Submerged macrophytes regulate diurnal nitrous oxide emissions from a shallow eutrophic lake: A case study of Lake Wuliangsuhai in the temperate arid region of China.

Submerged macrophytes can increase oxygen concentrations of water and promote diel oxygen fluctuations, and this phenomenon is hypothesized to play a vital role in regulating nitrous oxide (NO) emissions from eutrophic lakes. However, the effects of submerged macrophytes on NO emissions in shallow eutrophic lakes remain poorly investigated. In this study, Lake Wuliangsuhai, a typical shallow eutrophic lake, was investigated to study the role of submerged macrophytes in regulating NO emissions. We measured the NO fluxes and related parameters through continual 72-h in situ diel monitoring in two sampling sections that covered dense submerged macrophyte areas and open water. In this study, the dissolved oxygen (DO) concentration of the water in the submerged macrophyte area exhibited significant diurnal variations, with significantly higher water oxygen concentrations than the open water area during the daytime. The NO fluxes of Lake Wuliangsuhai ranged from 0.01 to 0.24 μmol m h, with an average value of 0.11 μmol m h. Moreover, significant diel variations in the NO flux and net NO production were observed in the submerged macrophyte areas, where the maximum NO flux occurred at midday. The molar ratios of NH-N to oxygen (N/O ratio) of the water were responsible for the diel variations in the NO production in the lake. However, the high oxygen concentration of the water was the major regulator of the NO flux of Lake Wuliangsuhai. Therefore, submerged macrophyte restoration is significant not only for water quality improvement in shallow eutrophic lakes but also for NO emission mitigation by increasing the DO concentration of the water.