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 222 Rn and salinity mass balance model for a whole bay scale. The resulting SGD rates showed large intra-annual variability from 0.05×10 6 to 0.77×10 6 m 3 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 0.4×10 6 m 3 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 0.36×10 6 m 3 day −1 . This value was similar to the annual groundwater recharge rate (0.33×10 6 m 3 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.

Nutrients supplied from submarine groundwater discharge (SGD) are generally thought to enhance primary production in coastal seas. However, there is little evidence for a direct association between SGD and phytoplankton primary productivity. To elucidate the response of in situ primary productivity to SGD, we conducted comparative experiments at 3 coastal sites in Japan with different SGD types (Site A: Obama Bay; Site B: coast along the western foot of Mt. Chokai; Site C: Beppu Bay) during the summers of 2013−2015. At Site A, which is characterized by seepage-type SGD, we found a significant positive relationship between in situ primary productivity and 222 Rn concentration. This was likely driven by nutrient-limited water column conditions. On the other hand, at the volcanic coastal Sites B and C, which are dominated by spring-type SGD, no clear relationships between in situ primary productivity and 222 Rn concentration were found. Although significant relationships between nutrient concentrations and 222 Rn concentration suggest that SGD acts as a primary nutrient source in these regions, the non-trivial influence of light availability complicates those relationships. Furthermore, lower biomass-specific primary productivity around submarine springs at both sites suggests that submarine springs have negative impacts on phytoplankton growth rates around vent sites, possibly due to changes in local environmental conditions. Our study clarified that the mechanism by which SGD affects phytoplankton production differs from one ecosystem to another because of variable hydrogeographical properties, such as the type of groundwater discharge (i.e. spring or seepage). KEY WORDS:In situ primary productivity · Submarine groundwater discharge · 222 Rn OPEN PEN

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