Spatial and seasonal variability of nitrous oxide in a large freshwater lake in the lower reaches of the Yangtze River, China.

Aquatic ecosystems are recognized as a source of NO in accordance with the flux estimations of rivers and estuaries; however, limited research has been conducted on large lakes. In this study, we report the annual NO dynamics of a large eutrophic freshwater lake located in the subtropical zone of East China. The dissolved NO concentrations in Lake Chaohu were observed to be between 8.5 and 92.3 nmol L with emission rates between 0.3 and 53.6 μmol m d, exhibiting considerable spatiotemporal variability. The average seasonal NO concentrations were obtained, with the highest value of 23.4 nmol L found in winter and the lowest value of 12.7 nmol L found in summer. In contrast to the NO concentrations observed, the highest NO emission rates occurred during summer, while the lowest emission rates occurred in autumn. The emissions of NO were substantially high in the western part of the lake, which suffers from serious eutrophication. In addition, the hotspots of NO emissions have been found around the inflowing mouth of the Nanfei River, which transports large amounts of nutrients into the lake. The results suggest that anthropogenically enhanced nutrient inputs may have a significant role in the production and emission of NO. However, the negative relationship between the surface water temperature and the NO concentration suggests that, NO fluxes might be influenced by other inconspicuous mechanisms. In the future the nitrogen dynamics of water and sediment in the lake should be collated to reveal mechanisms controlling NO emissions. In summary, Lake Chaohu acts as a source of NO with its most eutrophic part contributing 54.9% of the total NO emissions of the whole lake.