Analysis of black water aggregation in Taihu Lake

Water Science and Engineering, 2011, 4(4): 374-385doi:10.3882/jissn. 1674-2370.2011.04.002htp:/www.waterjourmal.cne-mail: wse2008@vip.163.comAnalysis of black water aggregation in Taihu LakeGui-hua LU*", Qian MA2, Jian-hua ZHANG'1. National Engineering Research Center of Water Resources Effcient Utilization and Engineering Safety,Hohai University, Nanjing 210098. P. R. China2. Hydrology and Water Resources Investigation Bureau ofJiangsu Province, Nanjing 210029, P. R. China.3. Water Resources Department of Jiangsu Province, Nanjing 210029, P R. ChinaAbstract: Black water aggregation (BWA) in Taibu Lake is a disaster for the lake environment. Itisa phenomenon resulting from water environmental deterioration and eutrophication caused byaccumulation of pollutants in the lake, according to research on the water quality, pollutants of BWA,and occurrence mechanisms of BWA. Dead algae arc the material basc of BWA, the pollutedsediment is an important factor for ithe formation of BWA, and hydrological and meteorologicalconditions such as sun light, air temperature, wind speed, and water flow are the other factors thatmay lead to the formation of BWA. Thioether substances such as dimethyl trisulfide are therepresentative pollutants of BWA. Parameters such as chlorophyll-a, DO, pH, and water temperatureare sensitive indicators of BWA. Measures such as algac collection, ecological dredging, pollutioncontrol, and water diversion from the Yangtze River to the lake, are effective, and strengtheningaeration is an emergency measure to control BWA.Key words: black water aggregation; water quality indicator; control measure; Taihu Lake1 IntroductionBeing one of the five largest freshwater lakes in China, Taihu Lake is of vital importancein regional flood control and water supply. Accounting for about 0.4% of the land area and3.5% of the population in China, the Taihu Lake Basin creates more than 11% of the nationalgross domestic product (GDP) and 20% of the national fiscal revenue. With the mostcentralized urban areas, a highly developed economy, and the greatest social productivity, thebasin plays a large role in the socio- economic development as one of the core areas in China.Black water aggregation (BWA) appeared at the Nanquan water source in Gonghu Bay inthe northerm part of Taihu Lake at the end of May in 2007. The drinking water supply formillions of people in Wuxi City near the lake had to be cut off because the water was sopolluted and cannot be purified to the required standard, causing a water supply crisis thatshocked the world. BWA appeared again in the coastal areas of Zhoutie Town and Dapu Townin Yixing City in Taihu Lake at the end of May in 2008, with a maximum area of up to 17 km2.It was scattered over the area and took a week to disappear gradually with the wind waves.This work was supported by the National Water Project of China (Grant No. 2008ZX07101-011).*Corresponding author (e-mail: lugh@hhu.edu.cn)Received May 31, 2011; accepted Oct.16, 2011中国煤化工YHCNMHG.Several small BWAs were also found in the western and northern parts of Taihu Lake in 2009and 2010. The occurrence of BWA resulted in rapid deterioration of water quality and severedestruction of the ecosystem, and posed a significant challenge to the water supply for urbanand rural residents. Since research on BWA is still in its preliminary stage currently, there existvarious views concerming the formation of BWA. Some have said that BWA is the result of ablue-green algal bloom; some have insisted that it is caused by exotic pollution sources, andothers think it is brought in through water transfer. For large shallow lakes like Taihu Lake, inorder to reduce the occurrence of BWA, protect the lake ecosystem, and secure the watersupply, it is of great importance to analyze the formation mechanism of BWA, to exploresensitive indicators of its occurrence, and to study prevention and control measures based ontracking and monitoring data.Analysis of tracking and monitoring data from recent years indicates that there are twopossibilities regarding BWA's formation. One is that BWA results from the biochemicalreaction