An Investigation On The Cause Of Water Bloom In Fishpond

The four newly opened shrimp ponds in the Xiao Nanhe country of Tianjin city wereinvestigated from June to September in 2006, the plankton carried on qualitative and thequantitative sampling. The main physico-chemical index of pond the porewater of bottom sedimentand nutrients of bottom sediment were investigated simultaneity. The effect of pH value,thetemperature and the light stress on the Microcystis aeruginosa was studied. The generantmechanism of the water bloom was discussed. The experiment results are as flollows:The phytoplankton has 6 phylum 64 species in total, Cyanophyta in primarily, Chlorophyta andBacillariophyta next, and other species are very few. The community structure is Cyanophyta-Chlorophyta, the dominant species were Chroococcus,Microcystis aeruginosa,Merisrnopediatenuissima, Selenastrum minutum,Scenedesmus quadricauda and Coelastrum sphaericum.The investigation discovered that the four shrimp ponds belonged to the chloride water type ofsalt alkaloid pond. The pond water nutrient index which was mainly the nitrogen and thephosphorus, discovered that the change tendency of TN was similar with NH₄⁺-N. In the first fourweeks, the nitrogen generally was lower, the NH₄⁺-N and NO₃^--N started to rising in the fifthweek. The phosphorus also increased from the forth week, The phosphorus increase dued to thecorpse of aquatic animals and plants,the waste of metabolism and bottom sediment released.The distribution and the change of the NH₄⁺-N content in the porewater of bottom sedimentmay reflect ecological stabilization and shrimp culture condition, also may reflect organiccontaminative degree of shrimp remnant feed,excretion,other remnant body of aquatic animalsand plants in the bottom shrimp pond. The concentration of PO₄^3-P change tendency was similar inthe porewater of bottom sediment of the four shrimp, and the fluctuant range was 0.004～0.065 mg/L.The NH₄⁺-N average content is 5.067 mg/L,the NO₃^--N average content is 0.177 mg/L and theNO₂^--N average content is 0.039 mg/L in the porewater of bottom sediment of the four shrimp. Theaverage percentage of the NH₄⁺-N occupied the inorganic nitrogen was 96 %, and occupied the TNwas 88%, indicated that the NH₄⁺-N content was the highest in the porewater of bottom sediment inthe entire cultivated process.The cyanobacteria bloom happened in the 1th,2th and 3th pond. Microcystis aeruginosa andMicrocystis flosaquae was found in the 1th pond and Merismopedia tenuissima mainly presented inthe 2th and 3th.Nutrients in upper layer of the water column, mainly loaded by land runoff, had close positivecorrelation with precipitation N and P concentrations in the reservoir were heavy enough for occurrence of cyanobacteria bloom. he ratio of the inorganic nitrogen and the concentration ofPO₄^3- P was 128.6, which was the foundation of the cyanobacteria bloom happening. During theanalysis of positive correlation, the sequence of correlation was: NH₄⁺-N＞NO₃^--N＞NO₂^--N.From July to September, the averge water temperature was 27.41±2.45℃in the pond, whichfited for the happening of cyanobacteria bloom, had significant correlation with abundance ofphytoplankton or cyanobacteria, indicating that water temperature was an important factor affectingcyanobacteria bloom.On different pH value condition, through significant test on Microcystis aeruginosa growth rateshown the Microcystis aeruginosa had the higher growth rate that pH value ranged from 8.5 to10, relative growth rate was highest in 30℃and pH 9.5, 20℃and pH 6.5 was lowest,respectively was 0.124d^-1 and 0.016d^-1.The result indicated Microcystis aeruginosa density was significantly differences on thecondition of 30℃,25℃and 20℃(P＜0.01), that indicated that the Microcystis aeruginosa on thehigh temperature and alkalinity condition grew well, the optimum temperature was 30℃, theoptimum pH was 8.5～10.In the light stress experiment discovered that average relative growth rate which the 3 days,5days and 7 days treatment was higher than 1 day treatment, was 1.8, 2.0 and 37.4 times of thecomparing group respectively. The result indicated that on the tolerance range of light stress, theperiod more longer stress, the effect of compensates to be more remakeable.