The Influence Mechanism Of Long-term Vertical Mixing On The Floating Function Control Factor S Of Microcystis

In recent years, eutrophication phenomenon in China’s lakes, reservoirs has been growing, leading to overproduction of algae and frequent "algae bloom" disasters, which has been a serious threat to drinking water safety, making big loss to industry and agricultural production. Microcystis is one of the most common water bloom algae in China, it has a special buoyancy regulation mechanism which makes it able to float and sink in water to get the best living environment. Many filed studies show that long-term vertical mixing can lead to a quick loss of Microcystis biomass in only few days. Earlier theories regard light limit is the main algae control mechanism of long-term vertical mixing. However, the latest research shows that sinking loss maybe is another important reason for the quick loss of Microcystis biomass. This thesis studies the influence of long-term vertical mixing on the floating function control factors and sinking loss of Microcystis, discusses the main course of Microcystis biomass loss at long-term vertical mixing. Three periods of experiments were developed: 1) the variation rule of the floating function control factors of Microcystis at short time illumination;2) the variation rule of the floating function control factors of Microcystis at long-term illumination;3) the variation rule of Microcystis population at long-term vertical mixing. The main results are as follows:1) Microcystis was able to proliferate at short time dark light, the cell density reduced along with the carbonhydrate concentration, the variation of cell density and carbonhydrate concentration was significant positive correlated. Microcystis proliferated most in 5000 lux light intens ity, carbonhydrate concentration increased along with the increase of illumination time and light intens ity, it led to the increase of cell density. Microcystis cell density was significant positive correlated to both light intens ity and carbonhydrate concentration. Besides, when the carbonhydrate was produced, it also brought additional turgor pressure, gas vesicle was easier to fracture. The increment of carbonhydrate at 5h illumination in 10000 lux light density could increase the turgor pressure for more than 0.1MPa, if the Microcystis was brought to deeper water by long-term vertical mixing, most gas vesicle would fracture, Microcystis cell density would increase together with Microcystis sinking velocity and sinking loss rate.2) 5000 lux was the best light intensity for Microcystis at long time illumination with the highest relative growth rate. Lower light intens ity would decrease the relative growth rate and increase the time to reach the highest relative growth rate. Carbonhydrate concentration decreased at 1000 lux illumination and increased at 3000 lux illumination. When the light intensity raised to 5000 lux and 8000 lux, carbonhydrate concentration remained almost unchanged over time. Microcystis cell density increased over time at long-term illumination in all light intensities, it reached the highest at 21 stday. Microcystis cell density was significant positive correlated with carbonhydrate concentration at 3000lux、5000lux and 8000 lux.3) Light shading, high light mixing and light shading with mixing conditions could remove Microcystis from water. Light shading with mixing condition had the best removal efficiency, it could not only promote the sinking loss of Microcystis but also decrease the total standing stock. High light mixing was slightly better than light shading with mixing condition to remove Microcystis, but both of the conditions could only promote the sinking loss of Microcystis, the total standing stock didn’t change over time. When Microcystis was cultured for 10 days at light shading condition and high light mixing condition, the growth biomass was higher than death biomass, sinking loss was the only solution of biomass loss. When the condition changed to light shading with mixing condition, the net death biomass was a bit lower than sinking loss, the contribution to biomass loss were 46.73% and 53.27% each. The variation of carbonhydrate concentration and group density of Microcystis group at light shading, high light mixing and high light shading with mixing conditions were conformed to that of pure culture experiments. The light intensities of light shading, high light mixing and high light shading with mixing conditions were equivalent to 1000 lux, 3000 lux and 0lux in pure culture experiments. Microcystis group diameter and density increased at light shading and high light mixing conditions, so the sinking loss rate also increased. When cultured at light shading with mixing condition, Microcystis group diameter increased while group density decreased, two contrary effects offset each other, brought little effect on sinking loss rate.