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Technoecosystem of the cooling pond of the South Ukrainian Nuclear Power Plant: group dynamics and transformation
Tatyana N. Novoselova*, Anzhelika A. Sylaieva, Yuliya F. Gromova, Tanita I. Menshova, Irina A. Morozovskaya, Aleksandr A. Protasov
Institute of Hydrobiology of NAS of Ukraine, 04210 pr. Geroev Stalingrada, 12, Kiev, Ukraine *labtech-hb@ukr.net
The results of long-term complex hydrobiological studies of the cooling pond of the South Ukrainian Nuclear Power Plant (SU NPP) are presented. The abundance of zoo- and phytoplank-ton increased until 1984-1985, and then in the summer of 1986, against a background of extremely high temperatures in the reservoir, there was a significant (32-fold) drop in the mean biomass of phytoplankton, coinciding with the minimum abundance of zooplankton. In subsequent years, the abundance of these groups recovered, but has not reached previous levels. At present, the zooplankton contains a considerable amount of thermophilic species. Research in recent years indicates that the zooperiphyton is dominated by the invasive gastropods Melanoides tubercuiata (Müller, 1774) and Terebia granifera (Lamarck, 1822). In the epi-liton, the number of LDT (lowest determined taxa) and groups of invertebrate was more than double that in the epiphyton (17 and 7, respectively). At the first stages of the development of the pond ecosystem, the periphyton communities were dominated by zebra mussel Dreissena polymorpha Pall. These communities were eliminated as temperature increased, and after the commissioning of the NPP, the second and third power units were not completely restored in the environment of a constantly high thermal load. Over years, with the formation of bottom biotopes, the abundance of zoobenthos increased, and with an increase in the technogenic load, it decreased. At the present stage, the zoobenthos is impoverished (9 taxa) and is dominated by mainly juvenile tubificids.
Keywords: technogenic succession, phytoplankton, zooplankton, zoobenthos, zooperiphyton, gastropods, invaders, river Southern Bug.
Received: 12.11.2019 Accepted: 14.01.2020 Published online: 17.02.2020
DOI: 10.23859/estr-191112 UDC 574.5 (28)
ISSN 2619-094X Print ISSN 2619-0931 Online
Translated by S.V. Nikolaeva
Novoselova, T.N., Sylaieva, A.A., Gromova, Yu.F., Menshova, T.I., Morozovskaya, I.A., Protasov, A.A., 2020. Technoecosystem of the cooling pond of the South Ukrainian Nuclear Power Plant: group dynamics and transformation. Ecosystem Transformation 3 (1), 40-54.
Introduction
The phenomenon of ecological succession, a process of orderly and predictable changes in the structurally functional organization of a community, is characteristic of all ecosystems. However, for tech-noecosystems, anthropogenic effects are at least as important as natural processes. The effect depends on design features, the nature of operation of various water supply and water management systems of industrial facilities, power plants, etc. (Beznosov and Suzdaleva, 2005; Protasov et al., 2011). The hyd-robiological regime of cooling ponds that are part of the technoecosystems of nuclear power plants and thermal power plants is influenced by such factors as changes in the hydrodynamic regime, the supply of additional heat, the influx of organic and inorganic substances with recharged water and the discharge of wastewater (Protasov and Silaeva, 2012; Protasov et al., 1991; 1994, 2011; Zdanowski and Protasov, 1998). In addition, processes related to climate change are currently of some importance. The abundant development of some aquatic organisms can cause biological hindrances in the operation of the technical systems of a power plant (Protasov et al., 2009).
The purpose of this work is to establish the direction in the changes in a technoecosystem of the cooling pond of a nuclear power plant under conditions of technological succession.
