Fresh and saline submarine groundwater discharge as sources of carbon and nutrients to the Japan Sea

Submarine groundwater discharge (SGD) is an important pathway for carbon and nutrients to the coastal ocean, sometimes exceeding river inputs. SGD fluxes can have implications for long-term carbon storage, ocean acidification and nutrient dynamics. Here, we used radium (Ra-223 and Ra-226) isotopes to quantify SGD-derived fluxes of dissolved inorganic (DIC) and organic (DOC) carbon, nitrate (NO3-), nitrite (NO2-), ammonium (NH4+) and phosphate (PO43-) in a spring-fed coastal bay in the Japan Sea. The average coastal water residence times using Ra-223/Ra-226 ratios was 32.5 +/- 17.9 days. Fresh and saline SGD were estimated using a radium mixing model with short- and long-lived isotopes. The volume of fresh SGD entering the bay (4.6 +/- 4.6 cm day(-1)) was more than twice that of the volume of saline SGD (1.9 +/- 2.1 cm day(-1)). Fresh SGD (mmol m(2) day(-1)) was the main source of DOC (2.7 +/- 2.6), DIC (13.9 +/- 13.7), PO43- (0.3 +/- 0.3) and NO3- (6.6 +/- 6.5) to the coastal ocean, whereas saline SGD was the main source of NH4+ (0.2 +/- 0.2). Total SGD-derived carbon and nutrient fluxes were 4 - 7 and 2-16 times greater than local river inputs. Positive correlations between chlorophyll-a, Ra-226 and delta C-13-DIC indicate that SGD significantly (p < 0.05) enhances primary productivity nearshore. Overall, fresh SGD of nitrogen and carbon to seawater drove chlorophyll-a, decreased DIC/Alkalinity ratios, and modified the carbonate biogeochemistry of the coastal ocean.

Automated summary

Submarine groundwater discharge (SGD) is a significant pathway for carbon and nutrient transport to coastal ocean, often surpassing river inputs. This study used radium isotopes to estimate SGD-derived fluxes of carbon and nutrients in a coastal bay in the Japan Sea. Fresh SGD was found to be the main source of carbon and nitrogen to the coastal ocean, while saline SGD dominated ammonium inputs. The study revealed that SGD significantly enhanced primary productivity and modified carbonate biogeochemistry of the coastal ocean.