Effects on water quality following water transfer in Lake Taihu, China

Effects on water quality following water transfer in Lake Taihu, China

Ecological Engineering 36 (2010) 471–481 Contents lists available at ScienceDirect Ecological Engineering journal homepage: www.elsevier.com/locate/...

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Ecological Engineering 36 (2010) 471–481

Contents lists available at ScienceDirect

Ecological Engineering journal homepage: www.elsevier.com/locate/ecoleng

Effects on water quality following water transfer in Lake Taihu, China Liuming Hu a,b,c,1 , Weiping Hu b,∗ , Shuhua Zhai d , Haoyun Wu e a

Graduate School of Chinese Academy of Sciences, Beijing 100039, China Nanjing Institute of Geography and Limnology, Chinese Academy of Sciences, 73 East Beijing Road, Nanjing 210008, China c State Key Laboratory of Lake Science and Environment, Chinese Academy of Sciences, 73 East Beijing Road, Nanjing 210008, China d Water resources Conservation Bureau, Taihu Basin Authority, MWR, 480 Jilian Road, Shanghai 200434, China e Taihu Basin Authority, MWR, 480 Jilian Road, Shanghai 200434, China b

a r t i c l e

i n f o

Article history: Received 18 July 2009 Received in revised form 29 September 2009 Accepted 11 November 2009

Keywords: Water transfer Water quality Observation Assessment Lake Taihu Yangtze River

a b s t r a c t To improve lake water quality, two experimental water transfers were conducted in winter–spring 2002 and summer–fall 2003 in Lake Taihu, a large shallow lake in China. Both observed data and estimated nutrient concentration with the elimination of effect from natural factors were used in this research to assess the spatial and temporal variations of water quality improvement induced by the two transfers. Clear improvement of water quality associated with deduction of TN, TP, and chlorophyll a (Chl-a) concentration was observed in many areas of the lake during the two water transfers. The over all reduction in TP concentration was notable in Southwest Zone, Centre Zone, and Dongtaihu Bay during the 2002 transfer, and was more pronounced in Meiliang Bay and Southwest Zone during the 2003 transfer period. However, the reduction in TN and Chl-a concentration was relatively weak. Results indicate a less impressive improvement of water quality from water transfer in large lakes than in small ones as the effectiveness of water transfer in large lakes is generally limited by large size, complex boundaries, and the difficulty of finding proper water source to be transferred. The comparison of observed and estimated water transfer effectiveness suggests a greater improvement of water quality derived from water transfer than appeared from the observation. © 2009 Elsevier B.V. All rights reserved.

1. Introduction Water transferred from one place to another has been used worldwide for irrigation, flood control, water supply, power generation, and so on. In many countries, transfer of a large quantity of low-nutrient water to a eutrophic lake is considered one approach to lake restoration. The theory behind this mechanism is that adding large amounts of low-nutrient water would reduce net nutrient loading and increase flushing rate in a lake, and consequently lower the steady state of nutrient concentration and the likelihood of algae biomass (Vollenweider, 1969). For several decades lake restoration has been a great concern due to the accelerated eutrophication (Qin, 2009). Numerous methods have been used in aquatic ecosystem restoration, such as reduction of nutrient load (Jeppesen et al., 2009), sediment dredging (Moss et al., 1996), wetland construction (Mitsch and Gosselink, 2007; Mitsch, 2009), and biomanipulation (Moss et al., 1996; Sierp et al., 2009).

∗ Corresponding author. Tel.: +86 2586882180; fax: +86 2557715759. E-mail addresses: [email protected] (L. Hu), [email protected] (W. Hu). 1 Current address: Nanjing Institute of Geography and Limnology, Chinese Academy of Sciences, 73 East Beijing Road, Nanjing 210008, Jiangsu, China. 0925-8574/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.ecoleng.2009.11.016

However, those methods usually show a delay in water quality improvement, especially in large eutrophic lakes. The advantages of water transfer are that they are low cost, easy to conduct, and show a quick response in nutrient deduction when suitable dilution water is available. Water transfer has been successfully used to improve water quality in many lakes, for example, Green Lake and Moses Lake in Washington (Oglesby, 1968; Welch et al., 1972; Welch and Patmont, 1980; Welch, 1981; Welch and Weiher, 1987; Welch et al., 1992), Lake Pontchartrain in Louisiana (Lane et al., 2001), Lake Valuwe in The Netherlands (Hosper and Meyer, 1986; Jagtman et al., 1992), and Lake Xihu in China (Yu, 1998). Since restoration by dilution requires a sufficient amount of suitable quality input water, most of the well documented research is on a small scale, either in lake size or in observation area. The surface areas of Green Lake, Moses Lake, and Lake Valuwe are less than 40 km2 . Although the surface size of Lake Pontchartrain is large, the assessment was localized in the Bonnet Carre spillway and the mouth of the Bonnet Carre in Lake Pontchartrain (Lane et al., 2001). The effectiveness of water transfer in large scale is unknown. Lake Taihu is the third largest freshwater lake in China, with a total water surface area of 2338 km2 , an average water depth of 1.9 m, and a volume of 44 × 108 m3 (Fig. 1). Bounded by Jiangsu, Zhejiang, Anhui Province, and Shanghai, its basin area of about

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Fig. 1. Site map of Lake Taihu, showing the distribution of seven subzones and the monitoring sites (stars). The bolded arrows indicate the direction of water transferred from the Yangtze River to Lake Taihu. The black lines along lakeshore are the major influents and effluences of the lake. Gray areas represent inlands.

