Stressors in a bottle: A microcosm study on phytoplankton assemblage response to extreme precipitation events under climate warming

The climatic stressors that are affecting lake ecosystems, especially phytoplankton, are projected to become more intense with continued climate change (e.g., heatwaves, precipitation events). Concerns over the combined effects that multiple, coinciding stressors can have on phytoplankton necessitates investigating the impacts of different regional climate scenarios. A microcosm study was conducted to assess the responses of a phytoplankton assemblage containing a cyanobacterium (Anabaena flos-aquae), a green alga (Chlorella vulgaris) and a diatom (Synedra) to a northwestern European summer scenario. Eutrophic microcosms were exposed to a full-factorial design including a press temperature treatment scenario (ambient or warm) and a pulse precipitation treatment (no runoff simulation or runoff simulation). Warming scenarios had significant effects on the phytoplankton assemblage biomass, which supports our first hypothesis (H1: higher water temperatures under eutrophic conditions will support larger phytoplankton biomasses, especially cyanobacteria). By contrast, the extreme precipitation runoff event had minimal and short-lived effects on the microcosm assemblage. Overall, the interaction between the two climate stressors was antagonistic. In contrast with our second hypothesis (H2: nutrient additions from extreme precipitation runoff will promote more productivity in higher temperature microcosms), the precipitation runoff event was not amplified by temperature. Our results indicate that the combined effect of two climate stressors on a phytoplankton community are not necessarily synergistic or multiplicative. Our findings on antagonistic interactions between climatic stressors necessitate future studies assessing variations of intensity and duration of the climatic stressors.