Impacts of Nitrogen Deposition on China’s Lake Ecosystems: Taking Lake Dianchi as an Example

Elevated atmospheric nitrogen (N) deposition has greatly influenced aquatic ecosystems, especially in N budget and phytoplankton structure. Compared with considerable studies for oligotrophic lakes, estuaries, and open ocean, few evidence for the importance of N deposition have been directly provided for eutrophic lakes. Our high-resolution systematic observations of N deposition over Lake Dianchi (the sixth largest freshwater lake in China), along with simultaneous measurements of riverine N exports, permit new insight into the contribution of N deposition to total N loads. Annual atmospheric N deposition accounted for 15.7–16.6% of total N loads under changing precipitation regime, twofold higher than previous estimates (7.6%) for the Lake Dianchi. It increased to 27–48% in the critical stage (May and June) when toxic blooms of the ubiquitous non-N2-fixing cyanobacteria Microcystis spp. are initiated and proliferate. Our observations reveal that reduced N (59%) contributes a greater amount than oxidized N to total N deposition, reaching 56–83% from late spring to summer. Progress toward mitigating eutrophication in part of lakes will be difficult without reductions in ammonia emissions and subsequent N deposition.