Nitrogen addition effect overrides warming effect on dissolved CO and phytoplankton structure in shallow lakes.

Shallow lakes are numerous in all climate zones, but our knowledge about their dissolved carbon dioxide (CO) response to future climate change and nutrient enrichment is rather limited. Here we performed a mesocosm experiment with four treatments to investigate how warming and nitrogen addition will impact the partial pressure of CO (pCO) and phytoplankton community individually and combined. We found that warming alone had no significant effect on pCO, while nitrogen addition increased pCO significantly. The combined effects of nitrogen addition and warming on pCO level were prevalent, indicating that eutrophic shallow lakes would be double-jeopardized in the future climate. Warming and nitrogen addition together also showed to have changed the phytoplankton community structure, suggesting a potential shifting of biological system in shallow lakes under changing climate. These findings highlight the importance of reducing nitrogen pollution to shallow lake systems for sustainable development goal.