MARINE ECOLOGY PROGRESS SERIES

Mar Ecol Prog Ser Vol. 229: 1–10, 2002 Published March 20

INTRODUCTION

Coastal power stations generally use seawater as

cooling water and thus release seawater with elevated

temperature into nearshore environments. In addition,

antifouling chemicals are injected into intake seawater

to prevent the growth of fouling organisms on the surface of the cooling systems. Chlorination of seawater is

mostly employed by coastal power stations and thereby causes formation of chlorination by-products which

might potentially inhibit microbes (Langford 1990).

During the passage through cooling systems, intake

seawater containing natural microbes is thus exposed

to both high temperature (rarely exceeding 40°C) and

antifouling chemicals for a short time (e.g. about 10

to 30 min). At the outfalls, thermal effluents become

mixed with receiving seawater, and elevated tempera-

© Inter-Research 2002 · www.int-res.com

*Corresponding author. E-mail: bccho@plaza.snu.ac.kr

Effects of thermal effluents from a power station 

on bacteria and heterotrophic nanoflagellates 

in coastal waters

Dong H. Choi1, Jong S. Park1, Chung Y. Hwang1, Sung H. Huh2, Byung C. Cho1,*

1School of Earth and Environmental Sciences and Research Institute of Oceanography, Seoul National University, 

Seoul 151-742, Korea

2Department of Oceanography, Pukyong National University, Pusan 608-737, Korea

ABSTRACT: To investigate effects of thermal effluents from a coastal power station on bacteria and

heterotrophic nanoflagellates (HNF) in adjacent coastal waters, surface distributions of bacteria and

HNF and interrelations between microbial and environmental variables were studied in November

1998 and July, August and November 1999 near Hadong Power Station, Korea. In addition, manipulation experiments with treatments of high temperature (40°C; temperature in the cooling system of

the power station) and additions of hypochlorite and dilution experiments of thermal effluents with

intake seawaters were carried out. Water temperature in the discharge-channel was always 5 to 10°C

higher than the ambient temperature, but bacterial production, HNF abundance and grazing rates on

bacteria, as well as chlorophyll a concentrations, were always lower there and increased with distance from the power station. Manipulation experiments showed that addition of hypochlorite had

much more deleterious effects on bacteria and HNF (95 to 98% inhibition of bacterial production and

25 to 45% decrease in HNF abundance at 0.13 ppm of residual chlorine) than high temperature (9 to

39% inhibition of bacterial production but no inhibition of HNF abundance at 40°C). Dilutions of

thermal effluents from the outlet and condenser tube with intake seawater would bring <0.03 ppm of

residual chlorine, but inhibitory effects of thermal effluents on bacterial production and HNF grazing

activity (23 to 69% inhibition of bacterial production and 31 to 36% inhibition of HNF grazing) were

shown, indicating inhibitory potential of chlorination by-products in the discharged water on coastal

microbes. Analyses of horizontal distributions of microbial variables, manipulation and dilution

experiments gave consistently similar results of inhibitory effects of thermal effluents on bacteria and

HNF. Our work suggests that a microbial ecological approach is useful in estimating influences of

thermal pollution on microbes in aquatic environments.

KEY WORDS: Thermal effluents · Chlorination · Bacteria · Heterotrophic nanoflagellates

Resale or republication not permitted without written consent of the publisher
Mar Ecol Prog Ser 229: 1–10, 2002

ture and chlorination by-products are always observed

near the discharge area (Jenner et al. 1997, Allonier et

al. 1999). Therefore, thermal discharges from a coastal

power station have 2 main components of pollution to

coastal waters, i.e. high temperature and chemicals

formed during antifouling procedures.

Micro-organisms are numerically abundant in coastal waters and carry out many ecologically important

roles in coastal ecosystems. Changes in microbial

activities caused by changes in environmental conditions will thus confer significant impacts on functions

of coastal ecosystems. In fact, marine bacterial responses to additions of inhibitory substances and organic pollutants have been extensively studied (Azam

et al. 1977, Siebers et al. 1981, Hudak & Fuhrman 1988,

Riemann & Lindgaard-Jørgensen 1990). Responses of

marine bacteria and heterotrophic nanoflagellates

(HNF) to temperature changes have also been studied

in detail (Marrasé et al. 1992, Shiah & Ducklow 1994).

