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

Abstract

Nitrogen deposition plays an important role in both air and water quality. Nitrogen emissions in cities are high mainly due to transportation and industrial activities and are predicted to further increase in the future. As nitrogen is often the limiting nutrient in coastal waters, excess nitrogen deposition may enhance phytoplankton growth, resulting in eutrophication and leading to a cascade of ecological impacts. Thus, understanding atmospheric nitrogen sources and the potential impacts of excess nitrogen deposition are crucial to mitigate nitrogen pollution in coastal waters. In this thesis, I measured stable isotopic ratios of nitrogen from wet- and dry-deposited nitrogen species in Hong Kong to identify atmospheric sources of nitrogen and their seasonal trends. These values are useful for further characterizing the source apportionment of nitrogen sources in seawater. Next, I examined the impact of nitrogen deposition on the oxygen budget within Chinese coastal waters. Specifically, I used a biogeochemical model to estimate the extent of future hypoxic areas resulting from such expected changes in nitrogen deposition in China. My results suggest that controlling nitrogen deposition can potentially reduce hypoxia. In summary, this thesis provides a new approach for studying the trends and impacts of nitrogen deposition in Hong Kong, which will shed light on the element’s present and future significance in coastal ecosystems.