36,900 km2 is considered as the most developed area in China (Hu et al., 2006; Hu et al., 2008). Possessing multi-functions, such as water supply, flood control, irrigation, water transport and recreation, Lake Taihu is greatly important to the regional development of economy and society. To date, there are 12 main water plants dispersed along east shore of Lake Taihu and these take approximately 1.19 billion m3 of water annually from the lake to support drinking water to the surrounding cities, such as Shanghai, Suzhou, and Wuxi. However, the rapid spread of water pollution and the speedup of lake eutrophication in Lake Taihu basin in recent year are becoming primarys factor that restrict sustainable development of the economy in that region (Wang et al., 2007; Shen et al., 2007). Significant efforts have been made to reduce external nutrients loading in Lake Taihu basin since January 1998 when the national ninth-five plan on water pollution prevention and control in Lake Taihu was approved. Although the trend of water quality deterioration was under control after the restriction of waste water discharge, no clear improvement in water quality was detected. In 2000, extensive blue-green algae bloom burst out. Since Lake Taihu has an access to the Yangtze River, where algae is absent and nutrient concentration is relatively low, transfer water from the Yangtze River into Lake Taihu to reduce severe algae bloom and improve water quality was proposed after the national tenth-five plan on water pollution prevention and control in Lake Taihu was approved in August 2001. Prior to the conduction of long-term water transfer in the lake, two experimental water transfers were carried out, in winter–spring 2002 and summer–autumn 2003 (Hu et al., 2008). This research investigated the impact of water transfer on water quality in a large scale. Lasting for 165 days, the two water transfers pumped 11.9 × 108 m3 water from the Yangtze River to Lake Taihu. A total of 24 sites spread in the lake were sampled from 2000 to 2003 once a month except more frequently during the transfer periods. In general, application of the water transfer technique to large lake requires long transfer period, in which the natural factors such as precipitation, temperature, runoff, and wind would change and consequently affect nutrient concentration. To assess the influence of water transfer objectively, the effect of natural factors should be eliminated. In this research, both observed data comparison and estimated effectiveness of water transfer (with the elimination of

the effect from natural factors) were used to assess the spatial and temporal variations of water quality improvement during the two experimental water transfers in Lake Taihu. 2. Material and methods 2.1. Study area Lake Taihu is a large shallow lake with most of its influent rivers distributed along west and northwest shore and effluent rivers located in the eastern and southern part of the lake. Water quality and the distribution of aquatic plants are uneven in different parts of the lake. According to its hydrological characteristics, aquatic plant distribution, water quality condition, and topography, Lake Taihu is divided into seven subzones, namely Gonghu Bay, Meiliang Bay, Northwest Zone, Southwest Zone, Centre Zone, East Epigeal Zone, and Dongtaihu Bay (Fig. 1). Special location of the influent and effluent rivers in Lake Taihu forms a normally north to south and west to east current in the lake, and results in much better water quality in the southeast of the lake than in the northwest (Table 1). Gonghu Bay is located in the northeast of Lake Taihu. In the northeast of the bay, the Wangyu River is the channel where the water was transferred from the Yangtze River to Lake Taihu. Except during water transfer, the Wangyu River is an effluent river of the lake. Water quality is relatively good in the bay after flowing a long way from northwest of Lake Taihu to the northeast. Meiliang Bay is located in the north of the lake, and together with Northwest Zone accounts for about 2/3 of the total runoff into Lake Taihu and these are considered the severely polluted zones in the lake. Phytoplankton is most abundant in the two subzones, whereas few aquatic macrophytes presented. Centre Zone is in the centre of Lake Taihu with highest wind-wave and no aquatic macrophytes observed. Southwest Zone is situated in the southwest of the lake. It accounts for about 1/3 of total runoff into the lake. Located in the east and southeast of the lake, East Epigeal Zone and Dongtaihu Bay are the areas with higher aquatic macrophytes coverage, higher water quality, and more water plant intake distribution (Gu et al., 2005; Liu et al., 2007; Jin et al., 2007). Dongtaihu Bay is also the area where the dilution water flows out off the lake.

