Submarine groundwater is richer in nutrients compared to surface (river) water and therefore has been considered to be an essential component of biological production in marine coastal ecosystems. However, there has been no information on the effects of submarine groundwater on animals at high-order trophic levels such as fishes. Here, we show the first direct evidence that fish feeding and growth are elevated by submarine groundwater discharge (SGD) by on-site experiments and quantitative sampling. An experiment using cages moored on the sea bottom confirmed that juvenile marbled sole Pseudopleuronectes yokohamae obtained elevated levels of nutrition in the vicinity of SGD. Quantitative sampling at three sites with different hydrodynamic properties in coastal waters of the western North Pacific showed correspondence of high SGD with high biological production or biomass from producer to secondary consumers. These findings demonstrate that nutrients of terrestrial origin provided via submarine groundwater in coastal areas promote marine fish production.

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Submarine groundwater discharge (SGD) consists of fresh submarine groundwater discharge (FSGD) and recirculated submarine groundwater discharge (RSGD). In this study, we conducted simultaneous 25-hour time-series measurements of short-lived 222Rn and 224Ra activities at two sites with differing SGD rates in the central Seto Inland Sea of Japan to evaluate SGD rates and their constituents. At both sites, we also quantified the total SGD, FSGD, and RSGD using a seepage meter to verify the water fluxes estimated with 222Rn and 224Ra. SGD rates estimated using 222Rn and 224Ra at the site with significant SGD approximated the total SGD and RSGD measured by the seepage meter. However, SGD rates derived using 222Rn at the site with minor SGD were overestimated, since 222Rn activity at the nearshore mooring site was lower than that in the offshore area. These results suggest that the coupling of short-lived 222Rn and 224Ra is a powerful tool for quantification of FSGD and RSGD, although it is important to confirm that tracer activities in coastal areas are higher than those in offshore.

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External nutrient supply from the land and ocean is crucial for sustaining high primary productivity in coastal seas. Submarine groundwater discharge (SGD) is recognized as one of the most important sources of terrestrial nutrients. However, the relative importance of SGD-derived nutrients from different sources in coastal ecosystems controlled by offshore exchange has not been well quantified. Here, we assessed water and nutrient budgets in the semi-enclosed bay along the Sanriku ria coast, where the intrusion of nutrient-enriched oceanic water is substantial. We conducted seasonal sampling campaigns and monitored the groundwater level throughout the year. Water and nutrient fluxes from fresh groundwater, saline groundwater, river water, and oceanic water were estimated using a hydrological method and radium (Ra) mass balance model. The results indicated that oceanic water was a dominant source, accounting for 99.5%, 86%, 97%, and 84% of the total influx of water, dissolved inorganic nitrogen, dissolved inorganic phosphorus, and dissolved silica, respectively. Although the mean fluxes of land-derived nutrients were small, the contribution increased to 28-59% in October, when nutrient fluxes of oceanic water weakened. Of the terrestrial sources, SGD dominated (41-94%), particularly saline SGD (>99% of total SGD). We concluded that an efficient supply of the primary limiting nutrient from land to the coastal ecosystem can accelerate coastal primary production during certain seasons, even if oceanic nutrients are typically the dominant source.Coastal seas are among the most productive regions of the Earth's biosphere, wherein land and ocean physical and biogeochemical processes interact (Jickells 1998). The external supply of nutrients is essential for sustaining the high primary productivity of coastal seas. Offshore seawater serves as a significant nutrient source in coastal seas (Kobayashi and Fujiwara 2008;Sugimoto et al. 2016). The magnitude and timing of oceanic nutrient flux is greatly influenced by physical processes, such as the movement of ocean current paths (e.g., Yoder et al. 1981;Sugimoto et al. 2009), wind intensity (e.g., Graham and Largier 1997;Carstensen and Conley 2004), and river discharge (e.g., Schubel and Pritchard 1986;Watanabe et al. 2017). Nutrient inputs from land via rivers and groundwater can also fertilize coastal ecosystems (Valiela et al. 1990).