Stressor fluctuations alter mechanisms underpinning seagrass responses to multiple stressors

Multiple anthropogenic stressors degrade ecosystems globally. A key knowledge gap in multiple stressor research is how variability in stressor intensity (i.e., fluctuations) and synchronicity (i.e., timing of fluctuations) affect biological responses, and the mechanisms underpinning responses. We evaluated the mechanistic effects of reduced light and herbicide contamination on seagrass, and determined how variations in stressor intensity and synchronicity influence the underlying mechanisms of responses. We used structural causal modelling and structural equation modelling to elucidate direct and mediating effects. Out-of-phase introduction (i.e., asynchronous fluctuations) altered the mechanistic pathways of how stressors affect seagrass relative to static stressors, and resulted in the greatest biomass loss (under the most intense stressor combination, ∼50 % reduction). Therefore, previous experiments that predominantly test only static stressor intensities might underestimate detrimental impacts in nature. Future experiments should explore mechanistic effects across realistic stressor intensities and synchronicities to improve our understanding and management of multiple stressors.