The city has issued a “watch” advisory for parts of Lake Mitchell where algal blooms are visible due to microcystin levels reaching above the respective threshold of 4 parts per billion (ppb).

According to Parks and Recreation Director Nathan Powell, recent tests over the past weekend detected the lake had microcystin levels at 5 ppb. Powell emphasized there is a big difference between a watch alert and warning alert. The city’s policy for a public health warning at Lake Mitchell discourages direct contact with the lake but doesn’t call for the closure of the lake, while a public health watch only discourages the specified portion of the lake where microcystin levels are above 4 ppb.

“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.”

The two toxins that exist in Lake Mitchell -- which the city of Mitchell tests for -- 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 human and animal illness.

The city tests the lake for the respective cyanobacterias 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.

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Powell said the cylindrospermopsin levels must be at or above 15 ppb for the city to issue a public health warning.

A public health warning will only be issued when microcystin toxin concentrations are greater than or equal to 8 ppb in any testing location, which used to be 20 ppb, 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.

“If we reach the warning level as well, we place signage all along the lake and do a press release,” Powell said.