of accumulated and dead algae after algal blooms in eutrophic lakes with organicsfom sediment under appropriate meteorological and hydrological conditions. The other is thatBWA is directly caused by the contaminated sediment under certain meteorological andhydrological conditions (Lu and Ma 2009).2 Occurrence of BWA in Taihu Lake since 1990sBWA has been observed many times in westerm Taihu Lake, Meiliang Bay, and GonghuBay since the 1990s. According to the records of the Wuxi Water Resources Department, BWAtook place after algal blooms in Meiliang Bay during the periods of July 1 to 25, 1990, July,1994, July 5 to8, 1995, August 1 to 10, 1998, and August, 2003. At the end of May, 2007,BWA occurred following a large-scale algal bloom in the Nanquan water source and caused adrinking water supply crisis in Wuxi City. From May 26 to June 9, 2008, BWA, with amaximunn area of 17 km', occurred in the coastal area of Taihu Lake in Yixing City. In 2009,BWA occured 11 times within the area of 22 km2 along the western and northem parts of thelake: once in May, in June, and in Septermber, and five and three times respectively in July andAugust. In 2010, BWA was again found four times in the western and norther areas of thelake, the area of BWA on July 23 reaching nearly 6 km2 in the westerm part of the lake. Fig. 1is the location map of the occurrence of BWA in Taihu Lake from 2008 to 2010.According to the study of Kong et al. (2007), there are four key stages in the occurrenceof BWA, which have been confirmed by simulation experiments, reviews of the formationprocess, and field investigations. Based on observation, tracking, and monitoring of thephenomenon of BWA in Taihu Lake, we note details of the four key stages: (1) Stage ofsubstance accumulation or the early stage: the algal bloom provides a material base for BWA,and the combination of accumulated algae and organics in the sediment is likely to be thesubstance factors of BWA formation. (2) Stage of aerobic decomposition: the accumulation ofGui-hua LU et al. Water Science and Engineering, Dec. 2011, Vol. 4, No. 4, 374-385375中国煤化工MYHCNM HG.algac is closely related to the process of aerobic decomposition. With the decreasing of oxygen,the hypoxic process begins to appear. On the other hand, the amount of oxygen in water has aclose relationship with the flow patterm and weather conditions. (3) Stage of anaerobicdecomposition: with the depletion of oxygen in the water, the decomposition of organicstransforms into anaerobic decomposition by utilizing small- molecule organic matter aselectron aceptors with microbial action. (4) Stage of formation of BWA: with thedecomposition of large amounts of anaerobic organics, malodorous black substances emerged.The combination of ferromanganese materials and reduced sulfur compounds in the sedimentfloats upward to water under the effects of volatile organic compounds, resulting in BWA,which moves along with the water flow and wind, leading to ecological disasters.Meiliang Bay◎Wuxi CilyZhushan BayNanquanwguer sourcesurceZhoutie ]◎Yixing CityGonghu BayDapu T--- May 26, 2008May27, 2008--- July, 2010Fig. 1 Location of BWA occurrence in Taihu Lake from 2008 to 2010From the four key stages in the occurrence of BWA in Taihu Lake, it can be seen that thechemical and physical characteristics and the hydrological and meteorological factors affectthe formation of BWA in each stage. Therefore, they should be given more atention in thefollowing study.3 Basic characteristics of BWA3.1 Appearance of BWABWA ocrring in the coastal area of Taihu Lake near Yixing City from May 26 to June10, 2008 is taken as an example. The BWA took place in the water area near Zhoutie Town andDapukou Town in Yixing City, and is named BWA526. The area of BWA526 changed with adaily average area of 7.5 km'. Bubbles appeared sometimes, and the water was turbid andblack, showing clear boundaries with other water in the lake. There were smells of sewage andhydrogen sulfide, and some dead fish, mainly lake loaches, were found in water surface.Table 1 shows the monitoring results of BWA526. Compared with the pollutants in the376Gui-hua LU et al. Water Science and