Thus, if thermal effluents have deleterious effects on

natural marine microbes, they should be easily detected. Some studies have examined effects of chlorination on marine natural phytoplankton (Carpenter et

al. 1972, Eppley et al. 1976). However, studies on

effects of thermal effluents on both natural marine

bacteria and HNF using ecologically quantitative

approaches are rare. To compare bacterial activities,

earlier studies either used the heterotrophic potential

approach to calculate maximum uptake velocity (Vmax)

(Davis & Coughlan 1978) or measured uptake rates of

3

H-glucose added as a tracer (Larrick et al. 1981).

However, these approaches do not quantitatively

assess the bacterial utilization of organic matter or in

situ growth rates of bacteria in the sea (Azam &

Fuhrman 1984). Further, Vmax has been shown to

increase gradually as the range of added glucose concentration is increased (Azam & Hodson 1981). Plate

counts have also been used to determine the effect of

thermal effluents on bacteria (Solski 1974, Guthrie et

al. 1978). It is now known that plate count approaches

grossly underestimate total bacteria, total viable bacteria or bacterial diversity in samples (Ferguson et al.

1984, Giovannoni & Rappé 2000). Thus, global effects

of thermal effluents on marine bacteria occurring

naturally might not have been accurately measured.

Based on earlier studies, however, Langford (1990)

concluded that short-term temperature exposure had

little effect on bacteria over the range normally found

in thermal discharges and that the total effects of a

thermal discharge would mainly depend on the concentrations of chlorine residuals present.

The goals of the present study were to assess the

effects of thermal discharge on coastal bacteria and

HNF using well-established, quantitative, microbial

ecological techniques and to confirm the conclusions of

Langford (1990). To analyze effects of the thermal

effluent on coastal bacteria and HNF, we measured

bacterial abundance and production, HNF abundance

and grazing rates on bacteria along with environmental parameters in coastal waters around a coastal

power station. We also carried out manipulation and

dilution experiments to evaluate the effects of thermal

effluents on coastal bacteria and HNF.

MATERIALS AND METHODS

A coastal power station at Hadong. Hadong Power

Station is located between Stn I (intake) and D (discharge) in a coastal area of Kwang-yang Bay, Korea

(Fig. 1). The inlet for intake of seawater is located

at 10 m depth, and thermal effluents are discharged

to the receiving seawater at the outlet. At present,

4 power plants are operating, and a direct cooling

system is employed. The time taken for the cooling

water to travel between the dosing point and the outfall is ca 20 min. To prevent growth of biofilms on the

surface of condenser tubes, sodium hypochlorite produced from electrolysing of seawater is applied continuously. The initial dose of sodium hypochlorite is

ca 0.35 ppm chlorine. However, the residual chlorine

concentration of seawater in the undiluted effluent

stream is kept at ca 0.05 to 0.10 ppm. No significant

difference was found in pH between intake and outlet

areas (Huh 2000).

Study area and sample collection. During 4 investigations in November 1998 and July, August and

November 1999 in the Hadong area (Fig. 1), seawater

samples were collected with 5 l Niskin bottles from 0, 3

and 5 m depth. Stn D was located at the center of the

outlet, which was connected to an adjacent coastal

area. At this station, surface seawater samples were

collected with a clean 1 l polycarbonate bottle. Each

sample was used for determining chlorophyll a (chl a),

HNF abundance, HNF grazing rates on bacteria and

bacterial abundance and production.

Bacterial abundance and production. Samples for

measurements of bacterial abundance were fixed with

0.2 μm filtered, borate-buffered formalin (final conc. of

2%). Bacteria, stained with DAPI (4’,6’-diamidino-2-

phenylindole) and collected on black 0.2 μm polycarbonate filters, were counted under UV excitation using

an epifluorescence microscope (Porter & Feig 1980). Bacterial production was measured by the 3

H-thymidine and 14C-leucine incorporation method (Ducklow et al. 1992).

