A bison wanders past Grand Prismatic Spring in Yellowstone National Park. The blue color of the spring results from extremely hot geothermal water that doesn't allow bacteria or other organisms to grow. However, as the water cools when it flows out of the spring, the orange and green pigments of cyanobacteria and other bacteria make for a vivid outline of the pool.
Limnology - The study of inland waters
Photo: 21 October 2012