Approaches to Mechanisms and Control of Eutrophication of Shallow Lakes in the Middle and Lower Reaches of the Yangze River

Abstract

The middle and lower reaches of the Yangtze River are one of the central areas of freshwater shallow lakes in China. Most of the lakes have been under mesotrophic or eutrophic conditions, the latter are especially common in urban lakes. Human activities have played a key role in the eutrophication process from at least two aspects: continuous effluents of industrial, agricultural and domestic wastewater could add nutrient to the lake; and reclamation, stone built dikes, cultivation could damage natural habitats and reduce nutrient output from the lake. The seriousness of eutrophication has aroused attention of national government. Two of the Three Lakes Water Pollution and Prevention Control Program that give priority to the Tenth Five Year Plan of China are situated in the plain, i.e., Taihu Lake and Chaohu Lake. The nutrient status has been under a relative high level in most lakes. The eco environmental questions in lakes are conspicuous even though local governments have taken some measures to prevent further environmental deteriorations. Steps dealing with external nutrient loadings have achieved good results in deep lakes internationally, while they are no longer so effective in shallow lakes, because we lack a through understanding of mechanisms of eutrophication in shallow lakes.World acknowledged experiences of shallow lakes eutrophication control have proved that eutrophication still prevails even though area source nutrient loading to the lake could be reduced to a minimum level. Internal sources from the sediment may certainly relate to the maintenance of eutrophication. The kinetics of water could re suspend sediments, and effect the nutrient release process, underwater light intensity and primary production as well. To control eutrophication in shallow lakes must take both external loading of nutrient and internal loading into consideration. One of the effective measures for internal loading is the restoration of aquatic plants in order to inhibit the re suspension and sediment release. Researches on the experimental ecological restoration and functions of ecosystem deterioration are needed. As a result, external control on nutrient source pollution is not enough in shallow lakes; internal nutrient within lakes must be emphasized. Two techniques have been applied in internal loading control, i.e., dredging and macrophytes restoration. The former requires a complete knowledge on the trophic history in lake ecosystem so that an exact schedule on the scope and depth of dredging could be determined lest newlyexposed sediments become fresh internal release source. The latter could inhibit sediment suspension under dynamic conditions, and absorb nutrient within lakes so as to diminish nutrient load as well. Macrophytes flourishing and algae bloom outbreak are two aspects of eutrophication in lakes. Why macrophytes dominated lake could be evolved into an algae dominate one? How to revolve the process? Future macrophytes restoration must answer them. Researches abroad prove that a macrophytes dominated lake could be turned into an algae dominated one under strong external disturbances and steady high lake level; once algae dominated, the lake would come to another steady state for nutrient cycling. To restore macrophytes and get cleaner water, this cyclic process must be broken down.Consequently, it is strongly recommended that researches on the mechanisms and control of eutrophication in shallow lakes be initiated, so that a theoretical and technical basis could be provided for future launching national programs on eutrophication control in China.