Buoyant cyanobacteria, previously mixed throughout the water column, float to the lake surface and form a surface waterbloom when mixing subsides. At the surface, the cells are exposed to full sunlight, and this abrupt change in photon irradiance may induce photoinhibition; at the same time, temperature rises as well. This study investigated the damaging effects of this increase in temperature as well as the ecologically more relevant combination of both an increased temperature and a high photon irradiance. Analysis of surface blooms with oxygen microelectrodes showed that integrated oxygen contents that are dependent on the balance of photosynthetic oxygen evolution and respiratory oxygen uptake decreased when temperature was raised about the lake temperature. Gross rates of photosynthesis were unaffected by temperatures up to of 35 degrees C; hence, a moderate increase in temperature mainly stimulated oxygen uptake. Preincubation of cells of the cyanobacterium Anabaena flos-aquae (Lyngb.) de Brebisson at temperatures up to 35 degrees C did not affect the subsequent measurement of rates of net photosynthesis. Another 5 degrees C rise in temperature severely damaged the photosynthetic apparatus. Failure to restore net rates of photosynthesis was coupled to a strong quenching of the ratio of variable to maximum fluorescence, F-v/F-m, that was the result of a rise in F-0. A combination of high temperature and high photon irradiance was more damaging than high temperature alone. In contrast, low photon irradiances offered substantial protection against heat injury of the photosynthetic apparatus . I conclude from this study that because cyanobacteria usually are acclimated to low average irradiance prior to bloom formation, there is a reasonable risk of chronic photoinhibition. The increase in temperature will enhance the photodamage of cells in the top layer of the bloom. Low photon irradiances in subsurface layers will offer protection against heat injury. If the high temperatures extend to the deepest, dark layers of the bloom, damage in those layers is likely to occur.