Dynamic Changes And Inlfuence Factors Of Algal Boom In Landscape Water Body Of Subcritical Flow

Nowadays，the algae blooms happen in the subcritical flow of landscape water bodyof city frequently，it not only makes the urban landscape water lost its aesthetic value oforiginal water body，but also the algae toxins produced by abnormal proliferation of algaewill be directly or indirectly harm the health of the general public. But the formationmechanism and process of blooming in urban landscape water was not understood clearlyby many people. So it was so important to search for main influencing factor and effect ofblooming in urban landscape water, and basic condition of visible blooming appearing forwarning the blooming, then measures can be taken to minimum the social andenvironmental damage which was brought by blooming.. Set the river in Science andTechnology of Suzhou University as an example, the research object was four rivers withdifferent characteristic in campus. The main difference in four rivers lies in there was waterexchange or not, bottom sediment or not, foreign sewage or not. The research was carriedon by long-time on-site observation for landscape water, sampling, monitoring,experimental analysis, and processing data.Determination of temperature, watertemperature, transparency, and pH, turbidity, DO, and chlorophyll a （Chla）, algae,photosynthetic activity and other parameters on the spot; Sampling and analysis todetermine the main water quality objectives for TN, TP, NH4-N, when significant algalblooms happen in the water, seasonable adjustments to determine the frequency. Throughthe above research tools on the following research:（1） master the main influencing factorsand their effect in the subcritical flow of landscape water body；（2） to determine the basicconditions of visible bloom in the subcritical flow of landscape water body.The main conclusions are as follows：（1） Through Long-term field observation on scenic water of city,finding that theconcentration of Algae of different river-course can reach relatively high levelsimultaneously; the concentration of Algae differfrom the time bucket in a certainriver-course, Algae grow easily in summer and can reach the peak;the water exchange andinlet of sewage have a negative effect on the growth of algae.（2） Combining monitoring，sampling，mensurating in lab with data analysis,finding that the key elements affecting the growth of algae are rainfall,temperature,thephotosynthetic activity of algae,turbidity,and the ratio of N:P.（3） The impact of rainfall on visible algal blooms is significant. Chla concentration ofwater will reduce the daily rainfall is more than5mm，the precipitation of moderate intensity （daily rainfall10²5mm） has a certain inhibition to cyanobacteria developmentin short-term，the probability of Chla concentration reduction in water is more than75%；the heavy rain or more （daily rainfall>25mm） precipitation process do have a very obviousinhibition of cyanobacteria development，the probability of Chla concentration reduction inwater is more than95%. Continuous rainy weather and concentration of heavyprecipitation have a significant dilution effect on algal blooms， the inhibition ofcyanobacteria development is very obvious，and there is a certain continuity.（4） the water temperature have a significant effect on growth and reproduction ofalgae. The water temperature of visible bloom in city slow flow river set between23.4℃³4.4℃，the higher the water temperature is more likely to produce visible algal blooms，visible algal blooms occurrence in a low water temperature need high nutrientconcentrations. Mutation of the water temperature （rise） will promote the occurrence ofalgal blooms.（5） The algae of light activity （Fv/Fm） have a significant effect on algae. When theFv/Fm <0.55，algal blooms are not easy happen. Fv/Fm ratio of algal blooms in water ismore than0.6; The turbidity and transparency affect the growth of algae. Algae is not easyto grow when water turbidity>20NTU， algal blooms occurred in the turbidity of about10NTU.; Suitable ratio of nitrogen to phosphorus have a promotion to visible algal blooms.when the ratio is about23，and when meet other conditions，the water algae will multiplyrapidly，prone to visible algal blooms.（6） When concentration of chla reach Chla[VBT]（Chla[VBT]=50μg/L,visible bluethreshold,VBT），if there is no outside interference （such as rainfall，a large number ofwater quality exchange，etc.），visible algal blooms will happen in a short time（2-7days）.the study draws the basic conditions of the slow flow landscape water algal bloom： thewater temperature is17.1℃-34.4℃，transparency <74cm，TN>1.26mg/l，NH₄⁺-N>0.91mg/l，TP>0.02mg/l，the algal light activity>0.557.