Longitudinal and Transverse Distribution of Plankton Rotifers in the Potamal of the River Elbe (Germany) during Late Summer

In July and August 1999, the longitudinal and transverse spatial dynamics of plankton rotifers and phytoplankton, the chlorophyll a content and abiotic parameters, were investigated in the potamal of the River Elbe, Germany during low river discharge. Longitudinal changes in composition and abundance were studied at 9 sampling stations over a distance of 537 river kilometers. In total 42 rotifer taxa were identified. Species composition and abundance were remarkably distinct from those of other large lowland rivers. Rotifers exhibited a steady downstream increase in abundance, with a maximum up to 12135 individuals l–1 at the most downstream station with Trichocerca pusilla dominating (37.0–77.7% of total abundance) the community at all stations. Phytoplankton abundances peaked in the middle part of the potamal (km 317) with 224 · 106 cells · l–1. At all stations small centric diatoms (<25 μm) and green algae (coccales) dominated the phytoplankton community. Small-scale transverse variations of abundance were investigated by sampling 6 transects of the main stream and the corresponding groyne fields in the central section (km 422) of the stretch of the river that was examined. Statistical analysis revealed no significant differences in abundance of rotifers and phytoplankton between main stream and groyne fields.