Distribution of phytoplankton in the Three-Gorge Reservoir during rainy and dry seasons

Science of the Total Environment 367 (2006) 999 – 1009 www.elsevier.com/locate/scitotenv

Distribution of phytoplankton in the Three-Gorge Reservoir during rainy and dry seasons Hui Zeng a,b , Lirong Song a,⁎, Zhigang Yu b , Hongtao Chen c a

State Key Laboratory of Freshwater Ecology and Biotechnology, Institute of Hydrobiology, The Chinese Academy of Sciences, Wuhan, Hubei 430072, PR China b Graduate School of Chinese Academy of Science, Beijing 100049, PR China c Ocean University of China, Qingdao 266003, PR China Received 21 October 2005; received in revised form 1 March 2006; accepted 2 March 2006 Available online 19 April 2006

Abstract After damming of the Yangtze River, in order to explore the impacts of the Three-Gorge Dam (TGD) on the aquatic ecosystem, phytoplankton composition, abundance and biomass spatial distribution were studied in the Three-Gorge Reservoir (TGR), and the closest upstream anabranch Xiangxi River, which is 38 km away from the Three-Gorge Dam (TGD) during August (rainy season) 2004 and April (dry season) 2005. In surveys, 6 transects (2 downstream and 4 cross-stream) and 25 stations have been investigated and 314 samples were collected from the surface to the river bed with water samplers. In TGR, 63 taxa and 60 taxa were identified in the rainy and dry seasons, respectively. In the Xiangxi River, 39 taxa were observed in the rainy and dry seasons. Algal blooms occurred in the Xiangxi River and at the influx region of the Yangtze and Xiangxi in both seasons, but had not occurred prior to damming. In the rainy season, the dominant species was Chroomonas acuta with 1.84 × 107 cells l− 1, and in the dry season the dominant species were Asterionella formosa and Cryptomonas ovata with 1.34 × 107 cells l− 1 and 1.79 × 106 cells·l− 1, respectively. In the main channel of TGR, there were no significant correlations between phytoplankton abundance and the concentrations of the main soluble nutrients. In the Xiangxi River, significant negative correlations were observed between phytoplankton abundance and nitrate (Spearman, p < 0.01, n = 21), phosphate (Spearman, p < 0.05, n = 21) and silicate (Spearman, p < 0.01, n = 21) in the rainy season, and similar correlations were also observed with nitrate (Spearman, p < 0.05, n = 28) and silicate (Spearman, p < 0.01, n = 28), but not with phosphate in the dry season. Since the damming of the Yangtze River, eutrophication in the anabranch within the backwater has occurred and become severe, and the frequency of algal bloom within TGR and anabranches is expected to increase. © 2006 Elsevier B.V. All rights reserved. Keywords: Phytoplankton; Distribution; Three-Gorge Reservoir; Rainy and dry season; Eutrophication

1. Introduction Large dams have been built in more than 150 countries and most of the 45,000 dams are in the ⁎ Corresponding author. Tel.: +86 27 68680806; fax: +86 27 68780806, +86 27 68780123. E-mail address: [email protected] (L. Song). 0048-9697/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.scitotenv.2006.03.001

developing countries. The Three-Gorge Dam (TGD), in China, is the world's largest dam, measuring 2335 m long and 185 m high, and the reservoir created by it will have an area of 1080 km2 in 2009 (Wu et al., 2003). The construction of the Three-Gorge Project (TGP) can be divided into three stages: preparation and the first stage (1993–1997), the second stage (1998–2003) and the third stage (2004–2009).

