File:Phytoplankton bloom in Lake Ontario (9673767360).jpg

In late August, 2013 Lake Ontario, situated on the boundary of Canada and the United States, wore a palette of milky blue-green swirls when seen from space, the result of a heavy late summer phytoplankton bloom. The Moderate Resolution Imaging Spectroradiometer (MODIS) aboard NASA’s Aqua satellite captured this true-color image on August 24 as it passed over the region.

When conditions are right microscopic cyanobacteria, or blue-green algae, will grow explosively, resulting in dramatic blooms that can be seen from space. Harmful algal blooms or HABs, have been observed in all of the Great Lakes, and are caused by a variety of factors, including change in precipitation, drought, nutrient loading from runoff (nitrogen and phosphorus) and warmer than average temperatures.

Some blue-green algae produce toxins – the most common which is microcystin - that could pose a health risk to people and animals when exposed to them in high quantities. The toxins can cause various problems when swallowed, when they come in contact with skin, or when airborne droplets containing toxins are inhaled. The toxins can cause a rash, stomach pain, nausea, diarrhea, runny eyes, cough, sore throat, asthma-like symptoms and allergic reactions, depending on the route of exposure. Large algal blooms can also cause hypoxia in the lake – a condition of very low oxygen content – which can lead to large fish kills.