Abstract
The Division Avenue Filtration Plant of the Cleveland Division of Water and Heat has undertaken almost daily phytoplankton counts of water samples from Lake Erie since 1919. Data exist for 25 full years and for 7 additional partial years between 1919 and 1963. There has been a consistent increase in the average quantity of phytoplankton. The vernal and autumnal phytoplankton maxima have consistently become more intense and have lasted longer. The periods of minimun phytoplankton development in winter and summer have become shorter and less well marked, until the winter minimum failed to develop at all in some of the latest years. Certain marked qualitative changes also have occurred. These effects are thought to have been caused by an increasingly rapid eutrophication of the water in Lake Erie.