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As the largest water conservancy in the world, TGP will have a significant impact on biodiversity and ecosystems in the region. Some large-scale dams can even influence the bioactive elements of the geochemical cycle at the global scale (Humborg et al., 1997; Jickells, 1998; Friedl and Wüest, 2002; Ragueneau et al., 2002). Phytoplankton is sensitive aquatic organisms, and their composition and abundance can reflect the eutrophic situation in a short time (Büsing, 1998; DíazPardo et al., 1998). Thus, phytoplankton is often used to estimate the impact of large-scale dams on the aquatic ecosystem (Humborg et al., 1997; Lancelot et al., 2002). Although the Yangtze River is the third longest river in the world, there were very few studies on phytoplankton in the Yangtze River before damming by the TGD (Boruzkij et al., 1959; Wang and Liang, 1991). This present paper aims to study: (1) the phytoplankton composition and abundance in TGR and the trend of eutrophication in this region after damming; (2) the responses of the phytoplankton to the changes of the hydrological conditions within TGR in the rainy and dry seasons. 2. Methods 2.1. Area description The Changjiang (Yangtze) River is 6300 km long, has a catchment area of 1.96 × 106 km2 (Chen and Yu, 2001) and an annual water discharge of 9.8 × 1011 m3 (Beardsley et al., 1985). The Three-Gorge Reservoir (TGR), sited at 29°16′–31°25′N, 106°–111°10′E, has a catchment of the typical gorge type and its normal water

level is currently maintained at 135 m. Mountains around the reservoir are made of limekiln, granite and shale. Hydrologic conditions of the Yangtze River have changed markedly since impounding in July 2003. The water depth in TGR has increased 65 m. Sands concentration in the dam has dropped from 0.578 kg·m− 3 to 0.155 kg·m− 3; average flow velocity in the main channel has dropped from 0.85 m·s− 1 to 0.20 m·s− 1. The retention time of the water within TGR has been prolonged by 77 days (Du et al., 2004). The Xiangxi River, which lies 38 km upstream from the Dam, is 94 km long, has a watershed of 3099 km2, with an annual discharge of 1.96 × 109 m3, and a natural fall of 1540 m. Since damming the water level in the Xiangxi River has increased 40 m and the water flow velocity has dropped from the original 0.43–0.92 m·s− 1 (Tang et al., 2004) to 0.0020–0.0041 m·s− 1 (Wang, 2005). 2.2. Sampling Water Samples were collected along six transects (A– F) and at two stations: transects A and B were lengthways transects along the Xiangxi River and the Yangtze River, respectively, and transects C–F were transverse transects across the Yangtze. Stations Z1 and Y2 were additional stations in the Xiangxi River (Fig. 1). Stations Z1, Y2, A1and A2 are on the Xiangxi River. Station A3 was located at the influx of the Xaingxi River and the Yangtze River. Station A4 is on the Yangtze River. Samplings were performed during August 2004 (rainy season) and April 2005 (dry season). Water and

Xiangxi River Z1

31°N

Gui Zhou

A1 Y2 A2 A3

B0 A4

B1 B2 B3

River

Yangtze B4 B5

30°50´N

C3 D3 C2 C1 D2 D1

Tai Pingxi E3 E2 Three-Gorges F3 E1 F2 Dam F1 B6

Zi Gui

110°50´E

San Douping

111°E

Fig. 1. Location of sampling stations during August 2004 and April 2005.

H. Zeng et al. / Science of the Total Environment 367 (2006) 999–1009 Table 1 List of algal taxa and their representative used for comparisons among the regions Taxa

Description

Representative (genera)

C-Diatom M-Diatom R-Diatom

Valves discoid Cylindrical filaments Ribbon-forming pinnate diatom Elongate valves, swollen poles

Cyclotella Melosira Fragilaria, Synedra

A-Diatom Chlorophyta Chrysophyta Cryptophyta Cyanophyta Euglenophyta Pyrrophyta

Asterionella formosa (species) Chlamydomonas, Pandorina Mallomonas Cryptomonas, Chroomonas Dactylococcopsis, Microcystis Euglena Peridinium, Ceratium

phytoplankton samples were collected by using 5l Niskin bottles on a steel wire at 0, 5, 10, 20 and 50 m depth and at the bottom (river bed). Phytoplankton samples for qualitative identification were collected at the water surface with 25# phytoplankton net; phytoplankton samples for quantitative analysis were collected from the surface to bottom (river bed) with 1-l sampler and 5-l Niskin bottles. Transparency was measured with a Secchi disk. 2.3. Qualitative and quantitative investigation of phytoplankton

