Relationship between diatom thanatocoenoses and anthropogenically-induced environmental changes in the Seto Inland Sea, Japan

Abstract:

The Seto Inland Sea (SIS), which is surrounded by Honshu, Kyushu, and Shikoku islands, is the largest enclosed sea in Japan. Water and bottom environments there deteriorated due to various anthropogenically-induced environmental changes from 1955 to 1973 (e.g. Yanagi, 2008). Then, several effluents have been regulated since the 1980s. Diatoms are one of important unicellular algae as a primary producer in waters. As diatoms respond rapidly to the nutrient supply in waters, they are good indicator of eutrophication. Thus, we clarified the spatio-temporal changes of diatom thanatocoenoses in Harima-Nada, eastern part of the SIS, and compared them with previous results in other areas in the SIS (Hirose et al., 2008; Hirose and Gotoh, 2009; Sako et al., unpublished data) to discuss the relationship between diatom thanatocoenoses and degree of anthropogenically-induced environmental changes in the SIS. The surface and/or core sediments were collected from the northern, northwestern, and southern parts of Harima-Nada. We conducted ¹⁴C, ²¹⁰Pb and ¹³⁷Cs dating of cores, diatom analysis, CHNS analysis, and grain size analysis of sediment samples. The results showed that marine environments in all areas of Harima-Nada deteriorated recently and the abundance of planktonic diatoms increased due to intense eutrophication. The comparison with the present and previous studies lead that Neodelphineis pelagica, small Thalassiosira spp., and resting spores of Chaetoceros spp. dominated other taxa in the most areas of the SIS since the mid-20th century, and the relative frequencies of the latter two taxa seem to be useful indicators for evaluating modern marine conditions.

References: Yanagi (2008) Kouseisha-Kouseikaku Co., pp. 130; Hirose and Gotoh (2009) Diatom, 25, p. 21–36; Hirose et al. (2008) The Quaternary Research, 47, 287–296.