Study On The Toxicity Of Effective Chlorine On Organisms In Prawn Ponds And Attenuation

Chlorine disinfectant is a common disinfectants for the prevention of disease in the process of prawn aquaculture and water treatment, and the effective chlorine as for its main components not only can kill some pathogenic microorganisms and other harmful organisms, but also can cause certain damage to the prawn and other beneficial microbes,and then affects the stability of the farming ecological system. The shrimp pond is a simple ecological system, and planktonic micrialgae, zooplankton and microorganism are playing an important role in maintaining the balance of the whole system. So toxicity test method was applied in our experiment to study the toxicity effects of effective chlorine on the six kinds of common microalgae(Chlorella pyrenoidosa, Oocystis borgei, Chaetoceros muelleri, Cyclotella meneghiniana, Microcystis aeruginosa, Chroococcus minutes), the four kinds of bacteria(Vibrio parahaemolyticus, Bacillus subtilis, photosynthetic bacteria,nitrifying bacteria), zooplankton(Pseudodiaptomus dubia, Artemia,) and Litopenaeus vannamei post larvae. Moreover, the relationships among temperature, light intensity and the chemical oxygen consumption(COD) and the attenuation of available chlorine in water were also investigated. The experimental results are as follows.1. The 96 hours EC50 of available chlorine on Chlorella pyrenoidosa, Oocystis borgei,Chaetoceros muelleri, Cyclotella meneghiniana, Microcystis aeruginosa, Chroococcus minutes were 0.71 mg/L, 0.73 mg/L, 1.45 mg/L, 2.42 mg/L, 0.32 mg/L and 1.02 mg/L,respectively. The safe concentration in 96 hour were 0.071 mg/L, 0.073 mg/L, 0.145 mg/L,0.242 mg/L, 0.032 mg/L and 0.102 mg/L, respectively. The toxic effect of available chlorine for different categories of algae is different. According to the 96 h EC50, the tolerance of Biatom was the strongest followed by Green algae and Cyanobacteria.2. When the concentration of available chlorine were 1.00 mg/L, the fatality rate of disinfectants on Vibrio parahaemolyticus, Bacillus subtilis, Photosynthetic bacteria,Nitrifying bacteria reached above 90% in 60 minutes; and 5.00 mg/L, all the bacteria were killed in 10 minutes.3. The 24 h LC50, 48 h LC50,72 h LC50 and 96 h LC50 of available chlorine on Pseudodiaptomus dubia were 2.31 mg/L, 1.78 mg/L, 1.31 mg/L and 1.17 mg/L,respectively. The 24 h LC50, 48 h LC50, 72 h LC50 and 96 h LC50 of available chlorine on Artemia nauplii were 2.88 mg/L, 1.99 mg/L, 1.14 mg/L and 0.97 mg/L, respectively. As forArtemia adults, the 24 h LC50, 48 h LC50, 72 h LC50 and 96 h LC50 of available chlorine were3.69 mg/L, 2.79 mg/L, 2.12 mg/L and 1.64 mg/L, respectively. The safe concentration in96 hours was 0.017 mg/L, 0.097 mg/L and 0.164 mg/L, respectively. There is a big difference in the tolerance of different kinds of zooplankton. And the tolerance to available chlorine of different sizes of the individual is also different. The tolerance to available chlorine of Artemia adults was stronger than Artemia nauplii.4. The 24 h LC50 、 48 h LC50 、 72 h LC50 and 96 h LC50 of available chlorine on Litopenaeus vannamei post larvae respectively were 3.94 mg/L, 3.10 mg/L, 2.23 mg/L and1.70 mg/L. The safe concentration in 96 hours was 0.17 mg/L. And the tolerance of Litopenaeus vannamei post larvae was the highest among all treatments.5. There were positive linear relationships between the attenuation of available chlorine and temperature, light intensity and COD. When the concentration reached 20 mg/L and temperature was 40℃, attenuation rate of available chlorine was 94.9% in 96 hours. When light intensity was 10000 lux, the attenuation rate reached 78.9% in 48 hours at the same concentration. But when concentration reached 100 mg/L,COD was 5 mg/L, the attenuation rate reached 97.5% in 96 hours. The effect of temperature, light intensity and COD on the attenuation of available chlorine with the change of time, concentration decayed rapidly in the first 4 hours and then declined slowly.The research shows that available chlorine decay in the water under the influence of many factors and it is essential to take attenuation effect into account when using the available chlorine. The function of available chlorine changed with the aim species, so it is important to consider the characteristics of the biological community in shrimp ponds, then according to the needs of farming and rationally use the chlorine disinfectants, accurately control the concentration of available chlorine. The results of this study provided a theoretic foundation for reasonably using chlorine disinfectants in the process of prawn aquaculture.