The Baltic Sea ice biota (March 1994): A study of the protistan community

The diversity of the protist communities of the water column and ice along the Finnish coast of the Baltic Sea was studied during March 1994. The preliminary identification of the organisms was made light microscopically on live material, while further identification of e.g. scale-bearing flagellates was based on whole mounts using light and electron microscopy. The ice biota and the winter plankton was dominated by diatoms. Other abundant groups were choanoflagellates, chrysophytes, chlorophytes, dinoflagellates and protists of uncertain affinity. The highest brine salinities (up to 30 permill ) and the most diverse ice biota were found at two stations in the Bothnian Bay, where the number of recorded taxa was 71 and 74. In the water column the numbers were much lower (34 and 42). In the Bothnian Sea and along the southern coast of Finland the ice biota was less diverse, and the number of protist taxa ranged from 21 to 47. A few taxa showed a special preference for the ice habitat. New distribution records to the Baltic Sea are Navicula pelagica (Bacillariophyceae), Cryothecomonas armigera (Protista incertae sedis), and the genus Polytomella (Chlorophyceae). The abundance and vertical distribution of C. armigera, other flagellates, Monoraphidium contortum (Chlorophyceae), and diatoms in the ice at station I was studied using inverted light microscopy and sedimentation chambers. M. contortum was found to be the cause of a distinct green colouration of the ice. With the exception of C. armigera, all other protist taxa formed well-defined maxima within the ice interior. The number of cells in such abundance peaks varied from approximately 2.5 times 10-5 M. contortum cells l-1 to 2 times l0-6 diatoms l-1 of melted sea ice.