Engineering, Dec. 2011, Vol. 4, No. 4, 374-385中国煤化工MHCNMH Gwater without BWA, the dissolved oxygen (DO) concentration of BWA526 was 6.7 mgLlower, and the concentrations of permanganate index (CODMn), chemical oxygen demand(COD), total phosphorus (TP), ammonia nitrogen (NH3-N), and total nitrogen (TN) were4.1 mg/L, 29.0 mg/L, 0.327 mg/L, 4.54 mg/L, and 2.03 mg/L higher, respectively. The lowestconcentration of DO was almost 0 mg/L, and the maximum CODMn, COD, TP, NH-N, andTN were, respectively, 2.13, 2.19, 3.79, 4.72, and 1.64 times of the background value.Table 1 Monitoring results of BWA526 in Taihu LakeConcentration of parameter (mg/L)ltemDCODmaCODTlNH3-NTNMcan value1.415.30.5776.46Concentration range0.14.00.5-23.943.1-126.00.360-0.9474.76-9.068.23-13.40Background value8.11.257.60.2501.928.173.2 Monitoring results of BWATable 2 provides the monitoring data and the statistics of major water quality indicatorsof BWA in Taihu Lake in recent years, which shows the following results:(1) The DO concentration has been trribly low, close to zero, while, the concentrationsof CODMm, TP, NH-N, and TN have been high. Compared with Grade II in EnvionmentalQuality Standards for Surface Water in China, the concentration was three to four times higherfor CODmn, about 100 times higher for TP, and about ten times higher for NH3-N and TN.(2) The concentrations of CODMm, TP, NH3-N, and TN of BWA were quite high on May11 and July 20, 2009. The concentrations of major water quality indicators of BWA on May 12and July 21 in 2009 were close to those in 2007 and 2008.(3) Comparing the water quality data of BWA occurring on May 26, 2008 with those onJune 2, 2007, when BWA occurred 1.5 km east of the Nanquan water source, the results wereroughly similar to each other. The DO concentration in the former was slightly higher, theCODMn concentration was roughly equal, the TP and NH3-N concentrations were respectively,0.25 mg/L and 0.65 mg/L higher, and the TN concentration was 5.8 mg/L lower. Dimethyltrisufide (DMTS), geosmin, and other materials were found in both water samples of BWA.Therefore, the composition of BWA occurring near Yixing City in 2008 was similar to thatnear the Nanquan water source of Wuxi City in the lake, and the causes were consideredbasically the same.3.3 Main pollutants of BWAAccording to the monitoring data, the odor of BWA is significantly different from thesmell caused by algal bloom. Smell from an algal bloom is mainly produced by secondarypollutants like 2-Methylisobormeol or geosmin, directly generated by the metabolism of algae,while the odor of BWA is very distinct. Yang et al. (2008) pointed out that the main cause of.Gui-hua LU et al. Water Science and Engineering, Dec. 2011, Vol. 4, No.4, 374-385377中国煤化工MHCNMH G .Table 2 Results of main water quality indicators of BWA in Taihu Lake in recent yearsValue of main water quality parameterMaximumDateLocation Temperature>HCODCODManNH3-N TNarea of(C)(mg/L)mg/L) (mg/L)Nanquan2007-06-02water27.581.016.20.446.10154.0source2008 05-2624.6107.0.1160.6910.12008-05-2725.996.9 0.523.90.509.0612.92008-05-28Near110.00.3612.0Zhoutie),42008-05-2923.691.0 0.915.1 .0.56 6.89.Yixing2008-05-3023140.549.02008-05-3124.1102.0 0.317.70.83 8.6611.82009-05-11East of257.3305.42.8515.1026.4Fudu2009-05-12Port in25.87.6 78.49.50.66.406..02009-05.13 Yixing24.4137.4221.092009-07-2032.5.2 162.0 0.345.00.824.932009-07-21Inlet of33.87.412.80.533.17.1Xiaoxi0.22009-07-23Port28.6.467.9 0.316.2 0.664.792009-07-2428.436.47.0.332.4.2010-07-23 East of32,9 1148.4155.0 10.4036.50120.02010-07-24Jiudu30.6.311.020.01.177.2016.42010-07-26 Yixing121.0 2.020.6 0.325.6113.85.22010-07-23Eastof32.26.7 1679.4152.014.5045.30 123.0Portin76.610.3 0.513.01 .9.92010-07-2627.23.066.90.116.3the odor of BWA in Taihu Lake near the Nanquan water source of Wuxi City in 2007 wassulfide material, particularly DMTS, produced by the decay of organisms in the anaerobicenvironment. Those materials can often be detected in contaminated river water. Thconcentration of DMTS in BWA was up to 0.0114 mg/L, far more than 2-Methylisobormeolnd geosmin. However, DMTS cannot be produced by the metabolism of algae and can onlybe generated in