Zooplankton Communities of Inter-Connected Sections of Lower River Oder (NW Poland)

Cent. Eur. J. Biol. • 8(1) • 2013 • 18-29 DOI: 10.2478/s11535-012-0110-8

Central European Journal of Biology

Zooplankton communities of inter-connected sections of lower River Oder (NW Poland)

Research Article

Robert Czerniawski*, Małgorzata Pilecka-Rapacz, Józef Domagała

Department of General Zoology, University of Szczecin, 71-412 Szczecin, Poland

Received 22 June 2012; Accepted 13 September 2012

Abstract: The aim of this study was the determination and comparative analysis of the zooplankton communities between the inter-connected sections of the lower Oder river in relation to physicochemical factors. The study was performed at five sites of Oder. Two sites were localized in the main channel of Oder (East Oder), other sites were localized in the west arm of Oder and at the beginning of the canal carrying the post-cooling water from the power plant, and the last site was below at the shallow channel joining the Western Oder with the Eastern Oder. At the channel site in which the two arms of the river are connected a significantly higher taxa number, abundance and biomass of crustaceans was observed than at the other sites. The taxonomic similarity index between all sites was at a rather low level. The Pearson’s coefficient, multiple regression analysis and CCA showed that temperature, conductivity and content of nitrates had the strongest impact on the abundance of zooplankton. Thus, in lower, slowly flowing section of River Oder the physico-chemical variables influenced zooplankton density. Post-cooling water from the power plant influenced the zooplankton communities only in the channel discharging the waters into the river, while its influence on the zooplankton in the Oder is insignificant.

Keywords: Potamozooplankton • Large river • River ecology • Post-cooling water © Versita Sp. z o.o.

1. Introduction best sources of organic matter, including zooplankton, in rivers are limnetic basins, such as lakes, impounding According to Hynes [1], the factors that affect the reservoirs, floodplains and slackwaters in the river zooplankton communities in rivers can be divided course [10-13]. Another important source of zooplankton into two categories: (1) those affecting transport of can be basins with post-cooling water in which various zooplankton from the source to the downstream, and (2) taxonomic composition and densities of zooplankton those affecting the generative and vegetative behaviour have been noted, depending on temperature [14-18]. of zooplankton in the river. On the basis of the findings Along the course of the river the trophic conditions of several authors [e.g. 2-4] it seems that the main vary. Individual sections of rivers, even those at a close factors which affected zooplankton communities, current distance can show low similarity in zooplankton, both velocity and discharge, determine the water residence in the qualitative and quantitative aspects. E.g. the time. Smaller current, velocity, and discharge influence density of zooplankton in outflows from stagnant basins not only the reproduction of zooplankton but also the is much higher than downstream, which is mainly a presence and accessibility of food (phytoplankton) consequence of fry predation [4,9]. Changes in the [5]. Possible factors regulating plankton biomass zooplankton density in the main channel are also related in rivers may be physical (light), chemical (nutrient to the character of the riparian zone, e.g. the presence of concentrations), hydrological, and biotic [6]. However, slackwaters, floodplains or pools and vegetation cover only a few authors [4,7-9] have indicated a significant [10,13,19]. The quantitative and qualitative communities correlation between the concentration of inorganic of zooplankton in the lower course of the river also nutrients and communities of riverine zooplankton. The depend on the hydrological conditions, especially on the

* E-mail: [email protected] 18 R. Czerniawski et al.

