Internat. Rev. Hydrobiol. 94 2009 5 542–559 DOI: 10.1002/iroh.200811107 SHUCHAN ZHOU1, 2, XIANGFEI HUANG1 and QINGHUA CAI*, 1 1Institute of Hydrobiology, Chinese Academy of Sciences, Wuhan, Hubei 430072, P.R. China; e-mail: [email protected] 2Institute of Biodiversity Science, Fudan University, Shanghai 200433, P.R. China; e-mail: [email protected] Research Paper Temporal and Spatial Distributions of Rotifers in Xiangxi Bay of the Three Gorges Reservoir, China key words: Rotifera, temporal and spatial dynamic, new impoundment, longitudinal distribution, Yangtze River Abstract From July 2003 to June 2005, investigations of rotifer temporal and spatial distributions were car- ried out in a bay of the Three Gorges Reservoir, Xiangxi Bay, which is the downstream segment of the Xiangxi River and the nearest bay to the Three Gorges Reservoir dam in Hubei Province, China. Thirteen sampling sites were selected. The results revealed a high species diversity, with 76 species, and 14 dominant species; i.e., Polyarthra vulgaris, Keratella cochlearis, Keratella valga, Synchaeta tremula, Synchaeta stylata, Trichocerca lophoessa, Trichocerca pusilla, Brachionus angularis, Brachionus caly- ciflorus, Brachionus forficula forficula, Ascomorpha ovalis, Conochilus unicornis, Ploesoma trunca- tum and Anuraeopsis fissa. After the first year of the reservoir impoundment, the rotifer community was dominated by ten species; one year later it was dominated by eight species. The community in 2003/2004 was dissimilar to that in 2004/2005, which resulted from the succession of the dominant species. The rotifer community exhibited a patchy distribution, with significant heterogeneity observed along the longitudinal axis. All rotifer communities could be divided into three groups, corresponding to the riverine, the transition and the lacustrine zone, respectively. 1. Introduction Zooplankton heterogeneity and community structure at a range of spatial and temporal scales is an important focus of aquatic ecological research (CLARK et al., 2001). Studies on temporal variations in zooplankton distribution are common, but research on spatial patchiness has received relatively less attention. Zooplankton spatial distributions are as heterogeneous as those of terrestrial and aquatic plants and animals (FERNÁNDEZ-ROSADO and LUCENA, 2001). Now, it is a well-known phenomenon (SEDA and DEVETTER, 2000) and has been shown to be an ecologically important feature of freshwater ecosystems (PINEL- ALLOUL et al., 1999). The structure and composition of organisms and the spatial pattern of a community are of crucial importance for understanding ecosystem functioning (ROSENZWEIG, 1991; ROMARE et al., 2003) because they can affect ecosystem processes, functioning and stability (MAESTRE et al., 2005). * Corresponding author © 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1434-2944/09/510-0542 Rotifers in Three Gorges Reservoir 543 Reservoirs are complex systems, considered as transitions between rivers and lakes. Sev- eral important differences between lakes and reservoirs are observed such as geological age, maximum depth location, shape and water retention time (STRAŠKRABA and TUNDISI, 1999). These differences are mostly pronounced in canyon-shaped reservoirs, which are spa- tially highly heterogeneous because of their relatively short retention times and longitudinal heterogeneity (MAŠÍN et al., 2003). A canyon-shaped reservoir presents three very distinct zones: the riverine zone, in the upper reservoir, which is subject to the influence of either the tributaries or river of origin; the transition zone, downstream from the reservoir, which functions as an intermediate river-lake ecosystem; and the lacustrine zone, located further downstream (MATSUMURA-TUNDISI and TUNDISI, 2005). These zones vary widely in flow and depth, making characterization of the reservoir more difficult than that of most natural lakes (BERNOT et al., 2004). The Three Gorges Reservoir of China (a newly impounded canyon-shaped reservoir), is a world-famous hydroelectric project (Fig. 1). This reservoir is undergoing a process of Figure 1. The location of Xiangxi Bay in China (above) and sampling stations (below). © 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.revhydro.com 544 S. ZHOU et al. eutrophication, which can strongly affect numbers, standing crops, population dynamics, production and community structure of zooplankton (CAJANDER, 1983). It is reported that man-made lakes present ecological features that lead to the establishment of a very dynamic system in which the plankton communities play an important role (BRANCO et al., 2002). However, zooplankton studies within the Three-Gorge Reservoir are scarce, especially at the initial stages of impoundment. Therefore in