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Table 1 Annual mean concentration of TN, TP, and CODMn in the seven subzones of Lake Taihu (unit: mg/L). Nutrients

Gonghu Bay

Meiliang Bay

Northwest Zone

Centre Zone

Southwest Zone

East Epigeal Zone

Dongtaihu Bay

TN TP CODMn

2.3 0.06 4.3

7.2 0.16 6.3

4 0.1 5.7

2.2 0.08 4.7

3.2 0.11 4.5

1.8 0.06 4.4

1.6 0.05 4.2

2.2. Water transfer measure Water was transferred from the Yangtze River to Lake Taihu on two occasions, in winter–spring 2002 and summer–autumn 2003. Two major rivers, the Wangyu River and the Taipu River, acted as inflow and outflow channels, respectively. Practically, the Wangyu River is an outflow river of Lake Taihu at all times other than the water transfer periods. A floodgate at the inlet of the Wangyu River was regulated to control the quality and quantity of the dilution water. Another floodgate at the Taipu River was set to control the rate of outflow water based on the water level in Lake Taihu. The water transfer in 2002 started on January 30th followed by 26 days of water storage in the Yangyu River and ended on April 4. The average input rate was 0.1066 × 108 m3 /d. The net input of water to Lake Taihu due to the transfer increased slowly before February 21st, and reached the maximum volume of 1.464 × 108 m3 on March 8. With the rising of pressure on flood prevention, the outflow via the Taipu River was increased after March 8. The net budget of water added to the lake therefore decreased gradually. The average input rate of dilution water was 0.1446 × 108 m3 /d between February 21 and March 8, while it was 0.0916 × 108 m3 /d for the rest of the days. The maximum water input rate of 0.2012 × 108 m3 /d was observed on March 2nd. By the end of the transfer, a total amount of 6.804 × 108 m3 of water from the Yangtze River was added into Lake Taihu through the Wangyu River, and 6.706 × 108 m3 of water flowed out the lake through the Taipu River (Fig. 2). The water transfer in 2003 started on August 6 and ended on November 17 with an average input rate of 0.1066 × 108 m3 /d. Higher water input rate appeared within the periods of August 6–18 and August 25–October 15 with the average water input rate of 0.1414 × 108 m3 /d. The highest water input rate of 0.243 × 108 m3 /d was observed on August 29. On August 21 and November 4–5, the transfer was stopped for the reasons of flood prevention and unsuitable water quality, respectively. The maximum net budget of water added to Lake Taihu was 4.859 ×108 m3 during this water transfer (Fig. 2). A total amount of 11.09 × 108 m3 of water was added into Lake Taihu through the Wangyu River, and 6.604 × 108 m3 of water flowed out the lake via the Taipu River during summer–autumn 2003 water transfer. 2.3. Water renewal time variation and contaminant exchange during the water transfers The regular water renewal time varies with location in Lake Taihu and changes from time to time (Table 2). In January–April 2002 without water transfer, the longest renewal time of 807 days was observed in Meiliang Bay after the closing of the Zhihugang floodgate. Due to the decrease of the rain runoff and water level during that period, the water renewal time in Northwest Zone and Dongtaihu Bay would be 526 and 214 days, respectively. In August–November 2003 without water transfer, the water renewal time in Southwest Zone was as long as 1115 days with the influences of both influent river runoff and wind-driven current. Relatively high water level and strong lake current would also shorten the renewal time in Gonghu Bay and Dongtaihu Bay at that time period. The 2002 water transfer greatly increased the flushing rate in all areas of the lake especially in Gonghu Bay and

Dongtaihu Bay, where the water renewal time decreased by about 4.9 and 10.9 times, respectively (Table 2). Widely divergent result, however, resulted from the 2003 transfer. Only three out of seven sub-zones in Lake Taihu showed increase in flushing rate. The water renewal time decreased only by about 11% (24 days), 25% (7 days), and 33% (79 days) in East Epigeal Zone, Dongtaihu Bay, and Centre Zone in August–November 2003, respectively (Table 2). Appearing out of the path of main inflow, the water renewal time in Northwest Zone did not change much during the two water transfers. The average concentrations of total nitrogen (TN), total phosphorus (TP), and permanganate index (CODMn ) in the transferred water were 2.3, 0.1, and 2.8 mg/L in winter–spring 2002 and 2.8, 0.13, and 3.4 mg/L in summer–autumn 2003, respectively (Table 3). The input rates of dilution water were similar in winter–spring 2002 and summer–autumn 2003 water transfers (0.1047 × 108 m3 /d and 0.1066 × 108 m3 /d). However, contaminant loading was lower by about 20% during 2002 transfer than that in 2003. The flow-in mass of TN and TP, and CODMn were 1645.1, 72.2, and 1936.5 ton during 2002 water transfer and 665.4, 31.9, and 3735.1 ton during 2003, respectively (Table 3). Although TN and TP had net increases in Lake Taihu by the end of the two water transfers, it should not result a negative impact on water quality in the lake, as these nutrient loading are account for only 3.7%, 1.2%, and 1.5% of the total input of TN, TP, and CODMn transferred from 23 transects along east shore of the lake in 1988 (Hu, 1992). 2.4. Water sampling and data analysis Water samples were collected once every month from January 2000 to December 2003 at 24 sites in Lake Taihu (Fig. 1) for determination of main environmental constituents, such as, TP, TN, and chlorophyll a (Chl-a). The analytical methods for determining TP, TN, and Chl-a were according to Jin and Tu (1990), including Potassium Persulphate Oxidation-UV Spectrophotometer Method for TP and TN, and Spectrophotometer Method for Chl-a. Unfiltered water samples were used in TP and TN analysis. Whatman glass microfibre filters (GF/C, pore size 1.2 ␮m) were used to filtrate water samples for analyzing Chl-a. 2.5. Method to estimating the impact of water quality resulted from water transfer Many environmental factors, especially water level, have great influence on water quality in Lake Taihu (Bai and Hu, 2006). The observed data on TN, TP, and Chl-a concentration in 2002 and 2003 were compared with the estimated data under un-transferable situation to assess the impact of water quality induced by the two transfers. Based on the historical data from 1993 to 2002, Bai and Hu (2006) established the specific relationships on TN, TP, and Chla concentration to water depth for each subzone in Lake Taihu, respectively. To eliminate the effect of natural factor (the changes in water depth) on water quality, those relationships were used to calculate the likely concentrations of TN, TP, and Chl-a under un-transfer situation in 2002 and 2003. The effectiveness of water transfer, in terms of changes on nutrient concentration, was then calculated by subtracting the likely concentrations from the observation concentrations. The following equation was used in the

