Sensitivity of native and non-native mollusc species to changing river water temperature and salinity

Climate change may strongly affect the abiotic conditions in riverine ecosystems, for example by changing water temperature regimes and salinisation due to sea water intrusion and evaporation. We analysed the effects of changes in water temperature and salinity on the species pool of freshwater molluscs in the river Rhine. Species sensitivity distributions (SSDs) for maximum temperature and salinity tolerance were constructed for native and non-native species that are currently present in the river Rhine. The maximum temperature tolerance was significantly higher for non-native mollusc species than native ones. For salinity tolerance, no significant difference was found between the two groups. The SSDs were used to determine the potentially not occurring fractions (PNOFs) of each species group corresponding with the yearly maximum water temperature and salinity levels recorded in (1) different river sections for the extreme warm and dry year 2003, and (2) the river Rhine at Lobith (The Netherlands) over the period 1960-2009. Changing temperature and salinity conditions in the river Rhine over the past 50 years corresponded with a net increase in PNOF for native species. This was mainly due to rising river water temperatures, which had a larger influence than decreasing salinity levels. For non-native species no change in PNOF was found, indicating that future temperature rise will disproportionally affect native mollusc species. Validation of the PNOF estimated for Lobith with the not occurring fraction (NOF) of mollusc species derived from monitoring data revealed similar trends for native as well as non-native mollusc species richness. The increase in the PNOF accounted for 14% of the increase in the NOF. The construction and application of SSDs appeared a promising approach to address the separate and combined effects of changing abiotic conditions on native and non-native species pools.