Oceanological and Hydrobiological Studies

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1 Oceanological and Hydrobiological Studies International Journal of Oceanography and Hydrobiology Vol. XXXVI, No.3 Institute of Oceanography ISSN X (39-51) 2007 University of Gdańsk eissn DOI /v Original research paper Received: Accepted: January 03, 2007 July 17, 2007 Spatial distribution of zooplankton in a cascade system of Pomeranian dam reservoirs (Hajka, Rosnowo), northern Poland Agnieszka Pociecha 1,3, Tomasz Heese 2 1 Institute of Nature Conservation, Department of Freshwater Biology PAS ul. A. Mickiewicza 33, Kraków, Poland 2 Division of Environmental Biology, Technical University of Koszalin ul. Śniadeckich 2, Koszalin, Poland Key words: zooplankton, spatial distribution, rheolimnic, dam reservoirs, cascade system Abstracts The structure of the zooplankton community and its spatial distribution were examined in two stratified rheolimnic Pomeranian reservoirs, Rosnowski (7 sampling sites) and Hajka (4 sampling sites), in July These reservoirs are part of the cascade system situated on the Radew River in northern Poland. In the Rosnowski reservoir 34 species of zooplankton were identified and in Hajka, 32. The two dominant species of rotifers in both reservoirs were Keratella cochlearis f. tecta and Polyarthra vulgaris. The dominant copepod and cladoceran species in both reservoirs were Mesocyclops leuckarti and Daphnia cucullata, respectively. Along the longitudinal axis of both reservoirs, rotifers were the dominant group at all sampling sites, except for the hypolimnion layer in both reservoirs and the metalimnion layer in Hajka reservoir at site H1. This initial study was undertaken in order to determine the spatial distribution of rotifer, copepod and cladoceran communities in riverine reservoirs built in the cascade system. 3 Corresponding author: pociecha@iop.krakow.pl

2 40 A. Pociecha, T. Heese INTRODUCTION Physical and chemical variables and river inflows often create horizontal gradients in reservoirs that can affect the distribution and occurrence of organisms (Wiśniewski, Błędzki 1989; Hart 1990; Thoronton et al. 1990; Błędzki, Ellison 2000; Baião, Boavida 2000). The interaction between biotic and environmental variables of reservoir environments may be crucial for interpreting community dynamics (Betsill, Van Den Avyle 1994; Hayward, Van Den Avyle 1986; Lewis 1978; Threlkeld 1982, 1983; Urabe 1989, 1990). Zooplankton constitute an important link in the well-documented pelagic food webs of eutrophic lakes, but their spatial distribution in rheolimnic reservoirs of cascade systems remains poorly described. The aim of this initial study was to determine if zooplankton community structure differs in the limnetic and littoral zones and in the epi- and hypolimnion layers along the longitudinal axis of the cascade of two rheolimnic riverine reservoirs. Thus, this work was undertaken in order to analyze the species composition of rotifers and crustaceans at various sites of the reservoirs, with determination of dominant species, and to compare the spatial distribution of zooplankton communities in Rosnowski and Hajka reservoirs, as they relate to environmental conditions. STUDY AREA The two old rheolimnic reservoirs, Rosnowski and Hajka, were built on the Radew River, a tributary of the Parsęta River in northwest Poland (Fig. 1). Mean water discharge of the river is 5.6 m 3 s -1 and some characteristic parameters of the reservoirs are given in Table 1. The Radew River originates from a destratified, cool-water lake that is part of a pumping storage hydropower plant system. Both reservoirs, connected by a 3.5 km channel, are surrounded by forests and characterized by a stable water level with yearly amplitudes not exceeding 20 cm. This allows them to establish a well-developed complex of littoral vegetation zones, which is rather typical of natural lakes (Puchalski et al. 1999). The shoreline of the Rosnowski Reservoir is characterized by a few deeply incised bays. There are no bays in the Hajka Reservoir. There were 7 sampling sites in the Rosnowski Reservoir, of which 3 were situated in bays, and 4 sampling sites in the Hajka Reservoir (Figs. 2, 3). The Rosnowski and Hajka reservoirs showed a typical littoral plant zonation characteristic of lakes. Within these zones one can distinguish species belonging to helofits, nympheids and elodeoids. The macrophyte community of Hajka had been less diverse than that of the Rosnowski reservoir, which is

