Characterising sources and dominant cycling processes of inorganic nitrogen in a subtropical semi-urbanised catchment: impact on harbour deoxygenation

Abstract Rivers are significant sources of dissolved inorganic nitrogen, contributing to coastal eutrophication and hypoxia. While the impact of large rivers is well documented, less is known about small rivers that directly discharge into the sea after draining urban areas. Their aggregate biogeochemical significance should not be overlooked, given their potentially high loads of nitrogen. There are various kinds of human activities within a river catchment that may contribute different types and amounts of inorganic nitrogen. Identifying the relative contribution of each of these sources is important to facilitate the planning of environmental management measures that can effectively reduce riverine nutrient loading. Focusing on the subtropical Lam Tsuen River in Hong Kong, this study made use of dual nitrogen and oxygen isotopes to identify sources of nutrients within the catchment. Results indicate that nutrients were mainly released into the system in lowland areas in the form of sewage originating from anthropogenic activities. Downstream sites had a significantly higher proportion of nitrate originating from sewage than upstream sites. Nitrification generally accounted for more than 40% of the nitrite and nitrate found in the catchment. Assimilation in the river was found to be negligible, suggesting that most nutrients were transported into the seasonally hypoxic Tolo Harbour. Compared to streams worldwide that do not drain urbanised areas, the Lam Tsuen River generally discharges 10 times more inorganic nitrogen per catchment area. This nitrogen delivered to the coast ultimately consumes roughly 2% of Tolo Harbour’s oxygen. Together with four other similar-sized or even larger rivers, up to 10% of the harbour’s dissolved oxygen will be consumed. Overall, our findings highlight the need for a better accounting of the impact of these small rivers on marine nitrogen budgets.