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mixed. Phytoplankton was counted in a Fuchs-Rosental slide. Volume of each taxon was calculated by measuring morphometric characteristics (diameter or length and width) (Montagnes et al., 1994) and converted to carbon biomass (Strathmann, 1967), log10C = 0.758log10V − 0.422 for diatom and log10C = 0.866log10V − 0.460 for other algae. (C is cell carbon in picograms and V is cell volume in μm3.) Phytoplankton was grouped into taxonomic categories (Lovejoy et al., 2002) and diatoms were further categorized by the morphological criteria outlined in Table 1. The dominant genera Cyclotella, Melosira and species Asterionella formosa were listed individually. 2.4. Chemical analysis After collection, water samples were filtered immediately through pre-cleaned, 0.45-μm pore-size, acetate cellulose filters, presoaked in diluted hydrochloric acid (pH < 2) overnight and then rinsed with Milli-Q water. The filtrates for nitrate, nitrite, ammonia and phosphate determination were kept frozen (− 20 °C), whereas those for silicate were kept cool (4 °C) in the dark until analysis. Nutrients were determined by colorimetric methods, using a Bran+Luebbe Autoanalyser AAIII. Statistical analysis was carried out using the SPSS 11.5 package. 3. Results

Phytoplankton samples were preserved immediately with 1% Lugol solution. A sedimentation method was used for taxon identification and counting (Zhang and Huang, 1991; Eker et al., 1999). Phytoplankton was counted with a standard light microscope (OLYMPUS C41). Before counting, the whole sample was gently

3.1. Species richness and surface abundance of phytoplankton In the mainstream of TGR in August 2004 (rainy season), 46 genera (63 taxa) were recorded, belonging

Table 2 Summary of taxa and surface algal density Region season taxa

Xiangxi River Rainy

Dry

Rainy

Dry

Diatom Chlorophyta Chrysophyta Cryptophyta Cyanophyta Euglenophyta Pyrrophyta Total taxa Total genera Cell density

14 20 0 2 2 0 1 39 27 1.06 × 107–1.87 × 107

22 13 1 1 1 0 1 39 26 1.22 × 105–1.67 × 107

29 22 0 2 6 1 3 63 46 4.69 × 103–2.25 × 104

32 22 0 1 2 1 2 60 41 2.81 × 104–1.64 × 105

Note: density unit cells·l− 1.

Reservoir (TGR)

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2005 (dry season), 26 genera (39 taxa) were identified, belonging to 16 families. In additional to the five major phytoplankton groups recorded in August, chrysophyta were also reported in April (Table 2). Diatoms accounted for 56% of the total taxa. Numbers of cyanophyta taxa were 2 in the rainy season and 1 in the dry season. In August 2004 (rainy season), an algal bloom (density > 106 cells·l− 1) occurred in the Xiangxi River and the influx region of the Yangtze and Xiangxi. The dominant species was Chroomonas acuta with the maximum density appeared at station A1, and high values were also recorded at stations A2 and A3. At station A3, the total abundance of chlorophyta reached 1.12 × 106 cells·l− 1 (Fig. 2).

to 25 families. In April 2005 (dry season), the number was 41 genera (60), belonging to 28 families (Table 2). Taxa composed diatom, chlorophyta, cyanophyta, cryptophyta, euglenophyta and pyrrophyta; chrysophyta were not recorded. Diatoms were common in TGR. In the rainy and dry seasons, diatoms accounted for 46% and 53% of the total taxa, respectively. Numbers of cyanophyta taxa were increased greatly in the rainy season (six species) compare with the dry season (two species). In the Xiangxi River in August 2004 (rainy season), 27 genera (39 taxa) were recorded, belonging to 18 families (Table 2). Taxa composed diatom, chlorophyta, cryptophyta, cyanophyta and pyrrophyta; chlorophyta accounted for the most taxa (51%). In April August 2004

