Anthropogenic changes due to partial dike removal in the Honjo Area of Nakaumi Lagoon, Southwest Japan

Nakaumi Lagoon is a coastal brackish water lake formed by the Yumigahama peninsula in Shimane and Tottori Prefectures, Southwest Japan. In 1981 the Honjo Area, approximately the northern quarter of Nakaumi Lagoon, was almost completely isolated by the Moriyama, O-misaki and West Dikes constructed by the Nakaumi Reclamation and Desalination Project. A connection remained with Nakaumi Lagoon via one shallow channel in the West Dike. In 2002, the reclamation project was canceled and restoration work has been undertaken and planned during 2007-2009, such as the opening of 60 meters of the Moriyama Dike and the removal of West Dike. The inflow of high salinity water allowed by this restoration work is expected to have a large impact on the present ecosystem of the Honjo Area. In May 2009 the opening of Moriyama Dike was carried out and the water chemistry of the Honjo Area changed drastically. In this study, we document changes in the water environment resulting from the opening of Moriyama Dike. The monitoring program is made up of 60 water quality measurement sites and 12 detailed sampling localities in a traverse line through the lake system and observations were made at monthly intervals from May 2006 to present. At detailed sampling locations we measured standard water quality variables, collected water samples for biomass and stable isotope chemistry, and collected surface sediment samples for geochemistry and benthos censusing. Before the Moriyama Dike was opened, the salinity difference between surface and bottom water remained between 2 and 4 psu except during winter when strong winds mix the Honjo Area waters vertically. For most of the year the water was stratified and bottom water in the Honjo Area showed oxygen depletion. The dissolved oxygen of bottom water decreased early in the year on the Moriyama Dike side of the Honjo Area. This oxygen-poor water was easily up-welled by winds because the vertical salinity difference was small. At times the observed dissolved oxygen value of surface water was less than 1 mg/l, indicating that the shallow ecosystem was significantly affected. After the opening of the Moriyama Dike, Sakai Channel bottom water with high salinity and high dissolved oxygen moved into the Honjo Area. The large density difference set up a strongly stratified structure in the Honjo Area. After stratification, the bottom water of the Honjo Area became oxygen depleted, becoming an anoxic environment except for a small area near the Moriyama Dike. The strong stratification, however, prevents bottom water from up-welling into shallow areas. Shallow water organisms, such as Ruditapes philippinarum, do not seem to be threatened by deep water anoxia.