Quantitative discrimination of algae multi-impacts on NO emissions in eutrophic lakes: Implications for NO budgets and mitigation.

It is generally accepted that eutrophic lakes significantly contribute to nitrous oxide (NO) emissions. However, how these emissions are affected by the formation, disappearance, and mechanisms of algal blooms in these lakes has not been systematically investigated. This study examined and determined the relative contribution of spatiotemporal NO production pathways in hypereutrophic Lake Taihu. Synchronously, the multi-impacts of algae on NO production and release potential were measured in the field and in microcosms using isotope ratios of oxygen (δO) and bulk nitrogen (δN) to NO and to intramolecular N site preference (SP). Results showed that NO production in Lake Taihu was derived from microbial effects (nitrification and incomplete denitrification) and water air exchanges. NO production was also affected by the NO reduction process. The mean dissolved NO concentrations in the water column during the pre-outbreak, outbreak, and decay stages of algae accumulation were almost the same (0.05 μmol·L), which was 2-10 times higher than in lake areas algae was not accumulating. However, except for the central lake area, all surveyed areas (with and without accumulated algae) displayed strong release potential and acted as the emission source because of dissolved NO supersaturation in the water column. The mean NO release fluxes during the pre-outbreak, outbreak, and decay stages of algae accumulation areas were 17.95, 26.36, and 79.32 μmol·m·d, respectively, which were 2.0-7.5 times higher than the values in the non-algae accumulation areas. In addition, the decay and decomposition of algae released large amounts of nutrients and changed the physiochemical properties of the water column. Additionally, the increased algae biomass promoted NO release and improved the proportion of NO produced via denitrification process to being 9.8-20.4% microbial-derived NO. This proportion became higher when the NO consumption during denitrification was considered as evidenced by isotopic data. However, when the algae biomass was excessive in hypereutrophic state, the algae decomposition also consumed a large amount of oxygen, thus limiting the NO production due to complete denitrification as well as due to the limited substrate supply of nitrate by nitrification in hypoxic or anoxic conditions. Further, the excessive algae accumulation on the water surface reduced NO release fluxes via hindering the migration of the dissolved NO into the atmosphere. These findings provide a new perspective and understanding for accurately evaluating NO release fluxes driven by algae processes in eutrophic lakes.