Stability in Hypolimnion of Deep Rift Lkes, East Africa
Grant-in-Aid for international Scientific Research
|Allocation Type||Single-year Grants |
|Research Institution||Hokkaido University |
NAKAO Kinshiro Professor, Faculty of Science, Hokkaido University, 理学部, 教授 (40000830)
KWETUENDA Menga kuluki President, Uvira Station, Research Center of Natural Science, Zair, ウビラセンター, センター長
ZANA Ndontoni General Director, Research Center of Natural Science, Zair, 所長
TOMINAGA Hiroyuki Instructor, Water Research Institute, Nagoya University, 水園科学研究所, 助手 (10022625)
CHIKITA Kazuhisa Lecturer, Faculty of Science, Hokkaido University, 理学部, 講師 (70142685)
|Project Period (FY)
Completed(Fiscal Year 1990)
|Budget Amount *help
¥5,000,000 (Direct Cost : ¥5,000,000)
Fiscal Year 1990 : ¥5,000,000 (Direct Cost : ¥5,000,000)
|Keywords||Hypolimnion / Meromictic vertical profiles / Dissolved gases of carbon dioxide / Dissolved gases of methane / Stable stagnant layer / Tropical lakes / Thermohaline convection / Internal seiche|
Lake Kivu is situated in the equatorial region of the Rift Valley, East Africa, at latitude 2^ﾟS and longitude 29^ﾟE ; it discharges from the south end of the lake by the Ruzizi River into Lake Tanganyika. The lake basin is characterized by a tectonic unit separated from the Edward-Kivu Trough by active volcanoes including Mt. Nyamulagira, Mt. Nyiragongo and others. Formed by ponding which occurred by lava flows across the valley, the lake is deep and shows a submerged shoreline like a rias coast, reflecting the damming effect.
The lake is classified as a tropical lake from the thermal regime, and has by far the most meromictic vertical profiles of water temperature. Its morphometric features are : 2,300Km^2 in area of lake surface, 7,300Km^2 in basin area including the lake surface, 485m in maximum depth and240m in mean depth.
The sounding results of water temperature indicate the significant tendency of rising from the depth of 90m to the bottom at all sites, it is characterized that t
he density of lake water is increased by a dissolved matter and is meromictic. The water of lake Kivu has been stabilized by a slight increase, with increasing depth, in the amounts of dissolved gases of methane and carbon dioxide.
The points to which special attention should be paid to the profiles of temperature and conductivity obtained by sounding responses in Lake Kivu, are two convection layers in the hypolimnion shown vertically with the same values. It is pointed out that these convection layers have the step-like structure similar to a thermohaline convection.
In the case of a thermohaline convection, the spreading perturbations originate in a great difference between the molecular diffusivity of salt and temperature. But that of Lake Kivu is caused by a decrease in solubility in dissolved gases accompanying uplifting of water. It is satisfactory to consider that an internal wave and a sudden gas eruption from the bottom work as trigger of starting perturbation in the hypolimnion. Less
Research Products (2results)