Cellular responses of 1-, 2- and 4-d-old Fucus spiralis embryos subjected to a single dose of elevated photosynthetically active photon flux density (PPFD), with or without ultraviolet (UV) radiation, were investigated by measuring the effects on the effective quantum yield of photosystem II (ΔF / Fm′) and intracellular production of active oxygen species (AOS). Production of AOS was determined by the in vivo conversion of 5-(and-6)-chloromethyl-2′,7′-dichlorodihydrofluorescein diacetate (CM-DCFH2-DA) to the fluorescent compound dichlorofluorescein (DCF) using confocal laser scan microscopy (CLSM) and image analysis. The role of xanthophyll cycle pigments in photoprotection was also assessed. A rapid decline in ΔF / Fm′ was observed under all elevated light conditions. A correlation was found between non-photochemical quenching and the de-epoxidation ratio zeaxanthin/(zeaxanthin + violoxanthin). Active oxygen formation increased with PPFD and was higher in older embryos and when UVB was present. Two photoinhibition responses were recognized: (i) a rapid decline of the PSII yield due to the violoxanthin–zeaxanthin cycle (photoprotection), and (ii) a slower second-phase decline, correlated with active oxygen production. Electron transport rate (ETR) increased with embryo age, and was correlated with AOS production. As a result of enhanced AOS production, there was a slow recovery of the PSII yield, in particular with increased effective UV dose. In general, embryos were able to recover from the imposed light conditions, but UVB had a more damaging effect. Overall, our data suggest that under natural conditions, embryos of F. spiralis are susceptible to elevated light levels, and that UVB radiation is an important stress factor.