Pathways of anthropogenic nitrogen in the coastal environment of Hong Kong

Abstract

The nitrogen (N) dynamics in coastal environments has altered due to urbanisation and human activities in the connected catchments. Eutrophication due to excessive N discharge can lead to negative ecological impacts, such as harmful algal blooms and hypoxia. We wanted to identify the pathways of anthropogenic N upon reaching the coastal waters. Therefore, we measured d15N of nitrate and particulate matter in Hong Kong waters, which are under influence of the Pearl River and local sewage discharge. The samples were collected in 42 sites at 2 or 3 depths during the OCEAN-HK summer cruise in 2017. The d15N showed a gradual transition from west to east and spanned a broad range from 7 to 23 permille in nitrate and -9 to 14 permille in particulate matter. The application of the steady state equation pointed toward strong N transformations at all depths. Assimilation of N in phytoplankton was the dominant process in the surface waters in the south of Hong Kong. Nitrification explained the patterns in the middle and bottom waters. The fractionation factors obtained from the steady state plots were subsequently applied into a model to estimate the magnitude of the different N transformation processes. Based on this dataset, we concluded that discharges of N, often caused by human activities, are significantly transformed in the coastal zone. The substantial assimilation of N in the surface waters which we observed on the south of Hong Kong can lead to algal blooms and associated environmental impacts.