Multiple stressor impacts on freshwater ecosystems: biodiversity responses and functional consequences

Cayetano Gutiérrez Cánovas

Freshwaters ecosystems are exposed to multiple drivers of change and stress, which threaten their biodiversity and their vital contribution to people’s welfare and global biogeochemical cycles. As a consequence, to improve and adapt biomonitoring, management and conservation actions, a large body of research have started to identify and quantify combined of co-stressors on freshwaters. Multi-stressor combined effects can include the dominance of individual stressors, additive effects (sum of individual effects) or ecological interactions (joint effect is lower or higher than the sum of individual effects).

Recent meta-analyses have shown a variable picture in the prevalence of such combined effects. Dominant and additive effects tend to be more frequent in observational studies, while interactive effects are common in experimental studies.  Furthermore, multi-stressor effects can vary across organisms and ecosystem types. For example, lakes tend to be affected by nutrients as dominant stressor, while rivers showed a context-specific response where additive effects and interactions gained importance. However, despite these huge advances in multi-stressor research, our capacity to anticipate global change impacts is yet to be improved. There are three promising lines of research that can enable a better understanding of the biological and ecological mechanisms mediating multi-stressor responses. First, there is a lack of mechanistic understanding on how multi-stressors drive biodiversity changes.

Mechanistic approaches, such as those combining eco-physiology, functional traits and food web relationships, can help to predict and generalize biodiversity responses in response to multiple stressors. Second, natural stressors and disturbances, such as flow intermittence, fire disturbance, water salinity or elevation, are widespread and can alter the way in which anthropogenic pressures impact freshwaters. A better understanding of how natural stressors interact with those of anthropogenic origin can improve our predictive capacity. Third, to anticipate the functional consequences of global change on freshwaters, there is an urgent need to explore how multiple stressors affect freshwater biodiversity – ecosystem functioning relationships.