the process of protein decomposition of organisms under anoxic conditions(Yu et al. 2007).3.4 Indicators of BWAThe sensitive indicators and representative pollutants of the key stages in the occurrenceof BWA can be obtained through simulation experiments and statistical analyses. In the stageof substance accumulation, we need to focus on the meteorological and hydrodynamic factorsresulting in the agglomeration of algal blooms. These factors are water temperature, dissolvedoxygen, oxidation-reduction potential, pH, and the indicators of aerobe, as well as the contentsof chlorophyll-a and xanthophyll, which are closely related to the stage of aerobic378Guihua LU et al. Water Science and Engineering, Dec. 2011, Vol. 4, No. 4, 374-385中国煤化工MHCNMH G .decomposition. In the stage of anaerobic decomposition, the indicators are water temperature,DO, oxidation-reduction potential, pH, indicators of anaerobic microorganisms, typical smellysubstances, NH3-N, hydrogen sulfide, and manganese sulfide. The variations in DO,oxidation-reduction potential, pH, and NH3-N need to be given close attention in the stage ofBWA formation.Although the detected odor materials like sulfide can be used significantly as indicatorsof the occurrence of BWA, the collection and analysis of those materials are not practical,usually taking long time. Based on the simulation of BWA and statistical analysis; thefollowing parameters can be selected as indicators of BWA: chlorophyll-a, DO, pH, and watertemperature. In general, anoxic or anaerobic conditions can form easily under conditions ofwater temperature above 20"C, chlorophyll-a up to 2000 μg/L, and pH in water close to or lessthan 7.0. The creation of a great amount of odor materials like sulfide is indicated when theDO concentration drops below 2.0 mg/L.4 Causes of formation of BWAIt was believed that the cause of BWA was the lake eutrophication caused by increasingpollutant loads in recent years (Zhu 2008). Based on the tracking and monitoring of BWA,analysis of water quality, algal blooms, sediment distribution, and meteorological andhydrological data over a long period, a large-scale blue algal bloom and accumulation ofcontaminated sediment are mainly blamed for the occurrence of BWA, and the sun light, airtemperature (generally higher than 25"C), wind speed (3 m/s to 5 m/s), water flow, and othermeteorological and hydrological conditions are the trigger factors of BWA.4.1 Water quality and algal bloom in Taihu Lake4.1.1 Significant deterioration of water qualityAccording to analyses of the monitoring data of water quality in Taihu Lake, the waterquality was Grade I to Grade II in the 1960s, Grade II in the 1970s, Grade II to Grade IIIon average in the early 1980s, Grade II and partly Grade IV or V in the late 1980s, and GradeIV on average in the mid-1990s with Grade V water in one third of the lake area. Waterquality in the lake has declined by one grade level every ten years on average, and thedeterioration rate has been exacerbated noticeably in the past ten years (Xu and Qin 2005;Zheng et al. 2001).Since 2004, the water quality in Taihu Lake has been Grade V or worse than Grade V inthe whole lake except the eastern part of the lake where it has been Grade IV, and the mainpollution parameters have been TP and TN. From 2004 to 2009, the average concentrationthroughout the lake varied from 0.071 mg/L (Grade IHI) to 0.104 mg/L (Grade IV) for TP andfrom 2.25 mg/L (inferior Grade V) to 3.38 mgL (inferior Grade V) for TN. Values variedfrom 0.080 mg/L (Grade II) to 0.165 mg/L (Grade IV) for TP and from 3.01 mg/L (inferiorGrade V) to 6.57 mg/L (inferior Grade V) for TN in Meiliang Bay, and varied fromGui-hua LU et al. Water Science and Engineering, Dec. 2011, Vol. 4, No. 4, 374-385379中国煤化工MHCNMHG0.092 mg/L (Grade II) to 0.158 mg/L (Grade IV) for TP and 2.99 mg/L (inferior Grade V) to4.57 mg/L (inferior Grade V ) for TN in Gonghu Bay.4.1.2 Pollutant loads into lakeAccording to 2009 monitoring data from the Taihu Basin Authority of the Ministry ofWater Resources of China, the loads of TN, TP, and CODMn into Taihu Lake were, respectively,4.4x10* t, 1.9x10*t, and 5.6x10* t, which were 4.7, 3.8, and 