ACTA ZOOLOGICA BULGARICA Acta zool. bulg., 59 (2), 2007: 191-202 Long-Term Changes of Zooplankton and Dynamics of Eutrophication in the Polluted System of the Struma River – Pchelina Reservoir (South-West Bulgaria) Dedicated to the memory of Prof. Dr. Sc. Weselin Naidenow Dimitar Kozuharov 1* , Vesela Evtimova 2 , Diana Zaharieva 3 1 Department of General and Applied Hydrobiology, Biological Faculty, Sofja University, 8 Dragan Tsankov Blvd., 1164 Sofja, e-mails: mitko@biofac.uni-sofja.bg, mitko_bf@abv.bg; 2 Institute of Zoology, Bulgarian Academy of Sciences, 1 Tsar Osvoboditel Blvd., 1000 Sofja, Bulgaria; e-mail: evtimova@zoology.bas.bg; 3 Department of Fishery and Aquaculture, Ministry of Agriculture and Forestry, 17 Hristo Botev Blvd., 1606 Sofja, Bulgaria; e-mail: diana_zaharieva@mail.bg Abstract: The qualitative and quantitative parameters of zooplankton in the Struma River – Pchelina Reservoir system and the ecotone zone between them are compared for the periods 1990-1992 and 2001-2003. The effects of transition from lotic to lentic conditions and the anthropogenic impact on the formation of zoo - plankton communities were investigated. During the fjrst period the reservoir was a typical mesotrophic water body. More than 15 zooplankton species, that were present during 1990-1992, could not be found in 2001-2003. They are all pelagic elements. The number of species found only during the second period was 36. All of them are characteristic of eutrophic water bodies. Signifjcant changes in the quantitative pa - rameters of zooplankton were established in all parts of the system. The values of species diversity indices also have changed signifjcantly in a way which suggests that the process of eutrophication has advanced. Key words: river-reservoir ecotone, zooplankton, eutrophication, self-purifjcation Introduction b eshKOva , b Otev (1994), K OzUharOv (1995a,b). Changes in water quality of water bodies, as a No data could be found in the available literature result of the anthropogenic impact, affect structure concerning the long-term changes of zooplankton in of biocoenosis. These may result not only in the such a dynamic zone as the ecotone between river qualitative and quantitative zooplankton parameters, and reservoir. but also in changes of water communities in the Pchelina Reservoir (S. W. Bulgaria – Fig. 1) ecotone zone between the lentic and lotic water was built in 1975 to contribute to the deposition of bodies. Issues referring to ecotone effects and suspended materials with which the Struma River mutual effects between rivers and reservoirs are discussed by O br (1972), K Ovachev , U zUnOv (1979, was loaded. In the 1980s the bottom communities 1987), n aidenOw (1981), n aidenOw , b aev (1987), in the river were strongly infmuenced by heavy * Corresponding author 191
Kozuharov D. , V. Evtimova, D. Zaharieva Fig. 1. Scheme of Pchelina Reservoir and part of the fmow of the Struma River with positions of the sampling stations. pollution (U zUnOv , K Ovachev 1987). During the last 2003. Five stations in the Struma River, Pchelina decade water pollution with suspended materials and Reservoir and the ecotone zone between them were organic wastes has been decreasing in the region. sampled (Fig. 1). Their coordinates, determined by After 1992 some of the coalmines were closed and GPS receiver – tuned on WGS 1984 datum, from NE to SW, are: a waste depository for the power plant of the town of Pernik was built above the dam. Thus the loading • Station V – N 42.31.35.7, E 22.53.36.4 (river); of the river was partly reduced. Pchelina Reservoir • Station IV – N 42.31.38.0, E 22.53.08.3 (upper was also used as an industrial and agricultural water ecotone zone); source. Nowadays it is used only for agricultural • Station III – N 42.31.36.4, E 22.52.52.35 (lower irrigation of Pernik and Radomir districts. Long- ecotone zone); term changes in zooplankton communities in the • Station II – N 42.31.23.6, E 22.52.05.5 system of the Struma River – Pchelina