Algae blooms in Clear Lake are a public health risk and increase water treatment costs. Image credit: Brett Walton/Circle of Blue May 4, 2021 On a good day, usually in late winter and early spring, the magnificent waters of Clear Lake seem to live up to their name. Under the shadow of the volcano Mount Konocti, the oldest lake in North America and second largest in California sparkles in an array of blues while fishing boats ply the shallow nearshore, their anglers hoping to hook a trophy bass. From his office two miles inland, Frank Costner knows that the lake’s waters also shelter a treacherous occupant – potentially toxic blooms of cyanobacteria. As general manager of Konocti County Water District, Costner is responsible for supplying drinking water from Clear Lake to 4,500 people who live in this region a two-hour drive north of San Francisco.