Materials and methods
The cooling pond (CP) of the SU NPP was constructed on the Tashlyk River (the left tributary of
the Southern Bug River, 35 km south of the city of Pervomaisk) by building a dam and filling it in 19791980 with the water of the Southern Bug River. At a normal retaining level, according to design data, the area of the reservoir is 8.6 km2, the volume is 86.0 million m3. The reservoir is of canyon type; the greatest depth (up to 46 m) is measured near the dam. In the central part of the reservoir and in the upper reaches, depths of 13-15 m prevail. To reduce wave abrasion, the shores are reinforced with a stone dump. Additional pumping of water is carried out from the Southern Bug River. Since 2004, continuous flushing of the CP has been carried out with the discharge of water into the Southern Bug River (water flow in this case is 0.5 m3/s). The station operates three power units with a capacity of 1000 MW each with VVR-1000 reactors, put into operation in 1982, 1985 and 1989. Cooling water is supplied through intake channels (IC). To drain the heated circulating water there is a short discharge channel (about 600 m). In 2017, a dam and a "thermal curtain" of about 500 m long, made of polymer material, were constructed near its exit. The curtain is suspended on pontoons from the water surface to the bottom directing the flow of heated water toward the head of the reservoir.
Hydrobiological studies at water bodies of the power complex of the SU NPP have been conducted more or less regularly since the early 1980s. The last sampling and observation was in July 2018. For the retrospective analysis we used our own, literary and archival materials (WaCo hydrobiological database of
Fig. 1. Scheme of the cooling pond of the South Ukrainian NPP: 1 - dam area, 2 - area of entry into intake channel, 3 - area of outlet of discharge channel behind thermal jet deflection curtain, 4 - middle part of cooling pond, 5 - upper part of cooling pond, 6 - corner of large bay. Dashed arrow indicates direction of heated water discharge.
the Hydrobiology Technical Group of the Institute of Hydrobiology of NASU).
In the pelagic part of the reservoir, samples of phyto- and zooplankton were taken over the entire water area (Fig. 1): phytoplankton from the surface horizon with a bathometer, zooplankton using the total fishing method of Apstein net (mesh size 80 ^m) from a depth of 3 m to the water surface. At the time of sampling, we measured the water temperature and transparency using a Secchi disk. Zoobenthos sampling was carried out at the same stations at a depth of 6.0 to 8.0 m, as well as at coastal areas in the upper reaches (stations 5 and 5a) and in a large bay (station 6) at a depth of 0.6-1.1 m (Fig. 1). Soils in the South Ural NPP are currently represented by silted clay with a significant number of shells of the gastropods Melanoides tuberculata (Müller, 1774) and Tarebia granifera (Lamarck, 1822).
Zooperiphyton was studied on solid substrates (epiliton) on the stone enforcement structure of the dam (station 1a), the shore of the CP (1b), and near the exit of the discharge channel, as well as on the stalks of the southern reed Phragmites australis (Cav.) Trin. ex Steud, 1820 (epiphyton).
To determine the taxonomic composition, biomass, and abundance, generally accepted standard research methods were used (Arsan et al, 2006). When describing the taxonomic composition, the term "the lowest determined taxon" (LDT) was used, the designation of taxa of both species and higher rank, defined in accordance with the identification capabilities. The names and systematic affiliation of phytoplankton taxa are cited according to the Algae-base database (www.algaebase.org). The number and biomass of epiphyton were calculated over the area of the plant substrate.
The similarity of taxonomic composition was evaluated by S0rensen and Smirnov coefficients (Pe-senko, 1982). The Smirnov coefficient was also used to calculate the originality of the LDT composition (txx). Coefficient of variation (CV) was used to assess the heterogeneity of the distribution of plankton and zoobenthos indicators over the water area (Plokhin-sky, 1970). The species diversity (LDT diversity) was evaluated using the Shannon index (Pesenko, 1982).
The taxonomic diversity of hydrobiont groups (Pro-tasov, 2002) was calculated by the LDT number in taxonomic groups using Shannon's index.
