Dependence of coastal water pH increases on submarine groundwater discharge off a volcanic island

Abstract

During the past few decades, excessive input of nutrients and organic matter, in addition to global ocean acidification, has resulted in significant changes in the water pH of coastal ocean. In this study, we investigated the effect of submarine groundwater discharge (SGD) on pH variations in the coastal waters of Hwasun Bay off the volcanic island of Jeju, Korea, which is situated in the oligotrophic open ocean. In this region, salinities of all coastal waters depend primarily on SGD because of the lack of any contributions from the river or stream waters. We observed a significant increase in pH along the lower-salinity plume zone, extending 0.5 km horizontally from the bottom to the surface (< 15 m water depth). The observed data for the entire bay-water column showed a significant negative correlation (r2 = 0.82) between salinity and pH. A simple two-endmember (submarine groundwater and offshore seawater) mixing model showed that this pH increase was caused by an enhanced biological production, which resulted from the SGD-driven nutrient inputs, rather than from groundwater dilution itself. Since a number of local and regional studies showed that SGD-driven fluxes of nutrients are comparable to or higher than their riverine fluxes, our results from an SGD-dominated environment suggest that SGD may have a significant influence on the coastal biogeochemical changes such as acidification, deoxygenation, and eutrophication.