Kyong Ha

Prediction and elucidation of phytoplankton dynamics in the Nakdong River (Korea) by means of a recurrent artificial neural network

A recurrent artificial neural network was used for time series modelling of phytoplankton dynamics in the hypertrophic Nakdong River system. The model considered meteorological, hydrological and limnological parameters as input variables and chl. a concentration as output variable. It was trained and validated by means of a complex database measured from 1994 to 1998 at a study site 27 km upstream of the river mouth. The validation results for 1994 indicated that the recurrent training algorithm and a 3 days time lag of input data predict reasonably accurate the timing and magnitudes of chl. a. A comprehensive sensitivity analysis of the model revealed relationships between seasons, specific input variables and chl. a that correspond well with theoretical assumptions and literature findings.

Spatial and temporal dynamics of phytoplankton communities along a regulated river system, the Nakdong River, Korea

The longitudinal distribution and seasonal fluctuation of phytoplankton communities was studied along the middle to lower part of a regulated river system (Nakdong River, Korea). Phytoplankton biomass decreased sharply in the middle part of the river (182 km upward the estuary dam), and then increased downstream reaching a maximum at the last sampling station (27 km upward the estuary dam). In contrast, there was little downstream fluctuation in species composition, irrespective of pronounced differences in nutrient concentrations (TN, TP, NO3, NH4, PO4) as well as in algal biomass. In the main river channel, small centric diatoms (Stephanodiscus hantzschii, Cyclotella meneghiniana) and pennate diatoms (Synedra, Fragilaria, Nitzschia) were dominant from winter to early spring (November–April). A mixed community of cryptomonads, centric and pennate diatoms, and coenobial greens (Pediastrum, Scenedesmus) was dominant in late spring (May–June). Blue-green algae (Anabaena, Microcystis, Oscillatoria) were dominant in the summer (July–September). A mid-summer Microcystis bloom occurred at all study sites during the dry season, when discharge was low, though the nutrient concentration varied in each study site. Nutrients appeared everywhere to be in excess of algal requirement and apparently did not influence markedly the downstream and seasonal phytoplankton compositional differences in this river.

The phytoplankton succession in the lower part of hypertrophic Nakdong River (Mulgum), South Korea

Investigations were carried out to determine the mechanism of phytoplankton succession in the lower part of Nakdong River. Intensive monitoring was conducted from April 1993 to April 1995 at weekly or biweekly intervals. This river is the main source of drinking water for more than 8 million residents living in Pusan and the Southeastern region of Korea, and it is also important for industrial purposes. Due to the rapid urbanization and industrialization, this river has become hypertrophic and it is heavily regulated (mean chl. a, 79±232 µg l-1 n = 76; mean TN, 4.3±2.0 mg l-1 n=30; mean TP, 165 ± 108 µg l-1 n = 45). Bacillariophyceae were dominant year-round in 1993, with spring and late fall peaks. Dominant communities in 1994 were Bacillariophyceae in March and April, Chlorophyceae and Cryptophyceae in May, Cyanobacteria in July and August, Chlorophyceae and Cryptophyceae in October, and Bacillariophyceae in December. As drought persisted through the summer of 1994, elevated water temperature (over 30 °C) possibly triggered Cyanobacteria bloom (mainly Microcystis aeruginosa, maximum cell density, 1.6 × 107 cells ml-1). The most common diatom, Stephanodiscus hantzschii, repeatedly dominated from late fall to next spring (mean and maximum cell density, 7.5 × 104, 1.2 × 105 cells ml-1, respectively). This small centric diatom was favored by the low discharge and the cold water (water temperature, 3–10 °C). After the diatom bloom, this community shifted to peaks of colonial Chlorophyceae and motile Cryptophyceae, owing to the high rate of zooplankton grazing activity and increased water temperature. Overall, the phytoplankton periodicity was primarily governed by the hydrologic regime (discharge). Changes in silica concentration, water temperature and high zooplankton density might have played an important role in phytoplankton dynamics during the non-flooding periods. Compared to other large rivers, strong Microcystis bloom events in summer and Stephanodiscus bloom events in winter were noticeable in the dry year. The patterns of phytoplankton succession observed in this study may have a significance since most of the large rivers in Far Eastern Asian countries are subjected to eutrophication and regulation of discharge.

