Abstract
The distribution of protistan microplankton situated southwest (S09) and northeast (S18) of the Izu Ridge along the Kuroshio Current was revealed by 18S rRNA gene clone analysis. A total of 257 clones were identified, consisting of 65 phylotypes of dinoflagellates, 49 phylotypes of diatoms and 57 phylotypes of other protists affiliated with Ciliophora, Cryptophyta, Cryptophyta nucleomorph, Choanoflagellata, Chlorophyta, Cercozoa, and Heterokonta. The dinoflagellate phylotypes were affiliated with five genera and 14 uncultured groups, with Gyrodinium as the most frequently detected genus. The diatoms were also well represented and consisted of 13 genera and six uncultured groups. The clones belonging to the genus Pseudo-nitzschia were most frequently detected. The frequencies of dinoflagellate clones and phylotypes were higher at station S09 in the south than at station S18 to the north, with the frequency of diatom phylotypes being higher at the latter. The species richness (number of phylotypes) and diversity (Shannon-Wiener) of the protistan microplankton community were slightly higher at S18 compared to S09. When the Kuroshio Current encountered with the Oyashio Current at northwestern Pacific, it affects the water temperature and nutrients of the Kuroshio Current. The clone analysis results showed a difference in the protistan microplankton community at both stations due to the collision of both currents.