longer water residence time [20]. Thus, it is expected 50 L of water were collected from the surface drift with that even the close sections in the lower courses of large a 10 L bucket. The collected water was filtered through rivers can differ in zooplankton communities. Vadadi- a 25 µm mesh net. The sample was then fixed in 4–5% Fülöp et al. [21] who studied the zooplankton of the main formalin solution. For counting, a Glass Sedgewick channel of the River Danube and its arm, have reported Rafter Counting Chamber was used. Each sample was great differences in zooplankton densities. Similarly, divided into five subsamples. For identification, a Nikon significant differences in zooplankton have been found Eclipse 50i microscope was used. Species identification by Schröder [22] between the main channel of the River was made using the keys of Wagler [23], Kutikova Oder and its western arm. [24], Harding and Smith [25] and Radwan [26]. In each The aim of this study was determination and sample, the body length of at least 30 individuals from comparative analysis of the zooplankton between the each species was measured by the Pixelink Camera inter-connected sections of the lower River Oder and Kit 4.2 computer program. If the number of individuals checking whether in the lower section of this large representing a given species was lower than 30, the river are significant correlations between zooplankton body lengths of all individuals were measured. The body communities and physico-chemical parameters. To length conversion to wet mass was made with the use of better understand these parameters, the following the Ruttner-Kolisko [27], McCauley [28], and Ejsmont- questions were addressed: (1) what is the similarity in the Karabin [29] formulas. quantitative and qualitative composition of zooplankton Temperature, pH, conductivity and dissolved oxygen between the inter-connected parts of the river, (2) do content at the sites, were measured using an oxygen the physico-chemical factors have significant influence content meter and pH meter CX-401 made by Elmetron. on the communities of riverine zooplankton, (3) does The contents of nitrites, nitrates, ammonium nitrogen, the post-cooling water have a significant effect of the total nitrogen, orthophosphates and total phosphorus zooplankton composition in the main channel of the were measured with a Hach Lange DR-850 photometer. River Oder. The list and mean ± SD of the above environmental variables are shown in Table 1. 2. Experimental Procedures

The study was performed on selected sites of the lower Oder section (N53°13’50”, E14°27’22”). Five sampling sites were selected (Figure 1). Site 1, the width of the river at this site was of about 200 m; the river had fast current and regular channel whose banks were covered with a narrow band of rushes. Site 2 – the Western Oder, the channel width of about 100 m, the two banks overgrown with a narrow band of rushes. Above site 2, on the Western Oder there is a water impounding dam. Site 3 was at the beginning of the canal carrying the post-cooling water from the power plant, at the way out from the harbour basin (2 ha). At this site the width of the river channel was close to 35 m. Site 4 was on the Eastern Oder below site 3, the river at this site was of about 170 m in width, with regular channel above the sample collection site, the banks were grown with a narrow band of rushes. Site 5 was below at the channel joining the Western Oder with the Eastern Oder. At the sample collection site the channel was of about 150 m in width whose banks were grown with a broad part of rushes and the bottom of the channel was grown with submerged macrophytes. At this site the channel had the smallest depth and a much slower current. Zooplankton samples were collected monthly in April, Figure 1. Study area. August and October from 2009 to 2011. At each site,

19 Zooplankton communities of inter-connected sections of lower River Oder (NW Poland)

Temp O Cond N-NH3 N-NO N-NO P-PO TN TP pH 2 3 2 4 (ºC) (mg l-1) (µS) (mg l-1) (mg l-1) (mg l-1) (mg l-1) (mg l-1) (mg l-1)

Site 1 16.0 8.35 8.03 629.1 0.12 0.7 0.011 0.20 1.9 0.46

Site 2 16.5 8.17 7.44 669.5 0.18 0.9 0.013 0.20 2.1 0.48

Site 3 22.3 8.27 7.33 744.6 0.15 0.9 0.012 0.21 2.0 0.50

Site 4 16.8 8.30 7.50 660.9 0.14 0.8 0.010 0.18 2.1 0.42

Site 5 16.7 8.46 8.68 651.4 0.18 0.8 0.019 0.18 2.0 0.46

Table 1. Mean values of physico-chemical factors in sites examined of lower River Oder. Temp – temperature, Cond – conductivity