Ten milliliter samples, in triplicate, were filled into sterile polypropylene tubes, and both 3

H-thymidine (specific

activity = 84 Ci mmol–1) and 14C-leucine (specific activity

= 315 mCi mmol–1) were added to the tubes, each at

10 nM (final conc.). All the tubes were incubated in the

2
Choi et al.: Effects of thermal effluents on coastal microorganisms

dark for 0.5 to 1 h at in situ water temperature. Formalin-killed samples (final

conc. of 2%) served as blanks. After incubation, 3

H-thymidine and 14C-leucine

incorporation was stopped by adding 2%

formalin. The samples were filtered onto

0.2 μm cellulose nitrate filters (Whatman)

and rinsed according to Ducklow et al.

(1992). The filters were radio-assayed.

The incorporated radioactivity was converted to cell number produced using

a conversion factor of 2.65 × 1018 cells

mol–1 thymidine incorporated and 0.18 ×

1018 cells mol–1 leucine incorporated

(Ducklow et al. 1992).

HNF abundance and grazing rates

of HNF. Samples for measurements of

HNF abundance were fixed immediately with alkaline Lugol solution (final

conc. of 0.5%) and borate-buffered formalin (final conc. of 3%, Sherr et al.

1989). Primulin-stained HNF collected

on 0.4 μm polycarbonate filters (25 mm

diameter) were enumerated with UV excitation using an epifluorescence microscope (Caron 1983). HNF grazing rates on bacteria

were measured by using fluorescently labeled bacteria

(FLB) prepared according to Sherr et al. (1987). For detailed descriptions, see Cho et al. (2000). Briefly, FLB

were prepared by staining natural bacterial samples

collected from coastal waters with 5-(4,6-dichlorotriazin-2-yl) aminofluorescein (DTAF). The added concentration of FLB was from 4 to 14% of the bacterial

abundance. Grazing experiments were conducted in

the dark at in situ temperature. Subsamples of 30 ml

were collected at 0, 10, 20 and 30 min, fixed and kept

refrigerated until microscopic examinations. Subsamples were primulin-stained and filtered, and at least

150 microscopic fields were counted. Bacterivory rates

were measured over the linear portion of the uptake

curve. The ranges for the coefficient of variation (i.e.

100 × SD over the mean) of the slopes from duplicate

bottles were from 2 to 40% (mean ± SD: 13.7 ± 9.2%).

Grazing rates were calculated by multiplying the ratio

of bacterial abundance to FLB by the mean values of

ingested FLB (l–1 h–1).

Manipulation and dilution experiments. We carried

out manipulation and dilution experiments to assess

the short-term toxic effects of thermal effluents on bacteria and HNF in coastal waters. In the first manipulation experiment, using intake seawater, we assessed

influences of temperature and chlorine (added as sodium hypochlorite) on coastal microbes: (1) 0.5 ppm

(final conc.) of chlorine was added to 2 polycarbonate

bottles, which were incubated at in situ temperature;

(2) addition of 0.5 ppm (final conc.) of chlorine and increase of water temperature to 40 ± 1°C were applied

in 2 other bottles; (3) water temperature was increased

to 40 ± 1°C in the remaining 2 bottles. Another 2 bottles

were employed as controls and incubated at in situ

temperature. Before the treatments, 4 bottles were

pre-incubated for ca 40 min at 40°C, the temperature

inside the cooling system. Incubations of samples

lasted 2 h, and subsampling was carried out every

hour. In a second series of experiments, we only

focused on influences of varying concentrations of

chlorine on coastal bacteria and HNF. We added varying concentrations (0.13, 0.25 and 0.5 ppm chlorine

equivalent) of sodium hypochlorite to coastal seawater

samples and incubated samples for 2 h at ambient

temperature. Subsamples were collected every hour

for measurements of bacterial abundance and production and HNF abundance. In dilution experiments, the

effluents were cooled to near in situ coastal water temperature and then diluted with coastal seawater 2-

(or 5-), 10-, and 100-fold. After 30 min incubations

of coastal waters, the effluents and variously diluted

effluents, samples were collected for measurements of

microbial variables.