Microcystin production by Microcystis aeruginosa exposed to different stages of herbivorous zooplankton

Microcystin (MC) production by four monoclonal Microcystis aeruginosa strains was evaluated in response to infochemicals (indirect exposure) released from different stages of herbivorous zooplankton (neonate/juvenile and adult Daphnia magna and Moina macrocopa). The intracellular MC and extracellular MC concentrations were significantly different among the control and treatments with zooplankton culture media filtrates (p<0.05), and in most cases MC production was significantly higher (p<0.05) in strains exposed to infochemicals released from adult zooplankton rather than those of neonate/juvenile zooplankton in four strains of M. aeruginosa. Compared to intracellular MC (385.0-5598.6microg g(-1)DW), very low concentrations of extracellular MC (9.9-737.6microg ml(-1)) were released, but both showed similar temporal patterns over the course of the experiment. This result might be attributed to the fact that adult zooplankton produced more infochemical signals than equal numbers of smaller juveniles and neonates. It is the first study to provide evidence that MC production might be impacted by infochemicals released from different stages of zooplankton, mediated with physiological characteristics, body size, and feeding habits.

Colony Formation in Planktonic Algae Induced by Zooplankton Culture Media Filtrate

ABSTRACT Grazer induced colony formation was examined using the cyanobacterium Microcystis aeruginosa Kützing and the green alga Scenedesrnus dimorphus (Türpin) Kützing. Algae were cultured in a medium with or without filtered water taken from cultures ofDaphnia magna Straus or Moina rnacrocopa Straus. Exposure to the zooplankton culture media filtrate (ZCMF) promoted colony formation in both S. dirnorphus and M. aeruginosa, with the magnitude of this response being directly proportional to the relative volume of ZCMF that was added to the culture medium. However, the degree of colony formation of M. aeruginosa was weakly influenced by ZCMF by comparison to that for S. dirnorphus. The mean number of cells per colony and mean particle biovolume of the two algal species increased significantly from 8 hr to 72 hr for the D. magna-CMF treatment and the M. macrocopa-CMF treatment (p < 0.001), most likely due to the influence of chemicals released from D. magna or M. rnacrocopa. Such info-chemicals could act as signals on trigger to induce colony development.

Vertical distribution of Microcystis population in the regulated Nakdong River, Korea

Abstract The vertical distribution of a bloom-forming Microcystis population was studied based on the relevant limnological parameters obtained from the lower Nakdong River (Mulgum) during the summer of 1994. Over three months (late June to late September), a high abundance of Microcystis population (mean ± SD, 2.9 ± 8.4 × 105 cells ml−1, n = 40) and algal biomass (mean ± SD, chlorophyll a, 131 ± 346 μg l−1, n = 31) was persistent throughout the entire water column (0–5 m, n = 11). The vertical distribution of carbon content was uneven, with a high concentration near the surface zone (mean ± SD, total, 7.9 ± 7.8; Microcystis, 5.2 ± 8.3 μg C ml−1, n = 15). Incorporating limnological and meteorological factors, a diel study of the vertical distribution of Microcystis showed that the chlorophyll a concentration was highest near the surface zone on a calm night (wind velocity, <2 m s−1, 2300–700) but was evenly distributed on a windy day (>4 m s−1, 1100–1900). Among many possible factors, wind velocity may have played an important role in controlling the vertical distribution of Microcystis in the lower Nakdong River.

Toxin-Mediated Interaction between Cyanobacteria and Native Fishes in the Eutrophic Hoedong Reservoir, South Korea

ABSTRACT The microcystin-production responses of Microcystis spp. to the feeding activity of dominant native fishes (Hemiculter eigenmanni and Culter brevicauda) were examined in mesocosm experiments in the eutrophic Hoedong Reservoir and in the laboratory. Microcystin content in feces and toxin accumulation in whole body tissues of fishes were also measured. For fish treatments microcystins were not detected on day 0 but increased from day 1 of the experiment (mean value of 42.3 μg g−1 -dry wt), and were up to six times greater (194.3 μg g−1 -dry wt) than those of controls (33.6 μg g−1 -dry wt) on day 6. Both fish species fed on the alga as reflected in elevated levels of microcystin within their body tissues and feces. Microcystin levels were generally higher in feces than in body tissues. In the field experiment, microcystin was detected in fish body tissues in both of surface (0 m) and subsurface waters (3 m), even though it was not detected in the reservoir water itself in the course of experiment. These results suggest that microcystin production may be induced by exposure to native fishes.

Inter-annual variability of the summer Microcystis bloom in the lower Nakdong River (S. Korea)

Nuisance cyanobacterial blooms are a worldwide sign of advanced eutrophicarion in freshwarer ecosysrems. Since blooms are frequenrly observed in nutrienr-enriched lakes and rivers, rhey can ofren be predicted following changes in physico-chemical conditions (PAERL 1988, STEINBERG & HARTMANN 1988). The outbreak of cyanobacterial blooms becomes a serious problem in rhe managemenr of freshwarer resources since mosr cyanobacreria can be toxic to humans and livesrock.