Heat, drought causing spike in algal blooms at Lake Mitchell

“Historically, the dry, hotter summers have led to increased toxin and microcystin levels. And that’s what is happening with the drought and higher temperatures,” said Parks and Recreation Director Nathan Powell.

Algae on the banks of Lake Mitchell at Sandy Beach. (Matt Gade / Mitchell Republic)
Matt Gade

The heat waves and drought conditions that Mitchell has experienced this year is taking its toll on Lake Mitchell.

Parks and Recreation Director Nathan Powell said this summer has seen the most “warning” and “watch” advisory levels at Lake Mitchell since 2017, which is when the city began testing on a weekly basis. The first watch advisory was issued in late May, while the first warning came shortly after on July 8 and lasted until July 26, equating to a total of five weeks worth of advisories this summer.

“Historically, the dry, hotter summers have led to increased toxin and microcystin levels. And that’s what is happening with the drought and higher temperatures,” Powell said. “In 2017, we were at a watch and warning level advisory from early summer into October. That was also a dry year, so it shows the dry and hot weather has a big impact on the water quality.”

The city tests certain parts of the lake for respective cyanobacteria once a week, which adheres to the Environmental Protection Agency (EPA) guidelines. Parts per billion is the units of measure the Parks and Recreation Department uses to calculate and identify the concentrations of the two contaminants or toxins. The two toxins that exist in Lake Mitchell are cylindrospermopsin and microcystin, which are a form of cyanobacteria, also known as blue-green algae. They both can produce toxins in recreational waters and have been found to cause illness in humans and animals.

A public health warning will only be issued when microcystin toxin concentrations are greater than or equal to 8 ppb in the testing locations, which includes the public beaches, coves and the center of the lake. Previously, the microcystin levels used to be 20 ppb for a public health warning to be issued, Powell noted. In addition, if the cylindrospermopsin concentration is greater or equal to 15 ppb in any testing location, a public health warning will be issued.


“The specified portions of the lake that are discouraged to swim under the current watch advisory are anywhere where there is visible algal blooms,” Powell said. “When we reach levels for a public health watch, we place signage at all the access areas to let people know we discourage them from swimming in areas where algal blooms are visible.”

At its peak, parts of the lake saw microcystin levels reach 10 ppb in early July this year, marking a significant spike from the 2.5 ppb that the lake experienced in the same time span in 2020. Comparing this summer to 2017, the bacteria levels aren’t as bad, considering microcystin levels reached 29 ppb in early August.

In the past five years, the water quality was at its best in 2020 and 2018, both of which saw much more precipitation than this year’s dry spell. There was just one warning and no watch advisories in 2018. In 2020, there were a total of three weeks that the lake saw warning levels.

While the lake had been under a warning level advisory since July 8, on July 26 there was a significant improvement, as the bacteria levels fell below the 4 ppb mark for the first time since mid June. As of now, there are no advisory watches or warnings at Lake Mitchell, but Powell realizes there is plenty of summer left yet.

“We could still get some more watches and warnings because the summer is not over,” he said. “Conditions can change daily, so it’s wise to refrain from areas where algae is thicker than other parts of the lake, even if we’re not in an advisory.”

Working toward solutions

As the algae woes intensify this summer, Mitchell Mayor Bob Everson has been working with numerous entities to advance the city’s plan to create a wetland and dam along Firesteel Creek to filter out the phosphorus and sediment flowing into the lake.

While Ducks Unlimited finishes up the remaining design work of the wetland that’s going to be constructed roughly two miles west of the lake near the former Kelley property, Everson said he’s anticipating to see ground break on the project by the fall with the goal of bringing some long-awaited improvements to the lake.

“We are anticipating to see the wetland take off before the winter hits,” Everson said. ”Once the design is done, we will be able to get bids opened up for crews to begin building it.”


Powell said the wetland will have a “major impact” on the algal blooms, as he pointed out that 53 percent of the phosphorus entering into the lake is coming in from Firesteel Creek, while 47 percent of it is in the lake itself. Working upstream before dredging the lake, which the city is also in the process of working towards, is a key move to enhance the lake’s water quality, Powell said.

Looking at the historical data on the lake’s algal blooms, Everson said he isn’t surprised to see bacteria levels uptick this summer. With the wetland inching closer to becoming a reality, Everson hopes to reverse the course of algae woes that have hampered Lake Mitchell for several decades.

“The phosphorus absorbs in the lake, and when we don’t have any water flow and movement going over the spillway (dam), we see more algal blooms,” Everson said. “When the wetland is done, we hope to see some improvement on the bacteria levels early on but it will be a gradual process.”

Sam Fosness joined the Mitchell Republic in May 2018. He was raised in Mitchell, S.D., and graduated from Mitchell High School. He continued his education at the University of South Dakota in Vermillion, where he graduated in 2020 with a bachelor’s degree in journalism and a minor in English. During his time in college, Fosness worked as a news and sports reporter for The Volante newspaper.
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