Such a “whitening event” was in progress on August 24, 2013, when an astronaut aboard the International Space Station captured the image to the right.
The pale appearance of Lake Ontario was produced by changes in water temperature that allowed fine particles of calcium carbonate to form at various depths.
The warmth and increased sunlight of summer can also sometimes increase photosynthesis among phytoplankton and other tiny marine life to cause the calcium carbonate to form.
But NASA says that scientists working on the lake when the photo was taken found that the whitening was caused by changes in water temperature rather than blooms of tiny plant life, which can often be triggered by agricultural runoff and other pollution.
The late August whitening can be seen swirling around much of Lake Ontario, carried on currents mainly generated by differences in water temperature within the lake.
Other features visible in the image are the sprawling Toronto metropolitan area and the cities of Buffalo, Rochester and Kingston.