A cyanobacterial cultivation process, designed to mitigate impacts of thermal effluents by reducing their nutrient content, was simulated in bench-scale rests using semicontinuous cultures of the nitrogen fixing thermophile Fischerella (formerly Mastigocladus laminosus). The cyanobacterial strains were isolated from and grown in water from nuclear reactor cooling reservoirs at the Savannah River Site near.Aiken, South Carolina. Major limiting factors for biomass production under various scenarios included temperature, light, carbon and phosphorus. The amount of biomass recycling needed to maintain a stable biomass in the mitigation (nutrient removal) zone of the mock reservoir varied with different light and temperature scenarios that were tested. In the worst case scenario, when biomass was repeatedly recycled into heated water at the end of the light cycle rather than at the beginning, stable daily production could be maintained only when 75% of the biomass was recycled. Otherwise, the inoculated biomass was able to maintain one doubling per day allowing a daily recycle of 50% of the biomass. Overall, the growth rates and primary production capabilities needed for successful operation of a proposed process for nutrient removal from thermal effluents were demonstrated in these laboratory simulations.