Biofouling occurs because water is a favorable environment for many life forms. Flow characteristics of recirculating cooling water systems encourage colonization by sessile organisms to use the circulating supply of food, oxygen and nutrients. Temperatures may become high enough to support thermophilic populations. Biofouling of heat exchange surfaces can reduce heat transfer rates of the cooling system; and biofouling of cooling towers can alter flow distribution to reduce evaporative cooling rates. Biofouling may also create differential oxygen concentrations increasing corrosion rates. OTC and open recirculating systems are most susceptible to biofouling. Biofouling may be inhibited by temporary habitat modifications. Temperature differences may discourage establishment of thermophilic populations in intermittently operated facilities; and intentional short term temperature spikes may periodically kill less tolerant populations. Biocides have been commonly used to control biofouling where sustained facility operation is required.