With the rapid rise of atmospheric CO₂, ocean acidification has attracted increasing attention. As reported by earlier researches, seawater aragonite saturation state (Ω_arag) is crucial for growth of calcifying organisms. Water with an Ω_arag below 1.5 restricts marine shellfish growth and threatens the aquaculture industry’s health. So far seawater-acidification-related studies in the Guangdong-HongKong-Macao Greater Bay Area have mostly focused on changes in pH, rather than Ω_arag. This lack of understanding marine Ω_arag dynamics may threaten the healthy development of marine shellfish aquaculture industry in the Greater Bay Area.

Based on three summertime field surveys carried out in 2015 and 2023, we found that the low Ω_arag values of <1.75 are widespread in the Guangdong-HongKong-Macao Greater Bay Area. Both low salinity and high apparent oxygen deficit (AOD) can be used to predict the critically low marine Ω_arag values of <1.5. Central to the threats posed to shellfish growth was the swift decrease of Ω_arag instigated by the riverine diluted waters with a salinity of below 15 and the oxygen-consuming seawater showing an AOD exceeding 100 μmol/kg. Moreover, the combination of moderate salinity and a moderate AOD also led to critically low Ω_arag values, potentially threatening the coastal aquaculture industry of shellfish. Taking a relatively dry year in 2023 as the case, the freshwater-endmember alkalinity exhibited a relatively low value of 1326 μmol/kg before the Pearl River flood, and Ω_arag^Pre-flood = 0.0101 × (8.26 × salinity – 123/138 × AOD) + 0.02 in the estuarine mixing zone. After the Pearl River flood, the freshwater-endmember alkalinity was added to a relatively high value of 1510 μmol/kg, while Ω_arag^Post-flood = 0.0102 × (5.59 × salinity – 123/138 × AOD) + 1.20. The real-data-based verification shows that, these formulae perform pretty well in warning the ecological risk of the marine carbonate chemistry below and/or around the critical Ω_arag value of 1.5 in the Guangdong-HongKong-Macao Greater Bay Area, providing idea for publics to easily assess potential ecological risk of the coastal Ω_arag decline.The research has been published in Marine Pollution Bulletin in October 2025, entitled with “Combining summertime freshwater inputs and oxygen consumption impacts on coastal low aragonite saturation state: Case of the Pearl River Estuary, South China”. Zhai Wei-dong, the Senior Research Fellow of the Marine Carbon Cycle Group at the Southern Marine Science and Engineering Guangdong Laboratory (Zhuhai), serves as the corresponding author of this article. Shi Wen-ting, a Ph.D. candidate under Zhai’s supervision in Shandong University, is the first author of this article.

The study was jointly supported by the National Key Research and Development Program of China (grant 2020YFA0608301) and Southern Marine Science and Engineering Guangdong Laboratory (Zhuhai) (projects SML2024SP024 and SML2023SP206), within the context of the Ocean Negative Carbon Emissions (ONCE) Program.

The original paper refers to: https://doi.org/10.1016/j.marpolbul.2025.118293