The Cause of Harmful Algal Blooms

A green, swirling paint-like substance floats atop the surface of a pond. It's an example of a microcystis bloom.
A microcystis bloom shows how much Harmful Algal Blooms can look like swirling paint.

Have you ever looked into a pond and observed a green, swirling paint-like substance on the surface? Have you ever wondered what this is?

You are likely seeing cyanobacteria or blue-green algae. When tiny cyanobacteria cells multiply, they can become visible to the naked eye forming a Harmful Algal Bloom (HAB). With ample nutrients, plenty of sunlight and warm temperatures, these blooms have the potential to develop in all bodies of water, including those across the state of Ohio. The blooms most frequently occur in late summer or early fall.

Cyanobacteria blooms often arise in eutrophic conditions — eutrophication is a natural process caused by the accumulation of nutrients in lakes, reservoirs or other water bodies (United States Geological Survey). Unfortunately, this process can be accelerated by human activities. According to the Ohio Environmental Protection Agency (Ohio EPA), the primary sources of nutrient pollution include fertilizer and stormwater runoff, car and power plant emissions and failing septic tanks (Ohio EPA). 

Although plants require nutrients to grow, excessive nutrients (particularly nitrogen and phosphorus) can become problematic. Algae and cyanobacteria feed on the excess nutrients, multiply and often change the color of the water or produce an offensive odor. When the blooms die, the resulting decomposition process consumes oxygen. Significant oxygen depletion can create dead zones or areas in a body of water without enough oxygen to sustain fish and other aquatic life (USGS).

Under some conditions that are not entirely understood, blooms of cyanobacteria can also release toxins (USGS). Ingestion of or contact with water containing elevated concentrations of cyanotoxins can cause health effects to humans like allergic reactions, dermatitis or gastroenteritis. The toxins can also be harmful to pets, so pets should be prevented from drinking or bathing in water that may be experiencing a harmful algal bloom (Ohio EPA).

For more information on specific toxins, their effects and how to keep you and your pets safe, visit the Ohio EPA website or the Hamilton County Public Health website.

If you have any questions or believe you have seen an HAB at Great Parks, please contact Amanda Nurre, Watershed Specialist.

Amanda Nurre
Watershed Specialist