Harmful Algal Blooms


Harmful Algal Blooms occur in fresh and salt water. They can occur in rivers, smaller lakes and Great Lakes. If a bloom becomes large enough it can hurt fish, people and other mammals. Public health experts follow blooms in the Great Lakes for many reasons, but primarily to prevent threats to drinking water.

There are many factors that come together to encourage dangerous levels of algae. The temperature of the water, circulation, and the presence of excessive nutrients, such as phosphorus and nitrogen, can feed the simple plants. When they reach critical mass the blooms block the sunlight that other organisms require to live. When the algae itself dies and decomposes oxygen in the water is reduced, and that can cause fish kills and impact the ecology in general. Excessive algae can sicken people and animals through contact, and airborn bacteria can be hard on those with compromised respiratory systems.


Perhaps the best known bloom in the Great Lakes took place in Western Lake Erie. On August 3, 2014, the City of Toledo was forced to issue a “Do Not Drink” order. Chemical tests confirmed excessive amounts of the algal toxin Microcystin in the drinking water plant’s treated water. The advisory, spanning three counties in Ohio and another in Michigan, left almost a half million people without drinking water for days. The order closed restaurants and prompted runs on bottled water. The bloom that caused the trouble in Toledo was not the largest ever, but happened to occur near the city’s intake valve.

The Ohio Environmental Protection Agency added sensors, and many stakeholders joined together to increase the ability to predict and monitor harmful algal blooms. Today you can keep an eye on conditions through the Harmful Algal Bloom Data Portal.