1.7 times, respectively, of thepermitted loads in the Comprehensive Program of Water Environmental Rehabilitation in theTaihu Lake Basin ratified by the State Council of China in 2008. However, pollutant loads ofTN, TP, and CODMn flowing out of Taihu Lake were, respectively, 1.4x10* t, 455 t, and3.4x 104 t. This indicates that the pollutant loads discharged into the lake were far greater thanthose flowing out of the lake. It is estimated that there were 1.5x 10 t ofTP, 3.0x 10* t ofTN,and 2x 104 t of CODMn retained in Taihu Lake in 2009 (Ma et al. 2009). From 1998 to 2009,the average pollutant loads flowing into the lake were 5.6x 104 t for CODMm, 1.7x10* t for TP,and 4.0x 10* t for TN, values far beyond the environmental carrying capacity of Taihu Lake,34 369 t for CODMn, 514 t for TP, and 8510t for TN.4.1.3 Internal pollutantsOver a long period of time, a large amount of exteral sources of pollutants has beenbrought in by river water running into the lake, particularly the areas with massive breedaquatics in the east of the lake, and the accumulation of pollutants in the lake has beenincreasing (Wang et al. 2009). According to the Sediment Dredging Planning Program inTaihu Lake issued by the Taihu Basin Authority, the deposition area of sediment in the lake in2005 was 1 547 km' , accounting for 66% of the whole lake area. Accumulation of pollutants insediments in Zhushan Bay, Meiliang Bay, Gonghu Bay, the easterm part of the lake, and theinlets of the lake were most serious, with a deposition depth of 0.8 m to 1.5 m, consisting ofmajor internal sources of water pollution in the lake. The release of nitrogen from internalsources accounted for about 22.5% of the TN load of the whole lake, and the release ofphosphorus from internal sources accounted for about 25. 1% of the TP load of the whole lake.4.1.4 Algal bloomAccording to monitoring data from the lake from 1987 to 2007 (Ma et al. 2010), algalbloom in the lake occurred more frequently year by year. On the basis of the analysis ofMODIS satllite images, algal blooms occurred only in one month of all the years before 1998,and in at least two months each year after 1998. From 2005 to 2009, algal blooms occurred inmore than six months in each year. From 1987 to 1998, algal blooms generally occurred insummer, from May to July, less often in August, and never in spring and winter. But after 1999,the frequency of algal blooms increased and the duration was prolonged, especially in 2007,when algal blooms occurred in all months of the year except January and February. Therefore,whether in frequency ot in scale, algal blooms have been becoming more and more serious inthe last two decades. However, the scale of algal blooms in the lake has been reduced asGui-hua LU et al. Water Science and Engineering, Dec. 2011, Vol. 4. No. 4, 374-385中国煤化工MHCNMHGintensive rehabilitation measures have been implemented since 2007.The accumulated dead algae in areas along the lake shore after large scale algal bloom,which were confirmed by simulation experiments, are the material base of BWA and thereason why BWA has occurred frequently in these water areas.4.2 Distribution of sludge in Taihu LakeThe sediment in Taihu Lake is the main destination of pollutants (such as nutrients, heavymetals, and organic toxicants) from outside of the lake. Sludge containing abundant nutrientsmay become the potential pollution source of the lake, as nutrients can be relcased into thelake under appropriate conditions, increasing the nutrient load in the upper part of the lakewater. Thus, close attention should be paid to the siltation, distribution, and content ofpollutants in the sludge in the lake.Sediment exists in most areas of Taihu Lake, affected by the inflow, outflow,wind-induced current, and terrain of the lake bottom. Since the 1980s, with the accumulationof a large amount of pollutants in sediment due to severe water pollution, the concentrations ofnitrogen, phosphorus, and organic matters have been relatively high, and have beentransported, dispersed, and deposited with the lake current. In addition, lake utilization andcreature residues in the lake have also influenced the sediment.Sediment