Have you ever wondered why Lake Tahoe is so blue or why there is so much community focus on it, such as the bumper stickers promoting, "Keep Tahoe Blue"?Several factors are involved in determining how an observer perceives lake color. The color of a lake can tell us much about the biological status. Lake Tahoe is classed as an unproductive (ultra-oligotrophic) lake, because it produces very little floating or attached algae under undisturbed conditions. Tahoe could be expected to stay blue for hundreds of years if we do not increase nutrients and sediment entering it to above natural levels, and if no natural catastrophes occur.Oligotrophic lakes are a deep blue color because pure water transmits blue and absorbs green and red light. The blue light goes more deeply into the lake, but some of it is scattered by suspended particle material and returns to the surface. Scattered light that is blue is more likely to leave the lake than scattered green or red light. Our eyes perceive the blue wavelengths.
Lakes that produce a great deal of algae are called productive or eutrophic lakes. In these lakes, algal pigments absorb blue and red wavelengths, and green wavelengths travel relatively farther. The probability is greater that green light will be scattered back out of the lake, rather than red or blue light. This gives highly productive lakes their green color. Other materials in a lake can cause coloration, including dissolved organic material, sediments, and organisms. For example, a lake that is clouded with reddish clay will look red. High altitude lakes near glaciers often contain very fine sediment particles created by glacial action. These particles can lend a milky appearance to otherwise unproductive blue lakes. Organisms can also impart colors to a lake. Species of water ferns can be bright purple. Photosynthetic bacteria can reach high densities and yield purple, brown, yellow, or blue appearances. Dissolved metals can also color ponds. For example, high concentrations of copper can lead to metallic blue ponds or lakes.
Measurements of Tahoe's blueness are not recorded, but scientists of University of California Davis Tahoe Research Group publish average water clarity data each year. UC Davis researchers measure the lake's clarity every seven to 10 days by lowering a white disk, called a Secchi disk, into the water until it disappears from sight. Measurements taken throughout 2003 show an average clarity of 71 feet (21.6 meters). When recordings began in 1968, the disk could be seen at depths of 102 feet (31 meters). That is an extraordinary level of transparency.The cause of the steady loss in clarity over the last three decades is increased algal growth and fine soil particles suspended in the lake water. If this continues, the buildup of algae and sediment will affect the blueness of the lake. The lake could begin to change from oligotrophic to eutrophic.To keep Lake Tahoe from turning green, the Tahoe Regional Planning Agency (TRPA) and other agencies conduct extensive restoration programs to reduce water pollution and improve the health of Tahoe's ecosystem. The Lake Tahoe Environmental Improvement Program (EIP) includes a large number of projects aimed at reducing sediment and plant nutrients entering the lake. Many agencies and university researchers are also developing a Tahoe Total Maximum Daily Load (TMDL), which is a water pollution reduction program. Their work should help the TRPA and others to identify measures that will have the most positive impact on the lake.
Local agencies and organizations are working hard to "Keep Tahoe Blue" and encourage residents and visitors to do their part.To learn more about the prognosis for Lake Tahoe's water quality, see the Cooperative Extension fact sheet on the Web at http://www.unce.unr.edu/publications/FS03/FS0367.pdf.Watch for "The Lake Tahoe Report" in the North Tahoe Bonanza each Friday, and tune in to KOLO-TV News Channel 8 Tuesdays at 5 p.m. to learn more. "The Lake Tahoe Report" is a collaborative effort of the Lake Tahoe Environmental Education Coalition, University of Nevada Cooperative Extension, UC Davis and the USDA Forest Service. For more information, contact Heather Segale, University of Nevada Cooperative Extension, (775) 832-4138, or logon to www.lteec.org.