Satellites provide a view from space of changes on the Earths
surface. This series of images from the Sea-viewing Wide Field-
of-view Sensor (SeaWiFS) aboard the Orbview-2 satellite shows
the dramatic change in the color of Lake Michigan during the
summer. The bright color that appears in late summer is probably
caused by calcium carbonatechalkin the water. Lake Michigan
always has a lot of calcium carbonate in it because the floor of
the lake is limestone. During most of the year the calcium
carbonate remains dissolved in the cold water, but at the end of
summer the lake warms up, lowering the solubility of calcium
carbonate. As a result, the calcium carbonate precipitates out
of the water, forming clouds of very small solid particles that
appear as bright swirls from above. The phenomenon is
appropriately called a whiting event. A similar event occured in
1999, but appears to have started later and subsided earlier. It
is also possible that a bloom of the algae Microcystis is
responsible for the color change