The brown alga Fucus vesiculous is one of the few marine species in the Baltic Sea. F. vesiculosus shows high morphological and physiological variability as a response to its environmental conditions. The salinity in the Baltic Sea is 4-5 psu in comparison to 35 psu in the Atlantic. Photosynthesis of algae is usually measured after collection and transportation to constant culture conditions. However, in this study the relative photosynthetic electron transport rate (rETR), calculated from chl a fluorescence parameters were compared for algae from 1 and 4 m depths by SCUBA divers. Measurements of light response curves from the same individuals of F. vesiculosus at different depths and times of the year have to our knowledge never been made before. Measurements were performed at four different occasions during the spring (February 25, April 3 and 29 and May 26 in 2005) in the Baltic Sea, using rapid light curves (RLCs) generated with a Diving PAM. In addition, samples were collected for photoinhibition studies in the laboratory. The light response curves obtained in situ at 1 and 4 m depths for F. vesiculosus showed lower values of light saturation with depth. When algae from 1 and 4 m depths were exposed to high irradiances of PAR (1400 µmol photons m-2 s-1), algae from 1 m depth showed a higher degree of photoinhibition in comparison to algae from the 4 m depth.