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.