3 Spatial distribution of zooplankton in a cascade system 41 Fig. 1. Location of the Pomeranian reservoirs, Rosnowski and Hajka (northern Poland). Morphometric features of the Pomeranian reservoirs Rosnowski Res. Hajka Res. Year filled Length [km] Surface area [ha] Max. depth [m] 9 7 Capacity [mln. m 3 ] Mean retention time [in days] Table 1 characteristic of lakes with low trophic state. Some species were found solely in the Rosnowski bays (Puchalski et al. 1999). Epilimnetic phytoplankton communities were composed mostly of diatoms (Aulacoseira granulata syn. Melosira granulata, Diatoma elongatum, Stephanodiscus hantzschii) and flagellates of various taxonomic groups: Dinobryon sociale, Cryptomonas sp., Chlamydomonas sp., Rhodomonas minuta, Peridinium sp. The contribution of green algae was small but cyanoprokaryotes were not noted. The species composition of phytoplankton in both reservoirs was similar. Total counts of cells did not exceed and

4 42 A. Pociecha, T. Heese thousands of cells l -1 in the upper layers of Rosnowski and Hajka, respectively. The same community of phytoplankton was noted in the bays as in the reservoir, apart from D. sociale and Cryptomonas sp. (Puchalski et al. 1999). MATERIALS AND METHODS Samples were taken from 11 stations located in the Rosnowski and Hajka reservoirs (Figs. 2, 3). The investigation was carried out in July All samples were collected at the center of the elongated part of the reservoirs where the sampling site is deepest. Zooplankton samples were also taken from several water layers: the epi-, meta- and hypolimnion at the following sampling sites: R4, R3, H1 and H2. Zooplankton was collected using a 5-l Bernatowicz sampler with 3 replicates (total of 15 liters per sampling site). The samples were concentrated with a plankton net (mesh size 50 μm) and preserved with 4% formaldehyde solution. Taxonomic analyses of zooplankton were conducted according to the following keys: Dussart 1967, 1967; Flößner 1972; Ejsmont- Karabin, Radwan, Bielańska-Grajner 2004; Rybak, Błędzki 2005; Voight, Koste 1978a, 1978b. Quantitative analyses of zooplankton were completed according to standard methods used in hydrobiological studies (Wetzel 1991). Planktonic animals were identified and counted under a binocular microscope (magnification 10-20) in a chamber (volume 0.5 cm 3 ). The physical and chemical water column parameters (P tot., N-NO 3, N NH 4 ) were determined according to APHA (1992) and Hermanowicz et al. (1976). Water temperature was measured with a mercury thermometer and oxygen concentration was measured with an OXI oxygen electrode from WTW, Wilheim, Germany; both parameters were measured in situ. RESULTS Physical and chemical parameters In July, clear thermal and oxygen stratification was observed in both reservoirs. In the Rosnowski Reservoir, which was supplied with cool water from the Radew River, the epilimnion water temperature was 15.4 C and the hypolimnion temperature was C. In the Hajka Reservoir, supplied with surface outflow from the reservoir situated above, the water temperature in the epilimnion and hypolimnion was above 15 C. Oxygen stratification was characterised by the following features: oxygen saturation of the epilimnion of the Rosnowski reservoir, maximum concentration of oxygen in the metalimnion of the Hajka reservoir and oxygen deficit in the hypolimnion in both reservoirs

5 Spatial distribution of zooplankton in a cascade system 43 Fig. 2. Percentage of rotifers and crustaceans at different sites in the Rosnowski reservoir. Fig. 3. Percentage of rotifers and crustaceans at different sites in the Hajka reservoir.