Results and discussion
During the research period in 2018, two nuclear power units were operating. Near the exit of the discharge channel, the temperature reached 41.2 °C, while in the rest of the water area it varied within 32.735.4 °C. Water temperature in the Southern Bug River in this period was 26.6-27.5 °C. Water transparency indicators ranged from 0.65 to 1.50 m. The minimum values were recorded from the "cold" side of the thermal curtain near the exit of the discharge channel, the maximum values were recorded near the dam.
Water in the CP was characterized by relatively high pH values; near the dam (station 1), this indicator varied from 8.63 at the surface to 8.42 at a depth of 30 m. In the rest of the water area in the surface layer, its value varied from 8.60 to 8.77, with the maximum at the upper reaches of CP (station 5).
The amount of dissolved oxygen in the dam area at a depth of 0.5 to 30.0 m was quite high: 7.207.68 mg O2/dm3.
A characteristic feature of CP of the SU NPP is a high content of sulfate ions in water (Table 1). For comparison, their content in the Zaporizhia NPP was about 70 mg/dm3, and in the Khmelnitsky NPP it reached 130 mg/dm3 (2018, according to the ecological and chemical laboratories of the NPP).
High dry solids (Table 1) indicated high salinity. According to the data provided by the Ecological and Chemical Laboratory of the SU NPP, in terms of mineralization, the CP is borderline oligohaline to p-mesohaline brackish water basin, and in terms of the concentration of main ions it belongs to type II, sulfate class, and sodium group (SNaM).
The content of nutrients (nitrogen and phosphorus) in the CP was relatively low (mesotrophic to eu-trophic waters). The organic matter content in water (according to the indicators of permanganate oxidizability, PO) was also low, which corresponds to me-soeutrophic waters (Table 1).
In 2018, the phytoplankton of the CP included 52 LDT algae of five phyla. The richest phytoplank-
Table 1. Hydrochemical indicators in the cooling ponds of the South Ukrainian NPP, July 2018.
Station pH Sulfates, mg/dm3 Dry residue, mg/dm3 NH4+, mg N/dm3 NO", mg N/dm3 NO3-, mg N/dm3 PO43-, mg P/dm3 PO, mg O2/ dm3
1 8.63 350 1090 0.19 0.009 0.66 0.016 6.56
IC no. 1 8.75 354 1115 0.17 0.012 0.77 0.016 5.76
2 8.60 353 1089 0.18 0.009 0.62 0.023 7.52
4 8.67 353 1099 0.1 0.009 0.52 0.016 7.04
5 8.77 352 1084 0.1 0.009 0.54 0.016 7.52
ton occurred in the upper part, the least susceptible to heated discharges (Table 2). The floral spectrum was formed by green algae (59.15 ± 0.88%), diatoms (18.05 ± 1.85%), cyanobacteria (11.42 ± 1.06%), charophytes (5.80 ± 1.03%) and cryptophytes (5.58 ± 0.66%). The composition of phytoplankton at individual stations was quite similar: the values of the S0rensen coefficient ranged from 0.50 to 0.85 (an average of 0.68 ± 0.01). The taxonomic diversity was low (1.70 ± 0.03 bit/taxon) and did not change much over the water area of the reservoir (coefficient of variation CV = 3.83%). Relatively low values of taxo-nomic diversity of phytoplankton, associated, as a rule, with the pronounced dominance of one of the phyla in terms of the number of LDT, are a feature of cooling ponds (Novoselova and Protasov, 2015).
Quantitative indicators of phytoplankton were fairly evenly distributed over the water area of VO (CVN = 35.13%, CVB = 26.61%). The abundance level was determined mainly by cyanobacteria (58.38 ± 6.05% of the total) and green algae (31.70 ± 4.68%). The cyanobacteria Aphanocapsa incerta (Lemmermann) G. Cronberg & Komarek, 1994 and Merismopedia minima G. Beck, 1897 dominated throughout the water area; at some stations, the dominant assemblage included the green alga Binuclearia lauterbornii (Schmidle) Proschkina-Lavrenko, 1966. At all studied
stations, the dominant biomass assemblage included Cosmarium sp. (Charophyta). In the upper reaches of the reservoir (station 5), near the dam (station 1) and in the large bay (station 6), B. lauterbornii joined the dominant assemblage. Also at some stations, Nitzschia kuetzingiana Hilse, 1863 (diatoms) and Rhodomonas pusilla (H. Bachmann) Javornicky, 1967 (cryptophytes) were among the dominants.
