The effects of spectral exposure corresponding to normal and depleted stratospheric ozone concentrations on photosynthesis and mycosporine-like amino acids (MAAs) contents of different natural phytoplankton communities were studied in early austral summer 1995/1996 during the JGOFS ANT XIII/2 cruise in the Atlantic Sector of the Southern Ocean. The radiation conditions were simulated in a special solar simulator in which the same sample was incubated under 2 light regimes differing in UV-B doses. In all phytoplankton samples the quantum yield of electron transport in photosystem II (PSII) decreased after incubation under increased ultraviolet radiation (UVR) levels. Only samples outside of phytoplankton blooms showed a significant lowering of photosynthetic production rate due to enhanced UV-B. Phytoplankton cells within the blooms probably received protection from UV-absorbing MAAs, because only there cells, chains or colonies of phytoplankton communities were large enough to act in combination with MAAs as effective sunscreens. In addition, within the blooms, due to shallow upper mixed layers (UMLS) and stability within the water column, cells had probably enough light to maintain turnover rates of repair mechanisms at PSII and induce sufficient MAA synthesis; these processes were able to compensate for the negative effects of UVR. In contrast, the damaging effect on photosynthesis was much more severe on phytoplankton 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.