How anthropogenic modification of riverscapes reduce the resilience of floodplain waters to drought

Central Europe has experienced an extreme drought over the last five summers, which has led to a deficit in precipitation and discharge unlikely to be replenished quickly. Due to climate change, extreme weather events and accompanying droughts are likely to occur more frequently in the future putting pressure on aquatic ecosystems. In addition, rivers have been significantly modified over the years, with channelization and the construction of dams drastically altering the natural flow of the river. Floodplains have been cut off and natural habitats have been lost. This underlines the need to investigate the interactions of climate change and antropogenic alterations to rivers and to establish a safe operating space for floodplain areas to ensure their ecological function. To achieve this, we investigated 36 floodplain lakes near the Elbe River in Magdeburg, Germany, with varying connectivity to the main river and different characteristics of each lake. Water samples were taken from the lakes, the main river and the groundwater. Major ions and isotopes to determine the origin of the water. Further, chlorophyll a was sampled and parameters such as oxygen and hydrogen sulfide were taken. Along with recorded fish kills and measured water level, a scoring system was established to determine the degree of impairment and habitat loss of each lake. Connectivity, defined here as the frequency of an existing surface connection of the lake to the main river, was determined to provide a measure of the impact of anthropogenic modification and channelization of the river bed. The difference in deuterium excess between fall and spring served as a measure of evaporation and thus of the influence of climate change during the sampling campaign. Critical chlorophyll a concentrations were measured in surface waters in lakes with less than 50 % connectivity, critical oxygen concentrations in lakes with less than 10 % connectivity. Fish kills, hydrogen sulfide, siltation and dry-out occurred predominantly in lakes with a low connectivity. Finally, lakes with a small perimeter by area were found to exhibit fewer signs of degradation and habitat loss. Our results suggest that lakes that are connected to the main river are better able to respond to drought stress caused by climate change. Therefore, better connectivity to the main river may help to reduce habitat degradation or loss in the floodplain ecosystem.