Blue-green algae, technically known as cyanobacteria, are microscopic organisms that are naturally present in lakes and streams. Typically present at low number, blue-green algae can quickly become abundant in warm, shallow, undisturbed surface water rich in nutrients like phosphorous and nitrogen.
Some blue-green algae produce toxins that could pose a health risk to people and animals when they are exposed to them in large quantities. Health effects could occur when ingested, inhaled, or through contact with the skin. Ingestion of the algae affects the liver and the nervous systems and even causes death in livestock and pets when large amounts have been consumed. Vomiting, diarrhea, asthma, headache, fever, and muscle weakness are also potential health effects of the blue-green algae toxins.
Blooms of blue-green algae that last more than a few months can be harmful to lake/river ecosystems and cause fish kills because of the decrease in oxygen levels and direct ingestion in the food chain. Blue-green algae blooms block sunlight that feeds plants growing on the bottom of lakes, resulting in the loss of rooted aquatic vegetation, which is valuable fish habitat.
Algae use sunlight to make food and are eaten by microscopic animals (zooplankton). Small fish eat the zooplankton, and larger fish and other large animals eat the small fish. However, blue-green algae are often difficult to eat or are of poor nutritional value for zooplankton. Consequently, zooplankton, particularly types that are the best fish food, often decline during blooms. This decline can impact the number of desirable game fish.