Ecology
Cyanobacteria
Photosynthetic bacteria (commonly called blue-green algae) that can form harmful blooms and produce toxins dangerous to humans and animals.
Cyanobacteria is a term from limnology — the scientific study of inland waters. The detailed explanation below covers the definition, why the concept matters to lake water-quality interpretation, and how it intersects with the LakeGrade rubric. Reading Cyanobacteria alongside the LakeGrade rubric helps make the rubric's structure intuitive — each grading parameter maps directly onto an established limnological concept.
The per-lake pages on LakeQuality always show the specific Cyanobacteria value for that lake, so you can move from the general concept to the specific lake without leaving the site.
What It Means for Your Lake
Cyanobacteria are an ancient group of photosynthetic bacteria that are among the oldest life forms on Earth, dating back over 3 billion years. Despite being commonly called blue-green algae, they are technically bacteria, not true algae. In Minnesota and Wisconsin lakes, cyanobacteria are a natural part of the phytoplankton community, but they become problematic when they dominate the community and form dense blooms. Several characteristics give cyanobacteria a competitive advantage in nutrient-rich, warm lakes: many species can fix atmospheric nitrogen, giving them access to a nutrient supply unavailable to other algae; they contain gas vesicles that allow them to regulate buoyancy and position themselves at optimal light levels; and they thrive at water temperatures above 77 degrees Fahrenheit, which are increasingly common during Upper Midwest summers. The primary concern with cyanobacteria is toxin production. Common genera found in regional lakes include Microcystis (produces microcystins, liver toxins), Anabaena/Dolichospermum (produces both microcystins and anatoxins, nerve toxins), and Aphanizomenon (produces cylindrospermopsin and saxitoxin). Exposure to cyanotoxins through swimming, swallowing water, or inhaling spray can cause skin rashes, nausea, vomiting, diarrhea, and in severe cases, liver failure. Dogs are particularly vulnerable because they drink lake water and lick contaminated fur. The Minnesota Department of Health and Wisconsin DHS maintain monitoring programs and issue advisories when cyanotoxin levels exceed recreational guidelines.
Frequently Asked Questions
What is cyanobacteria?
Photosynthetic bacteria (commonly called blue-green algae) that can form harmful blooms and produce toxins dangerous to humans and animals.
Why does cyanobacteria matter for lake health?
Cyanobacteria are an ancient group of photosynthetic bacteria that are among the oldest life forms on Earth, dating back over 3 billion years. Despite being commonly called blue-green algae, they are technically bacteria, not true algae. In Minnesota and Wisconsin lakes, cyanobacteria are a natural ...
Related Terms
Blue-Green Algae
The common name for cyanobacteria, photosynthetic bacteria that form potentially toxic blooms in nutrient-rich lakes during warm weather.
Algal Bloom
A rapid increase in algae population in a lake, often visible as green discoloration or surface scums, triggered by warm temperatures and excess nutrients.
Fish Consumption Advisory
Official guidance from state health departments recommending limits on eating fish from specific lakes due to mercury, PCBs, or other contaminants.
Phosphorus
A nutrient that fuels algae growth in lakes, measured as total phosphorus in micrograms per liter.
Source: EPA National Aquatic Resource Surveys, 2026.