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Fig. 2. The inflow and outflow rates during winter–spring 2002 and summer–autumn 2003 water transfers. Table 2 The changes of water renewal time due to 2002 and 2003 water transfer in Lake Taihu’s seven subzones based on their boundary net water exchanges (unit: day). Subzones

Gonghu Bay

Meiliang Bay

Northwest Zone

Centre Zone

Southwest Zone

East Epigeal Zone

2002 (January–April) without transfer 2002 (January–April) with transfer 2003 (August–November) without transfer 2003 (August–November) with transfer

275 47 82.2 103.5

807 356 215.4 326

79 70 61.5 66.7

117 81 237.6 158.3

526 131 1114.9 1310.9

157 133 222.5 198.4

estimation: NE = Nt − (N0 + (f (ht ) − f (h0 ))) where Nt and N0 represent observed nutrient concentrations in transferred year and benchmark year, f(ht ) and f(h0 ) are estimated nutrient concentration based on water depths in transferred year and benchmark year, and NE is the estimated changes of nutrient concentration. Water depth in the seven subzones of Lake Taihu in 2002 and 2003 were given in Table 4. A normal precipitation year, year 2000 was set as the benchmark year. 3. Results 3.1. In situ observation on water quality improvement in Lake Taihu Water quality improvements among the seven subzones of Lake Taihu were uneven in 2002 and 2003. Greater improvement was detected in Southwest Zone, Centre Zone, and East Epigeal Zone during the 2002 water transfer, whereas, the improvement was more pronounced in Meiliang Bay, Northwest Zone, and Southwest Zone during the 2003 transfer. Among TN, TP, and Chl-a, the improvements in TP and Chl-a concentration were more impressive. The Chl-a concentration in summer 2002 was especially low. The improvements of TN, TP, and Chl-a concentration are described in detail as follows: 3.2. TN concentration The decline on TN concentration during 2002 water transfer was more pronounced in Southwest Zone, East Epigeal Zone, and

Dongtaihu Bay 214 18 28.6 21.6

Dongtaihu Bay, representing a reduction of about 1.5 mg/L or 43%, 1.07 mg/L or 33%, and 0.63 mg/L or 55% in comparison to the mean 2000–2001 values, respectively. TN concentration then increased to the 2000–2001 level after the cease of the 2002 water transfer (column 1 in Fig. 3). In Gonghu Bay, Meiliang Bay, and Northwest Zone, however, TN concentrations in February and March 2002 were about 0.6, 1.8, and 3.3 mg/L above the pre-transfer values, respectively. TN concentration in Centre Zone remained the same level during the transfer as that of in 2000–2001. During 2003 water transfer, slight reduction of over all TN concentration exhibited in Meiliang Bay (0.12 mg/L), Northwest Zone (0.22 mg/L), and Southwest Zone (0.21 mg/L) (column 1 in Fig. 3). The TN concentrations in Gonghu Bay, Centre Zone, and East Epigeal Zone, however, were 0.4, 0.35, and 0.2 mg/L higher, respectively, compared to the mean 2000–2001 values. At the beginning of the water transfer in August 2003, TN concentration in Meiliang Bay, Centre Zone, and Gonghu Bay increased to about 1 mg/L above the pre-transfer values as the current derived from water transfer held back eastward or southward flow of the water and increased the vertical mixing of water body in the three areas. Subsequently, a marked drop of TN concentration in September was detected in all areas of Lake Taihu when the transfer lasted. 3.3. TP concentration During 2002 water transfer, TP concentration was reduced significantly in Southwest Zone, Centre Zone, East Epigeal Zone, and Dongtaihu Bay by about 0.06 mg/L or 62%, 0.04 mg/L or 46%, 0.02 mg/L or 31%, and 0.02 mg/L or 41%, respectively, compared to the mean 2000–2001 values (column 2 in Fig. 3). Whereas, TP con-

