Water-level fluctuations and nutrient enrichment interact to alter ecosystem structure in distinct ways at different water depths

Aquatic ecosystems are often impacted by several human-derived pressures simultaneously. A key challenge for ecologists is to develop mechanistic understanding of the cumulative effects of multiple stressors on freshwater biodiversity. We established an outdoor mesocosm experiment to examine the individual and combined effects of two key stressors on lake ecosystems globally-nutrient enrichment and amplified water-level fluctuations-on benthic algal and macroinvertebrate assemblages along a gradient in littoral water depth. We found that nutrient enrichment and water-level fluctuations not only altered the structure of littoral communities when acting individually, but also interacted to dynamically alter biomass and community structure in different ways at different depths. As expected, enrichment increased benthic algal biomass significantly in very shallow waters. Yet, when enrichment co-occurred with water-level fluctuations, this pattern was reversed, with algal biomass increasing significantly in deeper waters, but not in the shallows that were exposed to fluctuating water levels. This pattern occurred across all algal groups that we examined except for green algae. Green algae biomass increased with nutrient enrichment, was greatest in the shallows, and remained unaffected by fluctuating water levels. Water-level fluctuations modified the functional feeding group structure of benthic macroinvertebrate communities, reducing relative densities of collector-gatherers and increasing those of predators. However, these changes occurred only in the shallow-most regions exposed directly to fluctuating water levels. Our results reveal how two globally important stressors of standing water systems can interact in different ways in different ecological contexts and show how stressors can combine to have unanticipated impacts that vary in space. Our findings underscore the potentially highly multifaceted and complex nature of stressor combinations that present a major and growing challenge to ecosystem management, particularly in light of accelerating global environmental change.