Eutrophication around a mega-city in a mega-estuary: the perspective of dual nitrate isotopes in Hong Kong

Abstract

The coastal waters around Hong Kong experience significant eutrophication, resulting in hypoxia and harmful algal blooms.Iit is presumed that nutrients from two anthropogenic sources, local sewage and the nearby Pearl River (second largest river in China), play a major role. We investigated the origin and processing of nitrate in Hong Kong waters by measuring the stable nitrogen and oxygen isotopes in nitrate (δ15N and δ18O) at 2 or 3 depths from 43 sites. δ15N values of the surface nitrate showed a strong spatial gradient with highest values over 20‰ in the south. A clear west-east gradient was visible in the δ18O values at all depths and increased with increasing distance from the Pearl River. Many samples which were outside of the mixing area of 5 potential nitrate sources, indicating that nitrate transformation processes had taken place. We inferred that assimilation was the principal process in the surface waters in the south. The distribution of the samples from the middle and bottom waters suggested that nitrification was the major process at depth. We did not find a clear correlation with sewage outfalls. From these data, we concluded that the river plume of the Pearl River supported biological production in the surface waters south of Hong Kong and can therefore contribute to hypoxia and algal blooms. In the east side, where algal blooms are also common, the Pearl River influence is unlikely and other sources such as groundwater are under consideration.