Other analyses. Water temperature was measured

with a digital thermometer (APPA Technology Co.).

Salinity was measured with a temperature-salinity

bridge (YSI 30). Samples for chl a measurements were

filtered on GF/F (Whatman) filters. The filters were

kept at –20°C and brought to the laboratory. Concen3

Fig. 1. Study area and sampling stations in Kwang-yang Bay, Korea
Mar Ecol Prog Ser 229: 1–10, 2002

trations of chl a were measured spectrophotometrically

according to Parsons et al. (1984). Concentrations of

residual free-chlorine were measured directly at the

sampling points using a portable photometer (YSI 9600)

and the recommended tablets (Palintest Ltd, Catalogue

No. YAP 021 containing DPD (diethyl-p-phenylene diamine)). The tablets were added to seawater samples,

and a pink color developed as a result of a reaction. Linear regression and t-tests were performed using SPSS

for Windows (SPSS Inc. Version 8.0).

RESULTS

Surface distributions of temperature, 

chl a, bacterial and HNF variables, 

and residual chlorine

Only data from surface waters were presented here, because data from the surface

more clearly showed the patterns of influences of thermal discharges than those

from subsurface waters. Surface water temperature at the outlet was between 27.8 and

29.9°C in summer and between 23.2 and

27.2°C in November, and always from 5 to

10°C higher at the outlet (i.e. Stn D) than at

any other station (Fig. 2A). Surface water

temperature was, however, almost uniform

within distances of 1 to 5 km from the power

station throughout the investigations. Chl a

concentrations were always low at Stn D,

but increased with distance (Fig. 2B), except in August 1999. Bacterial abundance

was also lower at Stn D than at other stations, except in August 1999 (Fig. 2C). Bacterial production was always lower at Stn D,

and increased with distance (Fig. 2D). Both

HNF abundance and HNF grazing rates on

bacteria were always lower at Stn D than at

other stations and increased with distance

(Fig. 2E,F). The increase of bacterial production, HNF abundance and HNF grazing

rates with distance from the power station

was more pronounced in summer than in

winter (Fig. 2D–F). Concentrations of residual chlorine were below 0.03 ppm in coastal

waters including Stn D during this study

(data not shown), with the exception of 1

sample that had 0.05 ppm (in August 1999

at the surface at Stn C2).

Microbial ecological interrelationships

In a temperature vs salinity diagram

(Fig. 3A), it was obvious that data from

Stn D distributed separately from the remaining data, due to the high temperature there. When data from Stn D were

4

Fig. 2. (A) Temperature, (B) chlorophyll a (chl a), and (C, D) bacterial and

(E, F) heterotrophic nanoflagellate (HNF) variables vs distance from the

power station (BP-TDR, bacterial production based on thymidine incorporation). Mean ± 1 SD of data are shown for stations at the same distance.

When error bars are not shown, 1 SD is less than the size of the symbol. Different symbols represent different investigation dates: (j) November 1998, 

(d) July, (m) August, and (✚) November 1999
Choi et al.: Effects of thermal effluents on coastal microorganisms

excluded, some interrelationships improved very significantly (Fig. 3B–D), illustrating the negative effects

of the effluents on the corresponding microbial variables (i.e. low bacterial abundance and production and

HNF abundance). The remaining interrelationships

(Fig. 3E–H) showed trends typical of coastal waters,

regardless of inclusion of Stn D data;

bacterial cell production increased with

increases in chl a (Fig. 3E), and bacterial protein synthesis rates were tightly

coupled with DNA synthesis rates

(Fig. 3F). HNF abundance and grazing

rates increased with increases in bacterial cell production (Fig. 3G,H).

Manipulation and dilution

experiments

One of the typical results is shown

(Fig. 4A), in which increased temperature (40°C) and/or hypochlorite addition were applied to nearshore waters.

Increased temperature had somewhat

inhibitory effects on bacterial abundance and bacterial production. After

1 h of exposure to 40°C, bacterial abundance decreased substantially (21%

reduction) compared to the control and

was still lower after 2 h of incubation.

