Study And Investigation Of Lakes' Inner Source Factors For Non-accident Large Death Of Fishes

After the investigation of the inner factors of the eutrophicated lake which have been used as cage fish farming, we find that not only the high content of phosphorus salts, nitrogen salt, sulphur salt contained in the sediment, but also large amount of gas, such as CO2,CH4,CO and even some strong reduction toxic and hazardous sulphur compound. Via the simulative poisoning experiment in lab and field, the sediment of the eutrophicated lake was found to be the inner factor of the second pollution. Phosphorus and nitrogen is not the first causes of the accident of large area death of fish, but the gas, the reduction sulfur compounds stored in the surface layer sediment and the structure of the water. The mechanism and the toxicity of the inner factors to fish are significantly varies. As follows:(1) There is a transparent mucus interface formed between the sediment and the up water system in the eutrophicated lake, so the content of the oxygen in the water would be decreased as the reduction and toxically compounds and inert gas released from the sediment when vertical motion or agitation happened within the water system, witch resulted by the suddenly change of the environment or climate conditions. The low oxygen content, tissue were damaged by toxic compounds and the branchia attached with transparent mucus grain were the most factors that caused the fish dead.(2) SO2 is a high toxic compound to fish; the lethal concentration is hLC100≥0.12 mg / L, and its toxicity to be the highest levels.(3) Different concentrations of H2S have different stimulation effect on the fish, low concentrations for activation, high concentrations performed a hypnosis effect, but exorbitant H2S lead fish to deaths by blocking the fish breath. The lethal concentration of H2S is 24 hLC50 = 234mg/L and its toxicity is at the mid level.The results show that: there exist a close connection between non-accident large area of dead fish and the inner source factors (sediment and overlying water intergovernmental structure, the amount of dissolved gases, and the types of toxic and hazardous material types) and the condition of the outside world. The incident occurred outside the seasonal restrictions and the reasons for the accident directly comes from the inner source factors of lakes.