At most of the stations studied, the LDT diversity was quite high. At the entrance of the intake channel (station 2) and in the large bay (station 6), where M. minima constituted 63.9 and 57.5% of the total population, the Shannon index was slightly lower. The evenness indices testified to a fairly uniform distribution of LDT in the phytoplankton (Table 2).
In the higher aquatic vegetation, southern reed prevailed. Reed beds of this plant were noted along almost the entire perimeter of the reservoir, with the exception of the intake and discharge channels. In the dam area, individual clumps and strips up to one meter wide were found, in the rest of the water area, the strip width was 3-10 m. The overgrowing density varied from 68 to 220 stems per m2, the average biomass in the reservoir was 3.40 ± 0.66 kg/m2 with a maximum of 5.37 kg/m2 in the corner of a large bay. According to our observations, reeds have always dominated the associations of higher aquatic vege-
Table 2. Structural indicators of the pelagic ecological groups in the cooling pond of South Ukrainian NPP, July 2018. NSp - the number of species/LDT; Htax - taxonomic diversity, bit/taxon; N - abundance, mln cells/dm3 (for phytoplankton), ind./m3 (zooplankton), ind./m2 (zoobenthos and zooperiphyton); B - biomass, mg/dm3 (for phytoplankton), mg/m3 (zooplankton), g/m2 (zoobenthos and zooperiphyton); HN - abundance diversity, bit/ind., HB - biomass diversity, bit/mg (for phytoplankton and zooplankton), bit/g (zoobenthos and zooperiphyton); J/N - abundance evenness; J/B - biomass evenness.
Station no. NsP Htax N B HN HB J/N J/B
Phytoplankton
1 25 1.70 20.02 2.53 3.48 3.45 0.75 0.74
2 24 1.80 45.60 4.34 2.09 3.10 0.45 0.68
3 23 1.61 15.76 2.69 3.60 3.46 0.79 0.77
4 21 1.69 25.64 2.61 2.70 2.90 0.61 0.66
5 30 1.66 36.29 3.64 3.16 3.54 0.64 0.72
6 19 1.75 30.74 1.98 2.21 3.03 0.52 0.71
Zooplankton
1 12 1.56 477841 3872.76 2.32 2.604 0.65 0.73
2 13 1.53 335957 2405.43 2.41 2.35 0.65 0.63
3 13 1.53 54130 520.52 2.44 2.78 0.66 0.75
4 11 1.57 310290 2988.29 2.32 2.07 0.67 0.6
5 12 1.56 701913 7246.79 2.48 2.15 0.69 0.6
6 10 1.30 159520 1023.64 0.64 0.3 0.19 0.09
tation in a given reservoir. This is probably due to the lack of shallow water zones, which are needed for the development of submerged taxa.
Zooplankton of the CP in 2018 was characterized by low richness, which is characteristic of cooling ponds with a high water temperature (Zhivotova, 2007). In total, 17 LDT of invertebrates were discovered in July 2018. Among them, eight LDT were rotifers, six were branched, and three were copepods. The taxonomic diversity was low (1.51 ± 0.04 bit/taxon) and did not change much over the water area of the reservoir (CV = 6.64%). The similarity of zooplankton in areas with different temperature regimes was high; the S0rensen index was 0.76-0.96.
