11:15 〜 11:30
[ACG42-07] 二枚貝貝殻の炭素安定同位体比を用いた、大槌湾と船越湾の湧水環境評価
キーワード：海底湧水、炭素安定同位体、マガキ、イガイ、大槌湾、船越湾
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. We investigated to what extent SGD contributed to the production of primary consumer by the sampling survey of oyster and mussel at two sites (Otsuchi Bay and Funakoshi Bay, Iwate Prefecture) along Pacific coast of northern Honshu island in August 2017. The Radon 222 (222Rn) concentration that is a useful tracer of SGD was measured. Pacific oyster (Crossostrea gigas ) and mussel (Mytilus coruscus ) were also sampled at 2 areas. The measurement of δ13C of ambient water (δ13CDIC) was conducted by an elemental analyzer-isotope ratio mass spectrometer with Gasbench. The edge of the oyster shell was grinded by micro drill and δ13C of carbonate powders was measured. In order to estimate the contribution rate of SGD, we used the classical two end member mixing model incorporating DIC concentration of both sea water and fresh underground water.
The average 222Rn concentrations were 228.9±22.4 (Bq/m3) in Otsuchi Bay and 115.0±18.4 (Bq/m3) in Funakoshi Bay, respectively. The contribution rates of SGD in coastal areas estimated from δ13CDIC were 17.4±5.7% in Otsuchi Bay, and 9.0±1.6% in Funakoshi Bay, and those in offshore area were from 0.2±0.3% to 4.0±0.6% in Otsuchi Bay. The contribution rates of SGD in coastal areas estimated from δ13CSHELL of C. gigas and M. coruscus were 21.9±2.4 and 10.5±11.4% in Otuchi Bay, respectively, and 3.2±3.3 and 6.9±8.8% in Funakoshi Bay, respectively. Thus, the contribution rates estimated from the bivalves in Otsuchi Bay are higher than those in Funakoshi Bay. The contribution rates of SGD in offshore area estimated from δ13CSHELL of M. coruscus was from 0±10.5% to 12.4±2.8% in Otsuchi Bay. These results suggested that SGD contributes to the biological products in not only coastal areas but also offshore area in Otsuchi Bay. However, we did not divide the source of freshwater into SGD and riverine water. It is necessary to develop the three source model to evaluate the contribution rate of SGD.
The average 222Rn concentrations were 228.9±22.4 (Bq/m3) in Otsuchi Bay and 115.0±18.4 (Bq/m3) in Funakoshi Bay, respectively. The contribution rates of SGD in coastal areas estimated from δ13CDIC were 17.4±5.7% in Otsuchi Bay, and 9.0±1.6% in Funakoshi Bay, and those in offshore area were from 0.2±0.3% to 4.0±0.6% in Otsuchi Bay. The contribution rates of SGD in coastal areas estimated from δ13CSHELL of C. gigas and M. coruscus were 21.9±2.4 and 10.5±11.4% in Otuchi Bay, respectively, and 3.2±3.3 and 6.9±8.8% in Funakoshi Bay, respectively. Thus, the contribution rates estimated from the bivalves in Otsuchi Bay are higher than those in Funakoshi Bay. The contribution rates of SGD in offshore area estimated from δ13CSHELL of M. coruscus was from 0±10.5% to 12.4±2.8% in Otsuchi Bay. These results suggested that SGD contributes to the biological products in not only coastal areas but also offshore area in Otsuchi Bay. However, we did not divide the source of freshwater into SGD and riverine water. It is necessary to develop the three source model to evaluate the contribution rate of SGD.