Spatiotemporal nutrient patterns, composition, and implications for eutrophication mitigation in the Pearl River Estuary, China

Abstract

Human activities and climate change have changed large river estuary ecosystems, which impacts nutrient enrichment in coastal waters. Enhanced nutrient levels lead to eutrophication, oxygen deficits, and frequent occurrences of harmful algal blooms. In this study, the spatiotemporal nutrient patterns and composition of dissolved inorganic nitrogen (DIN) and dissolved inorganic phosphorus (DIP), as well as the relationships between DIN, DIP, eutrophication index (EI), and environmental factors were explored. The spatiotemporal pattern and composition of DIN and DIP in coastal water based on samples from fifteen stations in the Pearl River Estuary (PRE) were investigated during the dry (April), wet (July), and normal (October) seasons in 2018. The results showed that the average concentrations of DIN and DIP were 65.35 ± 47.72 μmol L−1 and 0.94 ± 0.70 μmol L−1 in the PRE, respectively. The concentration of DIN at most monitoring stations failed to reach the Grade IV national seawater quality standard, while the concentration of DIP did meet them. The ratio of DIN/DIP ranged from 4.33 to 242.50, with an average of 42.60 ± 47.06, exceeding the Redfield ratio. EI was high in the PRE, ranging from 0.01 to 67.19, with an average of 8.64 ± 11.94. Hotspots with high DIN and DIP concentrations were located in the upper PRE during all seasons. In addition, there were significant negative correlations among DIN, DIP, EI, and salinity (P < 0.05), except for DIP and EI in the wet water flow period, implying an impact from riverine nutrient input. Moreover, the DIN, DIP, and EI also had negative relationships with dissolved oxygen in all seasons (P < 0.05), indicating that eutrophication led to oxygen deficits in the PRE. Therefore, land–ocean integration management of PRE-associated nutrient criteria, composition, and sources should be introduced to effectively and efficiently mitigate eutrophication in the future.