Distribution of eukaryotic plankton in the English Channel and the North Sea in summer

The distribution of eukaryotic plankton was investigated in the English Channel and the North Sea during the MICROVIR cruise in summer 2007. The size distribution of autotrophic, heterotrophic eukaryotes and species composition was analyzed with a focus on two major divisions, Haptophyta and Chlorophyta, targeted by 18S rRNA probes. Picoeukaryotes (< 2 μm) dominated over the larger eukaryotes at all stations. Eukaryotes larger than 5 μm were mainly composed of diatoms in the English Channel and of dinoflagellates in the North Sea. The contribution of Haptophyta was maximal in the 2 to 5 μm fraction and they appeared more abundant in the central region of the North Sea. Chlorophyta, especially Micromonas pusilla, generally dominated the picoplanktonic fraction in the English Channel. Micromonas contribution decreased between the South and the North-east of the North Sea and it was even absent at some stations. Although this species is dominant among the picoeukaryote community of the English Channel, other Chlorophyta species may also play an important ecological role in these temperate ecosystems.