Zooplankton as an indicator of hydrological connectivity of the main channel and the floodplain in a large river system

The connectivity of the river-floodplain system during floods plays a decisive role in organisms/matter contribution to the ecosystems of large lowland rivers. We hypothesized that the direction of water-level trends during flood was crucial for zooplankton release from the floodplain into the river channel. The seven-year study of zooplankton in the large plain Ob river suggest that at the inundated floodplain even a slight water level fall brings to plankton release from the floodplain into the river thus causing its abundance boom in the main channel. At water-level stabilization or rise, zooplankton communities of floodplain become isolated from the channel that ceases the recruits' inflow from the floodplain and brings to a sharp drop in number of river zooplankton. Therefore, it is necessary to take into account dynamics trends (i.e., fall or rise) in river water level when assessing zooplankton density in large lowland rivers. The analysis of such trends should become the basis for predicting of river zooplankton abundance and assessing changes in feeding conditions of fish during the growing season.