Seasonal dynamics of heterotrophic and plastidic protists in the water column of Lake Biwar Japan

摘要

We followed seasonal changes in the vertical distribution of protistan communities in Lake Biwa for 2 yr. Heterotrophic nanoflagellates (HNF) and plastidic nanoflagellates (PNF) were enumerated from different depths, and the seasonal community composition of protists was analyzed from the epilimnion and hypolimnion using 18S rRNA gene clone libraries. In the epil-imnion, HNF were dominant during early summer and autumn, whereas PNF, which includes mixotrophs, had a higher contribution during the latesummer and winter months. Changes in the abundance of HNF and PNF differed, and their peaks of abundance did not coincide, suggesting that these groups show contrasting responses to changes in environmental conditions. Protistan communities in the epilimnion and hypolimnion were represented mainly by cryptophytes and dinophytes, respectively, whereas cercozoans had a significant contribution in both (i.e. all libraries). We found high diversity and distinct protist communities in the epilimnion and hypolimnion, where the majority of the hypolimnion sequences had low similarity with related sequences in public databases. This indicates that little is known about protistan communities from deep freshwater lakes and that further studies from deep watersare needed to understand the role of these organisms.