| The mineralization of sediment in shoal farming area, especially in aging area produces methane. It can pollute sediment and water, impact the growth of animal and plant, and release methane to air. Therefore, studies on CH4 release and distribution in shoal farming area can provide theoretical basis for accurately calculating the sea-air exchange flux of methane and understanding the deterioration degree of water quality and the mineralization rate of organic matters.In this paper, an ultrasonic-head-space gas chromatography method is designed to determinate the methane concentration in the cultured water and pore water. The methane, sulfide and organic C in cultured area of Rushan Bay are measured; the relationship about methane with organic C and sulfide in different depth are discussed. The effect of relative factors on methane release is also studied. Conclusions by all studies above as follows:1. The ultrasonic-head-space gas chromatography method we designed requires only simple apparatus, is rapid in analysis, and the result of determination can meet the demands for CH4 determination in cultured water and sediment. The detection limit is 3.42 nmol/L, and the reproducibility is 5.78%.2. The variation of CH4 concentration in seawater in different density cultured areas is not significant with mean value of 59.90±7.75nmol/L. It, in terms of vertical distribution, decreases generally with the increase of sediment depth. The higher concentration is observed at top 0～5cm through sediment profiles. The difference of methane concentration decreases gradually with the increase of sediment depth. When the depth is more than 10cm the CH4 content trends to uniformity. At top 0～10cm CH4 concentration in medium density area is higher than in low density and high density areas. Methane in non-cultured (aging) area is more than that in cultured area at top 0～5cm, and there is not significant when depth is more than 5cm. The vertical distribution of CH4 has positive correlation with OM concentration and has negative correlation with sulfide concentration.3. Lab simulation shows:a. The CH4 concentration everyday: high density group﹥medium density group﹥low density group≥non-cultured (aging) group; there is significant difference between high density group and others. And the release amount of methane in all groups is becoming more and more with the change of time.b. The most optimum pH is 7.83 about CH4 release from sediment of culture area of RuShan Bay. When pH is 7.66～7.81, the release of CH4 is higher than that at other pH, and the effect of CH4 release is more sensitive.c. When added sulfide concentration is 0～320mgS/kg, the amount and rate of released methane reach to the maximum at 160mgS/kg. It is negatively correlated to AVS in sediment. Using Na2S and FeS as added sulfide, the results show that FeS﹤Na2S.d. When temperature is 21～24℃, the release amount of CH4 is excreta of Ruditapes philippinarum﹥sediment﹥sea water. From 7th day the methane release amount becomes more and more and its cycle is 14 days.4. Site simulation indicates that methane release amount in different cultivation density areas: high density area﹥medium density area≥low density area; non-cultured area﹥cultured area.5. There is some difference about emission and distribution of methane between cultured area and other sea areas. Because the influence factors are many and complex, the results have some difference among lab simulation, site simulation and survey. Multifactorial analysis, culture activity contributes to methane release. |
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