INTRODUCTION
Zooplankters play an important role in transferring primary production to higher trophic levels, i.e., ichthyoplankton and aquatic insects, etc. Thus zooplankton dynamics is affected by biological environments (structure of aquatic fauna and flora around them) as well as physical ones (water temperature and diunal length) (Raymont, 1984). Due to these characteristics, zooplankters have generally been used as indicator organisms for the water quality assessment (Hanazato and Yasuno, 1990;Nam, 1996).
The Han River is the one of major rivers in the Korean Peninsula, and flows westward to reach the Yellow Sea. Flowing through the capital city, Seoul, the river is the sole resource for the industrial and drinking water supply in metropolitan area.
Because of this necessity of the river, a number of studies has been carried out to understand about the zooplankton dynamics in the Han River, and a monographic study has recently been compiled (Lim, 1992). Water quatily of the Han River has been deteriorated gradually due to the increasing sewage input. Moreover the construction of underwater dams, and the lin- earization and adjustment of riverside disturbed the ecosystem of the lower part of the Han River seriousely.
The objective of the study was to clarify the spatial and temporal distribution of zooplankton in waters adjacent to Yˇoˇuido and Chungjido of the lower part of the Han River. This study has been carried out as a part of study which assessed the impact of thermal discharge of Seoul thermal power plant on surrounding waters such as water quality (Hahn et al., 1997), phytoplankton dynamics (Lee and Chang, 1997), sediments (Hahn et al., 1997), and benthos and fishes (cf. KORDI, 1996).
MATERIALS AND METHODS
Zooplankton samples were collected six times in June, July, October 1995, January, May and June 1996 at seven selected stations in the lower part of the Han River (Fig. 1). The upper most station, station 1 was located beneath the Map’o bridge, stations 2 and 3: the front of the intake and discharge of Seoul thermal power plant, station 4: below the Tangsan bridge, station 5: the enterance of Saetkang, station 6: below the Sungsan bridge and station 7: off the Kayang water supply.
Samples were vertically collected from the bottom to surface (4~12 m depth) with Kitahara net (mouth diameter: 24 cm, mesh aperture: 0.1 mm), and were fixed with 4% neutralized formalin with borax on board. Samples were moved to the laboratory and examined within 48 hours.
Zooplankters were examined and counted under a dissecting microscope (Mag. X40~80) (cf. Omori and Ikeda, 1984). Protozoans, rotifers, cladocerans and copepods were identified to species under a compound microscope with higher magnification (X100-400). Identification was carried in accordance with Shen (1979), Chang and Kim (1986) for copepods, Chiang and Du (1979) and Kim (1988) for cladocerans, and Mizuno and Takahashi (1991) for protozoans and rotifers, and Kim (1991) for rotifers. The number of each species was converted to number per cubic meter of water by using calculated volume of water through the net.
Species diversity index (Shannon and Weaver, 1963) as an ecological index was caculated with the computer program in statistical ecology (cf. Ludwig and Reynolds, 1988) at each station and compared with other stations.
RESULTS
Among 70 taxa of zooplankton observed in the study waters, 60 taxa were identified to species and consisted of three protozoans, 25 rotifers, 18 cladocerans, 13 copepods and one polycheat (Table 1). Rotifers showed a prosperity in species number which consisted of 35.7% of total zooplankton taxa, followed by cladocerans (25.7%) and copepods (20.0%).
Table 2 showed the seasonal change in the species number of dominant zooplankton taxa, consisted of rotifers. cladocerans and copepods. In May 1996 and June 1995, rotifers showed a prosperity in species number (14 and 18 species) while cladoceran and copepod species were observed to be somewhat constant during the study period.
Taxa number was recorded to be maximum (10~27 taxa) in June 1995 at all stations except station 6 (Table 3). Just after a flood in July 1995, zooplankton species decreased drastically. Zooplankton species were more or less constant at station 2, whereas other stations showed to be variable with varying season.
Maximum abundance (44,317 indiv./m3) was recorded at station 7 in June 1996, while no zooplankter was observed at the same station in July 1995 (Table 4). Zooplankters were found to be abundant in June both in 1995 and 1996, and low abundance in July 1995 and January 1996.
Three cladoceran species (Daphnia galeata, D. pulex and Bosmina longirostris) dominanted zooplankton community with high abundance (Table 5). Three copepod species (Canthocamptus carinatus, Cyclops vicinus and Thermocyclops hyalinus) also dominated with seasons and stations. Maximum population density of 16,964 indiv./m3 was recorded by the single species of D. galeata (station 2 in June 1996). D. pulex (9,375 indiv./m3), B. longirostris (7,250 indiv./m3), T. hyalinus (6,714 indiv./m3) and C. vicinus (5,772 indiv./m3) also established dense populations, respectively. The proportion of dominant zooplankton species to total zooplankton abunadance was recorded in Table 5. D. galeata consisted of more than 85% of total zooplankton abundance at station 5 in October and C. vicinus also recorded to be 80% of zooplankton abundance at station 6 in July.
Fig. 2 showed the spatial and temperal variations in the species diversity index in the lower part of the Han River. Except stations 6 and 7, all the stations were oberved to fluctuate between 0.6 and 2.2. Spatial and temporal variation in the index seemed to show little correlation with the environmental change.
DISCUSSION
Zooplankton dynamics has often been used to interpret the water quality and aquatic ecosystem (Hynes, 1966;Waterman, 1960). For example, biological mornitoring system has been developed by using of Daphnia magna for watching the pollutant input into aquatic habitats (Knie and Puzicha, 1991). Recently, a number of studies has been performed for revealing the influence of various pollutants on aquatic habitat on the basis of toxicity test by using a freshwater cladoceran Moina irrasa in Korea (Kim et al., 1994, 1996; Yoo and Nam, 1996;Yoo et al., 1998).
Daphnia galeata was revealed to be the predominant species in the lower part of the Han River. Although rotifers showed the prosperity in species number, their abundance was lower than cladocerans and copepods.
Lower number of zooplankton taxa (70 taxa) observed during the study when compared with Lim , s result (115 taxa) (1992) seems to be due to our study water being confined to the lower part of the Han river while Lim had covered from the P , altang Lake to Haengju Bridge. Despite the defiency of intensity of our filed work when compared with Lim , s study, we found somwhat interesting results which was not mentioned by Lim, viz., the influence of flood. The flood due to the heavy rain fall in early July 1995 appeared to sweep the ichthyofauna (Cyprinus carpio and Carassius auratus) of study waters to the river mouth of the Yellow Sea while only an upper water fauna (Siniperca scherzeri and Lepomis macrochirus) was observed to be dominant at the same waters (KORDI, 1996). Zooplankton also seemed to be drifted away to the downstream because no zooplankters were found at station 7 in July 1995. No such a phenomenon was ever reported in the Han River.
We found a carnivorous cladoceran Leptodora kindti establishing a large population at station 4 in June 1995 (1,200 indiv./m3). According to Lee and Chang (1997), stations 1 and 4 were recorded to be higher than other stations in phytoplankton standing crop in June 1995. High primary production might cause the prosperity of herbivorous zooplankton, e.g, Polyarthra vulgaris and Daphnia galeata, and in turn might give a preyrich enviroment to L. kindti (Herzig, 1995). A lack of aquatic plants at this station possibly let L. kindti relieve food competition with ichthyoplankters (Reede and Ringelberg, 1995).
During the study, hybrid individuals in cladocerans were ignored. The occurrence of hybrid form in cladocerans remains to be studied in zooplankton dynamics more precisely in Korean inland waters.
