Influence of salinity on the thermal tolerance of aquatic organisms

Abstract

Aquatic organisms are challenged by changes in the external environment, such as temperature and salinity fluctuations. The response of an organism to temperature changes can be modified by salinity, thus pointing at the potential interaction of both variables. In the present study, we tested this assumption for freshwater, brackish, and marine organisms, including algae, macrophytes, heterotrophic protists, parasites, invertebrates, and fish. We reviewed the existing body of literature on potential interactions between temperature and salinity and performed a meta-analysis that compared the thermal tolerance (characterized by the temperature optima, lower and upper temperature limits, and thermal breadths). The final database includes 90 relevant publications (algae: 15; heterotrophic protists: 1; invertebrates: 43; and fish: 31). Relevant publications for microphytes and parasites were not available. Overall, our results show that decreasing salinity significantly increased the lower temperature limits and decreased the upper temperature limits irrespective of the organism groups. These findings mainly reflect the response to salinity changes in brackish and marine systems that dominate our database. Although the number of studies on freshwater species was limited, they showed negative, although statistically nonsignificant, effects of an increased salinity on the thermal tolerance of these species (i.e. increased lower limits and decreased upper limits). In addition, our meta-analysis shows nonsignificant differences in the responsiveness of thermal tolerance to salinity changes among different groups of organisms, but the sensitivity of thermal tolerance to salinity changes generally followed the order: algae > invertebrates > fish. Facing the impact of climate change, our findings point at adverse effects of salinity changes on the temperature tolerance of aquatic organisms. Further studies that investigate the thermal performance of freshwater species at various salinity gradients are required to broaden the evidence for interactions between salinity and temperature tolerance. This also applies to the influence of parasitic infections, which have been found to modulate the temperature tolerance of aquatic invertebrates and fish.