Difference in Macrobenthic Community Structures at Thermal Effluent Discharge Areas of Two Nuclear Power Plants in Korea

1. Introduction

Benthic organisms are capable of actively responding to physicochemical variables in aquatic environments, and their spatial and temporal distributions are largely influenced by sediment properties such as sediment particle sizes and organic carbon contents (Gray[1974]; Pearson and Rosenberg[1978]; Weston[1990]). Their distribution and community structure are highly affected by various human activities including logging, mining, reclamation, dredging, dumping, water pollution, oil spills, fishing, fish farming and so forth (Gray[1997]; Pearson and Rosenberg[1978]; Thrush and Dayton[2002]; Weston[1990]). In addition, altered temperatures via thermal pollution by fossil- fuel and nuclear power plants have potentials to affect physiological processes of all aquatic life forms and thus have caused great concerns in residents and fishermen of affected areas (Krishnakumar et al.[1991]).

Kori Nuclear Power Plant (KNPP), constructed in 1978, was the first large-scale nuclear power plant in Korea (Fig. 1). Composed of four turbines, it uses 5.1 billion tons of coastal seawater per year as condenser coolant and directly discharges heated effluent onto sea surface (KHNP[2013]). This discharge system has been widely adopted by most thermal power plants in Korea because of its economic feasibility (GESAMP[1984]); however, it has caused considerable public concerns and debates on probable adverse effects on coastal ecosystems. Sinkori Nuclear Power Plant (SNPP), constructed in 2012, has four turbine generators and uses 2.8 billion tons of coastal seawater per year (Fig. 1). In an effort to minimize environmental impacts associated with heated effluent, it adopted a novel outfall design where heated effluent goes through pipe lines submerged into seabed and discharges through diffusers at 15-20 m depth into overlying water column (KHNP[2013]). This discharge system is expected to mitigate ecological disturbance by rapid dilution and heat dispersal of effluent through mixing with underwater currents. As a result, the extent of heated effluent plume at the SNPP discharge area is much narrower than that at the KNPP (Fig. 1).

In this study, we compared sediment properties and macrobenthic community structures at the thermal effluent discharge areas of KNPP and SNPP. This study will provide baseline data that will improve not only our understanding of the ecological effects of thermal effluent, but also our ability to take effective conservation measures in the impacted areas.

2. Materials and methods

2.1 Sediment sampling

Study areas for KNPP and SNPP were limited to areas with a delta temperature (Δt) of 3 °C, which indicates a 3 °C increase in seawater temperature caused directly by heated effluent plumes (Fig. 1). Sediment samples were collected from each of 21 stations in soft bottoms at the two discharge areas between June 11-13, 2013 (Fig. 1), after considering the tide which had considerable effects on thermal plumes. Roughly 30 g of surface sediment was collected with a van Veen grab sampler (surface area: 0.1 m²). To analyze macrobenthic community structure, tworeplicate samples were collected and pooled together (total area: 0.2 m²). Fine sediment particles were removed with a 1-mm metal sieve, and the remaining sample was fixed with 10% neutralized formalin. Fixed samples were kept in watertight plastic bags and transported to the laboratory on ice in a cooler.

Fig. 1.

Two study areas showing sampling stations at the thermal effluent discharge areas of Kori Nuclear Power Plant (KNPP), which has the surface discharge system, and Sinkori Nuclear Power Plant (SNPP), which has the submerged discharge system. All sampling stations are located within the heated effluent plume of delta 3 °C.

3. Results

3.1 Sediment properties

Sediment textures at the KNPP and SNPP discharge areas are shown in Table 1. At the discharge area of KNPP, gravel content ranged from 0.0-41.1% [6.5 ± 9.5% (mean ± SD)], while sand was 11.8-93.6% (47.6 ± 29.9%), silt was 0.1-37.3% (19.4 ± 15.3%) and clay was 0.1-48.2% (26.5 ± 19.5%). Their mean particle size (ϕ) at KNPP was 4.3 ± 3.2. At the SNPP discharge area, gravel content ranged from 0.0-15.1% (4.0 ± 3.8%), sand was 33.4- 94.9% (70.0 ± 17.0%), silt was 0.1-27.1% (11.4 ± 8.1%) and clay was 0.1-39.5% (14.7 ± 10.9%). Their mean particle size (ϕ) at SNPP was 3.1 ± 1.7. Overall, sediment composition was similar between the two discharge areas, with sand comprising the majority of each area, followed by clay, silt and gravel. However, the SNPP discharge area had significantly higher sand content (70.0% vs. 47.6%) and lower clay content (14.7% vs. 26.5%) than the KNPP (both P < 0.05).