Table 3 The nutrient concentration (mg/L) in transferred water and the exchanges in contaminant mass (ton) due to the two water transfers in Lake Taihu. Year (duration)

Description

TN

TP

CODMn

2002 (65 days)

Concentration in transferred water Flow into Lake Taihu via the Wangyu River Discharge from the Taipu River Net contaminant retained in Lake Taihu

2.3 1645.1 592.0 1053.1

0.1 72.2 19.4 52.8

2.8 1936.5 2076.2 −139.7

2003 (100 days)

Concentration in transferred water Flow into Lake Taihu via the Wangyu River Discharge from the Taipu River Net contaminant retained in Lake Taihu

2.8 3104.7 665.4 2439.3

0.13 141.4 31.9 109.5

3.4 3765.3 3735.1 30.2

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Fig. 3. Mean concentration of total nitrogen (TN, column 1), total phosphorus (TP, column 2), and chlorophyll a (Chl-a, column 3) from water samples in Lake Taihu’s seven subzones.

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Table 4 Water depth used to estimate the improvements of water quality in the seven subzones of Lake Taihu following the water transfer in 2002 and 2003. Note: The net increase of water level during the transfer periods were subtracted from the observation. Date

Gonghu Bay

Meiliang Bay

Northwest Zone

Southwest Zone

Centre Zone

East Epigeal Zone

Dongtaihu Bay

J-02 F-02 M-02 A-02 M-02 J-02 J-02 A-02 S-02 O-02 N-02 D-02 J-03 F-03 M-03 A-03 M-03 J-03 J-03 A-03 S-03 O-03 N-03 D-03

1.73 1.69 1.79 1.83 1.99 1.91 1.99 1.97 1.94 1.84 1.82 1.89 1.83 1.75 1.88 1.82 1.75 1.66 1.83 1.83 1.77 1.75 1.69 1.65

1.98 1.93 2.05 2.09 2.28 2.19 2.28 2.26 2.22 2.11 2.08 2.17 2.10 2.01 2.15 2.09 2.01 1.90 2.10 2.10 2.03 2.00 1.93 1.89

2.00 1.95 2.07 2.12 2.30 2.21 2.31 2.28 2.24 2.13 2.10 2.19 2.12 2.03 2.17 2.11 2.03 1.92 2.12 2.12 2.05 2.02 1.95 1.91

1.93 1.88 1.99 2.04 2.22 2.13 2.23 2.20 2.17 2.06 2.03 2.11 2.05 1.96 2.09 2.03 1.96 1.85 2.05 2.05 1.98 1.95 1.88 1.84

2.32 2.26 2.40 2.46 2.67 2.57 2.68 2.65 2.61 2.47 2.44 2.54 2.46 2.35 2.52 2.45 2.36 2.23 2.46 2.46 2.38 2.35 2.27 2.22

1.58 1.54 1.63 1.67 1.82 1.74 1.82 1.80 1.77 1.68 1.66 1.73 1.67 1.60 1.71 1.66 1.60 1.52 1.67 1.67 1.62 1.59 1.54 1.51

1.12 1.09 1.15 1.18 1.28 1.23 1.29 1.27 1.25 1.19 1.17 1.22 1.18 1.13 1.21 1.18 1.13 1.07 1.18 1.18 1.14 1.13 1.09 1.07

centration in Meiliang Bay and Northwest Zone showed a rapid increase in March 2002, representing about 0.05 mg/L above the pre-transfer values. The over all reduction in TP concentration during the 2003 water transfer period was detected in Meiliang Bay and Southwest Zone by about 0.014 mg/L or 10% and 0.019 mg/L or 29%, respectively (column 2 in Fig. 3). Similar to the change in TN concentration, TP concentration in Meiliang Bay, Centre Zone, and Gonghu Bay increased to about 0.036, 0.041, and 0.027 mg/L above the pre-transfer values at the beginning of the transfer in August, respectively. However, clearly decreases of TP concentration were found in the three areas as the day of water transfer increased.

3.4. Chl-a concentration In most areas of Lake Taihu, Chl-a concentration was below the mean 2000–2001 value in the first month of the 2002 water transfer, but then increased to above in March (column 3 in Fig. 3). This increase lasted till April in Meiliang Bay and Northwest Zone. The Chl-a concentration in the entire lake was reduced to below the pretransfer level during summer 2002. Significant decline was found in Meiliang Bay and Northwest Zone with an average reduction of 40 mg/m3 or 75% and 38 mg/m3 or 60% in June–September 2002, respectively. Although the TN/TP in Lake Taihu was much higher than the critical ratio of 14, which indicates a P-limited growth of algae (Koerselman and Meuleman, 1996), the decrease of Chl-a concentration in June–September 2002 was not consistent to the decline of TP concentration. A clear decline in Chl-a concentration was observed only in Dongtaihu Bay during the 2003 water transfer (column 3 in Fig. 3). Contrarily, Chl-a concentration increased notably at the beginning of the 2003 water transfer in August in most of the subzones except in Northwest Zone where a sharp increase to 115 mg/m3 above the mean 2000–2001 value was observed in September. As the duration of the water transfer increased, Chl-a concentration in Gonghu Bay, Meiliang Bay, Southwest Zone, and Centre Zone declined to the pre-transfer level.

