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An unexpected increase of phototrophic and heterotrophic picoand nanoplankton under anoxic conditions was encountered in the central Baltic Sea below 120 m depth in August 1991 and 135 m depth in July 1992, respectively. Cell abundances in anoxic layers were up to 10 times higher for chroococcoid cyanobacteria, 6 to 8 times for phototrophic nanoflagellates and 3 to 4 times for bacteria, as compared to the overlaying aphotic oxic layers. A characteristic ciliate community occurred at the oxic/anoxic interface. Phototrophic organisms of anoxic depths were capable of productivity when reoxygenated and returned into the light. Considerable grazing on bacteria (0.41 to 0.65 d ' ) and chroococcoid cyanobacteria (1.19 to 1.29d-') was revealed by serial dilution experiments. Cyanobacteria showed gross growth rates of 0.85 to 1.33 d' under aphotic/anoxic conditions. The origin of deep-water phototrophic picoand nanoplankton and the existence of a permanent anaerobic microbial food web are discussed. A number of investigations have emphasized the occurrence of procaryotic and eucaryotic picoand nanoplankton in the oxic deep-sea (Hamilton et al. 1968, Fournier 1970, 1971, Silver & Alldredge 1981, Platt et al. 1983, Davis et al. 1985, Silver et al. 1986, Lochte & Turley 1988). Little information exists, however, on picoand nanoplankton in anoxic depths. For nearshore areas, recent investigations revealed distinct populations at the oxic/anoxic interface and in anoxic near-bottom layers (Fenchel et al. 1990). Anoxic conditions are typical for the deep basins of the Baltic Sea (Matthaus 1986). The present paper describes our observations of the occurrenceof phototrophic and heterotrophic picoand nanoplankton in deep anoxic water layers of the Gotland Basin, central Baltic Sea. We discuss whether these unique biotas are of overall significance for the interpretation of the Baltic ecosystem. Material and methods. Data presented were collected during RV 'Alkor' cruises BAMBI '91 and BAMBI '92 (Baltic Microbial Biology Investigation) in August 1991 (8 d drift study) and July 1992 (9 d drift study) in the eastern part of the Gotland Sea, central

Phototrophic and heterotrophic pico- and nanoplankton in anoxic depths of the central Baltic Sea