[A-CG23] Coastal Ecosystems - 1. Water Cycle and Land-Ocean Interactions

[ACG23-05] Estimation of submarine groundwater discharge and associated nutrient fluxes in Otsuchi Bay, northeast Japan in summer

*Hisami Honda¹, Ryo Sugimoto², Jun Shoji³, Osamu Tominaga², Makoto Taniguchi¹ (1.Research Institute for Humanity and Nature, 2.Fukui Prefectural University, 3.Hiroshima University)

Keywords:submarine groundwater discharge , Otsuchi Bay

Otsuchi Bay is a small semi-enclosed bay along the Sanriku ria coast. Although the watershed of the bay has large amounts of groundwater resources, the rates of groundwater input into the bay and associated nutrient fluxes are still unclear. In this study, we estimated the rates of submarine groundwater discharge (SGD) and associated nutrient fluxes into Otsuchi Bay using the radon-222 (²²²Rn) and salinity mass balance model during the spring and neap tides in summer. As a result, SGD rates ranged from 0.10 to 1.07×10⁶ m³ day^-1 (1.29 to 13.16 cm day^-1) with a mean of 0.43×10⁶ m³ day^-1 (5.28 cm day^-1) during the four sampling periods. This value was similar to the annual groundwater recharge rate (0.63×10⁶ m³ day^-1) estimated by the water balance method in the whole basin. Estimated fluxes of dissolved inorganic nitrogen (DIN) and phosphorous (DIP) through the SGD were 50.2 to 511.6 kg day^-1 and 0.4 to 4.2 kg day^-1, respectively. In contrast, these fluxes through the river water were 127.9 to 336.6 kg day^-1, 1.7 to 7.8 kg day^-1, respectively. Nutrient fluxes through the SGD were approximately 40 and 33 % of all terrestrial fluxes of DIN and DIP, respectively. In addition, DIN flux of the SGD was approximately 7.7 % of that of the oceanic water even though DIP flux of SGD was only 0.2 %. In Otsuchi Bay, the N/P ratio in bay water during the stratified period was often lower than Redfield ratio. Therefore, DIN supply from the SGD would have a non-negligible contribution on primary production during the stratified period.