(KLZA)-- The state of Nebraska has extended health alerts for Cub Creek Lake in Jefferson County and Rockford Lake in Gage County due to the presence of blue-green algae.

Iron Horse Trail Lake in Pawnee County has been removed from the alert.

Samples taken earlier this week at the affected lakes measured above the threshold of eight parts per billion of total microcystin, a toxin released by certain strains of blue-green algae.

When a health alert is in efect, signs are posted to advise the public to use caution, and designated swimming beaches are closed.

Recreational boating and fishing are allowed, but visitors should avoid activities that could lead to swallowing the water.