Laguna : 汽水域研究

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Laguna : 汽水域研究 14
2007-12 発行

中海湖心部における湖底の水中映像(その1) : 風速変化に対する底層水の動態

Lake fl oor images in the central part of Lake Nakaumi(part 1) : wind speed impacts on bottom water motion
野村 律夫
堀江 譲
高田 裕行
A long-term monitoring waterproof camera was installed on the bottom of Lake Nakaumi at a depth of 6.5 m, in order to observe bottom water movements and the ecology of brackish organisms. Over a six day period, Nov. 4–Nov. 9 2006, brackish water dynamics changed from stable to unstable conditions. Under the prior stable conditions, the sediment surface was covered with a bacterial mat. Strong winds, over 15 m/s, mixed upper and bottom water masses and the halocline disappeared for a short time in the middle of this period.
We show that winnowing of bottom sediments occurs at wind speeds greater than 10 m/s at this location; a small piece of bacterial mat was stripped off from the bottom sediment. When wind speed increased to greater than 12–13 m/s, bottom water turbulence was increased, with larger pieces of bacterial mat and sediment particles several centimeters in diamerter being resuspended. Water clarity decreased at this time. The bacterial mat was completely stripped off the sediment surface when wind speed exceeded 16 m/s. Suspended sediment caused water clarity to go to zero at this time.
Fish appeared in bottom waters soon after the wind-driven disruption of stratified water conditions, suggesting that oxygen-rich upper water were well mixed into oxygen-poor bottom water.