The culture of the “Kuruma” prawn, Penaeus japonicas BATE, has been carried on in various districts along the coasts of the Seto Inland Sea. In the culture pond of the “Kuruma” prawn, remodeled from the unused salt field or enclosed with stretching wire net along the sea coast, it is frequently experienced that the bottom materials are in a reduced state, owing to the accumulation of organic matters such as food-remnants, excrements etc., and that the reduced materials produced in this state have sometimes an injurious influence on the prawns cultured there. The conditions of the materials are evidently determined by diffusion of oxygen from the sea water and sedimentation of organic matters, on the one hand, and by reduction processes of biochemical origin in the mud, on the other. Many studies concerning the bottom mud of the culture pond have been carried out along the lines of separate chemical or physical analysis of each factor forming the environment without systematic or causal connection. However, it may be of most importance to examine the quality of bottom material as a whole, for indicating the criterion about the degree of deterioration of the culture pond. No clear idea of the biochemical transformations in bottom materials can be gained without a consideration of the redox potential. From these points of view, the present study was designed to clarify the oxidized and reduced phase of the bottom materials by means of measuring the redox potential in the Aji Prawn farm (Fig. 1). However, this quantity will give only a measure of intensity. Undisturbed cores were obtained and potential, pH value, organic carbon content and total nitrogen content were determined from the mud in various layers down to 22cm below the mud-water interface. Comparisons of redox potentials in summer with those in winter were made and then the potential of various mud layers below the mud surface was related to the quantity of the organic carbon and the total nitrogen, respectively. The results obtained may be summarized as follows: (1) The redox potential (corrected to pH 7) in the top layer (0-8cm) corresponds to -0.01 to +0.09 volt in August, and to +0.18 to +0.21 volt in March. Potential at about 20cm below the interface lies at about +0.27 volt throughout the entire annual cycle (Table 1, Fig. 5). (2) Near the surface (0-8cm) the potential is correlated with the quantity of organic matter, but in deeper layer no correlation is recognized between both (Fig. 3, 4). (3) Except on occasion at its surface the mud at the bottom of pond has reducing pro-perties in August. There is a rise of potential as far as 20cm into the mud. In March, the potential falls slightly with depth and then rises from 0.18 volt at 6cm to +0.27 volt at 14cm. Within the range of the depth 14-20cm, there is little change (Fig. 5). (4) The high potential obtained at about 2cm below the interface in March suggests that the mud at this depth is recovered from the reduced state; the potential lying at about -0.01 volt in August rises till in March a value of +0.21 volt is reached (Table 1, Fig. 5). (5) A very thin, flocculent, brown precipitate, or oxidized microzone, was seen dimly in August and clearly in March, so that the potential in superficial layer of mud would have shown the high value.
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May 1, 1986
Richard T Lovell + 3
Open Access