Effective immediately, Prospect Lake in Memorial Park is off-limits to swimmers, boaters, water skiers and all bodily contact following a positive E. coli test, city officials said in a Thursday news release.

A test of the lake water on Wednesday yielded an elevated concentration of the bacteria, according to the release. The maximum acceptable level of E. coli is 235 organisms per 100 milliliters. Wednesday’s test returned a level of 307.6 organisms per 100 milliliters, officials said.

The lake was retested on Thursday and could reopen if the results show an acceptable level of the microbe. The results of Thursday’s test should be returned on Friday, the release stated. The city typically tests the water once a week.

“During the closure, the following activities are prohibited: swimming, bathing, paddle boarding, tubing, water skiing and non-motorized boating of any kind. No pets are allowed,” the release stated. “Fishing areas will remain open, though anglers are urged to clean fish well and remove guts.”

Event organizers, together with the Parks Department, will make a decision about the “Splash and Dash” — in which balloons lift off from Memorial Park and then dip the bottom of their baskets in the lake before taking off again — after Friday’s test results.

E. coli is a bacteria primarily associated with fecal matter. According to the Colorado Department of Public Health and Environment, the elevated levels in Prospect Lake could be a result of a number of factors ranging from malfunctioning septic systems to fecal waste from swimmers or animals.

Signs announcing the closure will be posted around the lake, officials said.

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