Weekly measurements were made of light intensity, dissolved oxygen concentration, and water temperature at selected depths in five earthen experimental ponds. Measurements were made on a given pond on the same day between 7:00 a.m. and 8:45 a.m. and again between 10:00 a.m. and 11 :45 a.m. The depth at which the average light intensity, as measured with submersible Weston Photronic photoelectric cells, was less than 1 per cent incident radiation varied from 2.5 to 7.5 feet among the ponds, depending on the degree of Microcystis infestation. Generally, at depths where the average light intensity was not in excess of 1 per cent incident radiation, the average dissolved oxygen concentration was not in excess of 1 ppm. The average dissolved oxygen concentration in the pond with the most dense growth of Microcystis was usually less than 1 ppm. below 5 feet and less than 1 ppm below 7.5 feet in the ponds with the least amount of Microcystis. Generally, the decrease in water temperature with increased depth was directly related to the abundance of Microcystis. This study suggests that dense growths of scum-forming algae, such as Microcystis, limit the water depth at which the dissolved oxygen concentration is in excess of 1 ppm by limiting light penetration and by contributing to thermal stratification by heat absorption in the dense blooms near the water surface.