Tracing atmospheric nitrogen deposition and its impact on Hong Kong and Chinese coastal ecosystem

Nitrogen deposition plays an important role in both air and water quality. Nitrogen emissions in cities increase are high due mainly due to transportation and industrial activities and predicted to further increase. As nitrogen is often the limiting nutrient in coastal waters, the excess nitrogen deposition may enhance the phytoplankton growth, resulting in eutrophication leading to a cascade of ecological impacts. Thus, understanding the nitrogen sources and its potential impacts are crucial to mitigate the nitrogen pollution in coastal waters. In this thesis, I used multi stable isotope analysis of wet and dry deposition in Hong Kong to identify the atmospheric sources of nitrogen and the seasonal trend. I characterized the δ15N values of wet deposition, which is useful for further analysis of the nitrate sources in seawater. Next, I examined the ecological impact of future trend of nitrogen deposition in China. Specially, I used a biogeochemical model to estimate the future hypoxia area of such deposition, suggesting that reducing nitrogen deposition can reduce the hypoxia area. Moreover, it indicates that nitrogen deposition will potentially affect the hypoxia area if we do not regulate emissions in the future. This thesis provides a new approach to study the trend and impact of nitrogen deposition, which will allow more understanding on its significance to the coastal ecosystem.