[A-CG33] 陸海相互作用－沿岸生態系に果たす水・物質循環の役割－

[ACG33-09] 海底湧水環境の代替指標としての貝殻中δ13C値の有効性評価

*富永 修¹、堀部 七海¹、西 沙織¹、杉本 亮¹、山田 誠²、小路 淳³、本田 尚美²、小林 志保⁴、谷口 真人² (1.福井県立大学、2.総合地球環境学研究所、3.広島大学大学院生物圏科学研究科、4.京都大学フィールド科学教育研究センター)

キーワード：溶存態無機炭素, 代謝由来炭素, 二枚貝貝殻, 炭素安定同位体比, 海底湧水

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. However, to our knowledge, no scientific information about the relationship between environmental conditions of SGD and the extent of biological productivity of the primary consumers is available. The carbon stable isotope composition of dissolved inorganic carbon (δ¹³C_DIC) is generally different between SGD (low δ¹³C_DIC) and sea water (high δ¹³C_DIC). Bivalves record chemical and biological environment-signal in their shell. In this study, to examine whether the δ¹³C of bivalve shell (δ¹³C_SHELL) reflect the δ¹³C_DIC of the ambient water or not, we conducted the rearing experiments of Manila clam Ruditapes philippinarum under laboratory and field conditions. Manila clam was reared at three different salinity in the laboratory; 100% sea water (100 % SW), 80% sea water + 20% underground water (80 % SW) and 60 % sea water + 40% underground water (60 % SW). Water temperature of three salinity groups was maintained at 25 ℃ and the same amount of diet (Chaetoceros gracillis) was fed every day during experiments (3 months). There was a high positive correlation between δ¹³C_DIC and salinity (r² = 0.997, n = 32, p<0.001). Although the significant difference among 100 % SW, 80 % SW and 60% SW was not found (ANOVA, p>0.05), the δ¹³C_SHELL of 80 % SW and 60 % SW was tend to be lower than that of 100 % SW. The field experiment was carried out at 6 sites under different SGD condition in Obama Bay, Japan from July to August 2013. Manila clam was reared in the small containers with bottom sand, which were kept at the depth of 2 m under natural condition. There was a high negative linear relation between δ¹³C_SHELL and Radon 222 (²²²Rn) concentration at surface layer of each rearing site ,which is a useful tracer of SGD (r² = 0.920, n = 6, p<0.01). Thus, it seems that the δ¹³C_SHELL shows some possibility of being proxy for environmental reconstitutions of submarine groundwater discharge. However, the value of the δ¹³C_SHELL was lower by about 5 ‰ than that of the δ¹³C_DIC of ambient water in the laboratory experiment. It was the possible reason that isotopically light metabolic carbon, derived from food, is incorporated into shell carbonated.