April 2005

20000000 18000000 16000000 14000000

18000000 16000000 14000000 12000000 10000000

Abundance (unit: cell • l-1 )

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R–diatom Pyrrophyta Euglenophyta

Fig. 2. Phytoplankton surface distribution in transect A, B, C and at stations Z1 and Y2 during August 2004 (rainy season) and April 2005 (dry season).

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Entering the Yangtze River, at station A4, the algal abundance dropped dramatically (Fig. 2), and the principle species was replaced by diatoms, which are more adapted to a fluvial environment. Along transect B, the downstream transect along the mainstream of the Yangtze River, phytoplankton abundance was low, and the dominant species were Chlamydomonas reinhardtii, Cyclotella meneghiniana and Melosira granulata. Further downstream, in transects C, D, E and F, the dominant species were C. reinhardtii, C. acuta, C. meneghiniana and some ribbon-forming pennate diatoms (Fig. 3). August 2004

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In April 2005 (dry season), an algal bloom also occurred in the Xiangxi River. The bloom-forming algae were A. formosa and Cryptomonas ovata. The maximum cell density was recorded at station A1; blooms were also recorded at stations Z1 and Y2 (Fig. 2). At station A2, the density of A. formosa dropped and C. ovata disappeared; at station A3, A. formosa disappeared. Algal density at stations A2 and A3 decreased dramatically compared with stations Z1, Y2 and A1 (Fig. 2). The algae density in the TGR on the Yangtze was lower much than that in the Xiangxi River. Algal 80000

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Euglenophyta

Fig. 3. Phytoplankton surface distribution in transect D, E, F during August 2004 (rainy season) and April 2005 (dry season).

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Fig. 5. Phytoplankton biomass vertical distributions at stations during August 2004 (D3–F3) and April 2005 (Z1–B3).

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abundance in transects B, C, D, E and F was quite similar, and the common species were A. formosa, C. meneghiniana, Melosira varians and Peridinium bipes (Figs. 2 and 3). 3.2. Vertical distribution of phytoplankton biomass (biomass carbon) In August 2004 (rainy season), in the Xiangxi River, at station Z1, the surface biomass reached 662.10 mg C m− 3 and significantly decreased below 5 m (p < 0.01) (Fig. 4). From stations A1 to station A3, the biomass at the surface was high, especially at station A3, reaching 1288.30 mg C m − 3 , and biomass at 5 m was significantly lower than the surface layer (p < 0.01) (Fig. 4). At station A4, which located on the Yangtze River, diatom became the principle contributor and algal biomass at the surface dropped significantly (p < 0.01). Along entire transect B, biomass was significantly lower than in transect A (p < 0.01). In transects C, D, E and F, biomass remained even (p > 0.05). At some stations, the biomass at the surface layer was most due to chlorophyta and pyrrophyta. With the increasing depth, diatom was the major contributor (Fig. 4). In April 2005 (dry season), in the Xiangxi River, at station A1, biomass at the surface layer reached 1404.19 mg C m− 3. The biomass at stations Z1 and Y2 (additional station) were lower than at A1. At stations A2 and A3, biomass decreased significantly (p < 0.05) (Fig. 5). At stations Y2 and A1 biomass under 5 m were lower than the surface. But at stations Z1, A2 and A3, the biomass at the bottom depth was higher than that at the surface, due to the higher biomass of diatom (Fig. 5). Entering the Yangtze River, at station A4, algal biomass was only 9.35 mg C m− 3 (Fig. 5). Along transect B (Figs. 5 and 6), biomass at the surface layer was low and the vertical distribution of biomass kept

