ABSTRACT: The effects of spectral exposure corresponding tonormal and depleted stratospheric ozone concentrations onphotosynthesis and mycosporine-like amino acids (MAAs)contents of different natural phytoplankton communities werestudied in early austral summer 1995/1996 during the JGOFS ANTXIII/2 cruise in the Atlantic Sector of the Southern Ocean. Theradiation conditions were simulated in a special solar simulator inwhich the same sample was incubated under 2 light regimesdiffering in UV-B doses. In all phytoplankton samples the quantumyield of electron transport in photosystem II (PSII) decreased afterincubation under increased ultraviolet radiation (UVR) levels. Onlysamples outside of phytoplankton blooms showed a significantlowering of photosynthetic production rate due to enhanced UV-B.Phytoplankton cells within the blooms probably received protectionfrom UV-absorbing MAAs, because only there cells, chains orcolonies of phytoplankton communities were large enough to act incombination with MAAs as effective sunscreens. In addition, withinthe blooms, due to shallow upper mixed layers (UMLs) andstability within the water column, cells had probably enough light tomaintain turnover rates of repair mechanisms at PSII and inducesufficient MAA synthesis; these processes were able tocompensate for the negative effects of UVR. In contrast, thedamaging effect on photosynthesis was much more severe onphytoplankton cells outside the blooms; most cells (70 to 90%)here were too small to receive protection from the MAAs present,and UMLs were deep and mixing rates high.