Saline groundwater discharge accelerates phytoplankton primary production in a Sanriku ria coastal embayment, Japan

Nutrient supply through submarine groundwater discharge into coastal seas can be critical to primary production. However, the influence of recirculated saline groundwater on phytoplankton primary production has not been fully elucidated. Here, we assessed how saline groundwater discharge stimulates phytoplankton primary production in a semi-enclosed embayment (Kesennuma Bay, Japan). We contrasted offshore nutrient supply conditions during summer (stratified condition) and autumn (vertical mixed condition). First, we measured phytoplankton primary productivity in surface seawater using the stable carbon isotope tracer method and identified water sources based on radium isotopes. Then, bioassay incubation experiments were conducted to determine the nutrients limiting phytoplankton primary production and ascertain the effect of saline groundwater discharge. The results showed that nutrients derived from saline groundwater accounted for 19.3 to 38.3, 7.1 to 19.0, and 12.7 to 21.0% of dissolved inorganic nitrogen, phosphorus, and silica concentrations, respectively, in surface seawater. Furthermore, chl a -normalized primary productivity was higher in autumn when the relative contribution of saline groundwater in surface seawater increased. The bioassay experiment showed that phytoplankton growth was most stimulated by a combination of nitrogen and phosphorus, but nitrogen addition enhanced phytoplankton growth in most cases, even at low phosphate concentration (≤0.04 µmol l -1 ) in seawater. Our findings suggest that input of nitrogen-enriched saline groundwater could mitigate the nitrogen limitation of phytoplankton growth, and can be more effective in the vertically mixed season due to the high contribution of offshore phosphorus to surface water. Overall, saline groundwater discharge can stimulate phytoplankton primary production, and should be considered a significant nutrient source for sustaining biological production in coastal seas.