Origin of the Suspended Particles with High Turbidity Area in Murasaki River Estuary

摘要

To identify the cause of the high turbidity of the Murasaki River estuary, we investigated variations in suspended particle (SS) concentration and SS composition during the tide cycle in the Murasaki River estuary in June 2001 (rainy season) and September 2001 (nonrainy season). Additionally, the stable isotope ratio of carbon and nitrogen in the sediment and SS concentration and chemical composition were determined in the Murasaki River watershed over the investigation period.In September, the turbidity and SS concentration were high in the bottom layer. The SS compositions in the surface and middle layers depended on the tide phase, although in the bottom layer, SS composition did not depend on the tide phase. Moreover, the average SS composition in the bottom layer showed relatively low particulate organic carbon (POC) and chlorophyll a (Chl a) contents and high C/N and C/Chl a ratios, and the sinking particle compositions was similar to the SS composition in the bottom layer. This suggests that the origin of SSs in the bottom layer differed from those in the surface and middle layers, and the resuspension of sediment was a significant source of SSs during the nonrainy season. In June, the variation in SS composition in the surface layer did not correlate with tide level, and the average SS composition was similar to that in the river. This finding suggests that surface layer SSs were predominantly river-borne SSs.Sediments in the upper and middle reaches had a high δ 15N (+4.5‰∼+5.2‰) because of artificial pollution (small reservoir, agriculture effluent and human sewage). However, the δ 13C and δ 15N contents of the sediment in the Murasaki river estuary suggest that terrestrial C3 plants were the main source of organic matter in the sediment (δ 13C=-25.8‰, δ 15N=+2.9‰). Additionally, the δ 13C and δ 15N of sinking particles in the Murasaki river estuary suggest that the contribution of particles from the forest was higher in June (δ 13C=-25.2‰, δ 15N=+3.2‰) than in September (δ 13C=-24.4‰, δ 15N=+3.9‰). This suggests that the load from the forest brought when a storm was the main source of the organic matter in the the Murasaki River estuary sediments, and that the high turbidity of the Murasaki River estuary was caused by the resuspension of the sediment originating from the forest.