Limnological Features of Lake Kaiike, a Small Coastal Lake on Kamikoshiki Island, Kagoshima Prefecture, Japan

Abstract

Limnological surveys were undertaken on a small coastal lake on Kamikoshiki Island, Lake Kaiike (surface area : 0.15 km2; maximum depth : 11.6 m). The lake and adjacent Lake, Namakoike (surface area : 0.50 km2; maximum depth : 24.4 m), are separated from the sea by narrow gravel bars. Lake Namakoike has a considerable exchange of water with the sea, because a part of the gravel bar has been worn away by wave action and through it sea water seeps freely into the lake or lake water into sea following the tidal cycle of sea level. Usually, water flows through a small ditch from Lake Kaiike into Lake Namakoike. But an reverse flow occurs intermittently on a rare occasion when tidal action becomes great enough to raise the lake level of Namakoike higher than that of Kaiike. Chlorinity of lake water of Namakoike ranges between 13-17 ‰. Chlorinity of surface water of Lake Kaiike is 7-9 ‰, and it increases downwards and attains 18.64 ‰ near the bottom. The deeper high saline water of Lake Kaiike originates in sea water which seeps through the gravel bar between the lake and sea. Unusually high temperature and dissolved oxygen maxima are observed in the middle layer, where the vertical gradient of chlorinity is the highest (halocline). Dissolved oxygen has been depleted in the deeper water and hydrogen sulfide is present.