6 44 A. Pociecha, T. Heese (1-4 mg O 2 l -1 ). Total phosphorus in the Rosnowski epilimnion ranged from 73 to 82 μg l -1, from 137 to 266 µg l -1 in the bays, and from 110 to 125 µg l -1 in the epilimnion of the Hajka reservoir. The concentration of total phosphorus in the hypolimnion of both reservoirs did not exceed 140 μg l -1, and the concentration of N-NO 3 ranged from 36 to 130 μg l -1 in the epilimnion and to 330 μg l -1 in the hypolimnion. N-NO 3 was present in the bays in trace amounts. The concentration of N NH 4 was not above 80 μg l -1. Secchi disc visibility in the Rosnowski reservoir was 1.1 m, and in Hajka, 1.7 m. Rotifer community Numerical abundance of the Rotifer community in the epilimnion of Rosnowski (site R1) reservoir was almost three times higher than in the Hajka reservoir (site H1), while in the epilimnion, at sites R2 and H2, the abundance of rotifers was similar. However, the density dropped gradually in the metalimnion and hypolimnion in both reservoirs. At site R3 the density of rotifers was twenty-two times lower than in the Hajka reservoir at site H3, but at site R4 the inverse situation was observed, where the density was forty-four times higher than at site H 4. A higher density of rotifers, from 2000 to 4000 ind. l -1, was observed in bays of Rosnowski reservoir (Fig. 4). The greater abundance of the rotifer community in the Rosnowski reservoir suggested a higher trophic state than in the Hajka reservoir. Rotifer abundance in the Rosnowski and Hajka reservoirs at all sites are shown in Tables 2 and 3, respectively. The rotifer community consisted of 22 species in Rosnowski reservoir and 20 species in Hajka reservoir. The dominant species was Keratella cochlearis f. tecta (Gosse 1851), and the subdominant, Polyarthra vulgaris (Carlin 1943), in both reservoirs. Along the longitudinal axis of Rosnowski reservoir, the distributional pattern of rotifers was similar at all sampling sites, including bays. We noted one exception at site Bay R3 where over 80% of rotifers were K. cochlearis f. tecta and K. cochlearis; however, the species composition of rotifers was similar to the other sampling sites (Table 2). In Hajka reservoir, the species composition of the rotifer community was similar to that observed in the Rosnowski reservoir. An exception was noted at site H4, where the structure and density of rotifers differed from the other sites (Table 3). This was likely due to the proximity of the river channel and hydropower pumping. Rotifers were the dominant zooplanktonic group in both reservoirs, except the metalimnion at site H1 and hypolimnion at sites: R1, R2 and H1, H2. High percentages of rotifers were noted in the Rosnowski bays: 75-95% of the total density of zooplankton (Figs. 2, 3).

7 Spatial distribution of zooplankton in a cascade system 45 Individuals per liter Fig. 4. Total density of rotifers (black circle), copepods (open square) and cladocerans (black triangle) in Rosnowski (A) and Hajka (B) reservoirs.

