Effects of salinity changes on aquatic organisms in a multiple stressor context

Abstract

Under global change, the ion concentration of aquatic ecosystems is changing worldwide. Many freshwater ecosystems are being salinized by anthropogenic salt inputs, whereas many naturally saline ones are being diluted by agricultural drainages. This occurs concomitantly with changes in other stressors, which can result in additive, antagonistic or synergistic effects on organisms. We reviewed experimental studies that manipulated salinity and other abiotic stressors, on inland and transitional aquatic habitats, to (i) synthesize their main effects on organisms' performance, (ii) quantify the frequency of joint effect types across studies and (iii) determine the overall individual and joint effects and their variation among salinity-stressor pairs and organism groups using meta-analyses. Additive effects were slightly more frequent (54%) than non-additive ones (46%) across all the studies (n = 105 responses). However, antagonistic effects were dominant for the stressor pair salinity and toxicants (44%, n = 43), transitional habitats (48%, n = 31) and vertebrates (71%, n = 21). Meta-analyses showed detrimental additive joint effects of salinity and other stressors on organism performance and a greater individual impact of salinity than the other stressors. These results were consistent across stressor pairs and organism types. These findings suggest that strategies to mitigate multiple stressor impacts on aquatic ecosystems should prioritize restoring natural salinity concentrations.This article is part of the theme issue 'Salt in freshwaters: causes, ecological consequences and future prospects'.

Keywords: additive effects; dilution; inland waters; meta-analysis; salinization; transitional waters.

Figure 1.

Frequency distribution of the interaction types across (a) stressor pairs, (b) habitat and (c) organism groups, estimated from effect size calculations and categorized following the classification of Crain et al. [12] (see electronic supplementary material S2). (Online version in colour.)

Figure 2.

Mean effect sizes (Hedge's d ± 95% confidence intervals), overall and by organism groups of (a) joint effect, (b) salinity individual main effect and (c) stressors B individual main effect. The number of observations (n) of each analysis is indicated in parentheses. Filled black squares indicate significant effects (p < 0.05).

Figure 3.

Mean effect sizes (Hedge's d ± 95% confidence intervals) on joint and individual main effects on inland saline invertebrates for (a) salinity increase and temperature, (b) salinity decrease and temperature, and (c) salinity increase and toxicants. Filled black squares indicate significant effects (p < 0.05).