Clear Lake is one of California’s oldest and most unique natural features. Nestled in Northern California’s coastal mountains, Clear Lake is the largest lake completely within California and is the oldest lake in North America with sediments dating back 480,000 years (Sims et al. 1988). Rich mineral deposits around the lake were historically mined for borax, sulphur, and mercury. Thus, Clear Lake continues to be polluted by mercury and methylmercury which bioaccumulates in the food chain (Suchanek et al. 2008). In spite of pollution, the lake boasts an impressive diversity of biological life. It is designated as an Important Bird Area by the Audubon Society, and has endemic species such as the Clear Lake hitch (Lavinia exilicauda chi, a planktivorous fish), the Clear Lake splittail (Pogonichthys ciscoides, now extinct), and Clear Lake gnat (Chaoborus astictopus)—the latter of which was targeted by heavy application of of the pesticide DDD to control large swarms (Lindquist et al. 1951). These pesticide applications earned Clear Lake a feature in Rachel Carson’s seminal novel Silent Spring for its negative impacts on Western Grebe populations.

Water Quality Issues in Clear Lake

Clear Lake continues to struggle with long-lasting impacts of nutrient pollution. High concentrations of nutrients such as nitrogen and phosphorus fuel large algal blooms and contribute to poor water quality in the lake. Phosphorus is particularly abundant in Clear Lake and its associated watershed. As a result, harmful phytoplankton known as cyanobacteria thrive here, some of which can produce toxins harmful to humans. Commonly known as blue-green algae, cyanobacteria are an ancient group of organisms that are actually unrelated to algae since they are considered bacteria and not plants. Perhaps the most important difference between cyanobacteria and algae is that some species of cyanobacteria have specialized cells called heterocysts that capture nitrogen gas from the atmosphere and transform it into usable forms through a process called nitrogen fixation–something that plants are not capable of. In fact, legumes like soybeans and clover actually have symbiotic relationships with other nitrogen-fixing bacteria in order to glean nitrogen for their own use.

Nitrogen fixation gives cyanobacteria a competitive advantage in waters rich in phosphorus and relatively deficient in nitrogen–the exact conditions present in Clear Lake. 

Read the Article