The present invention is directed to a composition for reducing the levels of microorganisms in recreational water systems, comprising: (1) a biocidal effective amount of dibromonitrilopropionamide (DBNPA); (2) optionally, a biocidal effective amount of an algaecide selected from the group consisting of didecyldimethylammonium chloride (DDAC), zinc, and copper; and (3) optionally, a compound capable of in situ activation to form an oxidizing agent; wherein the composition is effective for reducing the levels of microorganisms in recreational water systems. The present invention is also directed to a method of controlling the growth of microorganisms in recreational water systems, comprising the step of providing the above composition; and adding the composition to a recreational water system.