Effect of decomposition products of cyanobacteria on Myriophyllum spicatum and water quality in Lake Taihu,China

In order to reveal the mechanism on degradation of submerged macrophytes in the eutrophying freshwater ecosystems in shallow lakes, the effects of the decomposition products of cyanobacteria on the growth of submerged macrophyte and water quality were studied by laboratory experiment in this paper. In order to simulate the environment of Lake Taihu, the cyanobacterial bloom dominated by microcystis was collected from the surface water of Meiliang Bay in Lake Taihu, and the submerged macrophyte Myphyllum spicatum with the same vigor and the same size were collected from the aquatic planting areas near the Meiliang Bay. After one week degradation of algae bloom under dark and anaerobic condition, the bloom scum with the concentration about 0.47 g/mL was applied to Myphylum spicatum incubation system. The system contained about 8 -- 10 Myphyllum spicatum which was growing in 12 L water in the tank. Then we planted the macrophyte on ca. 8 cm-thick sediment on the bottom of the tank. The water quality index including pH value, turbidity (NTU), temperature, dissolved oxygen (DO), electrical conductivity (Ec), salinity, phosphate radical (PO super(3) sub(4) super(-)) in the system and the index Fv/Fm of M. spicatum spicatum were measured to monitor the water quality and physiological activity of Myphyllum spicatum in the system. The results showed that DO value and pH value in water of Myphyllum spicatum incubation system drop significantly after the application of bloom decompostion residuals, and the turbidity increased markedly, which possibly resulted from both the large amounts of reducing substances and suspended substances in the decomposition products of cyanobacteria. The phosphate (PO super(3) sub(4) super(-)) concentration in water almost keep stable during the experiment and showed no dinstinct trends of change in the groups, which indicated that phosphorus nutrient did not significantly affect the growth of M. spiatum. The temperature and salinity in experimental groups also insignificantly differed from the control group from the results. After two days cultivation, the values of Fv/Fm always maintained nearly 0.8 in the control group, but it decreased sharply to 0.3 in treatment groups. It meant that the growtn of M. spicatum in the control group was much better than the growth in experimental groups which was on the margin of death. In other words, M. spicatum was strongly affected by the decomposition products of cyanobacteria in Lake Taihu. Inte