in the lake consists of 88% silt and 12% sludge with various thickness, locatedin the upper layer of sediment. The sludge has a moisture content of 55% to 85% and a densityof 1.5 g/cm3 to 1.8 g/cm', and the silt has a moisture content of 85% to 150% and a density of1.2 g/cm’to 1.5 g/cm'.Taihu Lake is a typical shallow lake. Sediment is likely to suspend due to the agitation ofwind when the temperature is relatively high. Although the amount of sludge is much less thanthat of silt, the main direct material source of BWA is the pollutants from thsludge- concentrated region. The latest survey on the sediment in the lake reveals that thethickness of the sludge containing organic matter in the western part of the lake is 0.02 m to0.31 m, with a total amount of2.793x10* m'.Fig. 2 shows the distribution of sludge in the western and northern parts of the lake. Itshows that the areas where BWA commonly occurred in recent years almost match the areaswhere sludge was abundant. The water areas with a large amount of sediment are the placeswhere BWA occurred readily. Similar to the effect of the accumulation and death of algae, theexistence of sediment provides large amounts of organics, which are the material bases for theoccurrence of BWA.4.3 Meteorological and hydrological conditions of BWA occurrenceThe air temperature, water temperature, and wind speed are the main causes of BWA.Based on data from seven automatic monitoring hydrological stations around Taihu Lake,Xishan, Guangjingkou, Shadungang, the Taipu River entrance, Dapukou, Jiapu, and Xiaomeikou,Gui-hua LU et al. Water Science and Engineering. Dec. 2011, Vol. 4, No. 4, 374-385381中国煤化工MHCNMH GZhushanBayMeiliangBay.回Gonghu BayWestern lake\QfBoundary of Taibu Lake0 AreaofBWAFig. 2 Isolines of depth of sludge in northern part of Taibu Lake (unit: m)when BWA occurred, the air temperature in the westerm part of the lake was 25"C to 35"C, themean water temperature was 24°C to 32"C, and the wind direction changed frequently. Theaverage wind speed was about 3.1 m/s, and the maximum wind speed sometimes reached upto 15.7 m/s.4.3.1 Wind directionBased on the statistic analysis of the meteorological data in Yixing City, when BWA tookplace, the main wind direction was from the southeast and the south, and sometimes it wasfrom the west and the northwest. The wind was between three and four knots on the Beaufortscale, leading to the transportation of algae moving from the southerm and central parts of thelake to the westermn and northem parts of the lake, increasing the amount of algae in thenorthwestern part of the lake.4.3.2 TemperatureMetrological data around the lake show that seven to ten days before BWA was observedin the westem and northem parts of the lake, the sunshine was sufficient in Yixing City, withair temperature rising, and hence the water was heated, accelerating the death of algae andproducing a large amount of organic residue of algae. The water area had a severe lack ofoxygen, due to a large amount of oxygen being consumed by microbial degradation. On theother hand, toxic intermediates were produced in the course of the incomplete decompositionof algae fragments (Sun et al. 2007).4.3.3 Water levelBWAs both in the westem and northerm parts of the lake from the 1990s to 2008 were382Gui-hua LU et al. Water Science and Engineering, Dec. 2011, Vol. 4, No. 4, 374-385中国煤化工MHCNMH Ganalyzed, and the results show that the water level was lower than 3.30 m when they occurred,with only a few exceptions. It is assumed that a lower water level is favorable for theoccurrence of BWA. Nevertheless, based on analyses of 15 BWA observations from 2009 to2010 in the western and northern parts of the lake, only a minority of BWAs occurred whenthe water level was less than 3.30 m, and the highest water level reached during theiroccurrence was 4.10 m. It seems that the water level in Taihu Lake has no absoluterelationships with the BWA occurrence.4.4 Sensitive areas of BWA and investigationsBased on analysis, the water areas where BWA can be observed are (1) water areas withmany inlets ftom severely polluted rivers, (2) water areas with thick silt and sludge, (3) waterareas where