Statistical significance of the differences in the of Cladocera, nauplii and Copepoda at site 5 was zooplankton community between sites was tested using significantly greater than at all other sites (P<0.05). ANOVA test with the posteriori Duncan test (P<0.05). No statistically significant differences in density of any The relationship between environmental variables and taxonomic group were observed between the other zooplankton abundance was checked by the Pearson’s sites. From among Rotifera the dominant quantitative correlation. To find the best predictors for abundance contribution was brought by species of the smallest of zooplankton the multiple stepwise regression was size, at all sites, (Table 2). From among Cladocera no used. The percentage of variation explained by the particular taxa was found to be quantitatively dominant, pattern was based on R2. In order to determine the while among Copepoda the definite dominants were influence of environmental variables on the abundance their larvae – nauplii. of zooplankton the Canonical Correspondence Analysis The highest mean biomass of zooplankton was (CCA) [30] was used. noted at site 5, while the lowest at site 4. The highest biomass of crustaceans was found at site 5, while the highest biomass of Rotifera at site 3 (Figure 2). At each 3. Results site, except site 5, the biomass of Rotifera made at least 70% of the mean biomass of zooplankton. Only The total number of zooplankton taxa identified in the at site 5 the biomass of Copepoda brought the greatest samples collected at all sites over the period of three contribution of 65% to the biomass of zooplankton. years of study was 74, which included 56 Rotifera, From among all taxonomic groups only at site 5 the 10 Cladocera and 8 Copepoda (Table 2). The species of mean biomass of Cladocera, nauplii of Cyclopoida and the highest frequency were Brachionus angularis, Keratella adult Copepoda was significantly higher than at all other cochlearis, Keratella ticinensis and cyclopoid nauplii. sites (P<0.05). At all sites the dominant contribution to Throughout the three years of study, the taxonomic the biomass of Rotifera was brought by their smallest similarity index between all sites was at a rather low size representatives (Table 2), the same was true for level (Table 3). The highest qualitative similarity was Cladocera. At all sites, the nauplii of Cyclopoida had found between sites 1 and 2 as well as sites 3 and 4. dominant contribution to Copepoda biomass, adult The lowest similarity index was calculated for sites 2 copepods dominated only at site five. At all sites the and 4 as well as sites 4 and 5. seasonal changes in zooplankton communities were The highest mean number of zooplankton taxa was similar. The highest taxa number, abundance and noted at site 5, while the lowest at site 4 (Figure 2). From biomass of zooplankton were observed in August while among all taxa only the number of those belonging to the lowest in October (Figure 3, 4). Cladocera and Copepoda was significantly higher at site The calculated Pearson’s coefficient values show 5 than at all other sites (P<0.05). At site 4 no presence that five environmental variables had a significant of cladocerans and adult copepods was found. Among positive influence on the abundance of zooplankton crustaceans at this site only cyclopoid nauplii were (Table 4). Abundance of rotifers was correlated with recorded. The highest mean abundance of zooplankton temperature and conductivity. The content of dissolved was observed at site 3, while the lowest at site 4. The oxygen affected the abundance of crustaceans. highest density of crustaceans was established at site 5, Moreover, an increase in pH and the content of nitrates while the highest density of rotifers at site 3 (Figure 2). caused an increase in the abundance of nauplii. At each site rotifers accounted for at least 78% However, the multiple regression revealed of the mean zooplankton abundance. The density that temperature, the content of dissolved oxygen

