Keywords:Submarine Groundwater Discharge, carbon stable isotope, oxygen stable isotope, rock oyster shell, bayesian stable isotope mixing model
Submarine Groundwater Discharge (SGD) results in various biogeochemical changes in the coastal environments, such as coastal primary production, eutrophication, and benthic production. The SGD condition of 6 sites along volcanic mountain coast in northern Japan was evaluated by using carbon and oxygen stable isotope ratio (δ13C and δ18O) in the shell of sessile bivalves Crassostrea nippona. In order to estimate the proportion of fresh water flow rate in SGD (FR) from the respective δ13C and δ18O of C. nippona shell, we used the three end member (sea water, riverine water and fresh underground water) bayesian stable isotope mixing model. The significant positive correlation was found between δ13C and δ18O of the shell and those of ambient water, respectively. FRs estimated from three source model were 3.1 -8.6% at low 222Rn sites and 21.9-28.5% at high 222Rn sites. At the site (Nishihama) of river mouth, the δ13C and δ18O of the shell collected were low (estimated FR 10.6-25.0%) and 222Rn concentration was also low. The contribution of DIC in SGD and riverine water for shell formation was estimated to be 19.8 % and 7.8%, respectively. In the present study, we demonstrated the effectiveness of δ13C and δ18O of C. nippona shell as the tool for estimating FR.