First phase of the ecosystem study of the Nakdong River (1993- 2000)
The lower Nakdong River has become a "reservoir-river hybrid" since the construction of an estuarine dam in 1987. Due to the increased nutrient input from the middle part of the river and a longer hydrologic retention due to the estuarine dam, phytoplankton biomass (chl. a) was increased from 28±23 mg/l (1982-87) to 60±152 mg/l (1989-96) while phytoplankton blooms occurred frequently. Nutrient concentrations (mg/l) also increased during the same period (NO3-N: 0.9 to 2, TN: 1.9 to 3.6, PO4-P (mg/l): 40 to 200). Since 1992, Microcystis (blue-green algae) in summer and Stephanodiscus (diatom) bloom in the winter repeatedly occurred. Recently, overall loss of biodiversity is rapidly progressing including a strong dominance of top predator (bass). Further study should address biological interaction and also human induced impact on the river (water intake, channel modification, landuse change)
There have been a number of reports on the cause, progress and effect of eutrophication and ecological changes in lakes. However, less attention has been given to river ecosystems, although the issue has also been equally important. Except several European and North American rivers (e.g., Hudson R., Rhine R., Thames R.) (Whitton, 1984), systematic studies of biological monitoring and river water quality have been rare.
Large rivers around the world have often been physically transformed for various purposes. Constructions of lock and dam to facilitate water transportation and to control flood in the main channel are the usual ways of river modification in relatively large rivers. However, the construction of dam or other structures in the estuary for industrial, agricultural and drinking purpose is rare, except a few cases in Korea and Japan (Nakdong R., Youngsan R. and Kum R. in Korea and the Nagara R. and other eight rivers in Japan) (Joo, 1995, Murakami, et al., 1994, Saijo and Okuda, 1996).
The Nakdong River is an excellent example of the "regulated river" and it provides a good case study of the river with an estuarine dam. Since the 1970s, the necessity to secure water resources has been magnified, due to rapid industrialization and a sudden increase in population of the cities located within the Nakdong river basin. Almost 8 million people depend on the river for drinking and other purposes. For this reason, the construction of five dams in the upper part of the major tributaries as well as one estuary dam in the mouth of the river was completed. The estuarine dam was constructed in 1987 to prevent intrusion of salt water into intake facilities near Mulgum (27 km from the estuarine dam).
The changes of biodiversity in Nakdong River had been little understood in scientific context. The purpose of this paper is, first, to analyze and evaluate changes of the physico-chemical and biological characteristics during the last two decade and second, to review seven years ecological studies conducted by the group of limnologists in the Department of Biology of Pusan National University.
The Nakdong River is the second largest river system in S. Korea, which springs from Hwangji in the Taeback Mountains of north-eastern S. Korea. The river, which runs down to the estuary dam near Pusan, has a drainage area covering 23,817 km2 and 528.7 km in length. The drainage area is situated in 35-37°N and 127-129°E (Fig. 1).
Figure 1. Map showing the basin of the Nakdong River, major tributaries, and the study sites ·: Mulgum, ■: Multipurpose dam)
Estuarine dam is located in the mouth of the river and mean depth of the Mulgum area is approximately 4.5 m (river width: 250-350 m).
The Nakdong River basin lies in the monsoon region of the eastern Asia, with hot summers and cool to cold winters. Water temperature rarely drops below 1 °C during the winter (Fig. 2a). A mean annual precipitation for thirty years in Nakdong River basin is 1030 mm (Fig. 2b). More than 60% of rainfalls occurs during summer (June through September). Inter-annual variabilities in precipitation of the region were strongly affected by the total rainfall in summer. While rainfall events were recorded in the summer of 1993 (total annual precipitation: 1181 mm), the summer of 1994 recorded the lowest precipitation (total annual precipitation: 567 mm) since 1907. The mean discharge from 1980 to 1995 was 289 m3/s. The maximum discharge was 7763 m3/s in August, 1993, and the minimum was 30 m3/s in the summer of 1994 (Fig. 2c). The fluctuation of discharge in the lower Nakdong River after the construction of estuarine dam seemed to be affected by the precipitation in the middle region of the basin and by the discharge operations of the dams in tributaries and the estuary.
Figure 2. Annual changes of water temperature (a), precipitation (b) and discharge (c) at various locations from 1976 to 997.
Detailed information on method used in this study appears in Kim (1996, 1999), Ha (1996, 1999), Park (1998), Ju (1998), and Chang (2000).
There were obvious changes of the phytoplankton community, species composition and biomass in the lower part of the Nakdong River after the estuarine dam construction. Marine and freshwater Bacillariophyceae and Chlorophyta appeared in the early 1980s. However, since 1987, freshwater Bacillariophyceae, Cyanobacteria and Chlorophyta has increased. Since the year 1992 during the period of low discharge and drought season severe winter diatom blooms as well as the blooms of Microcystis (blue-green algae) were repeatedly observed (Ha, 1996, Ha et al., 1998 a, b).