20 R. Czerniawski et al.

Rotifera Polyarthra dolichoptera

Anuraeopsis fissaA: 3 Polyarthra longiremis

Ascomorpha ecaudis Polyarthra remata

Ascomorpha ovalis Polyarthra vulgaris

Asplanchna priodonta Pompholyx sulcata A: 1;2;3;4. B: 1;3;4;5 MF

Bdelloidea Synchaeta oblonga

Brachionus angularis A: 3 HF Synchaeta kitina

Brachionus budapestinensis Synchaeta stylata

Brachionus calyciflorus MF Testudinella truncata

Brachionus quadridentatus Trichocerca capucina

Brachionus rubens Trichocerca pusilla SF

Brachionus urceus Trichocerca rouseletti

Brachionus variabilis Trichocerca similis SF

Cephalodella bulla Trichocerca rouselleti

Cephalodella mus Trichocerca vernalis

Cephalodella sterea Total Rotifera 56

Colurella adriatica

Colurella colurus Cladocera

Conochilus unicornis Alona guttata

Euchlanis dilatata Alona costata

Euchlanis lyra Alona rectangula

Gastropus stylifer Alonella nana

Itura aurita Bosmina coregoni B: 1;3

Keratella coch. cochlearis HF Bosmina longirostris

Keratella coch. hispida Daphnia cucullata

Keratella coch. robusta A: 1;2;3. B: 2 Ceriodaphnia quadrangula B: 2;5

Keratella coch. tecta A: 5. B: 5 Chydorus sphaericus B: 5

Keratella quadrata MF Chydorus gibbus

Keratella ticinensis HF Total Cladocera 10

Lecne arcuata

Lecane closterocerca Copepoda

Lecane hamata Nauplii Cyclopoida A: 1;2;3;4;5. B: 3;4 HF

Lepadella ovalis Copepodites Cyclopoida B: 1;2

Lepadella acuminata Acanthocyclops robustus

Lepadella ovalis Cyclops vicinus

Mytilina crassipes Eucyclops serrulatus B: 2;5

Mytilina mucronata Mesocyclops leuckarti

Notholca acuminata Thermocyclops crassus

Notholca caudata Thermocyclops oithonoides

Notholca squamula Total Copepoda 8

Notholca foliacea

Notholca labis Total taxa 74

Table 2. Taxonomic composition of zooplankton in examined sites of lower River Oder in 2009-2011. HF – highest frequency (80-100%), MF – middle frequency (60-80%). A – dominant in abundance, B – dominant in biomass in its taxonomical group. The numerical index means a site number where the taxa dominated.

21 Zooplankton communities of inter-connected sections of lower River Oder (NW Poland)

and nitrates affected significantly the zooplankton The abundance of crustaceans correlated best with the abundance (Table 5). The analysis explained from 38% content of dissolved oxygen and nitrates. to 57% of the variability in zooplankton abundance. The main predictors (with the highest level of significance) that influenced the abundance of rotifers and nauplii 4. Discussion were temperature and the content of nitrates. CCA of the samples and some taxa abundance The composition of zooplankton in the River Oder revealed that temperature, conductivity, the content of below and above the power plant makes a pattern nitrates and dissolved oxygen correlated best with the that is typical of zooplankton structure in large rivers first axis (Figure 5). The content of total phosphorus [20,31,32]. Although not many statistically significant correlated best with the second axis (Figure 5). The two differences were noted in the qualitative and quantitative axes explained 37.2% of the variability in zooplankton abundance. The seasonal samples (in April, August Site 1 Site 2 Site 3 Site 4 and October) were divided into three groups according to their CCA-ordination. The April samples were Site 2 0.70 correlated with the content of dissolved oxygen, the Site 3 0.59 0.59 August samples were correlated with temperature and conductivity, while the October samples were correlated Site 4 0.57 0.54 0.65 with the content of TP. The majority of all correlations Site 5 0.57 0.59 0.64 0.54 between the zooplankton abundance and environmental

variables were noted for small pelagic rotifers that Table 3. Jaccard similarity of zooplankton between examined sites correlated positively with temperature and conductivity. of lower River Oder in 2009–2011.

Figure 2. Mean + SD taxa number, abundance ind l-1 and biomass mg l-1 of Rotifera, Cladocera, Copepoda (except nauplii) and nauplii at sites examined of lower River Oder. Sites with the same letters not differ significantly (P<0.05).

22 R. Czerniawski et al.

parameters of the zooplankton studied at different sites (except significant higher abundance and biomass of crustaceans at site 5), a more detailed analysis revealed some differences. Typical of running waters, especially large rivers, is a much higher number of taxa and much greater density of rotifers than crustaceans; the contribution of rotifers in zooplankton communities often exceeds 70% [3,7,20,32]. It has furthermore reported that the most often met taxa in rivers are small plankters, from among small zooplankters, like Keratella sp. (Rotifera), Bosmina sp. and Chydorus (Cladocera), and the highest frequency from among copepods have their larval stages – nauplii [3,7,20,32]. Thus it can be concluded that the taxonomic composition of zooplankton in lower course of the River Oder has a structure typical of large rivers. The highest number of taxa was found in the channel joining the western bed with the eastern one, which can be explained by a small water flow and availability of ecological niches, as the bottom of the channel was overgrown with macrophytes. In stagnant water reservoirs, a positive effect of vegetation on biodiversity of zooplankton has been found [33,34]. The indices of taxonomic similarity between the sites studied were rather low, despite relatively close distances between them. The probable reason was that the sites were localised at the sections of the river characterised by different environmental factors. According to Cottenie et al. [35] the connected water reservoirs have similar zooplankton structures if they are environmentally similar. The lowest number of taxa, abundance and biomass of zooplankton were noted at section 4, so in the low section of the East Oder at the greatest distance to site 1 and below the discharge of the post-cooling water from the power plant. The decreasing number of taxa, abundance and biomass of zooplankton communities downstream are typical of flowing waters [3,4,9,11]. The first reduced are Cladocera, then Copepoda and finally rotifers. At site 4 no presence of adult crustaceans was established, which can be attributed to the predatory activity of fish as the main cause of reduction in the abundance of zooplankton in rivers [4,36]. The predatory fish preferably feed on the largest plankters omitting the small rotifers which are left as dominating the zooplankton communities in rivers. According to the local fisherman, the greatest number of fish are caught below the mouth of the post-cooling water to the east bed of River Oder, where as expected they feed on zooplankton drifting in the river current. Another reason behind the lack of crustaceans at site 4 can be the water residence time, which is also an important Figure 3. Seasonal changes of zooplankton taxa number in sites factor structuring zooplankton communities in rivers [3]. examined of lower River Oder.

