| Liuxihe reservoir is a large water body located near Cancer; its zooplankton may be characterized by a mixture of tropical and subtropical features in biogeography. Species diversity of metazooplankton was analyzed based collection from 2000 to 2006. The spatial and temporal variations of metazooplankton in the reservoir were also investigated from May 2005 to April 2006. 77 species of Rotifera, 24 species of Cladocera and 10 species of Copepoda were identified. In Rotifera group, the majority of the species were monogononts, while bdelloids were represented only by Rotaria sp.. The richest fraction belonged to the following families: Brachionidae (16)＞Lecanidae (13)＞Synchaetidae (9)＞Trichocercidae (8), and most of them were cosmopolitan. Warm-water taxa were few, and no endemic taxa found. The most frequent species were Keratella cochlearis, Polyarthra remata, Trichocerca similis, Conochilus unicornis, Ascomorpha ovalis, Ploesoma hudsoni, Asplanchna priodonta, Pompholyx sulcata, Synchaeta oblonga and Collotheca libera. In Cladocera group, 11 limnetic species (primarily Daphniidae), 13 littoral species (primarily Chydoridae) were observed. Similarly to cladocerans in the tropics, most of the limnetic species were members of six genera, Diaphanosoma, Bosmina, Bosminopsis, Moina, Ceriodaphnia and Daphnia. However, three 'northern' species unreported before: Leptodora kindtii, Daphnia pulex and D. galeata, were also found. In Copepoda group, 1 Calanoida species and 8 Cyclopoida species were observed. Phyllodiaptomus tunguidus, Mesocyclops thermocyclopoides, Tropocyclops bopingi and T. jerseyensis were the most frequent Copepoda. P. tunguidus was endemic to the tropics and subtropics in China, and T. bopingi was a new species found in Liuxihe Reservoir.Rotifer abundance and biomass varied from 27 to 2134 ind.·l-1 and 3 to 494μg·l-1 respectively. According to feeding habit, rotifers were classified as generalists and specialists. Many of the most common rotifer genera such as Keratella and Conochilus were suspension feeders or 'generalist'. A considerable number of rotifer genera such as Polyarthra, Synchaeta and Asplanchna were 'specialised' rotifers, able to catch and consume larger sized food items. In the flood season, rotifers were dominated by 'generalist' (primarily K cochlearis) and by 'specialist' (primarily P. remata and A. priodonta) in the dry season. In the lacustrine zone and transition zone, rotifer biomass peaked in the late dry season (February-March), and in the riverine zone, peak biomass was found in the mid dry season (November-December). Along the reservoir's main axis, rotifer abundance decreased towards the lacustrine zone in the early and mid dry season (October-January), and in the late dry season and flood season (February-August), rotifers were significantly more abundant in the transition zone.Cladoceran abundance and biomass varied from 0.08 to 35.13 ind.·1-1 and 0.95 to 422.33μg·l-1 respectively. Diaphanosoma orghidani transamurensis, Bosmina fatalis and Bosminopsis deitorsi were the dominant species and their total relative abundance were generally more than 80%. Both the abundance of D. orghidani transamurensis and Bosmina fatalis showed apparent seasonal patterns: low in the dry season and early flood season, peak in the mid flood season (May), decline toward the late flood season, and then re-increase toward the early dry season (August-October). Differently, Bosminopsis deitorsi only peaked in the early dry season. The spatial distribution patterns of cladocerans showed different seasonally: in the early flood season and late dry season, cladocerans were relatively more abundant in the transition zone, in the early dry season, the maximum abundance of cladocerans was found in the riverine zone, and in the other times, maximum abundance was found in the lacustrine zone.Copepoda abundance and biomass varied from 1.5 to 134 ind.·l-1 and 0.8 to 432.4μg·l-1 respectively. Longitudinal gradients in the Calanoida/Cyclopoida relation were observed, with the predominance of Cyclopoida (Mesocyclops thermocyclopoides and Tropocyclops bopingi) in the riverine zone and Calanoida (Phyllodiaptomus tunguidus) in the lacustrine zone towards the dam. In the lacustrine zone, maximum biomass was observed in the mid flood season (May), and in the transition zone and riverine zone, the maximum was found in the mid dry season (October-November). Along the reservoir's main axis, Copepoda abundance increased towards the lacustrine zone in the late dry season and early and mid flood season (January-June), and in the early and mid dry season (September-December), Copepoda were significantly more abundant in the transition zone. Metazooplankton size ranged from 50 to 645μm equivalent spherical diameter (ESD). The smallest organisms were rotifers (numerically dominated by Keratella cochlearis and Polyarthra remata and copepod nauplii, those in the middle size were cladoceran (primarily Bomina fatalis, Bosminopsis deitorsi and Diaphanosoma orghidani transamurensis) and cyclopoid copepods and calanoid copepodites (early stages), and the largest were adult Phyllodiaptomus tunguidus and its copepodites (late stage). Sheldon-type size spectrum of metazooplankton was not uniform, but showed a unimodal distribution. Peak biomass was found in the size class of 323-406,um ESD in the lacustrine zone and 203-256μm ESD in the transition zone and riverine zone. The slopes of normalized biomass spectra were-1.10,-1.05 and-0.77 in the riverine zone, transition zone and lacustrine zone respectively. On average, small size organisms, rotifers and nauplii, contributed a relatively more percentage biomass in the riverine zone than in the lacustrine zone, whereas adult Mesocyclops thermocyclopoids and Phyllodiaptomus tunguidus, and their late stage copepodites contributed less percentage biomass in the riverine zone. As a result, the slope of normalized biomass spectrum was more negative in the riverine zone than that in the lacustrine zone. Seasonal patterns of slope also showed spatially different. In the lacustrine zone, maximum slope was found in the mid flood season (-0.54), and the minimum (-1.01) in the early dry season. In the riverine zone, the maximum was observed in the late flood season, and the minimum in the early flood season and early dry season.In Liuxihe Reservoir, temporal metazooplankton community dynamics was primarily controlled by temperature, and food quantity and quality and competition also played an important role. Retention time, food quantity and quality and predation were the main factors induced the spatial heterogeneity in community structure.
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