Structure of the heterotrophic flagellate community in the water column of the River Rhine (Germany)

The planktonic heterotrophic flagellates (HF) of the Lower River Rhine (Germany) were investigated over a period of 21 months with special attention on the taxonomic composition. Samples were taken in Cologne (Rhine-km 685) and Kalkar-Grieth (Rhine-km 845) from the same water parcel (Lagrangian sampling). A live-counting technique in combination with video microscopy was employed, allowing a quantitative analysis and an acceptable taxonomic resolution. The majority (99.4% of the annual mean abundance and 73.9% of the annual mean biovolume) of the HF were nanoflagellates (HNF, 2-20 mum in length). Large heterotrophic flagellates (LHF, >20 mum in length), which were mostly composed of flagellates of uncertain taxonomic position (Protista incertae sedis), peaked in spring and summer. Forty genera of HNF belonging to the chrysomonads, choanoflagellates, bodonids, Protista incertae sedis (including apusomonadids, cercomonads and thaumatomastigids), bicosoecids, euglenids, cryptomonads, dinoflagellates, hemimastigids and diplomonads (mentioned in decreasing order of their annual abundance) were recorded. The most abundant genus was that of the chrysomonad Spumella. Different seasonal patterns in the occurrence of the different genera and species could be demonstrated. Some species such as Bodo saltans and Stelexomonas dichotoma were only recorded in winter, whereas others such as Bicosoeca lacustris and Desmarella spp. were recorded mostly in summer. The HF assemblages in the River Rhine were demonstrated to be as diverse and with a similar composition to those in lentic waters.