8 46 A. Pociecha, T. Heese Table 2 Species composition and density (ind. l -1 ) of rotifers and crustaceans in the Rosnowski reservoir (bold-dominant and subdominant species) R1 R2 R3 R4 Bay 1 Bay 3 Bay 5 epilimnion metalimnion hypolimnion epilimnion metalimnion hypolimnion Total Rotatoria Anuraeopsis fissa (Gosse, 1851) Ascomorpha sp Asplanchna priodonta (Gosse, 1850) Brachionus calyciflorus Pallas, Brachionus diversicornis (Daday, 1883) Conochilus unicornis Rousselet, Euchlanis dilatata Ehrenberg, Filinia longiseta (Ehrenberg, 1834) Kellicottia longispina (Kellicott, 1879) 2 Keratella cochlearis (Gosse, 1851) Keratella cochlearis f. tecta Keratella quadrata (O. F. Müller, 1786) Mytilina mucronata (O. F. Müller, 1773) 2 Polyarthra sp. 11 Polyarthra euryptera Wierzejski, Polyarthra vulgaris Karlin, Pompholyx sulcata Hudson, 1885 Synchaeta sp Synchaeta oblonga Ehrenberg, Synchaeta pectinata Ehrenberg, Synchaeta stylata Wierzejski, Trichocerca sp. 2 Trichocerca capucina Wierzejski i Zacharias, Trichocerca cylindrica (Imhof, 1891) Trichocerca inermis (Linder,1904) Trichocerca rousseleti Voigt, Copepoda Nauplius+copepodits Cyclops sp. Cyclops abyssorum (Sars, 1863) 2 4 Cyclops scutifer G. O. Sars, 1863 Cyclops strennus Fischer, Diacyclops sp. 1 2 Eudiaptomus graciclis (G. O. Sars, 1863) Mesocyclops leuckarti (Claus, 1857) Microcyclops sp. 1 Thermocyclops sp Thermocyclops crassus (Fischer, 1853) Cladocera Allonela sp. 20 Alona rectangula G. O. Sars, Bosmina sp. 1 2 Bosmina longirostris (O. F. Müller, 1785) Ceriodaphnia quadrangula (O. F. Müller, 1785) Daphnia sp. 6 1 Daphnia cucullata G. O. Sars, Daphnia longispina O. F. Müller, Diaphanosoma braychurum (Lievin, 1848)

9 Spatial distribution of zooplankton in a cascade system 47 Table 3 Species composition and density (ind. l -1 ) of rotifers and crustaceans in the Hajka reservoir (bold-dominant and subdominant species) H1 H2 H3 H4 epilimnion metalimnion hypolimnion epilimnion metalimnion hypolimnion Total Rotatoria Anuraeopsis fissa (Gosse, 1851) 1 10 Ascomorpha sp Asplanchna priodonta (Gosse, 1850) Brachionus calyciflorus Pallas, Brachionus diversicornis (Daday, 1883) 1 Conochilus unicornis Rousselet, Euchlanis dilatata Ehrenberg, Filinia longiseta (Ehrenberg, 1834) Keratella cochlearis (Gosse, 1851) Keratella cochlearis f. tecta Keratella quadrata (O. F. Müller, 1786) Polyarthra sp. 1 Polyarthra euryptera Wierzejski, Polyarthra vulgaris Karlin, Polyarthra trigla (Ehrb., 1834) 1 Pompholyx sulcata Hudson, Synchaeta sp Synchaeta oblonga Ehrenberg, Synchaeta pectinata Ehrenberg, Synchaeta stylata Wierzejski, Trichocerca capucina Wierzejski i Zacharias, Trichocerca cylindrica (Imhof, 1891) Trichocerca inermis (Linder, 1904) 1 3 Copepoda Nauplius+copepodits Cyclops sp. 1 Cyclops abyssorum (Sars, 1863) Cyclops scutifer G. O. Sars, Cyclops strennus Fischer, Diacyclops sp. 2 2 Eudiaptomus graciclis (G. O. Sars, 1863) 1 Mesocyclops leuckarti (Claus, 1875) Microcyclops sp. 1 1 Thermocyclops sp Thermocyclops crassus (Fischer, 1853) Cladocera Alona quadrangularis (O. F. Müller, 1785) 1 1 Bosmina sp. 1 1 Bosmina longirostris (O. F. Müller, 1785) Ceriodaphnia quadrangula (O. F. Müller, 1785) Daphnia sp. 1 Daphnia cucullata G. O. Sars, Diaphanosoma braychurum (Lievin, 1848) Leptodora kindti (Focke, 1844) 1

