Long-Term Changes of Zooplankton and Dynamics of Eutrophication in - - PDF document

191

ACTA ZOOLOGICA BULGARICA Acta zool. bulg., 59 (2), 2007: 191-202

Long-Term Changes of Zooplankton and Dynamics

• f Eutrophication in the Polluted System of the Struma River

– Pchelina Reservoir (South-West Bulgaria)

Dedicated to the memory

• f Prof. Dr. Sc. Weselin Naidenow

Dimitar Kozuharov1*, Vesela Evtimova2, Diana Zaharieva3

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

Changes in water quality of water bodies, as a result of the anthropogenic impact, affect structure

• f biocoenosis. These may result not only in the

qualitative and quantitative zooplankton parameters, but also in changes of water communities in the ecotone zone between the lentic and lotic water

• bodies. Issues referring to ecotone effects and

mutual effects between rivers and reservoirs are discussed by Obr (1972), KOvachev, UzUnOv (1979, 1987), naidenOw (1981), naidenOw, baev (1987), beshKOva, bOtev (1994), KOzUharOv (1995a,b). No data could be found in the available literature concerning the long-term changes of zooplankton in such a dynamic zone as the ecotone between river and reservoir. Pchelina Reservoir (S. W. Bulgaria – Fig. 1) was built in 1975 to contribute to the deposition of suspended materials with which the Struma River was loaded. In the 1980s the bottom communities in the river were strongly infmuenced by heavy

* Corresponding author

Kozuharov D., V. Evtimova, D. Zaharieva 192 pollution (UzUnOv, KOvachev 1987). During the last decade water pollution with suspended materials and

• rganic wastes has been decreasing in the region.

After 1992 some of the coalmines were closed and a waste depository for the power plant of the town

• f Pernik was built above the dam. Thus the loading
• f the river was partly reduced. Pchelina Reservoir

was also used as an industrial and agricultural water

• source. Nowadays it is used only for agricultural

irrigation of Pernik and Radomir districts. Long- term changes in zooplankton communities in the system of the Struma River – Pchelina Reservoir and the ecotone zone between them are investigated with regard to these changes in the water quality. The main aims of this paper are:

• 1. To determine and discuss the long-term

changes in zooplankton structure, their qualitative and quantitative parameters, to assess the dynamics

• f eutrophication;
• 2. To study the changes in qualitative and

quantitative zooplankton parameters with regard to changes in anthropogenic impact and changes from lotic to lentic conditions.

Material and Methods

The investigation comprises two distinct periods: March 1990 – September 1992, and July 2001 – June

• 2003. Five stations in the Struma River, Pchelina

Reservoir and the ecotone zone between them were sampled (Fig. 1). Their coordinates, determined by GPS receiver – tuned on WGS 1984 datum, from NE to SW, are:

• Station V – N 42.31.35.7, E 22.53.36.4 (river);
• Station IV – N 42.31.38.0, E 22.53.08.3 (upper

ecotone zone);

• Station III – N 42.31.36.4, E 22.52.52.35 (lower

ecotone zone);

• Station II – N 42.31.23.6, E 22.52.05.5

(reservoir);

• Station I – N 42.30.45.4, Е 22.50.31.8 (reservoir).

A total number of 208 zooplankton samples were collected. “Apstein” qualitative net, mesh 38 mkm, and "Juday" quantitative net, mesh 100 mkm, were used. In shallow ecotone and river stations qualitative samples were collected by direct fjltering

• f 50, 40 or 30 dm3 of water. Quantitative samples

were counted using the method of Hensen, modifjed by dimOff (1959) and naidenOw (1972, 1976, 1977, 1981). Absolute abundance and biomass were calculated for every station or horizon, as well as the average values for the water column of stations I and II, and mean season values for the whole system. Biocoenological analyses were made on the basis

• f the following indices: frequency of occurrence (pF),
• Fig. 1. Scheme of Pchelina Reservoir and part of the fmow of the Struma River with positions of the sampling stations.

