THE CONTROL OF LAKE EUTROPHICATION: FOCUSING ON PHOSPHORUS ABATEMENT, OR REDUCING BOTH PHOSPHORUS AND NITROGEN?

In terms of eutrophication control, quite a lot of whole-lake experiments and lake rehabilitation practices have shown that reducing phosphorus only can reverse lake eutrophication; however, many people still argue that both phosphorus and nitrogen should be reduced. Since reducing both phosphorus and nitrogen costs 4—15 times as much as reducing P only, it is of the utmost importance to conclude whether dual nutrient control is a must. Focusing on such an essential question, we critically scrutinize all relevant viewpoints and their evidences. First, we systematically summarize the research history about nutrients driving and controlling eutrophication. Second, we retrospect and examine the concept evolution and the experiment criterion of limiting factors, which have been considered as the main basis to judge controlling nutrients for a long time, and clearly point out that the purpose of identifying limiting factors is to determine the factors to promote growth of organisms. Third, we introduce the new term of an abating factor, which has been defined as the most cost-effective factor that can reduce overgrowth of individuals, populations or communities in ecosystem managements, being either an essential environmental factor or a physical/chemical/biological means of destroying organisms; and explain the five steps to determine an abating factor, i.e. analyses of necessity, controllability, practicability and costs, and tests, through an example, demonstrating convincingly that a non-limiting factor can become an abating factor, and a limiting factor is not necessarily an abating factor. Fourth, based upon abating factor analysis, we show that the abating factor of lake eutrophication is phosphorus; further, summarize the systematic evidences from whole-ecosystem experiments in Canada and China and lake management. These fully prove that only phosphorus abatement can effectively mitigate eutrophication, whereas only nitrogen reduction cannot decrease phytoplankton biomass, even can induce massive growth of N-fixing cyanobacteria. Fifth, we refute logical and experimental bases of the opinion of reducing nitrogen one by one; such arguments either confused abating factors with limiting factors, or lacked support from large-scale experiments. Sixth, we critically analyze the ecological effects of high nitrogen concentrations, and preliminarily conclude that it is only when total nitrogen and ammonia are more than 5 mg/L that nitrogen can have some stresses on organisms (submersed macrophytes, fishes, etc.) and promote sediment phosphorus release; then suggest to loosen total nitrogen and ammonia standards for Class Ⅰ—Ⅴ surface water first from ≤0.2—≤2 mg/L (China: GB 3838-2002) to the same value of 2 mg/L, and gradually to ca. 5 mg/L. Last, we recommend a systematic strategy in eutrophication control, with the purpose to rehabilitate physical, chemical, hydrological and biological integrity. On the basis of maintenance of lake basin integrity, the most fundamental measure is intercepting external phosphorus loading; if internal loading is heavy, in-lake phosphorus inactivation is the first choice. Next, the key action in shallow lakes is regulating water level to recover aquatic vegetation and switch ecosystems from turbid-to clear-water states. In conclusion, we advocate loosening N control and focusing on P abatement in eutrophication mitigation so as to reduce costs substantially.