23 Zooplankton communities of inter-connected sections of lower River Oder (NW Poland)

Figure 4. Seasonal changes of zooplankton abundance ind l-1(left column) and biomass mg l-1 (right column) in sites examined of lower River Oder.

Variable Temperature pH O2 Conductivity N-NO3

Rotifera 0.71*** 0.41**

Cladocera 0.37**

Nauplii 0.36* 0.42** 0.32*

Copepoda 0.46**

Table 4. Significant Pearson’s correlations between environmental variables and abundance of zooplankton. 2O – content of dissolved oxygen,

N-NO3 – content of nitrates. Significance: *P<0.05, **P<0.01, ***P<0.001.

24 R. Czerniawski et al.

Coefficients Regression statistics

2 Variable Temperature O2 N-NO3 F P R

Rotifera 4.25*** 4.60 0.0001 0.57

Cladocera 2.14* 3.28** 3.19** 3.43 0.0034 0.50

Nauplii 4.37*** 3.48 0.0030 0.51

Copepoda 2.12* 2.19 0.0481 0.38

Table 5. Significances of the effects of environmental variables on the abundances of zooplankton based on multiple regression (stepwise procedure) with the following dependent variables: abundance of Rotifera, Cladocera, nauplii, Copepoda. Independent variables taken

for analysis were: temperature, O2 – content of dissolved oxygen, content of nitrates – N-NO3, content of nitrites, content of total nitrogen, content of orthophosphates, content of total phosphorus, conductivity and pH. Probability levels of t-values for coefficients are denoted as follows:. *P<0.05, **P<0.01, ***P<0.001.

Figure 5. CCA constrained ordination of the taxa and samples from sites in the lower River Oder. Environmental variables: Cond – conductivity, Temp – temperature, pH, O2 – dissolved oxygen, TN – total nitrogen, NH3 – ammonium nitrogen, NO2 – nitrites, NO3 – nitrates, PO4 – orthophosphates, TP – total phosphorus. Taxa: Anu fis –Anuraeopsis fissa, Asc eca – Ascomorpha ecaudis, Bdell – Bdelloidea, Bra ang – Brachionus angularis, Bra bud – Brachionus budapestinensis, Bra cal – Brachionus cayciflorus, Bra rub – Brachionus rubens, Bra urc – Brachionus urceus, Cep ste – Cephalodella sterea, Col adr – Colurella adriatica, Euc dil – Euchlanis dilatata, Ker coc – Keratella cochlearis, Ker qua – Keratella quadrata, Ker tic – Keratella ticinensis, Lec clo – Lecane closterocerca, Myt cra – Mytilina crassipes, Myt muc – Mytilina mucronata, Not squ – Notholca squamula, Pol dol – Polyarthra dolichoptera, Pol lon – Polyarthra longiremis, Pol vul – Polyarthra vulgaris, Pom sul – Pompholyx sulcata, Syn obl – Synchaeta oblonga, Syn kit – Synchaeta kitina, Syn styl – Synchaeta stylata, Tri pus – Trichocerca pusilla, Tri rou – Trichocerca rouselleti, Tri sim – Trichocerca similis, Bos cor – Bosmina coregoni, Bos lon – Bosmina longirostris, Chy sph – Chydorus sphaericus, Nau Cyc – nauplii of Cyclopoida, Cop Cyc – copepodites of Cyclopoida, Aca rob – Acanthocyclops robustus, The oit – Thermocyclops oithonoides.

