Nitrogen concentration response to the decline in atmospheric nitrogen deposition in a hypereutrophic lake

Atmospheric nitrogen (N) deposition is becoming an increasingly important factor affecting the nutrient level of lakes, especially considering the long-term control measures for external N inputs in developed regions. However, few studies have investigated the effects of atmospheric N deposition and the respective ecological significance in eutrophic waters. In this study, bulk and wet deposition rates of all N species and water N concentrations in Lake Taihu were determined based on the long-term (2010-2018) high-resolution (weekly or monthly) systematic observations. The results indicated that the decline in wind speed and change in land-use type likely decreased the N deposition rate. The bulk N deposition rates decreased from 45.77 kg N ha-1 yr-1 in 2012 to 22.06 kg N ha-1 yr- 1 in 2018, which could account for decrease of 1.01 mg N L-1 in the lake N concentrations via a rough estimation, and this value was close to the actual variation in N concentration in Lake Taihu. The correlation between N concentrations and atmospheric deposition fluxes was stronger than that between N concentrations and riverine N inputs or lake storage, which further indicated that change in atmospheric N deposition was the key reason for the variation in N concentrations. The direct bulk N deposition into Lake Taihu accounted for 17.5% and 51.4% of the riverine N inputs and lake N inventory, respectively. Moreover, atmospheric N deposition was concentrated in summer, which was dominated by reduced N, and it may be important for the duration of algal blooms. Therefore, external N inputs, including atmospheric N deposition, should be further controlled for an effective mitigation of eutrophication and algal blooms in Lake Taihu.

Automated summary

This study determined the rates of atmospheric nitrogen deposition and nitrogen concentrations in Lake Taihu, and found that atmospheric nitrogen deposition is the main factor affecting nitrogen concentration changes in the lake. Atmospheric nitrogen deposition is concentrated in summer and may be important for the duration of algal blooms. Therefore, further control of external nitrogen inputs is necessary to mitigate eutrophication and algal blooms in Lake Taihu.