The vertical extinction coefficient (VEC) measures optical properties of Lake Tahoe similar to the Secchi depth, however VEC measures beyond the depth at which the Secchi Disc is visible, providing information on biological properties of the lake down to the bottom of the photic zone (approximately 50 to 60 meters). Light extinction results from one or more of the following factors: colored dissolved organic material, organic and inorganic particles and the water molecules themselves. The decline in VEC is a result of additions of fine sediment particles and growth of phytoplankton (algae). Drivers influencing the delivery of fine sediment and nutrients include urban development, anthropogenic and natural disturbance in the undeveloped portions of the watershed, and local and regional climate. Many programs throughout the Tahoe Basin are aimed at improving VEC such as stormwater reduction and stream restoration projects. VEC has been measured at Lake Tahoe by UC Davis since 1968.