reported.