外文翻译缓解湖泊富营养化放开氮控制并专注于磷的减排节选Word文档格式.docx

1、原 文 Mitigation of Lake Eutrophication: Loosen Nitrogen Control and Focus On Phosphorus AbatementHaijun Wang, Hongzhu WangAbstractTraditionally, nitrogen control is generally considered an important component of reducing lake eutrophication and cyanobacteria blooms. However, this viewpoint is refuted

2、 recently by researchers in China and North America. In the present paper, the traditional viewpoint of nitrogen control is pointed out to lack a scientific basis: the N/P hypothesis is just a subjective assumption; bottle bioassay experiments fail to simulate the natural process of nitrogen fixatio

3、n. Our multi-year comparative research in more than 40 Yangtze lakes indicates that phosphorus is the key factor determining phytoplankton growth regardless of nitrogen concentrations and that total phytoplankton biomass is determined by total phosphorus and not by total nitrogen concentrations. The

4、se results imply that, in the field, nitrogen control will not decrease phytoplankton biomass. This finding is supported by a long-term whole-lake experiment from North America. These outcomes can be generalized in terms that a reduction in nitrogen loading may not decrease the biomass of total phyt

5、oplankton as it can stimulate blooms of nitrogen-fixing cyanobacteria. To mitigate eutrophication, it is not nitrogen but phosphorus that should be reduced, unless nitrogen concentrations are too high to induce direct toxic impacts on human beings or other organisms. Finally, details are provided on

6、 how to reduce controls on nitrogen and how to mitigate eutrophication.Keywords Lake eutrophication; Cyanobacteria bloom 1. IntroductionEutrophication of waters is the phenomenon of an ecosystem becoming more productive by nutrient enrichment, stimulating primary producers. It is usually characteriz

7、ed by algal blooms, causing water quality deterioration and fish kills. It is becoming a global environmental crisis. In China, the problem of lake eutrophication is extremely severe, with frequent cyanobacteria blooms threatening the ecology of waters, economic development and society stability. Th

8、e most representative case is the cyanobacteria bloom that occurred in Lake Taihu in 2007, resulting in a shortage of drinking water and domestic water for 5 million citizens in Wuxi, Jiangsu Province. Therefore, mitigation of lake eutrophication and cyanobacterial bloom is of great importance in Ch

9、ina and the world.To mitigate eutrophication and cyanobacterial blooms, nutrient control is a fundamental process. Traditionally, besides phosphorus control, nitrogen control is generally considered a necessary practice. Abundant funds have been spent on nitrogen removal during wastewater treatment

10、processes. However, recent researches in China and North America suggest a change to the traditional practice of nitrogen removal for inland waters: to mitigate eutrophication, it is not nitrogen but phosphorus that should be reduced. This finding is of critical importance for the practice of lake r

11、estoration.In this paper, the traditional viewpoint of nitrogen control is pointed out first to lack a conclusive scientific basis. Then, new researches are introduced to elaborate a general rule: reduction in nitrogen loading cannot decrease total phytoplankton biomass, but stimulates blooms of nit

12、rogen-fixing cyanobacteria. Further analyses proceeded to elaborate on the significance of this general rule for the strategy to control the source of nutrients. In the end, suggestions on lake restoration are proposed in detail.2. The traditional viewpoint of nitrogen control lacks conclusive scien

13、tific basesThe traditional viewpoint of limiting nutrient control on phytoplankton biomass is based mostly on considerations of N/P （ratio of total nitrogen to total phosphorus） and on bottle bioassay experiments. However, the following analyses indicate that both the hypothesis and the experiment a

14、re not conclusive.2.1. The N/P hypothesis is a subjective assumptionGenerally, lakes have been regarded as limited by phosphorus if TN/TP was relatively large （such as TN/TP 17）, limited by nitrogen if TN/TP was relatively small （such as TN/TP 10） and colimited by nitrogen and phosphorus when TN/TP

15、was intermediate （such as 10 TN/TP 17）. However, the thresholds of TN/TP to indicate nutrient limitation vary greatly in the literature, being for instance 1017, 1030, and 715. The variation itself implies that this method is not reliable. Some limnologists have used the ratio C:N:P proposed by Redf

16、ield as a criterion to assess nutrient limitation. Again, the Redfield ratio is not a universal optimum ratio, but an average of species-specific ratios. The optimum N/P ratios vary greatly among various freshwater phytoplankton species, from 4.1 to 133.3. Obviously, it is almost impossible to set a

