The warning from the Nebraska Department of Health and Human Services is the first of the year. Officials began conducting weekly tests at 50 public lakes across the state in early May, and the tests will continue throughout the summer.
Officials say Lone Star Lake has a high level of microcystin, a toxin released by strains of blue-green algae.
Skin exposed to the toxin can develop rashes and blisters. Someone who drinks water containing the toxin is at risk for headaches, nausea and muscular pain.
Lone Star Lake will remain under alert for at least two more weeks because it must have two consecutive weeks of safe tests before the alert is lifted.