Long-Term Changes of Zooplankton... 193 frequency of dominance (DF) and order of dominance (DT) after de vries (1937). In order to assess the community structure the indices of species diversity (H) after Shannon – Weaver (shannOn, weaver, 1963), of dominance (c) after simpsOn (1949) and evenness (e) after pielOU (1966, 1975) were used. Temperature and oxygen quantities were measured using Handylab Ox 1 Set or the method

• f Winkler after lUre (1973). The “Fridenger”

and “Hydrobios” PVS – 436 302 water samplers were used for the hydrological and hydro-chemical samples.

Results

Hydrological and Hydro-Chemical Data During 1990-1992 the mean seasonal quantities of dissolved oxygen varied between 2.7 and 7.6 mg/ dm3; and the absolute oxygen – between 0 mg/dm3 in the hypolimnion (October 1990) and 13.71 mg/ dm3 in the epilimnion (March 1990). The maximum vertical difference reached was 11.8 mg/dm3 in april

• 1991. The oxygen dynamic during the fjrst period

was typical for a mesotrophic basin. Within the second period (2000-2003) the oxygen dynamic was more characteristic of eutrophic water body. The differences between its maximum and minimum values were greater. For instance, during the summer stratifjcation the values of oxygen in the bottom horizon were zero or near zero (Fig. 2). At the same time, at the surface horizon 5-0 m, the dissolved

• xygen reached 25-28 mg/dm3 (300-350 %), values

typical of eutrophic water bodies. During the two periods of investigation the presence of H2S was signifjcant not only in the bottom sediments, but in the deepest water horizon as well. During autumn homothermy it probably reaches even higher water layers. Indirectly these results can be observed on the graphics of the oxygen dynamics (Fig. 2). Qualitative Composition A total number of 105 taxa (genus, species and subspecies) were found in the system, including Nauplii and Copepodites stages of Copepoda, as important zooplankton components (Table 1). Signifjcant change in the qualitative composition

• f zooplankton was established – 36 were the

new components for the second period and 15 species were recorded only during the fjrst period. In the ecotone zone an increase in number of zooplankton species and taxa from Varia group

• 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.

2 4 6 8 10 12 14 16 18 5 10 15

Depth (m) Oxygen [mg/m3] Mar 1990 May 1990 Nov 1990 Apr 1991 Jul 1991 Nov 1991 Feb 1992 Jul 2001 Apr 2002 Aug 2002 Oct 2002 Jun 2003

Kozuharov D., V. Evtimova, D. Zaharieva 194

Table 1. Qualitative composition of zooplankton in the system Struma River – Pchelina Reservoir during the periods 1990 – 1992 and 2001-2003. Legend: (X) – taxa found

Taxa Periods of investigation № 1990/ 1992 2001/ 2003 Protozoa 1 Vorticella sp. x x 2 Aspidisca sp. x 3 Carchesium polipinum L. x 4 Epistilis plicаtilis (ehrenberg, 1832) x 5 Opercularia coarctata (claperede et lachman) x 6 Stentor sp. x Rotifera 7 Brachionus angularis gOsse, 1851 x x 8 Brachionus calycifmorus pallas, 1766 x x 9 Brachionus calycifmorus amphiceros ehrenberg, 1838 x 10 Brachionus calycifmorus dorcas gOsse, 1851 x x 11 Brachionus leydigi cOhn, 1862 x 12 Brachionus leydigi leydigi cOhn, 1862 x 13 Brachionus leydigi quadratus rOUsselt, 1889 x x 14 Brachionus quadridentatus hermann, 1783 x 15 Brachionus urceolaris O. F. müller, 1773 x x 16 Keratella cochlearis (gOsse, 1851) x x 17 Keratella tecta (gOsse, 1851) x x 18 Keratella quadrata (O. F. müller, 1786) x 19 Keratella quadrata frenzeli ecKstein, 1895 x x 20 Keratella quadrata quadrata (O. F. müller, 1786) x 21 Notholca acuminata OlaffsOn, 1918 x x 22 Notholca acuminata acuminata ehrenberg, 1832 x 23 Plathias quadricornis quadricornis (ehrenberg, 1832) x 24 Euchlanis dilatata ehrenberg, 1832 x 25 Kellicottia longispina (KellicOtt, 1879) x x 26 Lecane luna vOigt, 1957 x 27 Lecane (Monostyla) lunaris (ehrenberg, 1832) x 28 Lecane (Monostyla) sp. x 29 Proales daphnicola thOmpsOn, 1892 x 30 Lepadella (s. str.) costata wUlfert, 1940 x 31 Lepadella (s. str.) patella O. F. müller, 1775 x 32 Lepadella (Xenolep.) haueri rOdewald, 1935 x 33 Lepadella sp. x 34 Asplanchna priodonta gOsse,1850 x x 35 Asplanchna sieboldi leydig, 1854 x x 36 Asplanchna sp. x x 37 Synchaeta sp. x x 38 Synchaeta cecilia rOUsselet, 1902 x 39 Polyarthra vulgaris carlin, 1943 x x 40 Polyarthra dolichoptera idelsOn, 1925 x x 41 Polyarthra remata sKOriKOw, 1896 x 42 Polyarthra sp. x