On the whole, zooplankton was copepod-clado-ceran dominated by individuals of the juvenile stages of copepods, as well as rotifers Conochiloides delta-icus Rudscu, 1960 and Moina micrura Kurz., 1875. The biomass was dominated by individuals of the juvenile stages copepods, the rotiforms M. micrura, ctenopods Diaphanosoma dubium Manuilova, 1964 and D. orghidani Negrea, 1982, and the copepod Thermocyclops oithonoides (Sars), 1863. The zooplankton abundance distribution, in contrast to composition, was less uniform across the water area (CVN = 67.74%, CVB = 80.30%).
The diversity and evenness indicators at most stations (Table 2) indicate a fairly uniform distribution of quantitative indicators in zooplankton groups. The exception was station 6 in the large bay, where low values of diversity and evenness reflect the overwhelming dominance of Copepoda (99.2% in numbers and 99.7% in biomass).
Zoobenthos of the CP showed extreme impoverishment of taxonomic composition. Only nine LDT of invertebrates from three groups were recorded: oligochaetes represented five LDT (four LDT of tubificids and Dero sp.), chironomid larvae -three LDT, and Ostracoda sp. were also recorded. Only juvenile tubificids (88%) and Leptochironomus tener (Kieffer, 1918) (75%) were characterized by high levels occurrence. Ostracodes were registered only in the upper reaches, however, in other areas, their carapaces were observed, according to a visual assessment, belonging to recently dead individuals. Probably, organisms of this group are reduced during the hottest summer period and resume their numbers during the fall - spring. The similarity of the taxonomic composition of zoobenthos (S0rensen index) in the stations of the CP main water area was quite high (up to 0.8), with the exception of coastal areas.
Quantitative parameters of zoobenthos were low: the abundance was on the order of a thousand ind./m2, and the biomass was on the order of g/m2 (Table 3). In terms of abundance and biomass, juvenile tubificids predominated in almost the entire water area (to a lesser extent in the upper reaches).
Abundance indices were distributed fairly evenly across the CP water area (CV values were 32%), biomass was more mosaic (55%). Mature individuals of the tubificid Limnodrilus claparedeanus Ratzel, 1868 and L. hoffmaisteriClaparede, 1862 were noted, this confirms the observations (Karataev, 1990) that these species are resistant to high temperatures. Among the chironomid larvae, L. tener was the most tolerant to the raised temperature, and was found not only in the upper reaches, but also in the central area and near the dam, where even bottom water layers were considerably heated. In the area with the maximum near-bottom temperature in the reservoir, on the heated side of the "thermal curtain" (36 °C, depth 8 m), only L. claparedeanus and juvenile tubificids were recorded in the zoobenthos; abundance indices amounted to 4400 ind./m2 and 2.24 g/m2 with tubificids dominating the assemblage.
Thus, the zoobenthos of the CP of the SU NPP has currently a very low total biomass and is average in number of specimens (in accordance with the grades proposed by O.P. Oksiyuk with co-authors (2006)). The taxonomic composition is impoverished, at least during the hottest summer period. This state of invertebrate zoobenthos can be relatively stable over time and is characteristic of cooling ponds with significant technogenic load, in particular, thermal, which in Ukraine are cooling ponds of SU NPP and Zaporizhia NPP (Protasov et al., 2013).
A distinctive feature of the zooperiphyton of CP of the SU NPP in 2018 was the presence of massive populations of the invasive tropical gastropod mollusk species Melanoides tuberculata and Terebia granifera in single-species and joint colonies (Fig. 2).
In the epilitone, the number of LDT and invertebrate groups was more than double that in epiphyton (17 and 7, respectively). At individual stations, the amount of LDT ranged from 2 to 8. The richest epili-ton was in the upper reaches of the CP (station 5a).
The abundance of epiliton was in the range 333-16795 ind./m2, the total biomass was 0.0011880.37 g/m2, and without mollusks, it was 0.0013.36 g/m2 (Table 3). Settlements of gastropod mollusks with significant abundance indices: 1021716795 ind./m2 and 1739.39-1880.7 g/m2 were recorded on a stone dump along the perimeter of the CP and on the dam.
