Seasonal Changes in Submarine Groundwater Discharge and Associated Nutrient Transport into a Tideless Semi-enclosed Embayment (Obama Bay, Japan)

We carried out a seasonal study of fresh submarine groundwater discharge (SGD) and associated nutrient fluxes in a semi-enclosed bay along a tideless coastal zone using a 222Rn and salinity mass balance model for a whole bay scale. The resulting SGD rates showed large intra-annual variability from 0.05 × 106 to 0.77 × 106 m3 day−1, which were controlled by seasonal changes in the interaction of multiple driving forces, including water table height and seawater level. The highest SGD rate in early spring was induced by heavy snow and low sea level, whereas the seasonal increase in sea level gradually suppressed fresh SGD rates. In summer, an elevated water table may induce higher SGD rates (approximately 0.4 × 106 m3 day−1) regardless of high sea levels. The highest SGD fraction in total terrestrial freshwater fluxes also occurred in summer (>40 %), due to the decreasing rate of surface river discharge. The seasonally averaged SGD rate was 0.36 × 106 m3 day−1. This value was similar to the annual groundwater recharge rate (0.33 × 106 m3 day−1) estimated by the water balance method in the basin. Nutrient fluxes from SGD were approximately 42, 65, and 33 % of all terrestrial fluxes of dissolved inorganic nitrogen, phosphorous, and silicate, respectively. The average fraction of SGD in the water fluxes including terrestrial and oceanic water was low (0.3 %), but that of nutrient fluxes increased to 20–38 %. Higher nutrient concentrations in groundwater compensated for the lower volumetric flux of groundwater. Because primary production was mostly restricted by phosphorous throughout the year, phosphorous-enriched nutrient transport via SGD would play an important role in biological production.