Characteristics of fluctuations in salinity and water quality in brackish Lake Obuchi

Abstract

Fluctuations in the salinity and physicochemical characteristics of water quality were surveyed in brackish Lake Obuchi on the Shimokita Peninsula in Aomori, Japan. The mean salinity in the surface layer in all regions of Lake Obuchi was about 10 psu, whereas in the basin region at depths of greater than 3 m it was 20 psu. Furthermore, all the year round the halocline was formed at depths of 1-4 m. The maximum density gradient along a vertical axis in the center of the lake was observed at depths of 1-2 m in summer and 2-4 m in spring and fall. The depth of the maximum density gradient fluctuated with the seasons. In summer the water in the bottom layer was anoxic, and Fe, Mn, PO 4 3Ϫ -P, and NH 4 ϩ -N supplied from the bottom sediment accumulated at high concentrations below the halocline. Thus, it was observed that the transfer of substances between the layers above and below the barrier formed by the halocline is suppressed. Although Lake Obuchi is small and shallow, the inflowing seawater easily resides, and a stable halocline readily forms because of the shape of its basin, which suddenly deepens on the Pacific Ocean side.

Effects of the halocline on water quality and phytoplankton composition in a shallow brackish lake (Lake Obuchi, Japan)

“…1) in the center of the lake, where it attains its maximum depth, we confirmed that a strong salinity gradient (halocline) was formed between the upper and lower layers (Ueda et al 2000). The periodic fluctuations in salinity at this sampling station did not correspond to the meteorological tide (Ueda et al 2002), indicating that the water at this station was not immediately affected by the tide.…”

Present environment of Dam Lake Sambe, southwestern Japan: a geochemical study of bottom sediments

“…They argue that this concentration is maintained due to the increase of As (V) in the accumulated arsenic compounds in the sediments. The commensurate increase in As, Fe and Mn suggests that the reducing condition exists in the bottom water layers during summer stratification as indicated by August dissolved oxygen profiles (Sohrin et al 1997;Ueda et al 2000).…”

Investigation of physicochemical form of uranium in sediment of brackish Lake Obuchi using sequential extraction procedure

“…Payne and Waite (1991) proposed that uranium was scavenged by iron(III) hydroxide or manganese(IV) oxide and settled on the bottom. The concentrations of iron and manganese in the wa- ters of Lake Obuchi are usually high in the bottom layer during the summer (Ueda et al 2000). Therefore, it can be presumed that uranium in the bottom water may be precipitated with iron(III) hydroxide or manganese(IV) oxide formed by oxidation with dissolved oxygen.…”

“…The water qualities of coastal lagoons and brackish lakes are influenced by interactions between marine and terrestrial factors, reflecting their hydrochemical complex and ecologically unique nature (Ohtake et al 1982). Lake Obuchi has a crescentshaped tray, and the bottom layer is in a reduced state in the summer because of the halocline (Ueda et al 2000). Therefore, we presume that uranium in the lake water in the bottom layer may be precipitated by reduction to insoluble U(IV).…”