Long-Term Changes of Zooplankton... 195

Taxa Periods of investigation № 1990/ 1992 2001/ 2003 43 Pompholyx complanata gOsse, 1851 x x 44 Filinia terminalis plate, 1886 x x 45 Filinia longiseta (ehrenberg, 1834) x 46 Trichocerca (s. str.) rattus (müller, 1776) x 47 Conochilus unicornis rOUsselet, 1892 x x 48 Rotaria rotatoria (pallas, 1776) x 49 Rotaria sp. x 50 Rotifera g. sp. x Cladocera 51 Diaphanosoma lacustris KOrineK, 1981 x x 52 Daphnia galeata sars, 1864 x x 53 Daphnia cucullata sars, 1864 x x 54 Daphnia longispina typica müller, 1785 x x 55 Daphnia hyalina leydig, 1860 x x 56 Daphnia hyalina lacustris sars, 1862 x 57 Daphnia parvula fOrdice, 1901 x 58 Daphnia sp. x 59 Daphnia sp. juv. x x 60 Moina dubia gUerne et richard 1892 x 61 Moina micrura KUrz, 1874 x 62 Macrotricx laticornis (JUrine, 1820) x 63 Ceriodaphnia quadrangula (müller, 1785) x x 64 Ceriodaphnia dubia lilJebOrg, 1900 x 65 Scapholeberis mucronata (müller, 1785) x 66 Simocephalus expinosus (KOch, 1841) x x 67 Simocephalus vetulus (müller, 1776) x x 68 Bosmina longirostris (müller, 1785) x x 69 Bosmina longilostris similis sars, 1890 x x 70 Bosmina longirostris typica sars, 1890 x 71 Bosmina longirostris cornuta JUrine, 1820 x 72 Bosmina coregoni baird, 1857 x x 73 Bosmina kessleri (UlJanin, 1872) x 74 Chydorus sphaericus (müller, 1785) x x 75 Pleuroxus aduncus (JUrine, 1820) x 76 Pleuroxus uncinatus baird, 1850 x 77 Alonella sp. x 78 Alona costata sars, 1862 x 79 Alona rectangula sars, 1862 x 80 Alona cf.retangula x 81 Alona sp. x Copepoda 82 Acanthocyclops robustus (sars, 1863) x x 83 Acanthocyclops sp. 1 x 84 Acanthocyclops sp. 2 x x

Table 1. Continued.

Kozuharov D., V. Evtimova, D. Zaharieva 196 (drifted zoobenthic components) were recorded compared to the fjrst period. A possible reason may be the increasing species richness and density of zoobenthic components on the river bottom above the reservoir, due to the decreased loading with suspended materials. As a natural process in the river stream a part of the benthic organisms are involved in the biological drift. Several subspecies from the genera Keratella, Brachionus, Bosmina, Eucyclops were found in the system (Table 1). There are several explanations for this phenomenon. Most of the subspecies were found during different seasons and years and in the different parts of the system, so they were separated in time and in space. The length of the system is 7.3 km with very different conditions from the river to the reservoir. The depth varied between 0.8 m in the river and in the upper part of the ecotone and 19 m near the dam of the reservoir (KOzUharOv 1995a,b). Another reason infmuencing the zooplankton species and subspecies distribution is the wide zone with macrophytes in the ecotone creating specifjc

• microhabitats. The most common amongst them

are Typha latifolia, T. angustifolia, Potamogeton natans and Myriophillum verticillatum. There are also several shallow large temporal ponds near the river above the reservoir. These ponds connect with the Struma River when the water quantities increase signifjcantly in spring. It is possible that part of these

