The size structure of the Mesodinium rubrum population in the Gdańsk Basin

The ciliate Mesodinium rubrum Lohmann 1908 (= Myrionecta rubra Jankowski 1976) is an important phototrophic organismin the Gdańsk Basin. In June 2002 the vertical distribution and size structure of the M. rubrum population were studied. Its presence was generally observed in the whole water column (one exception was the anoxic near-bottom zone in the Gdańsk Deep) at all stations studied. Maximum abundance (18 300 cells dm−3) was recorded at 26 m depth at the station located in the inner Gulf of Gdańsk. Analysis of the size structure of the counted organisms demonstrated the co-existence of small and large cells of M. rubrum in the upper layer of the water column and a gradual increase with depth of the prevalence of large specimens. This shows that at least two forms of M. rubrum exist in the region studied. Deep migrations are probably undertaken only by relatively large organisms.