Significant spatiotemporal variability of nitrous oxide emissions from a temperate reservoir experiencing intensive aquaculture disturbance

Abstract

Freshwater reservoirs are known as important sources of atmospheric N2O; however, the patterns and controls of N2O emissions from these reservoirs remain poorly understood, particularly for reservoirs severely disturbed by aquaculture activities. This study aimed to bridge this knowledge gap by determining the spatiotemporal and magnitude variations of N2O concentrations, dynamics of the drivers of N2O concentration and diffusive flux, and N2O emission patterns from reservoirs severely affected by human activities. We measured N2O concentrations and estimated diffusive fluxes in a temperate reservoir—including the upstream river inflow zone—in northern China, which was experiencing acute aquaculture disturbance. A thin boundary layer equation was employed, and we analyzed the thermal and dissolved oxygen stratification conditions and other environmental auxiliary factors to identify crucial factors and underlying mechanisms that regulate N2O emissions. Our results showed that aquaculture activities led to the accumulation of high levels of organic matter and nutrients in sediments. The annual average water-dissolved N2O concentration and diffusive flux across the water–air interface in the reservoir were 24.97 ± 11.16 nmol L-1 and 0.43 ± 0.36 μmol m-2·h-1, respectively. The water-dissolved N2O concentrations exhibited supersaturation, indicating the reservoir as a net source of atmospheric N2O. Both N2O concentrations and diffusive fluxes exhibited strong spatiotemporal variability during the study period. The lacustrine area of the reservoir was the hotspot for N2O emissions. The statistical results indicated that nitrogen loading, temperature, and dissolved oxygen (DO) were the key drivers of N2O emissions. Moreover, dam construction and intensive aquaculture disturbances have altered aquatic conditions and significantly affected N2O emissions. Our results indicated that N2O emission estimates for the entire reservoir should cover different reservoir zones and sampling seasons. We suggest measures such as sediment dredging to reduce sediment organic matter and nutrient loading for mitigating N2O emissions from the reservoir.