Mechanism and control of lake eutrophication

A review about lake naturally eutrophicating, the internal loading of nutrients from lake sediment as well as the mechanism of algal blooms and the control practices was made, especially the eutrophication problem of shallow lakes since seventy percent of fresh water lakes in China are shallow lakes. It was found that shallow lakes are apt toward eutrophication than deep lakes. Without any influences of human activity, shallow lakes in the middle and lower reaches of Yangtze River are still easily eutrophicated, which may be owing to the effects of flood in this area. In shallow lakes, sediments are frequently disturbed by wind-wave and resuspended, which result in huge nutrients release to overlying water. This may be the major reason for higher internal loading of nutrients in shallow lakes than in deep lakes. Algal bloom is an extreme response of lake ecosystem to the eutrophication. Appearance of algal blooms is related to physical condition of lakes, such as underwater radiation (or transparency), temperature, and hydrodynamic conditions, or related to geochemical conditions of lakes, like concentrations of nutrients and ratio of nitrogen to phosphorus, as well as the physiological advantage of cyanobacteria such as vacuole for moving towards the radiant energy-rich zone and the mycosporine-like amino acids (MAAs) for resisting the harm of ultraviolet radiation. In shallow lakes, these advantages of cyanobacteria are favorable in the competition than in deep lakes. Also being the shallowness, it is more difficult to reduce nutrient loading and to control algae blooms in shallow lakes. For the control of eutrophication, people should follow the sequence from pollution sources control, ecological restoration to catchment management. To control the internal nutrient release, physical, chemical, biological techniques, and even bionic techniques could be selected. The idea of ecological restoration for a eutrophic lake is to shift the ecosystem from phytoplankton-dominant state to macrophyte-dominant state. To realize the shift of ecosystem state, environmental condition improvement is the fundamental work. Nowadays, we should do more work on environmental condition improvement than on planting of macrophytes since we are lack of the knowledge about the relationship between macrophyte and lake ecosystem. Emphasizing the macrophyte planting, therefore, has blindness at present. Because all lakes have different characteristics of environment and ecosystem, applicable lake harness techniques should be selected based on the distinct ecosystem types and environmental problems.