**1. Introduction**

The Ariake Sea, which is located in the north-western part of Kyushu Island, is one of the best-known semi-closed shallow seas in Japan. Many rivers flow into the eastern coast area of the Ariake Sea and carry 4.4 x 108 kg of sediments per year (Azad et al. 2005).Coarse sediments accumulate in the eastern coast, and fine grains brought by the residual current accumulate in the bay head to form vast tidal flats with fine sediments (Kato and Seguchi 2001). The vast tidal flat mud of the Ariake Sea, which is almost 40% of the total tidal flat area of Japan, is famous for its rich fishery products and *Porphyra* sp*.* (sea weed) cultivation. Different types of shells like *Sinonovacula constricta, Atrina pectinata* and *Crassostrea gigas* are important creatures in the Ariake tidal mud. However, a dramatic decrease in the catch of these shells is observed in the tidal flat area. From Fig. 1 it is seen that the catch of *Crassostrea gigas* usually living in the near surface mud, dropped from 7.99 x 105 kg in 1976 to only 1.26 x 105 kg in 1999; that of *Atrina pectinata,* living in the upper 0.10-0.15 m of the mud, declined from 1.3395 x 107 kg in 1976 to 7.9 x 104 kg in 1999, and the situation in the case of *Sinonovacula constricta*, living in the depth of 0-0.7 m of the mud, was even worse: 1.7 x 105 kg catch in 1976 dropped to practically nil by 1992.

The acid treatment practice for *Porphyra* sp*.* cultivation is one of the major causes for this declination of the shells as this practice has made the geo-environmental condition of the Ariake tidal mud unfavorable for the living creatures of the tidal mud (Hayashi and Du, 2005, Moqsud et al. 2007). During the period of the cultivation (December -March), the acid (which is mainly organic chemicals) is used as the disinfectant acid to treat the *Porphyra* sp. cultivated in the sea and also to provide some nutrient phosphorus to it.

This organic acid provides ample of foods for the sulphate reducing bacteria living in the mud and consequently increase the sulfide content in the mud. The generation of sulfide is also influenced by the seasonal temperature and shows a higher value during the summer and the late autumn as bacteria becomes more active in the higher temperature. The higher sulfide content created by acid treatment practice is the main reason for the unfavorable condition for the benthos in the Ariake Sea. Moreover, the activities of the benthos depend strongly on the thermal environment near the sediment surface. Photosynthetic capacity of micro phytobenthos on an intertidal flat was strongly influenced by mud surface temperature (Blanchard el. Al, 1997). The filtration rate of bivalves was dependent on the water temperature (Hosokawa et al., 1996). As a result, to evaluate geo-thermal environment is important especially for the acid contaminated Ariake Sea. Thermal properties dictate the storage and movement of heat in soils and as such influence the temperature and heat flux in soils as a function of time and depth (Anandkumar et. al, 2001). In recent years, considerable efforts have gone into developing techniques to determine these properties (Ochsner et al, 2001). The propagation of heat in a soil is governed by its thermal characteristics (De Vries, 1963). Main factors influencing soil thermal properties are mineralogical composition, the organic content and water content (De Vries, 1952, Wierenga et. al, 1969). No study has been carried out before to get the information about the thermal properties as well as thermal environment of the Ariake sea mud. So the objective of this study is to assess the thermal environment of the tidal mud by getting the information of the temperature distribution in different depths and find a diurnal and seasonal profile of it in the tidal flat region, and finally thermal properties variation with respect to depth for the temperature distribution in different seasons. The thermal properties of the Ariake Sea mud collected from both tidal flat and inside the deep sea of the Ariake Sea were conducted as a part of thermal environmental studies of the Ariake Sea.

Fig. 1. The graph of catch vs year
