[A-CG51] [JJ] Coastal Ecosystems - 1. Water Cycle and Land-Ocean Interactions

[ACG51-09] Evaluation of the environmental condition of Submarine Groundwater Discharge (SGD) by using carbon stable isotope ratio recorded in the shell of sessile bivalve

*OSAMU TOMINAGA¹, Ryo Sugimoto¹, Katsuhiro Kitagawa¹, Masaru Takeuchi¹, Makoto Yamada², Jun Shoji³, Hisami Honda², Shiho Kobayashi⁴, Makoto Taniguchi² (1.Fukui Prefectural Univ., 2.RIHN, 3.Hiroshima Univ., 4.Kyoto Univ.)

Keywords:Submarine groundwater discharge, sessile bivalve, Rock-oyster, carbon stable isotope ratio, SGD flow rate, the proportion of fresh water flux in SGD

Submarine Groundwater Discharge (SGD) is often characterized by high concentration of nutrients and documented as an important pathway between land and sea contributing to the biological productivity in coastal waters. In the present study we investigated two subjects 1)whether the δ¹³C of ambient water reflect the proportion of fresh water flux in SGD, 2)whether the δ¹³C recorded in the shell of the sessile bibalve reflect the that of the ambient water. The SGD flux and the proportion of fresh water flux in SGD were measured by the seepage meter at Kamaiso beach along the Mt. Chokai volcanic coast in June and September 2016.The rock-oyster ( Crossostrea nippona ) was also sampled at Kamaiso Beach and adjacent 3 areas ( Mega, Takinoura, Torisaki). There was a positive significant relationship between salinity and theδ13C of ambient water (p<0.01). Theδ¹³C of the rock-oyster shell was high where the SGD flow rate was high and the positive significant correlation was found between the δ¹³C of the rock-oyster shell and that of ambient water. In order to estimate the proportion of fresh water flux in SGD from the δ¹³C of the rock-oyster shell, we used the two end member mixing equation including DIC concentration of both sea water and underground water. The average proportion of fresh water flux in SGD estimated from the mixing model ranged from 4 % (Torisaki) to 35 % (Mega). These values are higher than those estimated from the δ¹³C of ambient water (1.3 % Torisaki and 18.1 % Mega). The difference of these results are seems to be the difference of local SGD environment around rock-oyster.