Reservoir and (reservoir); the ecotone zone between them are investigated with • Station I – N 42.30.45.4, Е 22.50.31.8 (reservoir). regard to these changes in the water quality. The A total number of 208 zooplankton samples main aims of this paper are: were collected. “Apstein” qualitative net, mesh 38 1. To determine and discuss the long-term mkm, and "Juday" quantitative net, mesh 100 mkm, changes in zooplankton structure, their qualitative were used. In shallow ecotone and river stations and quantitative parameters, to assess the dynamics qualitative samples were collected by direct fjltering of eutrophication; of 50, 40 or 30 dm 3 of water. Quantitative samples 2. To study the changes in qualitative and were counted using the method of Hensen, modifjed quantitative zooplankton parameters with regard to by d imOff (1959) and n aidenOw (1972, 1976, 1977, changes in anthropogenic impact and changes from 1981). Absolute abundance and biomass were lotic to lentic conditions. calculated for every station or horizon, as well as the average values for the water column of stations I and Material and Methods II, and mean season values for the whole system. The investigation comprises two distinct periods: Biocoenological analyses were made on the basis March 1990 – September 1992, and July 2001 – June of the following indices: frequency of occurrence (pF), 192
Long-Term Changes of Zooplankton... frequency of dominance (DF) and order of dominance differences between its maximum and minimum (DT) after d e v ries (1937). In order to assess the values were greater. For instance, during the summer stratifjcation the values of oxygen in the bottom community structure the indices of species diversity horizon were zero or near zero (Fig. 2). At the same (H) after Shannon – Weaver ( s hannOn , w eaver , time, at the surface horizon 5-0 m, the dissolved 1963), of dominance (c) after s impsOn (1949) and oxygen reached 25-28 mg/dm 3 (300-350 %), values evenness (e) after p ielOU (1966, 1975) were used. typical of eutrophic water bodies. Temperature and oxygen quantities were During the two periods of investigation the measured using Handylab Ox 1 Set or the method presence of H 2 S was signifjcant not only in the bottom of Winkler after l Ure (1973). The “Fridenger” sediments, but in the deepest water horizon as well. and “Hydrobios” PVS – 436 302 water samplers During autumn homothermy it probably reaches were used for the hydrological and hydro-chemical even higher water layers. Indirectly these results can samples. be observed on the graphics of the oxygen dynamics Results (Fig. 2). Qualitative Composition Hydrological and Hydro-Chemical Data A total number of 105 taxa (genus, species and During 1990-1992 the mean seasonal quantities of subspecies) were found in the system, including dissolved oxygen varied between 2.7 and 7.6 mg/ Nauplii and Copepodites stages of Copepoda, as dm 3 ; and the absolute oxygen – between 0 mg/dm 3 important zooplankton components (Table 1). in the hypolimnion (October 1990) and 13.71 mg/ Signifjcant change in the qualitative composition dm 3 in the epilimnion (March 1990). The maximum of zooplankton was established – 36 were the vertical difference reached was 11.8 mg/dm 3 in april new components for the second period and 15 1991. The oxygen dynamic during the fjrst period was typical for a mesotrophic basin. Within the species were recorded only during the fjrst period. second period (2000-2003) the oxygen dynamic was In the ecotone zone an increase in number of more characteristic of eutrophic water body. The zooplankton species and taxa from Varia group 18 Mar 1990 16 May 1990 Nov 1990 14 Apr 1991 Jul 1991 12 Oxygen [mg/m 3 ] Nov 1991 Feb 1992 10 Jul 2001 8 Apr 2002 Aug 2002 6 Oct 2002 Jun 2003 4 2 0 0 5 10 15 Depth (m) Fig. 2. Changes in the values of dissolved oxygen in the different horizons at station I during both periods of investiga - tion 1990-1992 and 2001-2003. 193
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