School of Water Resources and Hydropower Engineering, Wuhan University, Wuhan 430072, China; School of Environment, Nanjing Normal University, No.1, Wenyuan Road, Nanjing 210023, China; State Key Laboratory of Hydrology-Water Resources and Hydraulic Engineering, Nanjing Hydraulic Research Institute, Nanjing 210029, China.
School of Environment, Nanjing Normal University, No.1, Wenyuan Road, Nanjing 210023, China.
Water Res. 2023 May 15;235:119857. doi: 10.1016/j.watres.2023.119857. Epub 2023 Mar 12.
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.
人们普遍认为富营养化湖泊对氧化亚氮（NO）排放有显著贡献。然而，这些排放物如何受到藻类形成、消失和富营养化湖泊中藻类爆发的机制影响，尚未得到系统研究。本研究考察并确定了太湖富营养化湖中时空 NO 生成途径的相对贡献。同时，利用氧（δO）和总氮（δN）与 NO 和分子内 N 位偏好（SP）的同位素比值，在野外和微宇宙中测量了藻类对 NO 产生和释放潜力的多方面影响。结果表明，太湖的 NO 生成源自微生物作用（硝化和不完全反硝化）和水气交换。NO 的生成还受到 NO 还原过程的影响。藻类积累前、爆发和衰退阶段水柱中溶解态 NO 的平均浓度（0.05 μmol·L）几乎相同，是未积累藻类的 2-10 倍。然而，除了中心湖区外，所有调查区域（有和没有积累藻类）都显示出强烈的释放潜力，并由于水柱中溶解态 NO 过饱和而成为排放源。藻类积累区前、爆发和衰退阶段的平均 NO 释放通量分别为 17.95、26.36 和 79.32 μmol·m·d，分别是未积累藻类区的 2.0-7.5 倍。此外，藻类的衰减和分解释放了大量养分，并改变了水柱的理化性质。此外，增加的藻类生物量促进了 NO 的释放，并提高了反硝化过程产生的微生物衍生 NO 占比达到 9.8-20.4%。通过同位素数据证明，当考虑反硝化过程中 NO 的消耗时，这一比例更高。然而，在富营养化状态下藻类生物量过多时，藻类分解也会消耗大量氧气，从而限制了由于缺氧或缺氧条件下硝化过程中硝酸盐供应有限而导致的完全反硝化以及由于完全反硝化导致的 NO 产生。此外，过多的藻类在水面上的积累会阻碍溶解态 NO 向大气中的迁移，从而减少 NO 的释放通量。这些发现为准确评估富营养化湖泊中藻类过程驱动的 NO 释放通量提供了新的视角和理解。
