ABSTRACT. Durlng the summer of 1985, a small (2 to 3 pm) chrysophyte, Aureococcus anophagefferens, bloomed and dominated the phytoplankton assemblage, and recurred each summer for the followlng 12 yr in bays of Long Island, New York, USA. Macronutrlents remained at h~g h levels throughout the years but Secchi d~s c depths were as low as 30 cm in somc arcds, corresponding to a 1 ",, light depth of about 80 cm. In batch culture, A. anophagefferens malnta~ned hlgh growth rates at low irrad i a n c e~. The ln~tial slope of the growth versus irradiance curve, a,, was 0.021 + 0.003 divisions d-' (pm01 quanta m-2 S-')-' which is similar to that for bloom formers such as Skeletonema costaturn. In continuous culture, constant and fluctuating irradiance regimes resulted in equivalent steady state growth rates at 0.46 divisions d.'. Short-term carbon fixation per unit chlorophyll a was about twice as high in the fluctuating irradiance regime compared with constant irradiance. Differences between short-term carbon fixation and cell growth could not be accounted for by dark fixation or exudation and are assumed to be due to photorespiration. The photosynthesis versus irradiance parameter, P,,,,, (light saturated photosynthetic rdte), was similar in both irradiance regimes; however, g, (initial slope) was consistently higher in the fluctuating regime, indicating that photosynthetic efficiency is higher in fluctuating irradiance.