Different taxonomic groups of potamozooplankton have biomass of zooplankton were smaller than at site 2, different responses to the water residence time. When localised in the west arm of River Oder. Above site 2 (at the water residence time is short, the small plankters the West Oder) there is an impounding dam regulating e.g. rotifers dominate in zooplankton communities, the flow of water in the west bed according to the needs while when the water residence time is long the of the power plant, simultaneously leading to slowing dominating taxa are larger crustacean species [20]. In down water current and extension of the water residence the present study, the section between site 1 and 4 was time. Thanks to the latter, zooplankton and in particular characterized by the highest current velocity, so it can crustaceans were able to reproduce as their generation be expected that the water residence time was short, time is longer than that of small rotifers [31,37]. Dam which could lead to the presence of small rotifers only reservoirs, like lakes, change the hydrological and at site 4. ecological conditions in flowing water and are a valuable Site 1 and site 4 were localised at the same channel source of zooplankton in rivers [4,31,37,38]. Moreover, of the river and at both these sites the abundance and in the west and east arm of Oder the zooplankton

25 Zooplankton communities of inter-connected sections of lower River Oder (NW Poland)

density could also be influenced by the inflow from the development of crustaceans communities and quantity channels joining the east and west bed. The channels of submerged macrophytes in limnetic and lotic waters are characterised by small water flow, high number of [19,33,34]. Cladocerans as well as copepods find wetlands and floodplains, from which zooplankton can refuges against predators in the submerged vegetation be washed out into both beds of River Oder. In a similar [33,34]. Moreover, in summer in site 5 were good food way zooplankton can reach the River Oder bed at site 1 conditions for zooplankton caused by the phytoplankton above which the river often overflows the banks making blooming. Phytoplankton density is positively correlated many small stagnant water basins. with the density of zooplankton in stagnant waters In the post-cooling water channel the greatest [41,42] and in slow flowing streams [5]. abundance of zooplankton was observed, mostly In running waters a significant impact of chemical composed of small plankters typical of high trophy waters factors on zooplankton communities has been rarely [39]. The source of zooplankton in the post-cooling water observed. In small streams the zooplankton abundance was the harbour basin with a stagnant water ensuring may correlate with the chemical conditions, but this better conditions for zooplankton reproduction. Limnetic applies to outlets from strongly eutrophicated lakes [9] or basins are the most important sources of zooplankton to slowly flowing streams [5]. Moreover, Kobayashi et al. in flowing water [4,11,12]. The greater the abundance [7] in the Havkesbury-Nepean River observed strong of zooplankton in such a basin, the greater amount is positive correlations between the potamoplankton carried out with the river flowing out of the basin [11]. It community, conductivity and total phosphorus. In the is reasonable to expect that similarly high amounts of present study, the impact of inorganic nutrients and zooplankton were in the harbour basin through which conductivity on zooplankton abundance, especially the post-cooling water was flowing. planktonic taxa, was observed. A similar pattern has been The post-cooling water channel rather had no observed in stagnant water bodies [e.g. 33,43]. It seems influence on the zooplankton communities in the east that abiotic conditions in riparian zone and landscape bed of River Oder (below the outlet of the post-cooling influenced the abiotic conditions in the waters of lower water channel), which is evidenced by very small River Oder. Moreover, Swan and Palmer [44] have taxonomic similarity between sites 3 and 4 and by reported evidence that population dynamics of stream the fact that despite the lack of statistically significant meiofauna is related to organic matter availability. Thus, differences in zooplankton communities between site 3 it can be expected that the stream microfauna is also and 4 the abundance of zooplankton was over threefold related to organic variables. reduced and biomass almost fivefold reduced. Such The post-cooling water has higher temperature than a strong reduction was most probably a result of the the natural water throughout the whole year, which results presence of feeding fry. Evans et al. [40] report that in the in an extended vegetation season and higher trophic post-cooling water discharge area the fry are intensely status [45-47]. Heated waters in a limnetic basin can feeding on zooplankton, which is one of the main reasons lead to an increased number of zooplankton generations for its elimination. In general the tributaries have small [14]. According to some authors, the zooplankton effect on zooplankton communities in the main channel. density and biodiversity in stagnant basins filled with Because of the hydrological and biological conditions, post-cooling water are usually lower than in the natural the tributaries bring small amounts of zooplankton to the conditions [15,18,48]. In such water, small rotifers, main river channel [5,8,19]. including those that are indictors of high trophic status, The greatest amount of crustaceans as well as are more abundant [18,39]. The factor co-determining a relatively great amount of rotifers were found in the elimination of many zooplankton species and the the channel joining the east and west beds of River decrease in the zooplankton density in post-cooling Oder, site 5. In the samples collected at this site the water can be high temperature [15,17,18], although, on contribution of large crustaceans was clearly dominant in the other hand, according to Leeper and Taylor [48], only the biomass of zooplankton, which can be related to the at temperatures higher than 45°C a rapid reduction in above mentioned observation of Baranyi et al. [20] on zooplankton density and elimination of the majority of the dependence of crustaceans communities on water species representing both rotifers and crustaceans are residence time. At site 5 the water current is very slow, observed. The observations of the above authors do not almost unnoticeable, which is favourable for crustacean agree with the results of the present study in which a reproduction. Another factor favouring crustacean significant positive effect of temperature (mean 22.3°C) development in that channel was much greater cover on the zooplankton density was evidenced. At site 3, of its bottom by submerged macrophytes. According to directly related to the post-cooling water and localised some authors there is a positive correlation between the directly below the outlet of the post-cooling water from