In the zone of the highest temperatures (41.2 °C), invertebrates were not found in samples of periphy-ton from rocks; filamentous cyanobacteria with a biomass of 62.9 g/m2 are recorded from here.
At the dam, the total abundance ranged from 333 to 10217 ind./m2, biomass from 0.46 to 24.09 g/m2, the proportion of mollusks in the groups was dominant. Outside the mollusk populations, in the areas encrusted with filamentous algae (depth up to 3 m), invertebrates of the epiliton were represented only
Table 3. Structural indicators of the contour groupings in the cooling pond of the South Ukrainian NPP, July 2018. h - depth, m; JDD - jet deflecting dam (Fig. 1). Other explanations are in Table 2. *Colony of mollusks with 90% substrate coverage.
Station no. h nsp Htax N B HN HB J/N J/B
Zoobenthos
1 7.3 4 0.81 5200 2.66 1.38 1.41 0.69 0.70
2 6.0 3 0 5100 5.02 0.56 1.14 0.35 0.71
3 8.0 2 0 4400 2.24 0.44 0.59 0.43 0.59
4 7.0 4 0.81 2000 0.90 1.52 1.29 0.75 0.64
5 0.7 7 1.59 4602 3.84 2.25 2.13 0.80 0.75
5a 7,0 3 0.99 2300 0.61 1.34 1.15 0.84 0.72
Epiliton
0.3 8 1.75 10217 24.09 1.92 0.99 0.64 0.33
Dam (1a) 0.5 3.0 4 2 1.50 0 1528 333 0.46 0.001 1.28 1.00 0.63 1.00 0.63 1.00 0.31 1.00
4.0 4 1.50 16795 1880.37 0.37 0.16 0.18 0.08
0 2 1.00 11905 1739.39 0.14 0.00 0.14 0.00
Shore filling (1b) 0.15 3 0.92 21333 283.57 0.34 0.20 0.21 0.12
0.15* 3 0.92 51833 5837.49 0.36 0.41 0.22 0.26
JDD 0 3 0.92 2037 0.64 1.24 1.39 0.78 0.87
5 0.1 6 1.79 2051 3.44 1.62 0.13 0.62 0.05
5a 0.15 12 1.95 5183 3.22 3.52 2.34 0.92 0.61
Epiphyton
1b 0.5 2 1 696 13.35 0.34 0.00 0.33 0.00
6 0.35 3 0.92 428 0.004 1.50 1.33 0.94 0.83
5 0.4 4 0.81 193 0.001 1.92 0.96 0.95 0.48
5a 0.8 4 0.81 694 0.003 1.42 1.62 0.70 0.80
by two species of oligochaetes Naididae: Pristina aequiseta Bourne, 1891 and P. longiseta Ehrenberg, 1828, with minimal abundance indices.
The populations of M. tuberculata and T. grani-fera were represented by variously-sized individuals, which may indicate the naturalization of species in the CP, while T. granifera significantly prevailed in the two-species populations in terms of abundance.
At the dam (station 1a) at shallow depths, monospecific populations of M. tuberculata were found, the size composition of which is represented by two groups: 1-5 and 6-10 mm, with the predominance of the former (96%). At a depth of 4 m, two-species populations were found where individuals of M. tu-
berculata were recorded in six size groups: from 1-5 mm to 26-30 mm, with dominance of groups of 6-10 mm (39%) and 16-20 mm in size (29%). The maximum height of M. tuberculata shell reached 27 mm, the minimum - 2 mm. At the same depth, individuals of T. granifera are represented by three groups: from 1-5 mm to 11-15 mm, while groups of 6-10 and 11-15 mm in size (38% each) dominated; the maximum height of the shell was 13 mm, the minimum was 4 mm.
On a stone dump at the coast (station 1b), both mono- and bispecific populations of gastropod mollusks were found in the epiliton. In the monospecific populations of M. tuberculata, the morphometric pa-
