We assessed the in situ daily primary production, photosynthetic efficiency, and xanthophyll cycling of a temperate coral, Cladocora caespitosa, during the summer using an in situ incubation chamber equipped with temperature, oxygen, light, salinity, and pH sensors. During sunny days, photosynthetic carbon (C) production rates of C. caespitosa can be as high as those measured for tropical corals and rapidly follow changes in natural irradiance (from 200 to 1000 mu mol photons m(-2) s(-1) within 2 h). As a consequence, daily production rates varied by a factor of 2 (from 150 and 260 mu g C cm(-2) d(-1)) depending on the irradiance received. Under high irradiance levels, corals can suffer from photoinhibition (light-induced reduction in the photosynthetic capacity), especially temperate species, which do not experience high irradiance levels most of the year. However, in C. caespitosa, photoinhibition at irradiances higher than 1000 mmol photons m(-2) s(-1) was reduced as a result of the involvement of the xanthophyll cycle, with a degree comparable to those measured for tropical species (de-epoxidation ratio of 0.12), that allowed C. caespitosa to maintain high production rates and a maximal autotrophic carbon acquisition during sunny days. However, as soon as irradiance conditions decreased below 200 mu mol photons m(-2) s(-1) because of cloudy weather, autotrophically acquired carbon could not sustain respiratory needs, indicating that C. caespitosa has to rely on other sources, such as heterotrophy, to meet its energetic needs.