Multispecies assemblages and multiple stressors: Synthesizing the state of experimental research in freshwaters

Abstract

Recent decades have witnessed a sharp biodiversity decline in freshwaters due to multiple stressors. The presence of multiple stressors is expected to affect community structure and interactions in freshwater ecosystems, with subsequent functional consequences. We synthesized the state of experimental, manipulative multiple-stressor studies that focused on multispecies assemblages in freshwaters. Compared to rivers and lakes, wetland and groundwater ecosystems have received much less attention in identified multiple-stressor research. Most of the identified studies investigated combinations of abiotic stressors (e.g., nutrients, pesticides, heavy metals, warming, altered flow and sedimentation) on microbes and invertebrates while biotic stressors and vertebrates have been largely overlooked. The responses of community structure (e.g., alpha diversity, biomass, and abundance), some community/ecosystem functions (e.g., photosynthesis and autotrophic activity, leaf litter degradation), and morphological traits like body size and growth forms were frequently investigated. We observed a clear gap in biotic interactions under multiple-stressor conditions, which, although difficult to study, could impede a deeper mechanistic understanding of how multiple stressors affect freshwater assemblages and associated ecological processes. Although information on ecosystem recovery pathways following restoration is critical for freshwater management, few studies were designed to provide such information, signifying the disconnections between multiple-stressor research and environmental practice. To bridge these gaps, researchers and environmental practitioners need to work together to identify key stressors and interactions at different spatial and temporal scales and prioritize stressor management. Such collaborations will enhance the translation of multiple-stressor research into efficient management strategies to protect and restore freshwater ecosystems.