• rganisms have been drifted in the system during

that time. Quantitative Composition Within the two periods the quantitative parameters of the zooplankton vary greatly. These differences can be observed for common quantitative zooplankton

Taxa Periods of investigation № 1990/ 1992 2001/ 2003 85 Cyclops sp. x x 86 Cyclops vicinus UlJamin, 1875 x x 87 Thermocyclops crassus (fischer, 1853) x x 88 Thermocyclops sp. x 89 Eucyclops macruroides (lilliebOrg, 1901) x 90 Eucyclops serrulatus (fischer, 1851) x x 91 Eucyclops serrulatus proximus (fischer, 1851) x 92 Macrocyclops albidus (iUrine, 1820) x x 93 Mesocyclops sp. x 94 Paracyclops fjmbriatus (fischer, 1853) x x 95 Eudiaptomus gracilis (sars, 1863) x x 96 Eudiaptomus sp. x 97 Copepodites x x 98 Nauplii x x Varia 99 Ostracoda sp. x 100 harpacticoida x x 101 Fam. Naididae x x 102 Nais communis x 103 Fam. Tubifecidae x x 104 larvae fam. Chironomidae x x 105 larvae Odonata x 106 Ephemeroptera - Nimpha x

Table 1. Continued.

Long-Term Changes of Zooplankton... 197 parameters in the system as a whole, and for values at the certain stations. Period I – During the fjrst period the absolute abundance of zooplankton was between 600 ind/m3 at station V in July 1991 and 416580 ind/m3 in the lower ecotone in November 1991. The biomass varied from 1 mg/m3 in the Struma River in February 1992 to 6718.3 mg/m3 in reservoir in October 1990 when the water level in the reservoir was low (Fig. 3a,b). The differences between the abundance and biomass of zooplankton in river, ecotone zone and lake part decreased during 2001-2003. The highest abundance (40259 ind/m3) in the hypolimnion was recorded in May 1990. The average seasonal abundance and biomass was lowest in winter – 25448 ind/m3 and 271 mg/m3, and highest in autumn – 64264 ind/m3 and 1285 mg/m3. The average abundance for the fjrst period reached maximum in July 1990 (66872 ind/m3) and in November 1991 (80756 ind/ m3) (Fig. 4a). Two peaks in the dynamics

• f zooplankton biomass were recorded:

in July 1991 (1010 mg/m3) and in autumn (KOzUharOv, 1995b) (Fig 4b). The abrupt drop in abundance (14282 ind/m3) and biomass (180 mg/m3) in April 1991 was due to temperature, which was lower by 3-3,5° C than the one measured in March 1990. Period II – The absolute zooplankton abundance and biomass for the period 2001- 2003 ranged from 380 ind/m3 (river station, July 2001) to 738290 (St. I., 5-10 m, April 2002), and from 2.45 (St. I., 0-5 m, August 2002) to 14101.86 (river station, October 2002) mg/m3 (Fig. 3a). The peaks in abundance (511383 ind/ m3) and biomass (8561.33 mg/m3) registered at St. I in April 2002 were due to the dominant Nauplii and Copepodites of Copepoda. These are average values for the water column of the station and not absolute abundance at a certain

• horizon. For the second period two maximums of average

annual abundance of zooplankton were registered (Fig. 4a) – in spring (147000 ind/m3) and in autumn (65000 ind/m3). The average annual biomass also has two maximums. The fjrst one corresponds to the peak of the average abundance in April 2002

• Fig. 3a. Changes in the abundance of the zooplankton on different

stations during both periods of investigation.

• Fig. 3b. Changes in the biomass of the zooplankton on different

stations during both periods of investigation.

