[A-CG40] Coastal Ecosystems - 1. Water Cycle and Land-Ocean Interactions

[ACG40-05] Evaluation of the environmental condition of Submarine Groundwater Discharge (SGD) using carbon and oxygen stable isotope ratio of the sessile bivalve Crassostrea nippona shell

*Osamu Tominaga¹, Takeuchi Masaru¹, Ryo Sugimoto¹, Ishida Tatsuhiro¹, Takeru Hirai¹, Toshimi Nakajima¹, Jun Shoji², Hisami Honda⁴, Makoto Taniguchi³ (1.Fukui Prefectural University, 2.Tokyo Univ. AORI, 3.RIHN, 4.Fukui Pref Gov)

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.