Submarine groundwater discharge strengthens acidification in the coastal areas

Coastal ocean acidification is a worldwide marine problem. In this study, a close relationship between submarine groundwater discharge (SGD) and coastal ocean acidification rate in Hong Kong’s coastal waters is discovered. We for the first time evaluated the direct influence of SGD on seawater pH decline. Results show that SGD can contribute to up to 45% of seawater pH decline through direct input of carbonate species. Local air-sea CO₂ exchange has negligible influences on the seawater pH, but the uptake of air CO₂ can alter open ocean pH and indirectly alter coastal seawater pH by bay-open ocean water exchange. Aerobic respiration is the major contributor to the seawater pH decline in most coastal waters, in which SGD plays a significant role as the major nutrient source. The findings highlight the importance of the investigation and management of groundwater to alleviate the fast coastal ocean acidification.