this article we reported: 1) the temporal dynam- ics of rotifers in Xiangxi Bay of the Three Gorges Reservoir since impoundment; and 2) the longitudinal and transverse distributions of rotifers in this bay. 2. Materials and Methods 2.1. Study Site Description The Xiangxi River is an important tributary of the Yangtze River in Hubei province, China. The river, with a catchment area of 3,099 km2, has a length of 94 km, and a natural fall of 1540 m (TANG et al., 2002). With the impoundment of the Three Gorges Reservoir, water depth in the outlet of the Xiangxi River has increased from about 20 to 70 m; water current velocity has decreased from 0.43–0.92 m · s–1 to 0.0020–0.0041 m · s–1. The water body was neutral to slightly alkaline at the time of sampling. The downstream segment (about 25 km) of this river is now called the Xiangxi Bay. It is the biggest bay near the dam of Three Gorges Reservoir in Hubei province, China. There is a depth gradient increasing from the riverine zone to the lacustrine zone (at the mouth of Xiangxi River, about 70 m). From July 2003 to June 2005, a monthly investigation of the rotifer community was conducted in Xiangxi Bay. Nine sampling sites were selected (Fig. 1), to provide a spatial characterization of the bay. Moreover, to analyze the transverse distribution of rotifers in Xiangxi Bay, two transects (ST 1 and ST 5) with three sites were sampled. These sites followed the flow direction, including the left littoral zone (indicated by L), the middle pelagic zone, and the right littoral zone (indicated by R). 2.2. Sampling Methods At each site, we sampled three layers: surface water (0.5 m), Secchi disk depth and two times Sec- chi depth with a 10 L modified Schindler-trap sampler. These discrete samples were then pooled and mixed, and subsamples for analyses of chlorophyll a and rotifer community were taken from the pooled samples. For chlorophyll a determinations, 0.6 L of pooled water were filtered through a WHATMAN GF/C glass-filter and analyzed spectrophotometrically after 95% acetone extraction. Then, 1.5 L of pooled water were taken and fixed with 5‰ non-acetic Lugol’s iodine solution for quantification and determination of rotifers. Forty eight hours later, the undisturbed water samples were concentrated to about 50 mL and preserved with 4% formalin. Rotifers were counted in two Sedgewick-Rafter subsamples. Rotifer densities were determined for discrete species, and identification of species was according to WANG (1961), KOSTE (1978), ZHANG and HUANG (1991), and ZHUGE (1997). Here, dominant species were those whose density amounted to 20% or more of the total rotifer community (HABERMAN, 1983). 2.3. Data Analyses Since our data did not meet the criteria of normality, we used nonparametric statistics, using SPSS 11.5 for Windows. Nonparametric tests, based on ranks, are alternatives to parametric statistics for test- ing hypotheses about relationships and differences for variables. Friedman ranking test, which could be used for analyzing three or more repeated measurements of ordinal data (SHELDON et al., 1996), was used for analyzing the variation of rotifer density among the sites and months. The Wilcoxon signed ranks test, which was found suitable for analyzing the difference between two correlated ordinal- level measurements, was used for analyzing the difference of data between the two years (HANSSON, 2000). © 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.revhydro.com Rotifers in Three Gorges Reservoir 545 Principal component analysis (PCA) was carried out to identify any underlying relationships between zooplankton taxa and temporal and spatial distribution, respectively. PCA has the advantage of reduc- ing the initial number of independent variables to a smaller number of composite variables which can explain a substantial percentage of the total variance (SOUISSI et al., 2000). It was performed by PC- ORD (4.0). The months or sites were variables: the last letter “F” indicated in the first year (from July 2003 to June 2004), while “S” indicated in the second year (from July 2004 to June 2005). The species data were used as cases. Lastly, cluster analysis, which was carried out by PRIMER v.5 software, was also used to explore the spatial characteristic of the sampling sites. Abundances were Log(x + 1) transformed with no stand- ardization. Bray-Curtis similarity matrices were computed to generate clusters. 3. Results 3.1. Composition and Dominance Altogether 76 rotifer species were recorded (Table 1), belonging to 20 families and 29 genera. 14 dominant species were observed; i.e., Polyarthra vulgaris (CARLIN, 1943), Keratella cochlearis (GOSSE, 1851), Keratella valga (EHRENBERG, 1834), Synchaeta tremula (O. F. MULLER, 1786), Synchaeta stylata (WIERZEJSKI, 1893),