3.5. The improvement of water quality in Lake Taihu with the elimination of effect from natural factors The estimated changes of TN, TP, and Chl-a concentration in Lake Taihu with the elimination of effect from natural factors in 2002 and 2003 are shown in Figs. 4 and 5, respectively. Clear positive influence on deduction of TP concentration and relatively weak improvement in TN and Chl-a concentration were revealed in the lake. Spatial variation was observed on all of the three environmental characteristics. The improvement of TN and TP concentration during the 2002 transfer was more pronounced in Southwest Zone, East Epigeal Zone, and Dongtaihu Bay by about 1.7 mg/L (64%) and 0.05 mg/L (46%), 0.73 mg/L (50%) and 0.02 mg/L (24%), and 0.93 mg/L (69%) and 0.03 mg/L (57%) in average, respectively (column a and b in Fig. 4). A clear decline in Chl-a concentration in February was detected in most areas of the lake except in Southwest Zone and Centre Zone. Moreover, significant drop of the Chl-a concentration was found during summer 2002 especially in Gonghu Bay, Meiliang Bay, Nothwest Zone, Southwest Zone, and East Epigeal Zone (column c in Fig. 4). During the 2003 water transfer, positive influence on deduction of TN and TP concentration was found in many subzones of Lake Taihu, especially in Gonghu Bay, Meiliang Bay, and Southwest Zone, where the decline of TN and TP concentration was 0.37 mg/L (14%) and 0.02 mg/L (23%), 0.41 mg/L (8%) and 0.03 mg/L (21%), and 0.51 mg/L (31%) and 0.03 mg/L (48%), respectively (columns a and b in Fig. 5). The decline of Chl-a concentration resulted from 2003 transfer were more pronounced in Gonghu Bay and Dongtaihu Bay by about 11.9 mg/m3 or 55% and 3.2 mg/m3 or 41% during the transfer period. Although Chl-a concentration in September increased dramatically in Meiliang Bay, Southwest Zone, and Northwest Zone, it decreased to the mean 2000–2001 level in October and November (column c in Fig. 5). The comparison of the estimated and observed changes in TN, TP, and Chl-a concentration showed similar trend in 2002 and 2003 (Figs. 4 and 5). Generally, the estimated improvements in water quality were greater in comparison to that of from the observation in most subzones during the two transfer periods, suggesting a greater effectiveness of water transfer on water quality than it appeared. Although the results were as the same magnitude,

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Fig. 4. Comparison of the estimated (diamond) and observed (triangle) changes in TN, TP, and Chl-a concentration in Lake Taihu’s seven subzones in 2002. The estimated values were calculated with correction for the natural factors, which was done with an empirical relationship between water depth and TP, TN, and Chl-a concentration.

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Fig. 5. Comparison of the estimated (diamond) and observed (triangle) changes in TN, TP, and Chl-a concentration in Lake Taihu’s seven subzones in 2003. The estimated values were calculated with correction for the natural factors, which was done with an empirical relationship between water depth and TP, TN, and Chl-a concentration.

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variation over many subzones of Lake Taihu was notable. Gonghu Bay and Southwest Zone are considered as the area with larger difference between estimated and observed value during 2003 transfer. In contrast to the appearance of increasing TN, TP, and Chl-a concentration in Gonghu Bay, the estimated values showed a clear decline on the concentrations in summer–fall 2003. The high observed nutrient concentration was mainly contributed by low water volume in 2003. The differences on Chl-a concentration were distinct than that of on TN and TP concentration in most areas of Lake Taihu. Huge difference presented in Centre Zone with an average of 53.5 mg/m3 more decrease on Chl-a concentration by estimation than by observation in 2002. The results from the comparison of estimated and observed improvement on water quality indicates that changes in natural factors could make notable influence on water quality in some subzones of Lake Taihu, thus, should be concerned when consider an area of especially interested. Furthermore, the effect of natural factors should not be ignored in drought or flood year when the water level is particularly low or high. 4. Discussion 4.1. Methods to impact assessment By far, the method to assess the impact of water transfer is mainly focused on the direct comparison of the water quality characteristics before and after the treatment, without consideration of the influence from natural factors. Since relatively intuitionistic and easy to understand, direct comparison has been widely applied in the assessment of water quality improvement. This method is considered to be effective especially for relatively small lakes with little interruption from surrounding environment and significant improvement due to water transfer. For example, Oglesby (1969) observed a dramatic improvement in water quality in Green Lake, Seattle, following the dilution by city’s portable water supply, with great reduction in the concentration of TP (70%) and Chl-a (90%), and summer water clarity increased nearly four-fold. Welch and Weiher (1978) and Welch et al. (1992) reported a marked improvement in water quality associated with the decrease of over 70% on TP and Chl-a concentration in Moses Lake, Washington, after dilution with Columbia River water and diversion of sewage effluent. As to a large lake, however, the changes of natural factors such as temperature, solar radiation, precipitation, wind, waste water discharge, and water level during the water transfer period would induce fluctuations on the concentration of TN, TP, Chl-a, etc., thus, affect the effectiveness of water transfer (Hu et al., 1998a,b, 2008; Cioffi and Gallerano, 2000; Bai and Hu, 2006). The direct comparison by itself may not be sufficient to give a proper result on water transfer effectiveness, rather an over all view under the combination of the effects from all environmental factors. Bai and Hu (2006) found that natural factors especially water level had strong influence on water quality in Lake Taihu. As a result, the effect of changes in water level should to be eliminated from the observation when assess the impact driven from water transfer. Although the estimated values showed a similar trend in comparison to the observed data, undervaluation in the improvement of TN and TP concentration was found in most of the subzones in Lake Taihu. Thus, the improvement of water quality derived from water transfer should be greater than it appeared from the observation. Indirect simulation by means of ecological model has been also applied to assess the effects of water transfer. Ecological models have an advantage to recur the variation of the key environmental parameters and have been applied to forecast and assess the effects of restoration of lakes (e.g., Cioffi and Gallerano, 2000; Ruley and