26 R. Czerniawski et al.

the harbour basin, the zooplankton density, especially below. (1) In spite of a rather small distance between the density of small plankton rotifers was the highest. the inter-connected sections of the lower River Oder, However, it should be emphasised that the above authors the similarity in the zooplankton composition between studied and compared the zooplankton of limnetic basins them is small, which is most probably a consequence to which the post-cooling water was discharged [48]. In of differences in the natural environmental conditions our study we compared the zooplankton flowing out of at the sites studied. (2) The greatest influence on the a limnetic basin filled with post-cooling water (in which zooplankton communities have physicochemical factors the residence time is relatively long) with the zooplankton including temperature, conductivity and content of communities from typical riverine ecosystems. Thus, it is inorganic nutrients, however, another factor determining reasonable that the zooplankton density is greater at the the physicochemical factors and zooplankton outlet of the limnetic basin. Besides, a high correlation communities is the water residence time. Thus, in between temperature and zooplankton abundance was lower, slowly flowing section of River Oder the physico- also noted in the summer, at the time when zooplankton chemical variables influenced zooplankton density, abundance peaked at each site. Vadadi-Fülöp et al. [21] similar to lake pattern. (3) Post-cooling water from the who studied zooplankton of the Danube river, have also power plant influences the zooplankton communities reported a positive correlation between temperature and only in the channel discharging the waters into the river, crustacean density. while its influence on the zooplankton in the East River The multiple regression and CCA explain a rather Oder is insignificant. Most probably along the channel small percent of diversity in zooplankton density at length the zooplankton reduction takes place as a result the sites studied, despite revealing many significant of predatory activity of fry, mechanical damage and correlations. It is supposed that some other variables unfavourable conditions for reproduction because of a not taken into account could decide about the status of strong water current. The effect of post-cooling water zooplankton communities. The majority of authors claim from the power plant on the zooplankton structure in that mainly physical parameters, especially discharge the East River Oder decreases with increasing distance and current velocity, determine the communities of from the channel outlet to the river, and downstream riverine zooplankton, [e.g. 2,4]. Probably in the study (site 3) this effect is insignificant. sites of the Oder river, the water residence time also As a result of hydrological and physicochemical belonged to the main factors determining the status of conditions the similarity in zooplankton communities zooplankton communities, which was well evidenced between the sites studied is rather small. From among at sites 2, 3 and 5, at which the current velocity and the physicochemical factors the greatest influence on discharge were the lowest as mentioned above. zooplankton communities had temperature, both in the Moreover, it should be also considered that the long water affected and not affected by post-cooling waters. water residence time could have positive effect on the It seems that an important factor determining the content of inorganic nutrients. zooplankton communities and physicochemical factors The conclusions following from analysis of our in interconnected sections of lower River Oder was the results compared with those of other authors are given water residence time.

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