Records of environmental and ecological changes related to excavation in varve sediment from Lake Hiruga in central Japan

Sub-annual-scale environmental and ecosystem changes since the mid-18th century were reconstructed in a semi-closed lagoon, Lake Hiruga, located along the Sea of Japan in central Japan. Lake Hiruga is one of the Mikata Five Lakes and is connected to Lake Suigetsu through an artificial tunnel, the Saga Tunnel. Analyses of siliceous phytoplankton (diatoms, silicoflagellates, and chrysophyte cysts) and elemental composition (carbon, nitrogen, sulfur) of laminated sediments were conducted to determine the impacts of irregular hazards and anthropogenic tunnel construction on the lagoon ecosystem. In this lagoon, sediment laminae were deposited after the first Saga Tunnel re-excavation in 1844–1848, and the dominant phytoplankton taxa have changed with each re-excavation. The behaviors of total nitrogen, total organic carbon, freshwater diatom taxa, and chrysophyte cysts in Lake Hiruga are strongly affected by tunnel excavation and water gate construction. In addition to anthropogenic effects, the deposits show extreme hazard impacts of the Lake Hiruga ecosystem. The peaks of marine diatom taxa, silicoflagellate abundance, and sulfur content of Lake Hiruga showed close relationships with the local earthquake event. A relatively high sea level in the coast of Sea of Japan by local subsidence induced active seawater exchange with Lake Hiruga around AD 1928–1930. Furthermore, the peaks of freshwater diatom taxa and chrysophyte cyst showed a freshening event of this lake caused by local extreme events, such as continuous flooding around AD 1900.