Longitudinal and seasonal dynamics of the planktonic microbial community along the length of the River Rhine
Bergfeld, Tanja; Scherwass, Anja; Ackermann, Barbara; Fischer, Helmut; Arndt, Hartmut; Schöl, Andreas
We examined the abundances, biovolumes and biomass of bacteria, heterotrophic nanoflagellates (HNF) and ciliates along a large river, the Rhine. Water samples were taken every two weeks at six sampling sites between km 173 (Weil, Germany) and km 1019 (Maassluis, Netherlands) from January to December 2000. Microbial abundances along this river stretch of 850 km were relatively low as compared to other large rivers. Bacterial abundances ranged between 0.3 and 38.6 × 106 bacteria ml-1. The highest abundances occurred in winter and spring. Bacterial sizes were very small during the whole year, most bacteria being smaller than 0.3 μm in diameter. HNF abundances varied from 7 to 1500 HNF ml-1. Ciliate abundance ranged between 5 and 1095 ciliates l-1. Bacteria reached their minimum biomass at km 590 (Koblenz) and increased further downstream, while HNF and ciliates showed no longitudinal trend. The simultaneous measurements at the six sampling sites showed that chlorophyll a and seston concentrations increased until km 865, which indicates a concurrent improvement in the availability of bacterial substrates during all seasons downstream. We postulate a food limitation of bacteria along the Rhine. The small bacterial size can be an indicator for starved cells or for an adaptation to low substrate levels. HNF and ciliates reached highest abundances and biovolumes in winter and spring together with their preferred food organisms bacteria and chlorophyll a, respectively. Whereas food limitation of HNF and ciliate biomass seems to prevail in winter and spring, we discuss that the protozooplankton is controlled by benthic grazing in summer and autumn along the Rhine. For all seasons, the planktonic microbial components were more regulated by site-specific factors than by longitudinal transport along the Rhine.