Long-term trends in summer hypoxia and associated driving factors in the Pearl River Estuary, China.

Abstract

In this study, we collected in situ water quality data during the summer months from 1985 to 2021 and surface sediment organic carbon and stable carbon isotope (δ¹³C) data from 2002 and 2020 in the Pearl River Estuary (PRE), to analyze long-term trends in hypoxia and explore changes in deoxygenation processes and their potential drivers. Our results showed that hypoxic events in the PRE transitioned from episodic in Lingdingyang Bay in the 2000s to periodic in the lower estuary by the late 2010s. During this transition, the dominant deoxygenation processes shifted from being caused by terrestrial and wastewater emissions to eutrophication. Increased wastewater discharge, reduced sediment load, weakened wind mixing and strengthened coastal upwelling are potential factors contributing to the changes in deoxygenation processes and the deterioration of hypoxic conditions. This study provides a scientific basis for formulating policies to mitigate hypoxia in large estuaries deeply affected by human activities.