Table 1

Sediment texture, composition and organic content [minimum-maximum values (mean ± standard deviation)] at the thermal effluent discharge areas of Kori Nuclear Power Plant (KNPP) and Sinkori Nuclear Power Plant (SNPP) (n = 21)

Spatial distribution patterns of the sediment types (Folk[1968]) at the KNPP and SNPP discharge areas are shown in Fig. 2. Sediment samples were divided into four types: sand, muddy sand, sandy mud and clayey sand. Samples from KNPP (n = 21) were predominantly sandy mud (n = 12), followed by sand (n = 7), muddy sand (n = 1) and clayey sand (n = 1). In contrast, sediment samples from SNPP (n = 21) were mostly muddy sand (n = 15), followed by sand (n = 5) and sandy mud (n = 1). At both KNPP and SNPP, sandy sediment was typically found at stations proximal to the outfalls under the direct physical influence of discharged effluent, and both sandy mud and muddy sand were found distal to the outfall.

Organic carbon content was significantly higher at the KNPP discharge area (range: 2.6-8.7%; mean ± SD: 4.9 ± 1.5%) than that at the SNPP (2.2-6.2%; 3.9 ± 1.0%) (Table 1; P < 0.05).

Fig. 2.

Spatial distributions of sediment types at the thermal effluent discharge areas of Kori Nuclear Power Plant (KNPP) (A) and Sinkori Nuclear Power Plant (SNPP) (B). Seawater depth contours are marked by gradients.

3.2 Macrobenthic community structure

Species richness, abundance and biomass of macrobenthos at the thermal effluent discharge areas of KNPP and SNPP are compared in Table 2. Species richness at the KNPP discharge area ranged from 5 to 52 species (St. 3 and 21, respectively), with a mean of 27 ± 12 species. A total of 153 species were found in the KNPP discharge area, of which abundances ranged from 50 to 1,517 (St. 3 and 2, respectively) with a mean of 743 ± 380 ind. m^-2. Total biomass ranged from 0.30 to 1,833.50 ww g m^-2 (St. 3 and 10, respectively) with mean of 156.14 ± 388.38 ww g m^-2. Meanwhile, at the SNPP discharge area, species richness ranged from 15 to 52 (St. 6 and 4, respectively) with a mean of 34 ± 10 species. A total of 195 species were found. Abundances ranged from 338 to 2,088 m^-2 (St. 7 and 15, and St. 6, respectively) with a mean of 884 ± 394 ind. m^-2, and biomass ranged from 16.75 to 636.80 ww g m^-2 (St. 16 and 18, and St. 20, respectively) with a mean of 127.39 ± 154.15 ww g m^-2. Species richness was significantly higher at the SNPP discharge area compared with KNPP (P < 0.05), but there were no statistical differences in abundance and biomass.

Table 2.

Species diversity, abundance and biomass of macrobenthos at the thermal effluent discharge areas of Kori Nuclear Power Plant (KNPP) and Sinkori Nuclear Power Plant (SNPP) (n = 21)

Spatial distributions of macrobenthic species richness at the KNPP and SNPP discharge areas (Fig. 3) were similar in pattern to the distributions of sediment types (Fig. 2). Species richness was low at the stations close to the discharge outfalls and high at the ones on the outer edges of both discharge areas. Fig. 4 shows macrobenthic community compositions in terms of species richness, abundance and biomass of the KNPP and SNPP discharge areas. Among the macrobenthos, Polychaeta was the most dominant taxon at both discharge areas, with the largest number of species (50.4% and 46.2% of species richness at KNPP and SNPP, respectively), followed by Arthropoda (17.6% and 23.1%), Mollusca (12.4% and 12.3%) and Echinodermata (7.2% and 6.7%). It was also the most abundant taxon at both KNPP and SNPP (62.8% and 65.9% of samples, respectively). At KNPP, the second most abundant taxon was Arthropoda (12.1%), followed by Echinodermata (9.8%) and Mollusca (5.8%), whereas Echinodermata was the second most abundant at SNPP (10.9%), followed by Arthropoda (7.3%) and Mollusca (6.4%). Composition ratios by biomass were distorted by the occurrences of large-bodied invertebrates, which included the cnidarian Cerianthus filiformis at the KNPP discharge area and the molluscan Siphonalia fusoides at SNPP.

