A Freshwater “Red Tide” on Clear Lake, California1

A short‐lived freshwater “red tide” of Peridinium pernardii (dinoflagellates) occurred in Clear Lake, California, between late March and mid‐May 1970, confined to a zone of high concentration in shallow water around the lake’s margin. Densities reached at least 5,000 cells ml−1 and at least 130 µg liter−1 chlorophyll a. The red tide is attributed to winter floods followed by a calm period with clear skies that provided dissolved organic material and high insolation. It is postulated that the high inorganic turbidity present at this time prevented rapid growth of the spring‐summer dominant alga Aphanizomenon flos‐aquae but did not preclude the bloom of P. pernardii because of its phototactic mobility. No toxic effects were noted on the flora and fauna of the lake.