DEVELOPMENT OF LONG-TERM OBSERVATION SYSTEM OF THE HOLOCLINE IN COASTAL LAGOON

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The survey system of long-term observation system of the halocline in coastal lagoon was newly developed and the resolution of underwater acoustic records in Lakes Nakaumi and Shinjiwere successfully checked. This survey system, which automatically provides a time-series of echo-sounding profiles, is dirrerent from the conventional type equipped on a boast. The tranducer is fixed on the lake bottom. The echo-sounding recordobtained by the new ststem is clear enough to distinguish the 5 permill halocline.By using this system, we observed the halocline behavior at central and marginal place in Lake Nakaumi. Salinity, pH,dessolved oxygen, watertemperature, and water flow rate and direction were checked by the other equipments. This results suggested that the halocline was easily moved and that the behavior was influenced by wind.The optical fiber sensor for measuring temperature distribution over long distance can produce simultaneous temperature data at up to 4000 points with an interval of 1 m along it. The experiments used jointly with the above echosounding system in Lake Nakaumi have resulted in two interesting discoveries. The one is the temporal change of warm spring activity on the lake bottom. The other is the movement of a mass of high saline water into low saline water lake bottom driven by meteolological forcing. The advantage of the optical fiber distributed temperature sensor is its ability to easily obtain the temperature data as function of time and space over a long distance.