Conservation Biological Study Based on Multistability and Material's Circulation in Eutrophicated Lake Ecosystems

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Population dynamics and stability in a model lake ecosystem have been studied by using a microcosm consisting of algae (M.aeruginosa), flagellate (M.guttula), and few bacteria (P.putida, and so on). The predatory effect of M.guttula on M.aeruginosa exhibited a trade-off with respect to the concentration of nutrient; The quality of food (M.aeruginosa) was increased with increasing the nutrient concentration, thus the growth rate of M.guttula was increased. However, high nutrient conditions increased the photosynthetic activity of M.aeruginosa, which increased the pH of the solution, and thus the growth rate of M.guttula was decreased. Under the conditions of intermediate nutrient conditions, M.aeruginosa was extinct or survived depending on the initial population of M.guttula, suggesting bistability in the model system.A micro-Fourier transform infrared (FTIR) spectroscopy was used to study changes in cell compositions of bio-molecules in M.aeruginosa depending on the nutrient concentrations. The amounts of glycogen and lipid have been found to increase in the logarithmic growing phase under the high nutrient conditions. This may suggest that M.aeruginosa absorbs and uses nutrients effectively under eutrophicated conditions.A new mathematical model of a lake ecosystem has been constructed taking into account of increase in water temperature. Analyses of the model shows that there exists a critical value of water temperature, and that algal bloom occurs if water temperature exceeds the value in summer time, suggesting regime shift in lake ecosystems. It was also pointed out that macrophytes have significant effects on suppressing algal blooms, and that management of lake ecosystems needs feed back control by taking into account of uncertainty of lake ecosystems.