10 48 A. Pociecha, T. Heese Copepod community The density of copepods in the Rosnowski and Hajka reservoirs was different and depended on the water layer and sampling sites. We could not identify any patterns characterizing both reservoirs. Copepod densities above 400 ind. l -1 were noted in the Hajka reservoir at site H2 (epilimnion layer) and at site H3. In Rosnowski reservoir, a high density of copepods (387 ind. l -1 ) was observed at site R2, in the metalimnion (Fig. 4). In general, nauplius and copepodite stages occurred in highest abundance at all sampling sites along the longitudinal axis of the reservoir Copepod abundance at all sites in the Rosnowski and Hajka reservoirs are shown in Tables 2 and 3, respectively. The copepod community consisted of 6 species, with Mesocyclops leuckarti (Claus 1857) the dominant in both reservoirs. Copepods dominated in the deep hypolimnion of both reservoirs and in the metalimnion in Hajka reservoir at site H1. A high proportion of copepods along the longitudinal axis of the reservoir was observed in Hajka rather than in Rosnowski reservoir (Figs. 2, 3). Cladoceran community The abundance of cladocerans in Rosnowski and Hajka reservoirs was very similar and ranged from 3-56 ind. l -1. Along the longitudinal axis of the reservoir, the highest density of cladocerans (above 50 ind. l -1 ) in Rosnowski was noted in the metalimnion layer and bays, while in the Hajka reservoir, the highest density was observed in the metalimnion at site H1 and the epilimnion at site H2 (Fig. 4). Cladoceran abundance at all sampling sites in Rosnowski and Hajka reservoirs are shown in Tables 2 and 3, respectively. The cladoceran community consisted of 6 species in Rosnowski reservoir and 6 species in Hajka reservoir. Daphnia cucullata was the dominant species (Sars 1862) in both reservoirs. Bosmina longirostris (O. F. Müller 1785) was the subdominant species in the Rosnowski bays. A low proportion of cladocerans was observed in the epilimnion layer in both reservoirs, the Rosnowski bays, and at one site in the Hajka reservoir (H3) (Figs. 2, 3). DISCUSSION The presence of diverse plant communities, the development of a plant system characteristic of littoral zones, effective utilization of resources through trophic food chains and good mechanisms of compensation in nutrient cycle processes are indicative of advanced stages of succession and ecosystem maturity of reservoirs. The structure and functioning of both reservoirs are closer to natural lakes than to typical dam reservoirs. This may be due to the age

11 Spatial distribution of zooplankton in a cascade system 49 of the reservoirs (75-87 years), as well as the stable water level that allows for the undisturbed development of littoral communities. These, in turn, buffer all interactions between abiotic and biotic variables occurring in the adjacent catchment area (Puchalski et al. 1999). The physical and chemical characteristics of Rosnowski and Hajka reservoirs are typical of riverine water bodies, and are similar to the youngest rheolomnetic lowland reservoir, Włocławski Reservoir (Żytkowicz et al. 1990). The high concentration of nutrients, rich littoral zones of macrophytes and the riverine character of these reservoirs structured the zooplankton community that is adapted to this environmental condition. The patterns of spatial distribution of zooplankton along the longitudinal axes in these two Pomeranian reservoirs showed dominance by the rotifer community. Short retention time, small surface area and riverine structure of the reservoirs, the presence of the river channel, and probably, the continuous hydropower pumping activity could explain this observation. The same phenomenon was observed in the shallow riverine rheolimnetic Włocławski Reservoir, where rotifers dominated and the zooplankton community was significantly dependent on water retention time, which ranges from days (Błędzki, Ellison 2000). The species composition of the rotifer community is related to the retention time of reservoirs, and genera such as Keratella, Brachionus and Synchaeta are characteristic indicators of short retention times (De Manuel, Armengol 1993). The occurrence, dominance and high density of two species, K. cochlearis f. tecta and K. cochlearis, in the Rosnowski and Hajka reservoirs confirmed this observation. However, the high density of K. cochlearis f. tecta, K. cochlearis and P. vulgaris were characteristic for bay sites in the Rosnowski Reservoir. Macrophytes, which primarily develop in bays, are a good microhabitat for rotifers. Kuczyńska-Kippen & Nagengast (2006) showed that rotifer communities revealed the greatest similarity and highest density in the following macrophytes: Myriophyllum verticillatum, Chara hispida, Typha angustifolia and Nymphaea alba. These macrophytes were observed in the Rosnowski bays during the study period. The low density of cladocerans observed in the Hajka and Rosnowski reservoirs are related to the short retention time: similar results were reported from lowland dam reservoirs (Ejsmont-Karabin, Węgleńska 1990; Żytkowicz et al. 1990; Błędzki, Ellison 2000). The species composition of cladocerans (D. cucullata, B. longirostris) and copepods (M. leuckarti) were typical for temperate and rheolimnic dam reservoirs (Błędzki, Ellison 2000; Fleituch, Pociecha 2000). The dominance of two crustacean species (D. cucullata, M.