Bacterial protein synthesis rate, after

2 h of exposure to 40°C, was 61% of the

control value. Additions of hypochlorite

(giving 0.5 ppm chlorine equivalent)

caused almost complete inhibition of

bacterial production. Addition of

0.5 ppm chlorine as hypochlorite also

caused a ca 50% decrease in bacterial abundance. But, an addition of

0.13 ppm chlorine as hypochlorite did

not cause any change in bacterial abundance (see Fig. 4B). Double treatments

of high temperature and chlorine

(0.5 ppm) caused similar decreases in

bacterial abundance and production as

chlorine (0.5 ppm). Thus, the results

indicate that effects of low levels of

chlorine are dominant over the effects

of high temperature (40°C). Interestingly, high temperature had no inhibitory effect on HNF abundance. After

2 h of incubation, HNF abundance, in

contrast, increased significantly (24%

increase compared to control). After 2 h

of hypochlorite additions, HNF abundance significantly decreased to 31% of the control.

Combined effects of high temperature and chlorine

(0.5 ppm) were statistically similar (p > 0.05) to the

chlorine effect on HNF abundance during the 2 h of incubation, suggesting no synergistic effect of high temperature and chlorine on HNF. Overall, these results

5

Fig. 3. Interrelationships between temperature and (A) salinity, (B) bacterial

abundance (BA), (C) bacterial production based on thymidine incorporation

(BP-TDR) and (D) heterotrophic nanoflagellate (HNF) abundance; between

chlorophyll a (chl a) and BP-TDR (E); between bacterial production based on

leucine incorporation (BP-LEU) and BP-TDR (F); between BP-TDR and HNF

abundance (G); and between BP-TDR and HNF grazing rates (GR) (H). D: data

from discharge-channel. Solid and dashed lines: regression lines for all data

and data without outlet, respectively (the statistics and equations for solid and

dashed lines are shown at the top and bottom, respectively, on the right side of 

each panel)
Mar Ecol Prog Ser 229: 1–10, 2002

suggest that the effect of chlorine would be more dominant during a short-term period than that of high temperature to coastal bacteria and HNF in the study area.

In other experiments, various concentrations of chlorine were added to seawater (Fig. 4B). It was clearly

shown that a chlorine concentration as low as 0.13 ppm

almost completely inhibited bacterial cell production

and protein production activities. Decreases in bacterial and HNF abundances were detected at concentrations >0.13 ppm chlorine after 1 h of incubation.

We further tested the effects of thermal effluents by

diluting discharged waters with nearshore waters

(Fig. 5). In one experiment (Fig. 5A) in which outlet

water was used, bacterial production showed somewhat lower values than those expected from mixing

of the nearshore waters and effluents. In another

experiment, in which condenser water was used

(Fig. 5B), the effluent showed no inhibitory effect on

bacterial abundance at 10- and 100-fold dilutions,

similar to the first experiment, but there were significant inhibitory effects on bacterial production, HNF

grazing rates and HNF abundance. The inhibitory

effects were the greatest at the lowest dilution.

Although drastic decreases in bacterial production

were observed in both outlet and condenser waters,

substantial amounts (4 to 21% of bacterial production

found in inlet seawater) of bacterial production were

observed (Fig. 5).

6

Date Dilution Bacterial production- Bacterial production- HNF HNF

factor TdR Leu abundance grazing rates

at outlet (×107 cells l–1 h–1) (×107 cells l–1 h–1) (×106 HNF l–1) (×107 cells l–1 h–1)

