Brightening of Lake Michigan
September 24, 2001
The above series of visible satellite images shows changes in the appearance of Lake Michigan over the course of several weeks. The images are from the Sea-viewing Wide Field-of-view Sensor (SeaWiFS) aboard the Orbview-2 satellite. The bright color that appears in late summer is probably caused by calcium carbonate chalk in the water. Lake Michigan always has an abundance of calcium carbonate because the floor of the lake is composed largely of limestone. During most of the year, this calcium carbonate remains dissolved in the cold water, but at the end of summer the lake water warms up, causing dissolved carbon dioxide to leave solution to the air, raising the pH of the lakewater (making it more basic) thus lowering the solubility of the calcium carbonate overall. As a result, it precipitates out of the water, forming clouds of very small solid particles. From space these particles appear as bright swirls. The phenomenon is referred to as a whiting event. A similar event occurred in 1999. It's also possible that a bloom of the algae Microcystis is responsible for the color change, but unlikely because of Lake Michigan’s depth and size.