100000 200000 300000 400000 500000

I/IA II III/IIIA IV V stations Abundance of zooplankton [ind/m3]

• Mar. 1990

May 1990

• Jul. 1990
• Sep. 1990
• Oct. 1990
• Apr. 1991
• Jul. 1991
• Nov. 1991
• Feb. 1992
• Jul. 2001
• Apr. 2002
• Aug. 2002
• Oct. 2002
• Mar. 2003
• Jun. 2003

1000 2000 3000 4000 5000 6000 7000 8000 9000 I/IA II III/IIIA IV V stations Biomass of zooplankton [mg/m ³]

• Mar. 1990

May 1990

• Jul. 1990
• Sep. 1990
• Oct. 1990
• Apr. 1991
• Jul. 1991
• Nov. 1991
• Feb. 1992
• Jul. 2001
• Apr. 2002
• Aug. 2002
• Oct. 2002
• Mar. 2003
• Jun. 2003

Kozuharov D., V. Evtimova, D. Zaharieva 198 (2255.54 mg/m3), while the second one was registered in August (1742.56 mg/m3). Dynamics of the Structural Indices The only zooplankton components with frequency of

• ccurrence pF=100 (for both periods) or above 90 %

(only for period II) were Nauplii and Copepodites of

• Copepoda. Some local dominants like Nais elinguis

and Nais communis had the highest values of order

• f dominance DT=50 %. These elements are typical

components of the drifting process in the river and the upper part of the ecotone. The highest values

• f the Frequency of Dominance (DT) for the fjrst

period had Keratella quadrata (83.33%), because

• f its dominance at the stations in the ecotone

zone during autumn, winter and spring. In summer Copepodites and Nauplii (Copepoda) dominated with values of 50% and 25% (KOzUharOv, 1995b). The highest values of DF during the second period had Copepodites and Nauplii (correspondingly 50 and 43.8%). H values (H=0.61-0.5) decreased in the direction of the river during both periods

• Fig. 4a. Changes in the annual abundance of the zooplankton during both periods of investigation.
• Fig. 4b. Changes in the annual biomass of the zooplankton during both periods of investigation.

20000 40000 60000 80000 100000 120000 140000 J a n F e b M a r A p r M a y J u n J u l A u g S e p O c t N

• v

D e c Months Annual abundance [ind/m³]

1990 1991 1992 2001 2002 2003

500 1000 1500 2000 2500 Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Months Annual biomass [mg/m³]

1990 1991 1992 2001 2002 2003

Long-Term Changes of Zooplankton... 199

• f investigation, but for the second period of

investigations the values of that index in the river stations were considerably higher than during the fjrst period (Fig. 5).

Discussion

The number of species and taxa of Protozoa has increased during the period 2001-2003. Most of these organisms are typical of the activated sludge

• f Waste Water Treatment Plants (WWTP) with

most probable origin from the biobasins of WWTP “Radomir”, situated above the dam. Several species

• f Rotifera, which are characteristic inhabitants of

the Activated Sludge (AS), for instance Proales daphnicola, Rotaria rotatoria, Rotifera g. sp., were recorded during the second period. One of the explanations of the presence of these rotifers could be the hydrological regime of the system, namely the prolonged rainfalls in the spring of 2002. Other new rotifers for the system, compared to 1990-1992, were found during the second period. They belong to genus Lecane and Lepadella (Table 1), which are both characteristic of eutrophic water bodies. Typical

• f such water conditions are also the Cladocerans

from genus Pleuroxus and Alona that have been recorded during 2001-2003 (Table 1). Species like P. uncinatus, P. aduncus, A. costata and A. rectangula usually inhabit swamps and bogs. The aggravated oxygen regime in the deepest water horizons and the presence of H2S in the bottom sediments and the deepest water horizons affect the vertical distribution of zooplankton. The quantitative parameters indicate the epilimnic character of the community, especially during the second period of

• investigations. Differences in the density for the

stations in the lake part, ecotone and river were not as signifjcant as during the fjrst period. The abundance and biomass at the ecotone and river stations were higher than the values during the fjrst period due to the decrease of the loading with suspended materials. Peaks in zooplankton abundance for both the periods were observed at lower ecotone station and Station

• II. The only exception (concerning the number of the

series) was in April 2002 when the maximum was at

• St. I (Fig. 3a). A possible explanation could be the

wind effects or the explosive growth of zooplankton after the melting of the ice cover of the reservoir. The differences in the dominant complexes indicate changes in water conditions in the system as a whole and in the ecotone zone, where the processes of accumulating and removing of wastes are very intensive (naidenOw, baev, 1987). Changes in diversity index values are connected to the hydrological and hydro chemical regimes of the system. The tendency in values of the Shannon-

• Fig. 5. Changes in the values of the species diversity index (H) after shannOn-weaver (1963) in the system during both

periods of investigation with position of the sampling station (st).

