Total organic carbon and nitrogen contents in surface sediments of Harima Nada,eastern Seto Inland Sea,Japan: a comparison under two different trophic states |
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| Authors: | Hitomi Yamaguchi Naoto Hirade Miho Kayama Kazuhiko Ichimi Kuninao Tada |
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| Institution: | 1. Faculty of Agriculture, Kagawa University, 2393 Ikenobe, Kita, Miki, 761-0795, Japan
2. Seto Inland Sea Regional Research Center, Kagawa University, 4511-15 Kamano, Aji, Takamatsu, 761-0130, Japan
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| Abstract: | Phytoplankton abundance in the surface waters of Harima Nada, eastern Seto Inland Sea of Japan, decreased after around the late 1970s following a reduction of terrestrial phosphorous inputs, and relatively low levels persisted thereafter. This symptom of de-eutrophication in recent decades might have reduced organic matter storage in surficial sediments. To test this possibility, we examined total organic carbon (TOC) and nitrogen (TN) contents of Harima Nada sediments in 2011, and compared the results to past data from more eutrophic states. In 2011, the TOC and TN varied horizontally within the basin, in relation to physicochemical properties (water and mud content) and phytoplankton remnants (pheo-pigments) of the sediments. These horizontal patterns were fundamentally similar to those of the early 1980s (1982–1983), although discrepancies between the periods were observed in some areas. Differences of mean TOC and TN in the 0–2 cm layer at 63 stations from 1982–1983 to 2011 were consequently <10 %. Hence, the recent de-eutrophication in Harima Nada was associated with little overall change of TOC and TN storage in the sediment. In Harima Nada, changes in water clarity and particle size composition of the sediment seem to have occurred during recent decades. The aforementioned paradoxical phenomenon may therefore be explained by factors that are affected by these environmental changes. Such factors include the ratio of export flux to the seafloor to phytoplankton production, phytoplankton biomass and production in the entire euphotic zone, and accumulation rate of surface sediment. |
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