Meas Est Meas Meas Est Meas Meas Est Meas Meas Est Meas

Est Est Est Est

Nov 1998 3.20 0.66 1.26 0.52 0.34 1.44 0.23 0.08 0.23 0.33 – – –

Jul 1999 3.82 nd 12.09 – nd 14.63 – 0.38 1.30 0.29 0.20 0.74 0.27

Aug 1999 3.20 0.20 2.10 0.10 0.02 2.92 0.01 0.10 0.36 0.28 0.18 0.16 1.09

Nov 1999 2.45 0.29 0.73 0.39 0.09 0.65 0.13 0.08 0.12 0.68 0.06 0.07 0.78

Table 1. Comparisons of measured (Meas) and estimated (Est) values of bacterial production, HNF (heterotrophic nanoflagellate)

abundance and grazing rate at the outlet. Dilution factor of condenser water (F) with receiving seawater at the outlet was calculated on the basis of water temperatures at the outlet, coastal waters (within 2 km from the outlet), and condenser tubes. Estimated value equals (average value for the coastal waters) × [(F –1)/F ], assuming that microbial variables in condenser water were 

zero. nd: not detected; –: not determined

Fig. 4. (A) Effects of increased temperature (40°C) and hypochlorite on microbial variables. Different symbols represent different

treatments: (s) control, (✚) addition of 0.5 ppm chlorine as hypochlorite, (d) increased temperature, (j) increased temperature and

addition of 0.5 ppm chlorine. (B) Effects of different concentrations of chlorine added as hypochlorite on microbial variables. Different symbols represent different concentrations of chlorine: (s) control, (j) 0.13, (✚) 0.25, and (r) 0.5 ppm chlorine. Experiments 

were done in August 1999. Error bars represent 1 SD. When error bars are not shown, 1 SD is less than the size of the symbol
Choi et al.: Effects of thermal effluents on coastal microorganisms

DISCUSSION

Analyses of horizontal distributions of

microbiological variables, interrelationships between microbiological variables and temperature, and manipulation and dilution experiments gave

consistently similar results: thermal

effluents seemed to have inhibitory

effects on bacterial production and on

HNF grazing rates and abundance.

Treatments of full-load chlorination and

high temperature in the cooling system

seemed to be highly effective in reducing activities and abundances of natural marine bacteria and HNF. Further,

our results suggest that inhibitory effects are overwhelmingly due to chlorination and not to thermal loading of the

effluents.

A noteworthy observation of this

study was that bacterial and HNF activities were significantly inhibited on

diluting the effluents with intake seawater 10- to 100-fold. Since such dilutions of the effluents with intake seawater (<0.03 ppm chlorine) will lower

the chlorine concentration below 0.03

ppm, which is the natural background

level (Eppley et al. 1976), no inhibitory

effects on micro-organisms would occur.

Thus, the inhibitory effects of the effluents on bacteria and HNF at such dilutions strongly suggest the presence of

inhibitory substances (e.g. chlorination

by-products) formed during the chlorination process in the effluents.

The presence of inhibitory chlorination by-products in the thermal effluents also helps explain the strikingly low values of bacterial production and HNF grazing and abundance in

the outlet, where the residual chlorine concentration

was already at natural background levels. Mixing of

the thermal effluent with receiving seawater at the

outlet is calculated to bring a 3.2 ± 0.6-fold dilution of

the effluent, based on temperature data from the outlet, coastal seawater and condenser tubes. If chlorination treatments were assumed to have a completely

lethal effect on marine microbes in the effluents and

dilution of the effluents with receiving water did not

cause any effect on micro-organisms, ca 2/3 of the values for microbial variables of the coastal waters were

expected to be found at the outlet (this might be a conservative estimate). If some bacteria and HNF survived

during the chlorination treatment (see Fig. 5), the values would be somewhat higher. In reality, measured

values were much lower than the expected ones

(Table 1): bacterial cell production and protein production was on average 34 ± 22% and 12 ± 11% of the expected values, respectively (Table 1). HNF abundance

and grazing rate was, on average, 41 ± 23% and 71 ±

41% of the expected values, respectively. If the positive effects on metabollism of temperature increases at

the outlet are considered, the estimated values would

be much higher than those in Table 1. Therefore, the

ratios of measured to estimated values could be much

lower than those in Table 1. This implies that inhibitory

substances (i.e. presumably chlorination by-products)

formed during the chlorination process were present in

the thermal discharges and further inhibited bacterial

and HNF activities in the outlet. Interestingly, the in7

Fig. 5. Dilution experiments. (A) Discharged waters from outlet or (B) freshly discharged waters from condenser tubes were diluted in different ratios, with

intake seawater. Shaded bars: the difference between the measured (bars without shading) and predicted values (total height of bars) when 2 types of waters

were mixed with given ratios. BA: bacterial abundance; BP-TDR and BP-LEU

(hatched): bacterial production measured by thymidine and leucine incorporation, respectively; HNF: heterotrophic nanoflagellate; GR: HNF grazing rate.