Fig. 3.

Spatial distributions of species richness (species 0.2 m-2) at the thermal effluent discharge areas of Kori Nuclear Power Plant (KNPP) (A) and Sinkori Nuclear Power Plant (SNPP) (B).

Fig. 4.

Taxonomic compositions of macrobenthos in terms of species richness, abundance and biomass at the thermal effluent discharge areas of Kori Nuclear Power Plant (KNPP) (A) and Sinkori Nuclear Power Plant (SNPP) (B).

BPI and AMBI values were calculated to evaluate the health status of the benthic habitat at the two thermal effluent discharge areas of KNPP and SNPP based on the macrobenthic community structures (Table 3). BPI values ranged from 47 to 92 with a mean of 66 ± 13 at KNPP and from 18 to 76 with a mean of 61 ± 11 at SNPP. According to Choi et al.[2003]), both discharge areas resembled offshore environments without organic pollution. Similarly, AMBI values at KNPP (0.3-3.0; mean ± SD: 1.9 ± 0.7) and SNPP (1.2-2.7; 1.9 ± 0.4) indicated that the ecological status of both discharge areas was ‘good’ (Borja et al.[2003]; five classes: high, good, moderate, poor and bad).

Table 3.

Minimum and maximum values (mean ± standard deviation) of the Benthic Pollution Index (BPI) and Azti’s Marine Biotic Index (AMBI), which indicate pollution and ecological status of benthic habitats at the thermal effluent discharge areas of Kori Nuclear Power Plant (KNPP) and Sinkori Nuclear Power Plant (SNPP) (n = 21)

4. Discussion

This study investigated and compared the sediment properties and macrobenthic community structures at the thermal effluent discharge areas of two nuclear power plants with differing discharge systems. Sediments of the discharge areas of KNPP and SNPP were both composed primarily of sand, followed by clay, silt and gravel. However, there was significantly less sand and more clay at the KNPP discharge area (47.6% and 26.5%, respectively) than those in the SNPP (70.0% and 14.7%, respectively). These differences in the ratio of sand and clay contents were also reflected in a larger mean particle size at the KNPP discharge area (ϕ = 4.3) than that at the SNPP (ϕ = 3.1). Furthermore, sediment texture differences were well aligned with those of sediment types; the KNPP discharge area was mostly composed of sandy mud, whereas SNPP was dominated by coarser muddy sand. Correlation analysis confirmed that most sediment properties (sand, clay and organic carbon content, and mean particle size) were significantly correlated with each other at both discharge areas (data not shown).

In agreement with previous studies (Kim et al.[2011]; Loi and Wilson[1979]), species richness and abundance of macrobenthos were higher at the SNPP discharge area where the sediments have higher sand and lower organic carbon contents than those at the KNPP, although only species richness was significantly higher. Sediment properties play a pivotal role in driving spatial distributions of macrobenthos (Gray[1974]; Pearson and Rosenberg[1978]; Weston[1990]). Though our findings cannot be directly compared with previous studies because of differences in seasonality, sampling volume and expertise in taxonomy, the species richness of the KNPP and SNPP discharge areas (153 and 195 species, respectively) were quite similar to those found previously at the KNPP discharge area (157 species, Kim et al.[2011]), as well as in unaffected eastern coastal areas (136 species, Choi et al.[2000]; 117 species, Shin et al.[2001]). They were also quite similar to those of Wolseong Nuclear Power Plant on the eastern coast of Korea (163 species, Seo et al.[2009]), but substantially lower than the species richness in uncontaminated coastal areas in Korea (319 species, Paik et al.[2007]; 284 species, Yoon et al.[2009]). These comparisons suggest that species richness was not significantly decreased (e.g., Krishnakumar et al.[1991]) by thermal effluent at the KNPP and SNPP discharge areas.

