What is causing blue-green algae grow in Lake Shawnee?

TOPEKA (KSNT) - State health officials are keeping an eye on a local lake due to the presence of damaging algae that is feeding on an abundance of nutrient sources in the water.

Lake Shawnee is regularly impacted by blue-green algae alerts which sometimes result in the closure of recreational water activities and its swimming beach to protect people from the budding hazard. While the lake is certainly not the only body of water to be impacted by blue-green algae in Kansas each year, state health officials report that existing conditions near the lake are helping contribute to its growth during the summer months.

27 News reached out to the KDHE this week to learn what factors are playing a part in the growth of blue-green algae in Lake Shawnee. Mitchell Osterlund with the KDHE provided several responses from the agency's HAB team regarding the issue.

What is blue-green algae?

The Kansas Department of Health and Environment (KDHE) issues public health advisories each year starting in the spring and running into the fall for blue-green algae which appears as a paint-like or scummy material in the water. These alerts are classified by severity status: watch, warning and hazard.

Blue-green algae can harm people, pets and livestock that come into contact with it. If conditions are right, the algae may grow out of control and result in a harmful algal bloom (HAB) that can be toxic to the point of causing severe of fatal illnesses for those who come close to it. A "perfect storm" of heat, low movement in the water and ample nutrients can result in the shutdown of recreational activities at lakes by giving harmful algae the right environment to expand in.

"Harmful Algal Blooms (HABs) are composed of cyanobacteria, also known as blue-green algae," said the KDHE's HAB team. "Cyanobacteria are a natural component of a healthy ecosystem and normally present at lower densities within the freshwater systems of Kansas, including lakes, ponds, streams, and rivers. When specific conditions exist, for example ample sunlight, warmer water temperatures, minimal wind/wave mixing, and high nutrient levels, cyanobacteria densities can increase rapidly, leading to the formation of a bloom."

The KDHE said HABs can be unpredictable due to other influences ranging from local conditions to chemical factors. Lakes that enter warning or hazard status are deemed unsafe for people and animals to enter. Blue-green algae in a lake can cause harm to those who touch it, inhale it or consume it.

Blue-green algae in Lake Shawnee

The KDHE listed Lake Shawnee as being at the warning level for blue-green algae at the outset of its monitoring season in 2025. It last reached warning status in June 2022 and was under a watch for much of 2024, hampering the enjoyment of recreational water activities there.

Health officials lifted the blue-green algae warning at Lake Shawnee on May 23 this year just prior to Memorial Day weekend and the opening of Adventure Cove after testing showed the algae threat had subsided. Had the alert remained in place, the lake's swimming beach would have remained closed as it had the year prior due to an overabundance of algae.

Shawnee County Parks and Recreation (SCP+R) officials will reduce or shut down recreational water activities at Adventure Cove when the algae threat becomes prominent. Sean Zears with SCP+R said he hopes the algal levels remain low throughout the summer and that park officials will take action if necessary.

"It is out of our control as to controlling blue-green algae," Zears said. "The costs to prevent it are way over budget."

What is causing blue-green algae to grow in Lake Shawnee?

The KDHE's HAB team reports that Lake Shawnee is listed as an "impaired surface water body" because of something called eutrophication. This means nutrients are abundant within the water that provide fuel for the growth of blue-green algae and HABs.

KDHE staff created a Total Maximum Daily Load (TMDL) report for Lake Shawnee that was approved by the Environmental Protection Agency (EPA) in 2012 to address the input of local nutrient sources into the lake. The TMDL refers to the maximum amount of a pollutant that a body of water can receive while still meeting water quality standards. Lake Shawnee was listed as "Very Eutrophic" in the 2012 TMDL, just below the highest level of "Hypereutrophic."

The KDHE lists several possible nutrient sources that contribute to the growth of blue-green algae in Lake Shawnee. These include agricultural and lawn fertilizer runoff, wastewater discharge, erosion, decaying grass clippings and other vegetation, stormwater and more.

"While efforts to control and reduce external sources of nutrient enrichment are recommended to continue in the watershed, legacy inputs of nutrients that have accumulated in the lakebed sediment over time may continue to serve as a source of internal nutrient loading for years to come," the KDHE's HAB team said. "For example, extended periods of limited wind mixing in lakes that display mild stratification and eutrophication can lead to inadequate oxygen levels in the lake’s deeper waters during the summer. This condition can allow phosphorus to enter the upper water column, sustaining an active bloom."

The TMDL goes on to state that the lake's large goose population may contribute to the addition of nutrients to the water through the droppings they leave behind. The presence of invasive zebra mussels in Lake Shawnee may also be a contributing factor to the growth of blue-green algae.

Several recommendations are listed in the TMDL for people to follow to help keep the lake clean and reduce the influx of nutrients into the water. These include:

• Educating locals on appropriate lawn fertilizer application.
• Installing grass buffer strips along drainage channels in the watershed.
• Promoting proper management of construction sites to minimize sediment and nutrient runoff.
• Promoting the installation of porous and concrete grid pavement in the watershed.

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