Phytoplankton biomass at the lower Nakdong increased several folds since 1987 (Fig. 3a). The algal biomass sporadically examined from 1983 to 1986 was relatively low (annual mean of chl. a: £ 33 mg/l, n=32). According to OECD, (1982) criteria, trophic status of Nakdong River was indicated as a meso-eutrophic state at that time. Since the early 1990s, algal biomass has sharply increased, and currently the river shows hypertrophic state. The mean chl. a from January 1990 to April 1997 is 62±153mg /l (n=195) (Fig. 3b). The annual mean of chl. a concentration is 3-5 folds higher than that of some larger rivers with similar drainage area (Rhine R.: 20-30 mg/l, Hudson R.: 20 mg/l, R. Meuse: 35 mg/l) (Descy, 1987, Descy and Gosseloin, 1994).
Figure 3. Changes of chl. a concentration at Mulgum from 1984 to 1997 (a), and from 1993 to 1997 (b).
Since the 1970s, the necessity to secure water resources has been magnified, due to rapid industrialization and a drastic increase in population of the cities located within the Nakdong River basin. The Nakdong River has become a "reservoir-river hybrid" since the construction of an estuarine dam in 1987 (Joo et al., 1997). Due to the increased nutrient input from the middle part of the river and a longer water retention time due to the estuarine dam, acceleration of eutrophication has been observed in the lower Nakdong River since the early 1990s. Trophic status (chl. a and TN) changed meso-eutrophic to hyper-eutrophic with frequent bloom developments since 1992 (Kim et al., 1998).
3. Ecological studies since 1993
Figure 4. Schematic diagram of the research areas in the Nakdong River since 1993.
1.Climates, hydrodynamics, and nutrient loads
· Inter-annual variability of nutrient loadings in the lower Nakdong River, Mulgum, Korea (Lee, S. K., S. H. Choi, H. W. Kim, K. Ha, and G. J. Joo, 1999, Acta Hydrobiologia Sinica, 23: 17-23)
· Basic water quality of the mid to lower part of Nakdong River and the influences of the early rainfall during monsoon on the water quality (Park, S. B.,1998, M.S. thesis. Pusan National University, Pusan)
2. Water resources: quality
· Long-term trend of the eutrophication of the lower Nakdong River (Joo, G. J., H. W. Kim, K. Ha, and J. K. Kim, 1997, The Korean Journal of Limnology, 30: 472-480)
· Eutrophication of the lower Nakdong River after the construction of an estuarine dam in 1987 (Kim, H.W., K. Ha, and G. J. Joo, 1998, International Review of Hydrobiology, 83: 65-72)
3. Structural classification and heterogeneity of plankton community
· Characteristics of physico-chemical parameters and zooplankton dynamics in the Nakdong River (Kim, H. W., 1996, M.S.thesis. Pusan National University, Pusan)
· The mechanism of phytoplankton succession in the lower part of Nakdong River (Ha, K., 1996, M.S.thesis. Pusan National University, Pusan)
· Bacterioplankton in the lower part of Nakdong River: distribution and relationships with phytoplankton biomass (Ju, Y. H., 1999, M.S. thesis, Pusan National University, Pusan)
· The community dynamics of zooplankton in a regulated hypertrophic river (Nakdong River, S. Korea) (Kim, H. W., S.-J. Hwang, and G. J. Joo, 2000, Journal of Plankton Research (submitted)
· The longitudinal distribution and community dynamics of zooplankton in a regulated large river: a case study of the Nakdong River (Korea) (Kim, H. W., and G. J. Joo, 2000, Hydrobiologia (accepted)
· Zooplankton Dynamics in the Hyper-eutrophic Nakdong River System (Korea) regulated by with Estuary Dam and Side Channels (Kim, H. W., G. J. Joo, and N. Walz, 2001, International Review of Hydrobiology, (in press)
4. Interannual variability in plankton community structure
· The phytoplankton succession in the lower part of hypertrophic Nakdong River (Mulgum), S. Korea (Ha, K., H. W. Kim, and G. J. Joo, 1998, Hydrobiologia, 369/ 370: 217-227)
· Microcystis bloom in the lower Nakdong River in South Korea: Development of the 1994 summer bloom (Ha, K., E. A. Cho, H. W. Kim, and G. J. Joo, 1998, Marine and Freshwater Research, 50: 89-94)
· Zooplankton community dynamics during the summer Microcystis bloom in the lower part of the Nakdong River, South Korea (Kim, H. W., G. J. Joo, K. H. Chang, and S. J. Hwang, 2000, Internat. Verein. Limnol. (in press)
· Vertical distribution of Microcystis population in the regulated Nakdong River (S. Korea) (Ha, K., H. W. Kim, and G. J. Joo, 2000, Japanese Journal of Limnology (in press)
· Phytoplankton community dynamics and Microcystis bloom development in a hypertrophic river (Nakdong River, Korea) (Ha, K., 1999, Ph D. dissertation, Pusan National University, Pusan)
· Silica-based size variation of planktonic diatom in hypertrophic regulated Nakdong River, Korea (Ha, K., S. B. Park, J. H. Hong, and G. J. Joo, 2000, Fisheries Science (submitted)
