Annual hypoxia causing long-term seawater acidification: Evidence from low-molecular-weight organic acids in the Changjiang Estuary and its adjacent sea area

In this study, components, concentrations, distribution characteristics, sources of low-molecular-weight organic acids (LMWOAs) and relationships among the annual hypoxia, LMWOAs and seawater acidification were investigated in the Changjiang Estuary and its adjacent sea area in July 2015. Lactic, acetic and formic acids were detected in the seawater samples in the study area, and their total concentrations (ELMWOAs) varied from 0 to 262.6 pmol.L-1, with an average value of 39.2 pmol.L-1. In the surface seawater, high concentration areas of ELMWOAs occurred in the sea area near the Changjiang Estuary and the Hangzhou Bay, and north of study area. In the sampling stations along transect A6, high concentration areas of ELMWOAs appeared in the bottom seawater of nearshore stations and middle seawater of offshore stations. The terrigenous inputs, especially the Changjiang runoff, were the dominant sources for LMWOAs in the sampling period. The consistency of hypoxia areas, high concentration areas of ELMWOAs and low pH value areas in winter and summer suggested that annual hypoxia could cause the long-term seawater acidification by producing LMWOAs in the Changjiang Estuary and its adjacent sea area.

Automated summary

This study investigates the components, concentrations, distribution characteristics, and sources of low-molecular-weight organic acids (LMWOAs) in the Changjiang Estuary and its adjacent sea area. The results show that terrigenous inputs, especially the Changjiang runoff, are the dominant sources of LMWOAs. The consistency between hypoxia areas, high concentration areas of LMWOAs, and low pH value areas suggests that annual hypoxia could cause long-term seawater acidification by producing LMWOAs.