Lake Ontario Algae Bloom: NASA Image of the Week

Blue-green swirls across northern and eastern Lake Ontario are blooms of algae caused by pollution flowing into the waterway.

Late-summer warmth and the relatively high sun angles of late August promoted a bloom of microscopic algae across much of northern and eastern Lake Ontario for several days.

The swirls of green clearly visible in the image to the right are actually caused by billions and billions of tiny phytoplankton, which use sunlight to convert water and carbon dioxide into food.

Such algae blooms have not always been such a dramatic feature of the lake, which is the smallest and easternmost of all the Great Lakes.

Runoff from farming and industrial activity during the first half of the 20th century resulted in an approximate doubling in the crop of phytoplankton off Toronto.

Pollution running off into the lake, such as that from fertilizers, sewage treatment plant discharges and even the atmosphere act as food for the tiny plants.

Researchers found that the depletion of oxygen that followed massive algae blooms over the lake resulted in dramatic declines in the number of lake trout between 1930 and 1950.

The volume of summertime algae blooms has dropped somewhat since the United States and Canada signed an international treaty in 1972 to curtail the level of pollutants flowing into Lake Ontario.

But, as can be seen in the image from NASA’s Aqua satellite on August 20, 2008, enough manmade nutrients still exist in the lake to feed the huge clouds of the colorful phytoplankton.

Satellite Image: NASA's MODIS Rapid Response Team