Experiments A and B were done in August and November 1999, respectively.

Error bars represent 1 SD. When error bars are not shown, 1 SD is less than the 

size of the symbol
Mar Ecol Prog Ser 229: 1–10, 2002

hibitory effects were apparently more pronounced in

summer than in winter (Table 1), although our data

were limited. Consistently, formations of halogenated

compound in the discharge area are known to be

higher in summer than in winter (Allonier et al. 1999),

probably related to the nature of the organic matter in

the seawater.

In seawater, humic substances, constituting substantial fractions of dissolved organic matter (Thurman

1985), could serve as precursors of toxic organo-halogenated compounds produced during chlorination in

seawater. Chlorination by-products are detected from

the thermal effluents of coastal nuclear power stations

in which low-level chlorination was routinely used.

Bromoform, dibromoacetic acid and dibromoacetonitrile are the main compounds (Allonier et al. 1999).

Further, the bromoform concentration in the plume of a

coastal power plant was measurable (0.26 μg l–1) at a

distance of 1.5 to 15 km from the outlet of thermal

effluents, although concentration of residual oxidants

was below 0.03 ppm in the field (Jenner et al. 1997).

Chlorinated humic substances from diverse sources

are known to genetically mutate a strain of Salmonella

typhimurium (Meier et al. 1987, Watt et al. 1996). As

Allonier et al. (1999) suggested the possible toxicity of

such compounds towards aquatic organisms, diverse

halogenated compounds formed during chlorination of

seawater might have induced toxic effects on bacteria

and HNF or even have lethal effects in our samples.

As Langford (1990) concluded, in the cooling water

system, chlorination seems to be the overwhelmingly

responsible reason for the inhibitory effects. Combined

treatments of chlorination and high temperature gave

similar effects as chlorination treatment (Fig. 4). All

the treatments of seawater with chlorination (0.13 to

0.5 ppm) caused drastic inhibition of bacterial production and significant reduction of HNF and bacterial

abundances, indicating some lethal effects on bacteria

and HNF (Fig. 4). Similarly, 14C-glucose uptake by

bacteria was reported to be null when chlorination was

in progress (Davis & Coughlan 1978), indicating total

suppression of bacterial activity by chlorination. Wood

(1979, cited in Langford 1990) also reported that

0.1 mg l–1 residual chlorine caused at least 90% reductions of culturable bacterial counts. In this respect,

observations of detectable bacterial production in condenser waters are interesting, suggesting the presence

of chlorination-resistant bacteria, presumably derived

from biofilms on walls of the cooling systems. Overall,

it seems that chlorination is lethal for a fraction of the

exposed natural marine bacteria and HNF in cooling

water and induces injury in a large proportion of the

remaining cells.

In contrast, the effects of high temperature were not

as pronounced as those of chlorination. In an experiment where water temperature increased to 40°C for 

1 to 2 h (Fig. 4A), bacterial cell production and abundance were only from 9 to 22% and 10 to 21% lower,

respectively, than the control incubated at in situ temperature. High temperature seems to have no negative

effect on HNF abundance at all: HNF abundance, in

contrast, increased ca 24% after 2 h incubation compared to control. Thus, short-term (<30 min) exposure

of coastal bacteria and HNF to 40°C would not seem to

cause detrimental damage to bacteria (or probably

HNF either) during passage through the cooling system. At the outlet, water temperature was 7 to 10°C

and up to 5°C higher than ambient seawater temperature in November and in summer, respectively. Bacteria and HNF in the discharge area could be released

from high-temperature (e.g. 40°C) stress, but still

be exposed to temperatures elevated above ambient

coastal seawater. Although we did not test the effects

of temperatures elevated 5 to 10°C above the in situ

temperature, such temperatures may not have inhibitory effects on bacteria and HNF (Marrasé et al.

