**4.2 Seasonal variation of temperature in tidal flat**

Figure 5 shows that the seasonal variation of temperature at different depths at Higashiyoka tidal mud in 2007. During the spring and summer the surface temperature shows a higher value than the subsequent depths. During this time, heat was absorbed by the tidal mud and heat was transferred from the surface to the deeper part of the tidal mud. On the other hand, during winter and autumn the surface temperature was lower than the subsequent depths. During this time heat is released at the surface. During April, the variation was not so prominent. It showed almost straight line graph. Iida site also showed the same trend as with Higashiyoka site during the summer and the winter.

The acid treatment practice started during the winter season (December-February). In winter, the temperature drop down about 50 C. Due to the lowering of temperature the tidal flat mud showed a contraction. The acid treatment practice and the contraction tendency of the tidal mud occur during the same time. As a result, the chemicals used in the sea laver treatment agent entered into the tidal mud. On the other hand, during the summer the surface temperature reached about 380 C. The high temperature results in an expansion of the tidal mud. The tidal mud expansion causes easy movement of some biogenic gases generated inside the tidal mud.

#### **4.3 Conceptual image of seasonal temperature effects and acid treatment practice on tidal flat**

Figure 6 shows the conceptual image of the seasonal temperature variation and the acid treatment practice in the Ariake Sea. The various chemicals which are inside the sea laver treatment medicine enter into the tidal mud during the winter season due to the contraction effect of tidal mud. The chemicals and organic acid supply a lot of foods to the sulfate reducing bacteria. With this ample of foods and the convenient temperature during the spring and summer the sulfate reducing bacteria becomes very active and consequently produces hydrogen sulfide, sulfur di oxide, and as a result, the acid volatile sulfide (AVS) content increased. The AVS content at the Iida tidal flat area shows much higher than the safe limit for the living creatures in the tidal mud. The AVS represents a complex and dynamic biogeochemical system which is not defined simply by analysis of acid volatile sulfide materials (Richard and Morse, 2005). Actually there are many factors which are liable to produce AVS in some specific regions. However, the laboratory test showed that due to the acid treatment practice the AVS value increased in the tidal flat mud (Moqsud et al. 2007). So the conceptual image of acid treatment practice and the seasonal variation of temperature are thought to be rational.
