Seafoor environmental changes during the last 100 years resulting from reconstruction of the artificial lake in Kojima Bay, Okayama Prefecture

Kojima Lake, an artificial lake in the western part of Kojima Bay in the southern region of Okayama Prefecture, was constructed in 1959. To examine the impacts of constructing this artificial lake, we demonstrated spatial and temporal variations of the seafoor environment in the lake and Kojima Bay, based on physical and chemical properties of surface and core sediments. The distribution of grain size in surface sediments indicated that the bay is in a stagnant condition, resulting in deposition of silt sediments. Sand is deposited in the coastal part of Kojima Lake, which is affected by waves whose influence decreases toward the lake center. Differences of chemical composition between fresh and marine water caused higher concentrations of total phosphorus (contributed mainly by iron and aluminum-bound phosphorus) in the lake than in the bay. Collected lake core sediment was divided into three sections - a light and bioturbated section deposited prior to the 1960s, a dark laminated section accumulated between the 1960s and 1990s, and a dark and bioturbated section deposited after the 1990s. These variations indicate that the lake environment developed eutrophic conditions, resulting in increased organic matter and nutrient reserves, and benthic activities decreased with lake formation. However, improvements to water quality slightly restored benthic activities after the 1990s. The decrease of grain size from sand to mud of the core in the bay implies that lake formation decreased tidal current velocity.