12 50 A. Pociecha, T. Heese leuckarti) in these reservoirs is typical for meso-eutrophic lakes and reservoirs (Fleituch, Pociecha 2000; Pociecha 2002). The spatial distribution, density and composition of rotifers and crustaceans in Pomeranian reservoirs are also related to zooplankton survival strategies. Pociecha & Wilk-Woźniak (2006) demonstrated that Keratella cochlearis representing an r life strategy and Mesocyclops leuckarti representing a K life strategy responded differently during dry (characterized by low river inflow) and wet years (characterized by high river inflow). Rotifers (r life strategy), which are the dominant group of zooplankton in the Pomeranian reservoirs, occur in high density and are adapted to high river inflow along the longitudinal axis of both reservoirs. Generally, the composition and density of the rotifer and crustacean communities was similar along the longitudinal axes of both Pomeranian reservoirs. The rotifer dominance was typical for rheolimnic riverine reservoirs. The physical and chemical variables that provide hydrographic context to this study did not give any insight into their impact on zooplankton communities. The above description of the spatial distribution of zooplankton and environmental factors are an attempt to understand the microscale temporalspatial organization of the ecosystem of riverine reservoirs. Further research could study biological relationships (e.g. food availability, phytoplanktonzooplankton interactions) and colonization processes (e.g. plant microhabitats) within the reservoirs. Certainly, the initial results of our study seem to be due to complex mechanisms, which require further research. REFERENCES APHA. (1992). Standard methods for the examination of water and wastewater (18 th ed.). Washington: American Public Health Association. Baião, C. & Boavida M. J. (2000). Environmental factors determining the structure of rotifer communities in a river-shed reservoir. Aquat Ecol, 34, Błędzki, L. A. & Ellison A. M. (2000). Effects of water retention time on zooplankton of shallow rheolimnic reservoirs. Verh. Internat. Verein. Limnol., 27, Betsill, R. K. & Van Den Avyle M. J. (1994). Spatial heterogeneity of reservoir zooplankton: a matter of timing?. Hydrobiologia, 277, De Manuel, J. & Armengol J. (1993). Rotifer assemblages: a contribution to the typology of Spanish reservoirs. Hydrobiologia, 255/256, Dussart, B. (1967). Les Copépodes des eaux continentals d Europe occidentale: Calanoides et harpacticopides. Paris: Boubée and Cie. Dussart, B. (1969). Les Copépodes des eaux continentals d Europe occidentale: Cyclopoides et biologie. Paris: Boubée and Cie. Ejsmont-Karabin, J., Radwan S. & Bielańska-Grajner I. (2004). Polish Freshwater Fauna. Rotifers (Rotifera). Łódź: University of Łódź. Ejsmont-Karabin J. & Węgleńska T. (1990). Zooplankton of Zegrzyński reservoir its abundance, structure and role in ecosystem functioning. In Z. Kajak (Eds.), Functioning of aquatic

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