Net effects of multiple stressors in freshwater ecosystems: a meta‐analysis

Abstract

The accelerating rate of global change has focused attention on the cumulative impacts of novel and extreme environmental changes (i.e., stressors), especially in marine ecosystems. As integrators of local catchment and regional processes, freshwater ecosystems are also ranked highly sensitive to the net effects of multiple stressors, yet there has not been a large-scale quantitative synthesis. We analysed data from 88 papers including 286 responses of freshwater ecosystems to paired stressors, and discovered that overall, their cumulative mean effect size was less than the sum of their single effects (i.e., an antagonistic interaction). Net effects of dual stressors on diversity and functional performance response metrics were additive and antagonistic, respectively. Across individual studies, a simple votecounting method revealed that the net effects of stressor pairs were frequently more antagonistic (41%) than synergistic (28%), additive (16%) or reversed (15%). Here, we define a reversal as occurring when the net impact of two stressors is in the opposite direction (negative or positive) from that of the sum of their single effects. While warming paired with nutrification resulted in additive net effects, the overall mean net effect of warming combined 2 with a second stressor was antagonistic. Most importantly, the mean net effects across all stressor pairs and response metrics were consistently antagonistic or additive, contrasting the greater prevalence of reported synergies in marine systems. Here, a possible explanation for more antagonistic responses by freshwater biota to stressors is that the inherent greater environmental variability of smaller aquatic ecosystems fosters greater potential for acclimation and co-adaptation to multiple stressors.

Evolutionary consequences of multidriver environmental change in an aquatic primary producer

“…The identity of dominant drivers in each regime partially explains the small differences in the response to selection in multidriver environments. The overriding effect of the dominant drivers is consistent with the acclimation response to these regimes (7), scenario-based experiments (8), and many physiological responses of microalgae to pairs of drivers (31)(32)(33)(34)(35). Our data are consistent with either small effects of driver interactions relative to the effects of dominant drivers or (nearly) zero-sum interactions among drivers.…”

Biotic and abiotic variables influencing plant litter breakdown in streams: a global study

“…However, substantial unexplained variation indicates that further comparative research is required to develop a comprehensive picture of litter breakdown in streams at the global scale. Such understanding is vital to appropriate management of these ecosystems in the face of multiple anthropogenic stressors [72,73]. …”

Thermal evolution offsets the elevated toxicity of a contaminant under warming: A resurrection study in Daphnia magna

“…Except for the internal zinc concentrations, which were log‐transformed, the assumptions of ANOVA (normally distributed errors and homogeneity of variances) were met for all variables. When an interaction effect between temperature and the nZnO treatment was detected, the type of interaction was identified by calculating the interaction effect size (estimated as Hedges'd with 95% confidence interval) (Jackson et al., 2016). An interaction effect size larger than zero indicates a synergistic interaction among stressors, while an interaction effect size smaller than zero indicates an antagonistic or reversal interaction.…”

“…80% of the studies showed a synergistic interaction between a contaminant and heat stress for traits such as survival, developmental rate, and hatching success. Jackson, Loewen, Vinebrooke, and Chimimba (2016) showed that ca. 30% of the studies testing combined effects of warming and contaminants in freshwater ecosystems showed a synergism for traits such as growth and reproduction.…”