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comparatively even (p > 0.05). Along transect C, the vertical distribution of biomass varied remarkably (p < 0.05) (Fig. 6). Along transects D, E and F, biomass did not changed much in vertical direction (Fig. 6). Cryptophyta, chlorophyta and pyrrophyta were usually distributed in the upper 0–10 m layer. With the depth increasing, diatom was the main component of the biomass. 3.3. Chemical variation The soluble nutrient concentrations were significantly different in the Xiangxi River and the Yangtze River in the same season (p < 0.05) and within same river there were significant differences between seasons (p < 0.01) (Table 3). Correlation analysis between the algal abundance and the main soluble nutrient concentrations (unpublished) in the Xiaingxi River indicated that in the rainy season the algal abundance had a significant negative correlation with nitrate (Spearman r = − 0.571, n = 21, p < 0.01), phosphate (Spearman r = − 0.47, n = 21, p < 0.05) and silicate (Spearman r = − 0.681, n = 21, p < 0.01); in the dry season, algal abundance had a significant negative correlation with nitrate (Spearman r = − 0.401, n = 28, p < 0.05) and silicate (Spearman r = − 0.857, n = 28, p < 0.01), but not with phosphate. In contrast, for the TGR in both seasons, there was no significant correlation between algal abundance and the main soluble nutrients. 4. Discussions Three-Gorge Project (TGP), the largest water conservancy project ever built, has greatly changed the original Yangtze River ecosystem (Wu et al., 2003), and the impacts of TGD on the environment and water ecosystem have become focuses of world attention. In the present study, we have investigated the dynamics of phytoplankton community structure and abundance in

Table 3 Physicochemical data for the Xiangxi River and the Three Gorges Reservoir Xiangxi River

Temperature (°C) Transparency (m) NO3-N NO−2 -N NH+4-N PO3− 4 -P SiO3-Si

Reservoir (TGR)

Rainy

Dry

Rainy

Dry

27.0–27.2 0.70–2.60 25.54 ± 4.618 3.29 ± 0.799 0.61 ± 0.099 0.17 ± 0.018 114.51 ± 2.466

22.2–22.4 1.40–4.40 75.35 ± 32.76 4.23 ± 2.086 1.66 ± 1.064 0.64 ± 0.240 49.42 ± 33.48

26.8–27.0 0.15–0.30 51.51 ± 20.635 1.97 ± 1.210 0.10 ± 0.044 0.48 ± 0.196 125.11 ± 1.479

22.0–22.2 3.40–4.45 99.68 ± 11.011 5.83 ± 0.815 1.49 ± 0.940 0.59 ± 0.278 80.33 ± 1.999

Note: chemical data are expressed as mean ± standard deviation; unit is μmol·l− 1.

