Changes in the plankton community along the length of the River Rhine: Lagrangian sampling during a spring situation

The development of the planktonic microbial community [bacteria, heterotrophic nanoflagellates (HNF) and ciliates] was investigated along a 660 km stretch of the River Rhine using Lagrangian sampling in May 2000, and by taking the influence of the main tributaries into account. The relative importance of the components of the microbial food web (HNF, ciliates) increased in the course of the German river stretch, whereas the absolute abundance of protozooplankton decreased and phytoplankton abundance increased. Bacterial abundance remained relatively constant throughout the river stretch investigated. The taxonomic composition of phytoplankton changed from a dominance of cryptophytes to a dominance of diatoms, whereas flagellates, ciliates and metazooplankton showed no shift in species composition. Some tributaries had higher abundances of planktonic organisms than the Rhine itself, but only the River Moselle contributed significantly to the plankton load of the Rhine. However, even after its confluence with the River Rhine abundances of the heterotrophic plankton (HNF and ciliates) remained low and decreased further downstream. The shifts in the composition of the pelagic microbial food web along the river stretch are discussed in the context of group-specific growth and loss processes.