中海浚渫窪地に堆積する年縞堆積物が示す浚渫後の水環境変化

Dredged channels, trenches, and pits are common in waterways throughout Japan. Lake Nakaumi, located in western Honshu, also has 8 million meter2 of channels that were dredged after the 1960s for coastal reclamation. Areas normally 3-4 m deep were dredged to depths of 6-15 m during construction. A sediment core, 17N1, was obtained from a dredged area in the southeast end of Lake Nakaumi in order to investigate environmental change after land reclamation and dredging. Core 17N1 is 57 cm in length, and the top 43 cm contains 35 laminated layers. Lithologies were clearly different between the laminated and lower sediments. The laminated layers have high water content (> 80 %) and are high in organic matter derived from plankton. The laminae in core 17N1 are inferred to be annual, meaning that the coring site was dredged in approximately 1982. After that time an anoxic water mass developed in the dredged channel and laminated sediments were preserved due to the absence of bioturbation and high seasonal variation in primary production. Although the total organic carbon and nitrogen contents have continuously increased in core 17N1, diatoms follow a different pattern. The total number of diatom valves increased until 1995, after which they gradually decreased. This pattern suggests that phytoplankton sourced in red-tide events shifted from diatoms to other taxa, perhaps dinoflagellates. In contrast, the C/S ratio has continuously increased in core 17N1, particularly in light laminae layers, suggesting that the anoxic water mass in this deep channel might be gradually dissipating.