5. Trophic structure: bottom-up or top-down?
· The role of zooplankton in controlling spring phytoplankton community dynamics in a regulated river (Nakdong River, Korea) (Kim, H.W., K. Ha, K. H. Chang, and G. J. Joo, 2000, Marine and Freshwater Research, (submitted)
· Zooplankton grazing on bacteria and phytoplankton in the regulated Nakdong River (Korea) (Kim, H. W., S.-J. Hwang, and G. J. Joo, 2000, Journal of Plankton Research 22:1559-1577)
· Zooplankton grazing on phytoplankton and bacteria along the different trophic conditions in the Nakdong River (Korea) (Kim, H. W., S.-J. Hwang, K.-H. Chang, J.-G. Kim, G.-J. Joo and N. Walz, 2000, International Review of Hydrobiology (submitted)
· Grazing rates of rotifers and their contribution to community grazing in Nakdong River (Kim, H. W., S. J. Hwang, and G. J. Joo, 1999, The Korean Journal of Ecology 22:337-342)
· Seasonal changes of zooplankton community and predation effect of age-0 juvenile fish in the Nakdong River (Chang, K. H., 2000, M.S.thesis, Pusan National University, Pusan)
· Water quality, plankton community dynamics, and trophic regulation in the microbial food web by zooplankton in a hypertrophic river (Nakdong River, Korea) (Kim, H. W., 1999, Ph D. dissertation, Pusan National University, Pusan)
· The current status of the distribution of introduced fish in large river systems of South Korea (Jang, M.-H., J.-G. Kim, S.B. Park, K.S. Jung, and G. J. Joo, 2000, International Review of Hydrobiology (submitted)
6. Modelling of plankton community dynamics
· The application of artificial neural network for predicting phytoplankton dynamics in a eutrophicated river with a estuary dam (S. Korea) (Jeong, K. S., G. J. Joo, H. W. Kim, K. Ha, and F. Recknagel, 2000, Ecological Modelling (submitted)
· Hybrid river modelling by the integration of deterministic equations and genetic programming (Whigham, P., F. Recknagel, K.-S. Jeong, and G. J. Joo, 2000, Ecological Modelling (submitted)
The limnology of the lower Nakdong River experienced distinct seasonality and inter-annual variability during the study period. In accordance with the theory on stream ecology (Moss, 1998) long-term research of the lower Nakdong River revealed that the plankton dynamics as well as basic limnology are mainly driven hydrological patterns. The change of flow conditions from fast to slow and vice versa causes seasonal stable states of water body. The ecology of the river in summer is dominated by transport processes as steady heavy rain accelerates river flow and diminishes underwater light (Park, 1998). In contrast, water retention can increase and stimulate growth processes during seasons with low flow. As Kim et al. (1998) reported, the lower Nakdong River became hypertrophic only after the construction of the estuarine dam that in turn increased the rivers retention time. In addition the flow regulation has accelerated eutrophication of the Nakdong River as the river-reservoir hybrid system (Joo et al., 1997; Kim, 1999).
Ecological studies show that throughout the year the phytoplankton population of Nakdong River is highly dominated by diatoms which rely heavily on the availability of silica (SiO2) (Ha, 1999). In 1993 when the frequent precipitation occurred, the average concentration of SiO2 in the water at Mulgum was high (7.4 mg L-1) during July and August (Joo, 1995). The high discharge caused by concentrated rainfall resulted in high loading of silica in the river (Park, 1998).
The chl. a concentration and Cladocera abundance did not show statistically significant differences along the four seasons. The equality among the seasons caused by the large variance of each seasonal average. Especially in case of the chl. a, though the river system was affected by the hydrological regime, the characteristics of river-reservoir hybrid system gave birth of high probability of algal proliferation due to the regulation of flow. Reynolds (1994) suggested several persistent regional factors (discharge, water velocity, etc.) and seasonal factors (light, grazing, etc.) governs the algal community structures in river systems. In conclusion, the limnology of the lower part of Nakdong River is characterized by hydrology impact, autogenic nutrient dynamics, and biological aspects. These factors combine with each other and determine the water quality and microbial ecology on time scale. The more controlling river flow, the larger opportunity of algal bloom might be given.