The potential effect of an increase in ultraviolet radiation on organic matter in the sea surface microlayer and subsurface bulk seawater was evaluated in spring during three sampling periods in 1997, 1998, and 2000. The phytoplankton concentration is high in the water of the southern Baltic during this period. The results of temporal changes in chlorophyll a, phaeopigments, and oxygen concentrations were taken into account. An inhibitory effect (expressed as a decline in chlorophyll a concentrations) on biological processes in the sea surface microlayer as well as in the subsurface layer occurred around noon and in the early afternoon. High levels of ultraviolet radiation intensified chlorophyll a reduction. The measurements conducted in the microlayer and the subsurface layer indicated the possibility of photodamage to phytoplankton under natural conditions when there is an excessive increase of UV-B radiation in comparison to PAR. A significant increase in the phaeophytin a concentration and a decrease in the chlorophyll a concentration was observed at noon when the UV-B/PAR ratio was high.