The invention provides a method for reducing aquatic organic contamination present in a volume of water, the method comprising: pumping the water from an open body of water contaminated with aquatic organisms through a reactor unit including a conduit system of varying diameter such that the pressure head in the water is caused to fall to a level below atmospheric pressure, and so to cavitate, at a point in the system by increasing the velocity head of the water at that point. The invention also includes apparatus for use in the method.