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[SCG55-P05] Characteristics of Diatom Species in Surface Sediments off the Abe River, Suruga Bay, Japan
Keywords:Diatom, Abe River, sediment
Suruga Bay is the deepest bay in Japan, measuring approximately 60 km from north to south, 56 km from east to west, with a maximum depth of about 2,500 m. Four major rivers-the Oi, Abe, Fuji, and Kano Rivers-flow into Suruga Bay. The Abe River, which runs along the west coast of Shizuoka (about 51 km in length), is a significant sediment-supplying river originating from the Odani Collapse. The continental shelf offshore of the Abe River extends about 20 km from northeast (offshore Miho) to southwest (offshore Yaizu), with a maximum width of approximately 5 km (off Kunozan), narrowing toward the Miho Peninsula. The Miho Peninsula, located in Shizuoka City, is a composite spit made up of an upper sand layer and a lower sand and gravel layer. The Abe River and the Udo Hills are the main sources of the sediments forming the Miho Peninsula (Kimura et al., 2005).
The Marine Geology Laboratory is studying the influence of the Abe River on Suruga Bay, focusing on sediment supply and transport processes. On August 23, 2024, mud sampling was conducted at various depths (10 m, 20 m, 40 m, 100 m, and 450 m) along the north-south (mouth-offshore) line (L-1) off the Abe River. Additionally, samples were taken at a depth of 10 m off Kunozan, between the Miho Peninsula and Miho Bay, and at 585 m and 635 m depths off Miho Peninsula for comparison. This study analyzed sediment composition and diatom processes to understand sediment transport from the Abe River mouth to offshore Miho Peninsula. Surface sediments (0-3 cm layer) were collected using a Smith McIntyre grab sampler.
At 10 m depth along L-1, sediments consisted of fine sand with shell and plant fragments. At 20 m depth, they contained dark gray, very fine sand to silt with small plant fragments. Dark gray silt was found at depths of 40 m, 100 m, and 450 m. Sediments off Kunozan (10 m depth) were medium- to fine-grained sand, while those from Miho Peninsula (10 m depth) were fine- to medium-grained sand. At 585 m and 635 m depths offshore Miho Peninsula, dark gray silt was observed.
Diatom analysis identified seawater, brackish water, and freshwater species. Seawater species included Thalassionema nitzschioides, Thalassiothrix frauenfeldii, Thalassiosira lineata and Rhizosolenia spp., with Thalassionema nitzschioides being dominant. Freshwater species such as Achnanthes lanceolata, Cocconeis placentula var. euglyputa, and Nitzschia palea were present at all sites. The brackish-water species Cocconeis scutellum, commonly attached to seaweed in brackish environments, was abundant at 10 m depth above L-1 near the Abe River mouth but rarely observed at 450 m depth.
At 10 m depth off Kunozan, sediments were coarser with fewer diatoms. The diatom species at 585 m and 635 m depths off Miho Peninsula were similar to those off the Abe River in both saltwater and freshwater species. This species was rarely observed at other depths, suggesting it is primarily found near the river mouth. The proportion of saltwater species offshore of the Abe River increased with depth: 17% at 10 m, 26% at 20 m, 35% at 40 m and 100 m, and 45% at 450 m. Offshore Miho Peninsula (585 m and 635 m depths), saltwater species accounted for about 70% at both depths, higher than the percentages in the Abe River samples.
These findings suggest that the Abe River's influence weakens offshore along L-1 as saltwater species increase. However, the similarity in freshwater species offshore Miho Peninsula (585 m and 635 m depths) to those offshore Abe River indicates that this site may also be influenced by the Abe River. Future studies will continue to investigate the sedimentary environment offshore of the Abe River and Miho Peninsula through further research and analysis. References:Kimura et al. (2005) Journal of Marine Development, 21. Shiba et al. (2016) Journal of Fossil Research, 49(1). Inaba et al. (2001) Deep Ocean Water Research, 2(1). Yoda et al. (2000) Journal of the Japan Society for Marine Surveys and Technology, 12(2).
The Marine Geology Laboratory is studying the influence of the Abe River on Suruga Bay, focusing on sediment supply and transport processes. On August 23, 2024, mud sampling was conducted at various depths (10 m, 20 m, 40 m, 100 m, and 450 m) along the north-south (mouth-offshore) line (L-1) off the Abe River. Additionally, samples were taken at a depth of 10 m off Kunozan, between the Miho Peninsula and Miho Bay, and at 585 m and 635 m depths off Miho Peninsula for comparison. This study analyzed sediment composition and diatom processes to understand sediment transport from the Abe River mouth to offshore Miho Peninsula. Surface sediments (0-3 cm layer) were collected using a Smith McIntyre grab sampler.
At 10 m depth along L-1, sediments consisted of fine sand with shell and plant fragments. At 20 m depth, they contained dark gray, very fine sand to silt with small plant fragments. Dark gray silt was found at depths of 40 m, 100 m, and 450 m. Sediments off Kunozan (10 m depth) were medium- to fine-grained sand, while those from Miho Peninsula (10 m depth) were fine- to medium-grained sand. At 585 m and 635 m depths offshore Miho Peninsula, dark gray silt was observed.
Diatom analysis identified seawater, brackish water, and freshwater species. Seawater species included Thalassionema nitzschioides, Thalassiothrix frauenfeldii, Thalassiosira lineata and Rhizosolenia spp., with Thalassionema nitzschioides being dominant. Freshwater species such as Achnanthes lanceolata, Cocconeis placentula var. euglyputa, and Nitzschia palea were present at all sites. The brackish-water species Cocconeis scutellum, commonly attached to seaweed in brackish environments, was abundant at 10 m depth above L-1 near the Abe River mouth but rarely observed at 450 m depth.
At 10 m depth off Kunozan, sediments were coarser with fewer diatoms. The diatom species at 585 m and 635 m depths off Miho Peninsula were similar to those off the Abe River in both saltwater and freshwater species. This species was rarely observed at other depths, suggesting it is primarily found near the river mouth. The proportion of saltwater species offshore of the Abe River increased with depth: 17% at 10 m, 26% at 20 m, 35% at 40 m and 100 m, and 45% at 450 m. Offshore Miho Peninsula (585 m and 635 m depths), saltwater species accounted for about 70% at both depths, higher than the percentages in the Abe River samples.
These findings suggest that the Abe River's influence weakens offshore along L-1 as saltwater species increase. However, the similarity in freshwater species offshore Miho Peninsula (585 m and 635 m depths) to those offshore Abe River indicates that this site may also be influenced by the Abe River. Future studies will continue to investigate the sedimentary environment offshore of the Abe River and Miho Peninsula through further research and analysis. References:Kimura et al. (2005) Journal of Marine Development, 21. Shiba et al. (2016) Journal of Fossil Research, 49(1). Inaba et al. (2001) Deep Ocean Water Research, 2(1). Yoda et al. (2000) Journal of the Japan Society for Marine Surveys and Technology, 12(2).