As part of the on-going study of water quality in the Nanticoke area of Lake Erie phytoplankton samples were collected at several stations from 1975 to 1978. An evaluation of the abundance and changes in seasonal composition of standing stocks of phytoplankton at these locations is related to complementary chemical and physical data for the same period. The report compares the phytoplankton data to that collected during pre-operational studies conducted from 1969 to 1974. This report, the third of a series includes a fourth year of data to finalize the first ten years of the Nanticoke study.