On the contribution of atmospheric reactive nitrogen deposition to nitrogen burden in a eutrophic Lake in eastern China

Although it has been demonstrated that atmospheric reactive nitrogen (i.e., Nr mainly including NH3, NH4+, NOx, NO3- and etc.) deposition has substantial impacts on nitrogen pools in remote and/or sensitive lakes, there is a scarcity of systematic evaluations regarding the impact on nitrogen burden in eutrophic lakes with riverine input as primary nitrogen source. Utilizing a regional atmospheric chemical transport model, combined with observation-based estimates of atmospheric nitrogen deposition fluxes and riverine nitrogen inputs, we investigate the contribution of atmospheric Nr deposition to the fifth largest freshwater lake located in eastern China, i.e., the Chaohu Lake which is facing frequent outbreaks of algal bloom. The results indicate that in the studied year of 2022, riverine total nitrogen (TN) input to the lake was 11553.3 t N yr(-1) and atmospheric TN deposition was 2326.0 t N yr(-1). For Nr species which are directly available for the biosphere supporting algae and plant growth, riverine NH4+ input was 1856.1 t N yr(-1) and atmospheric NHx (NH3 and NH4+) deposition was 824.5 t N yr(-1). The latter accounts for 30.8% of total NHx input to the lake. For NOy (HNO3 and NO3-), riverine NO3- input was estimated as 2621.7 t N yr(-1), while atmospheric NOy deposition was 629.3 t N yr(-1), accounting for 19.4%. In all, atmospheric Nr deposition accounts for 24.5 % of total Nr input to the lake. Our results suggest that even in regions with dense human activities with primary riverine N input, atmospheric deposition of Nr could also contribute significantly to the bio-available nitrogen in lake systems, and addressing eutrophication in Lake Chaohu and other eutrophic lakes will also need to consider the influence of atmospheric Nr deposition which is related to NH3 and NOx (i.e., NO + NO2, the precursor of NOy) emissions, in addition to the mitigation of riverine N input.