Direct and indirect effects of salinisation on riverine biota: a case study from river Wipper, Germany

Salinisation has direct and detrimental physiological effects on freshwater organisms, yet little is known about its indirect effects. Here, we present a study that investigated the primary and secondary effects of salinisation on aquatic macroinvertebrates. We analysed macroinvertebrate samples from nine sites in river Wipper, Germany against nine stressor variables (water quality, salinisation, hydromorphological degradation). A Principal Component Analysis showed water quality deterioration and physical habitat degradation to constitute the main stressor gradients. Two macroinvertebrate community metrics (index of general degradation and percent Ephemeroptera-Plecoptera-Trichoptera specimens) revealed strong and significant positive correlations with water quality, but only weak and insignificant positive correlations with hydromorphological quality. High-resolution temporal measurements revealed notable oxygen deficits at a salinised and macrophyte-rich site that was dominated by the salt-tolerant macrophyte species Stuckenia [ Potamogeton ] pectinata (L.) Börner. At the site, oxygen levels frequently dropped below 6 mg/L during nighttime, suggesting an interaction of osmoregulatory stress (through salinisation) and respiratory stress (through plant mitochondrial respiration) that is caused by the mass development of a salt-tolerant macrophyte species. This biologically driven interaction of direct and indirect salinisation effects adds another level of complexity to the influence of multiple stressors in lotic systems and requires consideration in river management and restoration.