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Rusch, 2004; Hu et al., 2008; Schauser and Chorus, 2009). However, the improper of parameter setting up could be the drawback to this method. Using Eco-Taihu model, and Chorus, 2009). However, the improper of parameter setting up could be the drawback to this method. Using Eco-Taihu model, Hu et al. (2008) estimated the percentage of area that water quality could be improved due to the 2002 and 2003 water transfer in Lake Taihu. Although the model result indicates a similar conclusion of partial improvement of water quality with our assessment, the areas with better improvement were different. Contrary to the result from this paper, Hu et al. (2008) suggests more positive effect on water quality in Meiliang Bay, Gonghu Bay, and Northwest Zone during winter–spring 2002 water transfers. 4.2. The effectiveness of water transfer on water quality in large lake Results from this research indicate a less impressive improvement of water quality from water transfer in large lakes than in small ones. Previous research reported significant improvement of water quality in small lakes after adding low-nutrient water (e.g., Oglesby, 1968; Welch et al., 1972). In Lake Taihu, however, not all of the seven subzones experienced notable deduction of TN, TP, and Chl-a concentration during the experimental water transfers. Moreover, clear increases in TN concentration were observed in some areas of the lake. The effectiveness of water transfer in large lake is generally limited by its large size, complex boundary, and the difficulty in finding the proper water source to be transferred. With large size and complex boundary, the path of the transferred water in Lake Taihu was impossible to cover all of the sub zones evenly. The observed water renewal time (Table 2) implies that Northwest Zone was more likely out of the main path of the inflow, therefore, less impact was revealed. Results suggest that higher increasing rates of water renewal time are generally associated with greater water quality improvement (Fig. 6). Nevertheless, no relationship was found between increasing rate of water renewal time and percentage of TN, TP, and Chl-a improvement during the two water transfers in Lake Taihu while the increasing rates of water renewal time were less than 200% (Fig. 6). The effectiveness of water transfer in large lakes is also limited by the source of the water being transferred. Applying water transfer method to a large lake requires large amount of low-nutrient water, which is normally difficult to find. Since the TN and TP concentration in Columbia River water was 1/7 and 1/18 of that in Moses Lake, respectively, marked improvement in water quality resulted from 1977 and 1978 dilution was detected in most area of the lake (Welch and Patmont, 1980; Welch and Weiher, 1987; Welch et al., 1992). During the two water transfers in Lake Taihu,

Fig. 6. The relationship between the increasing rate of water renewal time and the percentage of TN, TP, and Chl-a improvement during the two water transfers in winter–spring 2002 (black diamond) and summer–fall 2003 (gray triangle) in Lake Taihu.