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TGR and its anabranch, the Xiangxi River. Since erection of the TGR, algal blooms have been detected in the Xiangxi River and the influx region in the rainy and dry seasons. Before damming, algal abundance (0.25–32.70 × 104 cells·l− 1)⁎ in the mainstream of the Yangtze was similar to the density in 2004 and 2005, and the water body was in eutrophic condition due to the high concentrations of nitrate (62.43–71.64 μmol·l− 1), phosphate (0.74–0.86 μmol·l − 1 ) and silicate (40.03– 91.16 μmol·l− 1) (Liu, 2000). During our surveys, there was no significant correlation between algal abundance and concentrations of the main soluble nutrients in the mainstream. The vertical distribution of phytoplankton in TGR remained comparatively even from the surface down to the river bed, and phytoplankton was observed even below 80 m during our surveys. Similar phenomena were also reported in some deep reservoirs (Godlewska et al., 2003), due to intensive vertical mixing of water. On the whole, distribution of phytoplankton in the mainstream was influenced much more by hydrodynamics than by nutrient availability. A similar situation was observed in some reservoirs and rapidly flushed impoundments (Søballe and Kimmel, 1987; Wagne and Zulewski, 2000; Dubnyak and Timchenko, 2000). In the Xiangxi River, there were no algal blooms reported before the building of TGD. However, blooms were identified in the rainy season 2004 and dry season 2005 in the downstream of the Xiangxi and the influx regions; A. formosa, C. acuta and C. ovata were the dominant species. The algal abundances were much higher than the value before damming (1.18 × 105– 6.15 × 105 cells·l− 1) (Tang et al., 2004). The main nutrient concentrations in the Xiangxi River have not changed much since damming (Wang, 2005). It is interesting to find that in the rainy season the algal biomass in the Xiangxi River had significant negative correlation with nitrate (Spearman r = − 0.571, n = 21, p < 0.01), phosphate (Spearman r = − 0.47, n = 21, p < 0.05) and silicate (Spearman r = − 0.681, n = 21, p < 0.01); in the dry season, the algal biomass also had significant negative correlations with nitrate (Spearman r = − 0.401, n = 28, p < 0.05) and silicate (Spearman r = − 0.857, n = 28, p < 0.01), but not with phosphate (Spearman r = 0.178, n = 28). These data indicate the soluble nutrients may be effectively uptake by phytoplankton. However, soluble nutrients concentrations in the Xiangxi River were significantly lower than those in TGR (p < 0.05) (Table 3). After damming, eutrophic trend was also observed in backwater regions of many upper anabranches such as Daninghe, Shennvxi, Bao-

longhe and Daxihe, and the nutrient concentrations in these anabranches were also lower than in the mainstream of TGR (Zhong et al., 2004; Meng et al., 2005). Damming not only changes the hydraulic conditions of rivers but also causes the variations of the phytoplankton composition and biomass at the same time. After damming of the Asahi River in the dam phytoplankton increased due to the prolonged hydraulic retention time, and the water quality in the reservoir was usually eutrophic (Kawara et al., 1998). Nogueira (2000) also gave the similar report: higher phytoplankton diversity and abundance were associated with the contact zone between riverine and lacustrine systems towards dam in Jurumirim Reservoir (Paranapanema River). After damming of the Columbia River, phytoplankton biomass increased due to the combined consequences of reduced flow velocity, increased water retention time and clarity, and decreased vertical mixing intensity (Sullivan et al., 2001). After damming in TGR the reduced flow velocity, increased retention time may be expected to benefit the growth of phytoplankton in rivers, and phytoplankton have enough time to grow before they are exported from the river to its estuary and adjacent continental margin (Sullivan et al., 2001). Modeling predicts that, in 2009, with the completion of the third stage of TGP, the backwater scope of TGR will extend 665 km upstream. The average flow velocity in the mainstream will decrease to 0.17 m·s− 1 (Li et al., 2002) and the average flow velocity in the Xiangxi will decrease to 0.0012–0.0037 m·s− 1 (Luo and Tan, 2000). With the reduced velocity, prolonged retention time and high nutrient concentrations in the reservoir (Zhang et al., 2005), the eutrophic trend within TGR will continue. Acknowledgements This study has been supported by National Natural Science Foundation of China (No. 30490232) and the Chinese Academy of Science Project (KSCX-2-1-10). ⁎The algal density was provided by The Ecological and Environment Monitoring Information Center of TGP. Some hydrological data were provided by The Hydrological Information Center of the Chinese Hydrological Department. References Beardsley RC, Limeburner R, Yu H, Cannon GA. Discharge of the Changjiang into the East China Sea. Cont Shelf Res 1985;4:57–76. Boruzkij EB, Wang Q-L, Chen S-Z, Wang S-D, Liu Q-R, Wu X-W, et al. The aquatic life investigation and fishery utilization opinion. Acta Hydrobiol Sin 1959:1–32 (in Chinese, special issue).

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