17、 specific “cut-off” ratio to identify a limiting nutrient（s） for a multi-species community.By analyzing the regressions and scatterplots of phytoplankton Chlaorophyll with nitrogen and phosphorus, Sakamoto first proposed the N/P hypothesis. However, when probing into the evidence, we find that his c

18、onclusion is just a subjective judgment on the patterns of the scatterplots. In his analysis, he noted three points with a N/P ratio larger than 1517 deviating. He concluded that phosphorus became the critical limiting factor if the N/P ratio was larger than 1517. In fact, there are ten points with

19、this level of N/P ratio and the left seven points fell exactly on the regression line. Obviously, such a reasoning based on minor points is not right. Similarly, he concluded that phytoplankton was limited by nitrogen when N/P was less than 910 based on two outlier points, when analyzing the regress

20、ion relationship between log（TN） and log（Chla）. Therefore, this earliest empirical evidence does not support the N/P hypothesis.Many researchers have followed the principles espoused by Sakamoto to analyze the limiting nutrient and variations under different N/P ratios. However, all the analyses hav

21、e been performed without strict statistical tests, failing to prove that the dependence of phytoplankton on nitrogen and phosphorus are significantly different under different N/P ratios.In conclusion, through logical reasoning and empirical analyses, the N/P hypothesis is found to be just a subject

22、ive assumption without conclusive evidence.2.2. Bottle bioassay experiments are too small in scale to simulate the natural process of nitrogen fixationBottle bioassay experiments have also long been used to demonstrate the limiting nutrient of phytoplankton in a waterbody. The experiments are usuall

23、y performed in an enclosed container to discriminate the limiting nutrient by observing the growth of phytoplankton after nutrient addition. They generally last a period between several hours to 1 week. If growth of phytoplankton is stimulated by the addition of some nutrient, this nutrient is consi

24、dered to be the limiting factor. Due to their small scale in time and space, these experiments fail to simulate some important processes in real open systems such as biological nitrogen fixation. Therefore, bottle bioassay experiments cannot prove the long-term existence of nitrogen limitation in th

25、e field.3. New researches find that nitrogen control fails to restrain total phytoplankton, but stimulates blooms of nitrogen-fixing cyanobacteriaIn May 2008, our article disproved the N/P hypothesis based on regional comparative studies. In August 2008, an article by researchers from North America

26、indicated that nitrogen control might stimulate nitrogen-fixing cyanobacteria based on a long-term whole-lake experiment. Accordingly, it appears that reduction in nitrogen loading cannot decrease the biomass of total phytoplankton. It is also logical when comparing the cycling characteristics of ni

27、trogen and phosphorus.3.1. Regional research in the Yangtze BasinTo test the N/P hypothesis on a large scale, we analyzed the relationships of Chlaorophyll a with total nitrogen and total phosphorus based on multi-year investigations of more than 40 Yangtze lakes.Almost all the TP-Chla regressions h

28、ad higher R2 values and lower percentage predictive error than those of TN-Chla regressions, indicating the superiority of TP over TN as a predictor.Because different thresholds of TN/TP ratios have been proposed in the literature, we further compared the differences of R2 values between TP-Chla and

29、 TN-Chla regressions over the complete TN/TP spectrum. The results showed that 28 R2 values of TP-Chla regressions were higher than those of TN-Chlaregressions over the entire TN/TP spectrum. According to the traditional N/P hypothesis, these cases should be limited by phosphorus and have relatively

30、 high TN/TP ratios. However, they were not confined to any specific region of higher TN/TP ratios, but distributed evenly between 5 and 50. It further demonstrates the failure of TN/TP to indicate nutrient limitation.For a given amount of TP, Chla varied regardless of the changes of TN. That is, wit

31、hin the TN/TP range of the present research, Chla/TP tends towards a specific value. However, for a given amount of TN, Chlaincreased rapidly with the increase in TP. Therefore, in the field, the total phytoplankton biomass is not determined by TN but by TP.Our research disproved the traditional vie

32、wpoint of using N/P as an index to discriminate nutrient limiting phytoplankton. We are of the opinion that TP is the primary factor regulating phytoplankton Chla regardless of the concentration of TN.3.2. Long-term whole-lake experiments in CanadaIn Lake 227, a small lake in the Experimental Lakes Area （ELA） of Ontario, Canada, researchers from Canada and the USA have performed whole-lake fertilization experiments for 37 years. The lake has been fertilized with constant phosphorus and decreasing nitrogen addition. In t