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however, the mean concentrations of TN and TP in the transferred water were only slightly lower than that of in the lake as a whole, and were even higher than that of in Gonghu Bay. Without any buffering zone to allow self purification, input water with high nutrient concentration flowed directly into the bay, consequently, increased the nutrient level in the bay. Moreover, relatively higher TN and TP concentration in the transferred water compared to that of outflow water resulted in a net increase of TN and TP loading in Lake Taihu. The improvement in water quality, therefore, was not as notable in Lake Taihu as in Moses Lake. Hu et al. (2008) also reported the similar statement that in Lake Taihu the percentage of area with water quality being improved would decrease even to zero if the nutrient concentration in transferred water was higher than in the lake. Since the physical condition in the Yangtze River is not suitable to the growth of algae, Chl-a concentration was assumed to be zero in the transferred water. Consequently, the decline of Chl-a concentration was detected in most of the areas in Lake Taihu at the beginning of each transfer. Steady decrease of Chl-a concentration also appeared after the 2002 transfer has ceased in May 2002. However, as wind field is a driving force to the spatial accumulation of algae and the formation of water bloom in Lake Taihu (Hu et al., 1998b, 2008), wind field could contribute to the changes of Chl-a concentration in different zone in Lake Taihu as well. Southwestward wind in August 2003 might contribute to the high Chl-a concentration in the north of the lake. The great increases of Chl-a concentration in East Epigeal Zone in October and November 2003 were probably caused by the north and northwest wind in the 2 months. The quantity of water being transferred is considered another key factor which can affect the effectiveness of the transfer. Welch and Patmont (1980) suggest that 50% of a lake water input might result a sufficient improvement in water quality of the lake. During the two water transfers, water about 15% and 25% of the whole lake volume were added to Lake Taihu, respectively. Although this is only 3/10 and 1/2 of the critical volume, respectively, clear decrease of nutrient concentration present in many subzones in the lake. The improvement in TN and TP concentration during the two transfer periods was about 40% in the south and east of the lake in January–April 2002 and about 20% in north and southwest of the lake in August–November 2003. The locations of input water entrance and exit in Lake Taihu influence the effectiveness of water transfer by way of affecting hydrodynamic process and flushing rate in different subzone of the lake. The entrance of the inflow located in Gonghu Bay has made notable effect on water quality in the bay. As the Wangyu River in the bay is an outflow river of Lake Taihu except at water transfer periods, the water transfer caused a westward or southwestward water flow upset to its original current in Gonghu Bay. Water column mixing driven by this anti-current might be another possible reason to the nutrient concentration increasing in the bay. Although the two water transfers in Lake Taihu brought clear water quality improvement in many subzones of the lake, the improvement was insignificant when considering the lake as a whole. Ecosystems have ability of self-regulating from which to resist changes both with increasing and with decreasing nutrient loading in the ecosystems and this ability is even greater for a complex system (Odum, 1969). Large lakes are usually associated with more complex and stable ecosystem. It consequently weakens the effectiveness of water transfer on water quality improvement and slows down the lake restoration process. Unlike impressive trophic state change from hypereutrophic to slightly eutrophic in Moses Lake 10 years after dilution and diversion, restoration of Lake Taihu in recent year by only transferring water intermittently from the Yangtze River to the lake is insufficient.

4.3. Other potential impact on Lake Taihu ecosystem In addition to the improvement of water quality in some areas of Lake Taihu, the water transfers provided the flexibility in the reallocation of water resource between the Yangtze River and Lake Taihu. The water level of the lake could be controlled by changing the inflow and outflow rate. The increasing of outflow rate in March 2002 reduced the flood pressure in Lake Taihu. Whereas, in drought year 2003 the water transfer provide about 4.5 × 108 m3 of additional water to the lake and made a 19.19 cm increase in water level. This not only avoided possible water quality deterioration due to drought, but reinforced the water supply to Shanghai, Suzhou, Wuxi, etc. in 2003 as well. The water transfers in Lake Taihu did not eliminate the source of nutrient from the Yangtze River. Since the nutrient concentration in the transferred water was higher than that of in the Taipu River (the primary outflow river), water transfer caused a net increase of nutrient in the lake system. Thus, the decrease of Chl-a concentration would only be a short-term reaction if the nutrient concentration in the transferred water were not reduced to a reasonable level. 5. Conclusions Using both observed data comparison and estimated effectiveness of water transfer with the elimination of effect from natural factors, the paper assessed the impact of winter–spring 2002 and summer–fall 2003 water transfers on water quality in Lake Taihu. Results from the two assessments showed a clear improvement on water quality in some of the subzones in the lake during the water transfer periods, which suggest that water transfer can be used as one of the techniques to improve water quality in Lake Taihu. However, careful consideration on the quality of transferred water and the location of its entrance are needed while apply this technique. As Lake Taihu has great self-regulating ability, only transferring water intermittently from the Yangtze River is insufficient to restore the lake in recent years. The areas of water quality being improved due to the two water transfers were distributed unevenly in Lake Taihu. The 2002 transfer brought more positive effect in Southwest Zone, East Epigeal Zone, and Dongtaihu Bay, while 2003 transfer brought more positive effect in Gonghu Bay, Meiliang Bay, and Southwest Zone. Among TN, TP, and Chl-a, the decline of TP concentration was more pronounced during the two water transfers. The main negative impact of water transfer was the increase of TN concentration in the north of the lake during 2002 transfer. Besides water transfer, natural factors especially water level could influence water quality in Lake Taihu. Comparing with the estimated effectiveness of water transfer, the observed data showed a similar trend but an undervaluation in the improvement of TN and TP concentration. Thus, the improvement of water quality derived from water transfer might be greater than it appeared from the observation. Since the water transfers brought a net increase of TN and TP, it would increase the risk of water quality deterioration in Lake Taihu if the concentration of TN and TP in the transferred water was not cut down to a reasonable level. Acknowledgements This research was supported by the knowledge innovation major projects of the Ministry of Water Resource (MWR), Peoples Republic of China (SCXC2002-09) and the research project “A demonstrative research on lacustrine ecosystem structurally dynamic model in Lake Taihu” (NSFC 30670351) and also Basic

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