Algae: How Climate Changes Cyanobacterial Harmful Algal Blooms

Wisconsin's warmer and wetter climate fuels more algal blooms that can harm health

Blue-green algae floating by pier shore.

Wisconsin’s changing climate is bringing more rain and hot days, often topping 90 degrees Fahrenheit in summer. Cooling off in one of our state’s abundant lakes or waterways is usually a great way to beat the heat, but there can be danger lurking underwater on hot days—cyanobacterial harmful algal blooms or cyanoHABs.

CyanoHABs contain a type of bacteria that can harm the health of people, pets, and livestock if it's swallowed, rubbed on skin, or inhaled into lungs. Research shows that climate change fuels the growth of more algal blooms due to:

High temperatures that warm surface waters.
Heavy rainfall that flows over the land, carrying nutrients into our lakes and waterways.
Shorter winters and longer growing seasons that support earlier and longer-lasting bloom seasons.

All of these factors create ideal conditions for cyanoHABs to flourish in Wisconsin waters, increasing the chances of people and animals getting sick. It's important to look for the signs of blue-green algae and avoid any contact with any algal blooms. If you do come into contact with an algae bloom, know the signs, symptoms, and treatments for illnesses associated with cyanoHABs

Temperature

Cyanobacteria grow best in warm water and plentiful sunlight, naturally gravitating to the water's warm surface. As a result, increases in water temperature from climate change spur its growth and reproduction. The warmer surface water also pushes the cooler, denser water to the bottom, displacing the warmer water. That process is called lake stratification, which also causes the release of more phosphorus from lake sediments that float upward to the surface. Phosphorus is an essential food source for cyanobacteria, feeding its growth.

Precipitation

Pouring rain flooding a playground with a tree

Big swings in heavy periods of intense rainfall to long periods of drought are both features of Wisconsin's changing climate. Increased rainfall picks up more nutrients from the land as it flows into lakes and waterways, fueling more growth of cyanoHABs. When rain is followed by drought, waterbodies can retain those nutrients for longer, causing cyanobacterial species to spread and reproduce.

Wisconsin data

The Wisconsin Initiative on Climate Change Impacts (WICCI) is a group of scientists studying and documenting climate trends and their effects on Wisconsin using state data from 1950 onward. Their data show that Wisconsin is getting warmer and wetter. Average temperatures are projected to increase and become more like the current temperatures of states hundreds of miles to our south. Total precipitation and the amount of rainfall in each event is also expected to continue increasing. If these trends continue through 2050 and beyond, we can expect the frequency and intensity of blooms in Wisconsin to also increase.

Wisconsin maps showing changes in temperatures from 1950 to 2023. The annual average temperature increased 3 degrees Fahrenheit statewide from 1950–2023. — Figure A: Wisconsin is warming. The annual average temperature increased 3 degrees Fahrenheit statewide from 1950–2023.

Wisconsin maps showing changes in precipitation from 1950 to 2023. The average statewide precipitation has increased 17% (about five inches) since 1950. — Figure B. Wisconsin is getting wetter. Overall, the state experienced more rainfall from 1950–2023 with significantly wetter conditions in the southern half of the state. The average statewide precipitation has increased 17% (about five inches) since 1950.

Climate and health connection

An increase in cyanoHAB frequency and intensity across Wisconsin means more people and animals are likely to be exposed and get ill from higher concentrations of cyanobacteria and cyanotoxins. A 2021 WICCI report confirms the increase in cyanoHABs-related illnesses, but also predicts other climate-related health effects, including:

An adult and a child splashing water in a river.

More heat-related emergency department visits and hospital admissions from extreme weather and natural disasters.
An uptick in mental health disorders as heat and extreme weather events disrupt life and cause stress.
Worse air quality due to higher levels of ground-level ozone that need heat and sunlight to form. Fine particles, called particulate matter or PM2.5, caused by drought and industry also worsen air quality.
A rise in waterborne diseases from bacteria, parasites, and viruses from more flood events.
A rise in certain infectious diseases, such as West Nile Virus and Lyme disease, from a longer tick and mosquito season.

Visit our Health Concerns webpage for more information on signs, symptoms, and treatments of cyanobacteria illnesses for people and animals.

Resources

Looking for more information about health impacts from climate change? Visit the the Climate and Health Program website or email dhsclimate@dhs.wisconsin.gov.