Abstract: The treatment of water to remove gas vacuolate cyanobacteria is effected by subjecting the water to a predetermined pressure by being pumped down a centrally located pipe in a bore-hole, which is steel lined. This causes the gas vesicles in the cyanobacteria to collapse. The treated water is then stored in a settling tank where the cyanobacteria sink to the bottom and can be removed along with any other sediment in the water. The method can be applied to decrease the amount of cyanobacteria with gas vesicles in a lake or other water impoundment. (FIG.