How does summer environmental degradation influence native mussels in Corbicula invaded rivers?

Abstract

Abstract Droughts and heat waves are becoming more frequent, persistent, and intense in a global warming context. Although previous studies reported Corbicula mass mortality events associated with the occurrence of extreme summer conditions, knowledge of their effects lags behind ecosystem functioning. The goal of this study was to determine whether summer environmental degradation (droughts, extreme temperatures, and spikes in ammonium concentration) influences the survival and behaviour of native mussels in rivers invaded by Corbicula and, in consequence, their functioning in ecosystems. Unio delphinus mussels were exposed to increasing summer environmental degradation. Water column and pore water samples were collected at six sampling time‐points for nutrient analysis. Environmental conditions, survival and behaviour were monitored at 12‐hour intervals. Mortality risk was greatest under no‐flow conditions, reaching 100% of mortality after 84 ± 22 hours of exposure to anoxic (<2 mg O2 L−1) conditions. At the sub‐lethal level, quiescent conditions impaired filtration activity. In addition, when facing rising temperatures, native mussels buried deeply and vertical movements were depleted. There were significant differences between the water column and the pore water in physicochemical characteristics. Oxygen and nitrate concentration were generally higher in the water column. In contrast, ammonium and phosphate maximum concentrations were measured in the pore water. The results suggest that environmental degradation affects the role of mussels in ecosystem functioning. For instance, prolonged valve closure under poor water quality conditions may have impacts on particle removal from the water column. Furthermore, impairment of vertical movements may reduce nutrient release from sediments. The present study has key implications for water management and for the conservation of mussel populations. For instance, a responsible approach for maintaining ecosystem integrity should include the water needs of freshwater mussels in environmental flow recommendations and the conservation of riparian vegetation corridors to mitigate the effects of increased water temperature in rivers.