Submarine groundwater discharge strengthens acidification in the coastal areas 

<p>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&#8217;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.</p>