Abstract

Recycled water will play an increasing role in maintaining river flow in arid and semi-arid regions. However, its long-term effects on wildlife populations are still poorly studied. In this study we used trends in fish abundance, indices of biotic quality across taxa (diatoms, invertebrates and fish), and biomarkers (blood tests and scaled body condition index) to assess the ecological impact of treated sewage discharges in Ripoll River (NE Spain) over the period 2002-2013. To test the hypothesis that drought would increase the pollution stress, blood markers were sampled seasonally in the native fish Barbus meridionalis and Squalius laietanus in 2013. Barbus meridionalis consistently increased in abundance in polluted sites over the entire study period, but S. laietanus markedly declined in 2013. Both species had better conditions under pollution but with signs of stress (neutrophilia, monocytosis, and/or release of immature cells). Further, S. laietanus had higher values of specific pollutant biomarkers (% abnormal cells) than B. meridionalis and mostly in spring, suggesting that the former was the most sensitive species and the breeding season more stressful than summer. Nonetheless, contrasting hydro-morphological features and trends in fish abundance in reference sites suggest that changes in water level may shift the response of pelagic and benthopelagic species under similar pollution stress. Diatom and invertebrate indices captured better variations in water quality than the fish index. In conclusion, recycled water has a genotoxic risk for fish populations and potential to cause multi-trophic impacts in Ripoll River. Better sewage management strategies are needed in Mediterranean rivers under water scarcity, and blood tests are a cheap way to monitor their consequences for fish health.