Getting a Burn-Out: How Heatwaves Impact Eutrophic Freshwater Ecosystems

Abstract

Aquatic ecosystems are governed by complex interactions among meteorological, hydro-morphological, and biological components. However, these interactions are increasingly disrupted by anthropogenic stressors such as eutrophication and global warming. Understanding the combined effects of these stressors on ecosystem structure and function is critical. However, the complexity of these interactions and logistical challenges has limited understanding of stressors' interaction at the ecosystem level. Here, we investigated the impacts of nitrate enrichment (5, 50, or 200 mg/L) and heatwaves (absence or presence of a prolonged, intense heatwave) on semi-realistic mesocosm aquatic ecosystems. Our study assessed the responses of key biological components, including primary producers (algae, submerged macrophytes, floating macrophytes) and consumers (daphnia, fish), under integrated stress conditions. The findings revealed that heatwaves and nutrient loading, two common anthropogenic stressors, interactively influence ecosystems in complex ways. At the ecosystem level, we did not observe a synergistic positive effect of heatwaves and nitrate enrichment on algal growth, as is often reported in studies focusing on single organisms. Moreover, nitrate enrichment alone reduced submerged macrophyte growth while promoting floating macrophyte dominance. Combined nitrate and heatwave stressors led to a decline in both macrophyte types, though the reduction was not statistically significant for floating macrophytes. We conclude that neither stressor alone shifted the ecosystem to a phytoplankton-dominated turbid state; however, nitrate enrichment alone resulted in a floating macrophyte-dominated turbid state. Additionally, nitrate and heatwave interactions negatively affected both primary (daphnia) and secondary (fish) consumers, likely due to reductions in food quality, increasing complexity of the situation. This may eventually allow the ecosystem to shift to a phytoplankton-dominated turbid state under high nitrate and heatwave conditions. In conclusion, these findings underscore the species-specific and time-dependent nature of stressors' impacts and emphasize the importance of conducting longer and more diverse mesocosm studies to understand ecosystem-level responses under multi-stressor scenarios.

Keywords: Ecosystem, Mesocosm, Phytoplankton-dominated turbid state, Wetland, Stressors interaction