Abstract: A method of retarding the growth of algae in aqueous solution is disclosed. An aqueous medium is moved along a flow path in a direction terminating at an aqueous solution into which the aqueous medium is added. Before adding said aqueous medium into the aqueous solution, a gas stream having a nitrogen content in excess of 90% is injected into the aqueous solution in the same direction in which the aqueous solution is moved along the flow path. The gas stream is injected into the aqueous medium at a flow rate of at least 0.01 cubic feet per hour and said gas stream and said aqueous medium continue to intermix along said flow path prior to reaching the termination of the flow path at the aqueous solution.