Big Valley area. Agriculture impacts area groundwater sources through pumping, and is responsible for 82% of water usage in the watershed. This usage may contribute to early drying of stream beds, which impacts local wildlife and riparian vegetation. Pesticides In 1949, as part of an effort to boost tourism and improve the local economy, DDD (dichlorodiphenyldichloroethane) was applied to the lake in heavy doses to eradicate the gnats that were driving summer tourists away from the lake. The treatment succeeded in controlling the gnats that year and for the following year, however in 1953 the gnat population rebounded, prompting another application in 1954. The final application of DDD to Clear Lake was made in 1957 (Cook 1963). Numbers of western grebe were found dead, their tissues containing high concentrations of DDD. The effects were devastating to the local ecology. From 1962 to 1975 carefully planned applications of methyl parathion were made for Clear Lake gnat control. The gnat still occurs in Clear Lake, but at much lower numbers than in the 1940s-1970s. The Clear Lake gnat's population is believed to be kept in check now by two introduced fish species (the threadfin shad and the inland silverside) that compete with the Clear Lake gnat for its preferred zooplankton for food. Invasive species The ecology of Clear Lake has been significantly altered due to the presence of invasive species. Notable invasive species include hydrilla (Hydrilla verticillata), and the lake is actively monitored for quagga mussels and zebra mussels (Dreissena bugensis and Dreissena polymorpha). Hydrilla is a submerged aquatic plant, which notably out-competes both native and non-native species to create pure stands. The plant is so successful as establishment due to its ability to break into smaller propagules, which can survive in sediment for up to seven years The presence of hydrilla can affect fish stocks, and interfere with recreational boating or fishing. Hydrilla prevention is overseen by the California Department of Food and Agriculture’s Hydrilla Program. The goal of the program is eradication of existing hydrilla populations and prevention of future infestations of hydrilla, and the program has been highly successful Harmful algal blooms Due to its shallow depth and warm temperatures, Clear Lake is naturally eutrophic. Written records documenting cloudy discolorations in the lake exist from late 19th century. Cyanobacteria (blue-green algae) in the lake can produce blooms often beginning in the spring and lasting through the fall. Referred to