Nonlinear pattern and algal dual-impact in NO emission with increasing trophic levels in shallow lakes.

Shallow lakes are considered important contributors to emissions of nitrous oxide (NO), a powerful greenhouse gas, in aquatic ecosystems. There is a large degree of uncertainty regarding the relationship between NO emissions and the progress of lake eutrophication, and the mechanisms underlying NO emissions are poorly understood. Here, NO emission fluxes and environmental variables in different lakes along a trophic state gradient in the Yangtze River basin were studied. NO emission fluxes were -1.0-53.0 μg m h and 0.4-102.9 μg m h in summer and winter, respectively, indicating that there was marked variation in NO emissions among lakes of different trophic state. The non-linear exponential model explained differences in NO emission fluxes by the degree of eutrophication (p < 0.01). TN and chl-a both predicted 86% of the NO emission fluxes in shallow lakes. The predicted NO emission fluxes based on the IPCC EF overestimated the observed fluxes, particularly those in hyper-eutrophic lakes. These findings demonstrated that nutrient-rich conditions and algal accumulation are key factors determining NO emission fluxes in shallow lakes. Furthermore, this study also revealed that temperature and algae accumulation-decomposition determine an NO emission flux in an intricate manner. A low temperature, i.e., winter, limits algae growth and low oxygen consumption for algae decomposition. The environment leaves a high dissolved oxygen concentration, slowing down NO consumption as the final step of denitrification. In summer, with the oxygen consumed by excess algal decomposition, the NO production is limited by the complete denitrification as well as the limited substrate supply of nitrate by nitrification in hypoxic or anoxic conditions. Such cascading events explained the higher NO emission fluxes from shallow lakes in winter compared with summer. This trend was amplified in hyper-eutrophic shallow lakes after algal disappearance. Collectively, algal accumulation played a dual role in stimulating and impeding NO emissions, especially in hyper-eutrophic lakes. This study expands our knowledge of NO emissions from shallow lakes in which eutrophication is underway.