Dynamic behaviours of anoxic saline water in Lake Abashiri, Hokkaido, Japan

Abstract

Lake Abashiri (surface area 32.87 km 2 ; maximum depth 16.2‐16.5 m) is consistently stratified by the lower anoxic saline layer (salinity 14‐20%; dissolved oxygen less than 0.1% Sat.). Behaviours of the lower saline water are investigated by continuously and vertically measuring flow velocity, salinity and water temperature, and by observing meteorology, river discharge, lake level, sea level and water properties. During the period of flow measurement (June‐August 1997), the sea level remained higher than the lake level except for a neap tide of 12‐15 June and two large rainfall discharges of the upper Abashiri River of 10‐15 August. Consequently, seawater intrusion from the lower Abashiri River occurred almost daily except for these two periods. Longitudinal distributions of salinity, water temperature and turbidity in the lake indicate that a density underflow initiated from the seawater intrusion is bifurcated into an interflow and an underflow at the halocline. Meanwhile, in a shallow region on the opposite side of the seawater entrance, the halocline becomes unclear with the unstable density structure in the lower layer. This is probably due to the entrainment of saline water by downwelling currents in the lower saline layer, which coincide with wind-driven vertical circulation in the upper layer. Spectral analyses of flow velocity (1‐6 cm s ˇ1 ), temperature, and salinity in the lower layer, and of wind velocity, revealed that daily cyclic winds or diurnal southward winds with no nocturnal winds induce irrotational water motions in the lower layer with a period of 21. 3o r 25.6 hrs. It was also found that the internal seiche of the first mode is dominant with periods of 6 to 7 hrs during a blowing time of more than 2 days by a one-directional weak wind (maximum speed of 4 m s ˇ1 ) or after a blowing time of more than half a day by a relatively strong wind of more than 5 m s ˇ1 . Copyright # 2000 John Wiley & Sons, Ltd.