Structure, dynamics and control of the ciliate fauna in the potamoplankton of the River Rhine

Planktonic ciliate abundance and taxonomic composition were investigated from November 1998 to January 2000 in the River Rhine at Cologne, Germany. The composition of the ciliate fauna revealed the occurrence of both typical benthic (drifting peritrichs) as well as pelagic ciliates (oligotrichs) in the potamoplankton. Oligotrich ciliates prevailed in the potamoplankton community. Peritrichs, represented almost exclusively by sessile species, appeared to be washed out from the substrate at increased water discharge during flood events. Highest abundances of ciliates were registered during the phytoplankton spring bloom, indicating a bottom-up control of ciliates during this time, an observation which was supported by laboratory experiments. In summary, the ciliate potarnoplankton community of the River Rhine is controlled by a seasonally varying set of factors: 1) Flood events in early spring and autumn cause a short-term succession from drifting peritrichs to pelagic oligotrichs, 2) the phytoplankton spring bloom supports oligotrich population growth, and 3) the low water level and subsequently high grazing pressure by benthic predators (mainly the mussel Corbicula) reduce the abundance of ciliates. The latter was probably the major reason for the extremely low ciliate abundances observed during summer.