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The chemical components and benthic foraminiferal assemblages in core sediments from Notoro Lagoon in northern Japan revealed the effects of artificial inlet excavation and the role of regional climate cycles on the shift of oxygen conditions in the lagoon basin. The materials analyzed were four sediment cores obtained from three basins in Notoro Lagoon using pushing-piston corers. An artificial inlet was excavated at the lagoon in 1977. Immediately after excavation, CNS contents in the sediment began to decrease, and benthic foraminiferal assemblages shifted due to high-oxygen seawater inflow. There was a 7-year transition before the lagoon environment and ecosystem were stable. After inlet excavation, the bottom environment and benthic foraminiferal assemblage of Notoro Lagoon were strongly influenced by short-term climate oscillations such as the Pacific Decadal Oscillation and Arctic Oscillation. Frequent short-duration peaks of oxygen-proxy foraminiferal taxa and decreases in total organic carbon in sediment were confirmed between 1986 and 1998. This transition occurred because the spring air temperature and pressure over Notoro Lagoon were affected by the short-term climate oscillations, which have determined the timing of lake ice melt and the volume of oxygen-rich seawater inflow. As a result, bottom conditions and the benthic ecosystem of the Notoro Lagoon have shifted along with these regional climate changes. (C) 2018 Elsevier B.V. All rights reserved.