Resilience Approach for Assessing Fish Recovery after Compound Climate Change Effects on Algal Blooms

Abstract

In Europe, climate change will increase hydrologic extremes, resulting in shorter flood peaks and longer droughts. Extended low flows will significantly alter physico-chemical water quality, paving the way for compound, novel impacts. We analyze the Oder River catastrophe of August 2022, where the complex interplay of increased salinity, temperature, low flows, reduced water volumes and sunlight enabled Prymnesium parvum blooming. This brackish water alga grew to 100 million cells per liter and killed about 1000 tons of fish. We assess the impact on and the recovery potential of the fish population to guide both preventing future catastrophes and enhancing river resilience. Stock decline rates were assessed while accounting for natural population fluctuations. Significant relative declines in both fish and biomass density reached up to 76% and 62%, respectively. The mid-channel was more severely affected than littoral areas. Littoral shelter, depth variability, and especially lateral and longitudinal connectivity appeared essential for fish survival and recovery. The compound nature of this catastrophic event highlights the urgent need to rethink the present mismanagement of rivers. Resilient rivers are the backbone of climate change-resilient landscapes. Therefore, we argue for holistic approaches to water resource management, aiming to increase the resilience of aquatic ecosystems.