Zooplankton community structure along a trophic gradient in a canyon-shaped dam reservoir

The zooplankton community was surveyed along the longitudinal axis of Rimov Reservoir (Czech Republic) on seven occasions during the vegetative season of 1996. The dimictic Rimov Reservoir has a pronounced trophic gradient along its axis. In nearly all samples, rotifers were dominant by numbers and formed on average 60-95% of the total zooplankton (including copepod nauplii). There was a consistent pattern of increasing relative abundance of rotifers in the upper regions of the reservoir compared with the downstream parts. Very large rotifer populations could develop in the upper regions, often associated with flood events, but also coinciding with reduced abundance of crustaceans associated with stronger wash-out effects and the advent of turbid conditions. There was a greater similarity between two adjacent sites in the proportions of crustacean species than of rotifer species. This is partially due to the greater species diversity of rotifers than of crustaceans. The timing of the seasonal succession of zooplankton species showed a coherent pattern along the whole longitudinal profile. The site-specific zooplankton patchiness seems to be reduced as there was no site dependence if average data on zooplankton composition from downstream lacustrine sites were compared.