Riverine freshwater outflow enhanced ocean acidification in an urbanized subtropical estuary

Ocean acidification poses a growing environmental threat to estuarine ecosystems. Most research has focused on bottom water acidification driven by eutrophication from riverine nutrient inputs. In contrast, the impacts of other riverine components in estuarine systems have received less attention. This study investigates the impacts of riverine carbonate and organic matter input on acidification in the Pearl River estuary (PRE), using field data from April 2015 to January 2016. The results show that DIC and TA in the PRE are primarily governed by river-ocean mixing, while their seasonal variations are largely influenced by changes in the freshwater end-member. Compared to the marine carbonate system, the riverine DIC:TA ratio is significantly higher. Riverine carbonate input weakens the estuarine buffering capacity during mixing, thereby amplifying acidification from atmospheric anthropogenic CO₂. The pH reduction in the estuary due to anthropogenic CO₂ intrusion reached a maximum of 0.15 units at a salinity of around 15, exceeding the 0.12 reduction at the seawater end. With the urbanization of the Pearl River Basin, the increased input of anthropogenic organic matter enhanced aerobic respiration in the estuary, releasing CO₂ that may further intensify acidification. Unlike eutrophication-induced bottom acidification, which is closely associated with water column stratification and mainly occurs in the outer estuary during summer, organic matter-driven acidification is most pronounced in the upper estuary and affects the entire water column year-round. Moreover, it caused a maximum pH decline of up to 1.01 units in the PRE, surpassing that induced by eutrophication.