algae readily accumulates, (4) water areas or bays with lttle water exchange, and(5) water areas where BWA has been observed before. The sensitive areas of BWA in TaihuLake are shown in Fig. 3.Meiliang Bay,(Zhushan》BayGonghu Bay! Westerm lake》Boundary of Taihu Lake0 Sensitive area of BWA in Taibu Lake .Fig. 3 Sensitive areas of BWA in Taihu LakeIn order to obtain better information about BWA, measures such as combining randominvestigation with regular observation of the lake, analyzing the data both from fieldmonitoring in the lake and from laboratory testing, integrating daily reports with regularanalysis, and integrating data from hydrology departments and environmental protectiondepartments should be adopted. Water sampling and monitoring should be carried out, with afocus on the sensory quality of BWA and algae, transparency, water temperature (both insurface water and deep water), wind speed, wind direction, DO (both in surface water anddeep water), pH, the density of algae, and so on, of which DO, pH, and the density of algaewould be monitored in the field with multi-parameter water quality meters. In case the waterarea is covered by algae, we should note specific information about the location, and area, andsend the samples to the laboratory if necessary.Gui-hua LU et al. Water Science and Engineering, Dec. 2011, Vol. 4, No. 4, 374-385383中国煤化工MHCNMH G4.5 Control and prevention measures of BWAOn the basis of preliminary analysis of the causes and mechanism of BWA, measuressuch as algae collection, ecological dredging, pollution control, and water diversion from theYangtze River to the lake are effective in mitigating and controlling BWA. Meanwhile, moreefficient investigations in relevant BWA-sensitive areas and the setup of monitoring warningsystems are helpful for early alert and handling of BWA emergencies.It is possible to prevent and control BWA, as the frequency and areas of BWA havedecreased significantly since large-scale measures including algae collection and ecologicaldredging were conducted for lake rehabiltation.5 Conclusions(1) BWA is a phenomenon resulting from water environmental deterioration andeutrophication caused by accumulation of pollutants in Taihu Lake. Thioether substances suchas dimethyl trisulfide are the representative pollutants of BWA. Preliminary study shows thatchloroplyl-a, DO, pH, and water temperature are sensitive indicators of BWA.(2) BWA is caused by the integrated reaction of dead algae and polluted sediment, and theformer is the material base of BWA, while the latter is an important factor for BWA.Hydrological and meteorological conditions such as sunlight, air temperature, wind speed, andwater flow are inducements of BWA in Taihu Lake.(3) BWA can be prevented and controlled by pollution control and improvement of waterenvironment of the lake. Measures such as algae collection, ecological dredging, pollutioncontrol, and water diversion from the Yangtze River to the lake are effective, and strengtheningaeration is an emergency measure to control BWA.(4) Multidisciplinary and multi- domain studies are necessary for the analysis of thecauses of BWA. Nevertheless, there is :no mature theory and experience up to now to explainthe mechanism of BWA formation, which still needs further exploration and study.ReferencesKong, F. X., Hu, W. P, Gu, X. H, Yang, G s.. Fan, C. X., and Chen, K. N.2007. On the cause of cyanophytabloom and pollution in water intake area and emergency measures in Meiliang Bay, Taihu Lake in 2007.Journal of Lake Sciences, 19(4), 357-358. (in Chinese)Lu, G H, and Ma, Q.2009. Analysis on the causes of forming black water cluster in Taihu Lake. Advances inWater Science, 20(3), 438 442. (in Chinese)Ma, Q, Liu, J. J, Tong, J, Wen, L, Gao, M. W., and Tian, W.2009. Investigation on black water aggregationin Taihu Lake and analysis on water quality in water source in Jiangsu Province. Techniques andPractices in Hydrology and Water Resources. Nanjing: Southeast University Press. (in Chinese)Ma, Q, Liu, J. J, and Gao, M. Y.2010. Amount of pollutants discharged into Lake Taihu from JiangsuProvince, 1998 2007. 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