Abundance of macrobenthos at the KNPP and SNPP discharge areas (743 and 884 ind. m^-2, respectively) were comparable but slightly higher than abundances previously found at the KNPP discharge area (552 ind. m^-2, Kim et al.[2011]). They were also comparable to abundances in other eastern coastal areas (1,168 ind. m^-2, Choi et al.[2000]; 535 ind. m^-2, Shin et al.[2001]) and at the Wolseong Nuclear Power Plant discharge area (1,005 ind. m^-2, Seo et al.[2009]), but less than half those of uncontaminated coastal areas in Korea (1,972 ind. m^-2, Paik et al.[2007]; 2,202 ind. m^-2, Yoon et al.[2009]).

Compositions by taxonomic group of macrobenthos were similar between the KNPP and SNPP discharge areas both in terms of species richness and abundance. Nearly half the species found at both discharge areas belonged to Polychaeta. It was also the most abundant taxon, comprising slightly more than 60% of the total abundances at both discharge areas. Previous studies of other coastal areas of Korea similarly showed that Polychaeta dominated in terms of species richness and abundance (e.g., Kim et al.[2011]; Seo et al.[2009]; Yoon et al.[2009]). Although highly contaminated areas typically exhibit abnormally high biomass of opportunistic species (e.g., Bamber and Spencer[1984]), the KNPP and SNPP discharge areas showed no obvious detrimental effects of heated effluents in terms of macrobenthic community structures.

Because our study did not include replicated control sites that experienced only natural temperatures, it is not possible to determine the direct effects of heated effluent on macrobenthic Fig. 5. Result of cluster analysis and nMDS ordination plot based on square-root transformation of abundance data of macrobenthos at the thermal effluent discharge areas of Kori Nuclear Power Plant (KNPP) and Sinkori Nuclear Power Plant (SNPP). All stations were divided into four groups at 18% similarity level. communities (e.g., Kim et al.[2013]; Lardicci et al.[1999]; Riera et al.[2011]). However, agreements between our study and previous ones at nearby areas (Kim et al.[2011]; Seo et al.[2009]) to our study areas and foreign areas (Lardicci et al.[1999]; Loi and Wilson[1979]; Riera et al.[2011]) suggest no detrimental effects of heated effluent from either KNPP or SNPP on macrobenthic community structure. However, a few previous studies have reported detrimental effects of heated effluent (Bamber and Spencer[1984]; Kailasam and Sivakami[2004]). Kim et al.[2013] also reported that species richness and abundance of meiobenthic nematodes were significantly different between impacted and control sampling locations at the KNPP discharge area.

Fig. 5.

Result of cluster analysis and nMDS ordination plot based on square-root transformation of abundance data of macrobenthos at the thermal effluent discharge areas of Kori Nuclear Power Plant (KNPP) and Sinkori Nuclear Power Plant (SNPP). All stations were divided into four groups at 18% similarity level.

Spatial distributions of sediment types within the heated effluent plumes were similar between the KNPP and SNPP discharge areas. At both areas, stations close to the discharge outfalls were composed primarily of sand, while offshore stations were mostly sandy mud at the KNPP discharge areas and coarser muddy sand at the SNPP. In addition, spatial distribution patterns of macrobenthic species richness were aligned with the sediment types. At both discharge areas, species richness was low at the proximal stations to the discharge outfalls of which sediments were relatively coarse, and high at the outer stations of which sediments were characterized by fine sediment types. These distribution patterns are consistent with those of previous studies (Kailasam and Sivakami[2004]; Kim et al.[2011]; Riera et al.[2011]) and partially explained by strong flow intensity removing fine sediments near the discharge outfalls.

Acknowledgments

This study was conducted by Korea Environment Institute (KEI) grant support. We have to disclose that this study was based on the partial extract of ‘Studies on Marine Environment Impact Prediction and Minimizing Measures according to Thermal Effluent Discharge from Power Plants’ of KEI.

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