ABSTRACT: During the summer of 1985, a small (2 to 3 µm) chrysophyte, Aureococcus anophagefferens, bloomed and dominated the phytoplankton assemblage, and recurred each summer for the following 12 yr in bays of Long Island, New York, USA. Macronutrientsremained at high levels throughout the years but Secchi disc depths were as low as 30 cm in some areas, corresponding to a 1% light depth of about 80 cm. In batch culture, A. anophagefferens maintained high growth rates at low irradiances. Theinitial slope of the growth versus irradiance curve, alpha_g, was 0.021 ± 0.003 divisions d^-1 (µmol quanta m^-2 s^-1)^-1 which is similar to that for bloom formers such as Skeletonema costatum. Incontinuous culture, constant and fluctuating irradiance regimes resulted in equivalent steady state growth rates at 0.46 divisions d^-1. Short-term carbon fixation per unit chlorophyll a was about twice as high in the fluctuatingirradiance 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 versusirradiance parameter, P_max (light saturated photosynthetic rate), was similar in both irradiance regimes; however, alpha_p (initial slope) was consistently higher in the fluctuating regime, indicating that photosyntheticefficiency is higher in fluctuating irradiance.