1992, Shiah & Duckow 1994). Rather, they might have

stimulating effects on microbial activities (Massana

et al. 1994, Shiah & Duckow 1994, Vaqué et al. 1994).

Were the low values of bacterial production at the

outlet due to the effects of low-level chlorination on 3

Hthymidine and 14C-leucine? In other words, chlorination

might cause chemical transformation of the tracers so

that incorporation rates of the tracers are reduced.

Thus, bacterial production could be underestimated due

to the lowered concentrations of tracers at the outlet

and in condenser samples. We checked this possibility

by adding stock solution of tracers to double-distilled

water in which hyperchlorite was contained (final conc.

of 0.03 and 0.1 ppm chlorine equivalent). After 30 min

of incubations with hyperchlorite, the stocks were

added (vol. of 140 μl) to 10 ml seawater to measure bacterial production. There were no significant differences between hyperchlorite-treated and control tracer

stocks for bacterial production rate measurements (data

not shown). Thus, low values of bacterial production

were due to inhibitory effects of thermal effluents, not

due to chemical alterations of tracers by chlorination.

So far, we have presumed that toxic effects arise

mainly from chlorination and chlorination by-products.

However, any inhibitory substance might yield similar

results. Other possible toxic substances could be heavy

metals, antifouling paint, or ash discharges (Larrick et

al. 1981, Langford 1990). Distributions of total heavy

metals of various elements (Fe, Zn, Cu, Cr, Pb and Cd)

were not different among stations including the discharge channel (Huh 2000, S.-R. Yang pers. comm.).

Further, Hadong Power Station does not treat the cooling system with antifouling paint or other coatings.

Also, no ash effluents are discharged near the thermal

8
Choi et al.: Effects of thermal effluents on coastal microorganisms

discharges in the study area. Thus, it seems that the

observed inhibitory effects of thermal effluents on

coastal microbes were actually due to chlorination and

chlorination by-products.

Finally, our results suggested that the range in which

thermal effluents affect bacteria and HNF in receiving

coastal waters might be proportional to 100 times the

volume of the flux of condenser water. However, this

could be a maximal estimate because of potential chlorine decay due to volatilization of chlorine, progressive

chemical reactions of chlorine with seawater constituents, and the photochemical destruction of chlorine during mixing in the receiving water (Eppley et

al. 1976). Depressed activities of bacteria and HNF in

the affected region would slow down regeneration of

nutrients and flow of energy and material through the

microbial food web. Efforts in developing techniques

to reduce the toxic effects of thermal effluents are

increasing. Recently, with the aid of ‘mussel monitor’,

‘pulse-chlorination’ technology using short (i.e. a few

minutes), intermittent chlorination has reduced the

amount of chlorine by 50% compared to continuous

chlorination (Jenner & Polman 1998). As chlorination is

still the most widely applied method for antifouling,

pulse-chlorination would be preferable with fewer

subsequent environmental effects.

In conclusion, thermal effluents had inhibitory effects

on bacterial production and on HNF grazing rates on

bacteria. The inhibitory effects seemed to be mostly

due to chlorination of seawater and chlorination byproducts in the effluents. Our results suggest that natural marine micro-organisms seem to be sensitive to

thermal pollution, and microbial ecological approaches

are useful in assessing the magnitude of influences of

thermal effluents on coastal microbes.

Acknowledgements. We thank Korea Electric Power Corporation (KEPCO) for their cooperation during the study. We

thank Dr Jed Fuhrman for careful reading of an earlier version of our manuscript. We thank 4 anonymous reviewers for

helpful comments. The present study was supported (in part)

by the BK 21 Project of the Korean Government and a KEPCO

project.

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10

Editorial responsibility: Otto Kinne (Editor), 

Oldendorf/Luhe, Germany

Submitted: December 12, 2000; Accepted: August 16, 2001

Proofs received from author(s): March 4, 2002