中海湖心部における湖底の水中映像(その2) : 冬季の季節風に対する底層水の動態

We observed the bottom water movements and the ecology of brackish organisms over a fi ve days of Feb. 13- Feb.17, 2007, by means of a long-term monitoring waterproof camera that installed on the bottom of Lake Nakaumi at a depth of 6.5 m. The winter organisms are occupied with a hydrozoan and cionid community. These organisms indicate the bottom water is enriched with dissolved oxygen and the condition is quite different from our fi rst report that showed a bacterial mat covering the bottom fl oor.
Unstable condition was occurred by a strong south and westerly wind over the speed of 10 m/s in Feb. 14 and 15. Winnowing of bottom sediments occurred at wind speeds greater than 10 m/s, which supports the previous observation. Water clarity decreased in these days. However, we confi rmed that the winter bottom sediment is easy to blown up even in the speed of ～7 m/s, which is supposedly due to the absence of bacterial mat. In contrast to the quick response of bottom turbulence to wind stress, waters of bottom and the upper waters were not easily mixed, despite of the wind speed of over 15 m/s. A mixing of the bottom and the upper waters was temporally occurred after the 20 hours continuous blow over 11～16 m/s, but soon after that the waters from different depths were separated while the strong wind continuing.
A number of fi shes along with a school of unknown fry were frequently observed when the bottom water turbulence stopped. We are able to recognize the rich fi sh communities in this season.