Between 2018 and 2023, central Europe experienced a series of droughts—one manifestation of climate change expected to increase in frequency in the future. These events increasingly expose aquatic ecosystems, such as floodplain water bodies, to hydrological extremes. Simultaneously, anthropogenic river modifications like channelization and diking have been progressively disconnecting semi-aquatic ecosystems from the main river. Here, we examined how drought and hydromorphological modifications affect the water quality of floodplain water bodies.

Specifically, we investigated 36 floodplain water bodies near the Elbe River in Germany and analyzed water samples from spring and late summer 2022. We assessed connectivity (past frequency of surface connection to the stream) and effects of drought (evaporation measured by changes in stable water isotopes) using a scoring system. This system evaluated impairment and habitat loss, allowing us to quantify the impact of hydrological disconnection on floodplain water bodies.

Severe impacts, including low surface oxygen (<3 mg/L), products of anaerobic decomposition, fish kills, and dry-outs, primarily affected water bodies with low connectivity (<10 %). Multiple linear regression linked low connectivity and high evaporation to habitat degradation (r² = 0.487, p < 0.001). Furthermore, dissolved oxygen concentrations increased with increasing chlorophyll a in connected lakes (p < 0.001), as expected, but no relationship was observed in disconnected lakes (p = 0.626). We attribute this lack of correlation to the decomposition of fine-grained organic sediments that have not been resuspended and flushed out in disconnected lakes. We conclude that river modifications for navigation reduce floodplain resilience to droughts, amplifying the effects of climate change on semi-aquatic ecosystems.