Control mechanisms of planktonic food webs in a river-floodplan system

Dynamic floodplain systems are strongly influenced by hydrological exchange processes with the river in terms of productivity and chemical-physical conditons. High nutrient input induces a fast development of autochthonous organic material. Terrestrical carbon input is comparably low. The material transformation is thus intensified compared to river channels where constant flowing condions prevail. In this regard backwaters increase purification capacity of the river-floodplain system considerably. Rivers are interfaces in global geochemical cycles and consequently of special interest in material transport and transformation. The conditions are determined by the catchment area and longitudinal as well as lateral exchange processes. The high capacity of floodplain areas in material transformation became apparent in the last century when large rivers in Europe and North America have been channelized and connectivity to adjacent waterbodies has been interrupted. The floodplain system along the River Danube downstream of Vienna (Austria) is still characterized by a wide spectrum of hydrological exchange conditions. In spite of the major regulation scheme at the end of the 19th century some backwaters of high connectivity are still present. Re-opening efforts additionally enhanced the dynamic character. A backwater of reinforced dynamic became the main study site during a research project between 1997 and 1999. The location "Regelsbrunn" provided optimal conditions to examine the influence of varying connectivity and hydrological situation on water chemistry, plankton community and particulate organic and anorganic matter, basic informations to understand carbon dynamics in the water column of these heterogeneous systems. As reference a very isolated waterbody, "Lobau" as well as the Danube itself were chosen. The information gained by this study can furthermore serve as scientific backround for effective restoration and management efforts. The River Danube is characterized by high nutrient loading that is distributed in the floodplains during high floods and at locations of high connectivity. With increasing water retention the nutrients stimulate blooms of algae typical for rivers. Therefore, a maximal productivity is observed following episodic resource pulses. Heterotrophic organisms react with high production rates. High zooplankton biomass opens the carbon pool for higher trophic levels such as fishes with high biomass in the floodplains. Even the river fish profit by the high ressources, since they use backwaters as feeding grounds. Generally, we observe distinct succession patterns of the plankton community, both phyto- and zooplankton, depending on the hydrological situation with highest species richness in periods of ongoing lentic situations after a flood. This emphasizes the stimulating effect of changing hydrological conditions on diversity and productivity. Conclusively, our results accentuate the importance of reopening projects for a river-floodplain system for plankton dynamics and its role in the aquatic community. The immense heterogeneity and productivity that originally enriched huge areas beside streams can only be preserved by warranting the exchange between river and its backwaters.