An Unforeseen Long-Term Nitrate Accumulation Under Nutrient Mitigation Measures in the East Pearl River Estuary: Phenomenon, Drivers and Implications

Either NO3- or NH4+ is the dominant component of dissolved inorganic nitrogen (DIN) in most estuarine waters, but few studies have focused on the mechanisms of ongoing DIN transformations under various hydrologic regimes. In the Shenzhen Bay (SZB), a highly anthropogenic affecting bay in the east Pearl River Estuary, the dominated DIN component has been found to change from NH4+ to NO3- in recent years. Especially at the top bay, the concentration fraction of NO3- in DIN was unexpectedly promoted from 17-22% (1990-2017) to 65-66% (2018-2020), which would imply an increasing risk of red tide occurrence under current nutrient mitigation strategy. The spanning 31 years’ water quality data analysis and numerical simulations were performed to identify the roles of multiple driving factors. It was found that the contribution from decreasing runoff NH4+ input might mainly account for such change. Although NH4+ was further diluted through runoff and tidal current conditions, more NO3- was transported to the upper bay. In addition, the decreases of biochemical oxygen demand along with increasing Secchi disk depth and dissolved oxygen suggested a shrink in NH4+ production, but an improvement in nitrite oxidation process. These biogeochemical actions were well confirmed by simulations that strengthened the decrease of NH4+ and the increase of NO3-. Even without nutrient inputs from the territorial sources, the simulated biogeochemical processes still acted as decreasing NH4+ throughout the bay and increasing NO3- at the middle and lower bay. The new insights into DIN change mechanisms from the coupling biogeochemical and hydrodynamic processes would help to boost effective mitigation measures to deal with nitrate accumulation in estuarine waters for integrated land-ocean nutrient management.