0,5 1 1,5 2 2,5 3 Mar 1990 May 1990 Jul 1990 Sep 1990 Nov 1990 Apr 1991 Jul 1991 Nov 1991 Feb 1992 Jul 2001 Apr 2002 Aug 2002 Oct 2002 Mar 2003 Jun 2003 Months and Years Species diversity index (H)

/ st st / st V st V st

Kozuharov D., V. Evtimova, D. Zaharieva 200 Weaver index (H) showed the highest diversity in the lower part of the ecotone zone when the high water level was combined with low temperature in spring in the both periods of investigation (Fig. 5). The values of the index of dominance (c) after simpsOn (1949) were in the opposite correlation with the values of the (H) index for both periods of investigation (Fig. 6), and in a straight dependence with the values of the evenness index (e) after pielOU (1966) (Fig. 7). Relatively high values of H index were observed when high number of species was combined with relatively close values of the density index of all species in the community. Probably ice cover and low temperature in winter (1992, 2002 and 2003), suppressed the development of dominant

• species. This could be the reason for the highest

H values in the lake part of the system (H=1.8) at that time. There are two possible explanations

• f the decrease of H values in the river direction:

decrease of the suspended materials in the water

• f the river and of the anthropogenic impact from

the towns of Radomir and Pernik after the opening

• Fig. 6. Changes in the values of the dominance index (c) after simpsOn (1949) in the system during both periods
• f investigation with position of the sampling station (st).
• Fig. 7. Changes in the values of the evenness index (e) after pielOU (1966) in the system during both periods
• f investigation with position of the sampling station (st).

0,2 0,4 0,6 0,8 1 1,2 Mar 1990 May 1990 July 1990 Sep 1990 Oct 1990 Apr 1991 Jul 1991 Nov 1991 Feb 1992 Jul 2001 Apr 2002 Aug 2002 Oct 2002 Mar 2003 Jun 2003 Months and Years Index of dominance (c)

/ st st / st. V st

0,2 0,4 0,6 0,8 1 Mar 1990 May 1990 Jul 1990 Sept 1990 Oct 1990 Apr 1991 Jul 1991 Nov 1991 Feb 1992 Jul 2001 Apr 2002 Aug 2002 Oct 2002 Mar 2003 Jun 2003 Months and Years Eveness index (e)

/ st st / st. V st V st

Long-Term Changes of Zooplankton... 201

• f the WWTP; and signifjcant quantities of species

from the Activated sludge of Radomir WWTP. The drifted zoobenthic components may also infmuence the values of the index. Seasonal changes in the ecotone zone varied, but always depended on the shape of the river valley, water quantities in the river and on water levels in the reservoir. The ecotone is wide and shallow (KOzUharOv, 1995b). The changes in water quality in the river above the reservoir during the last 5-10 years have infmuenced the plankton community in the ecotone zone between the reservoir and the river. As a result of the accumulation of pollutants in the sediments of the reservoir and the ecotone zone the processes of eutrophication are developing in the system.

Acknowledgements: The authors are grateful to engineer- chemist Ivan Botev who made the chemical analyses of the wa- ter samples from the fjrst period of investigations and for the technical support in the fjeld work to the students Lubomir Ko- zlov and Julian Jorev.

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Received: 13.11.2006 Accepted: 08.02.2007

Kozuharov D., V. Evtimova, D. Zaharieva 202

Дълговременни промени на зоопланктона и динамика на еутрофизацията в натоварената система р. Струма – яз. „Пчелина“ (Югозападна България)

Д. Кожухаров, В. Евтимова, Д. Захариева

(Резюме)

Сравнени са качествените и количествените параметри на зоопланктона в системата р. Струма – яз. „Пчелина“ и екотона между тях за периодите 1990-1992 г. и 2001- 2003 г. Изследвани са ефектите от прехода от лотични към лентични условия и антропогенното влияние върху формирането на зоопланктонните съобщества. През първият период на изследване язовирът бе типично мезотрофен. Над 15 зоопланктера, установени през периода 1990-1992 г. не са намерени през 2001-2003 г. Всички те са пелагични организми. Само за втория период са установените 36 вида. Всички те са характерни за еутрофни водни басейни. Значими промени в количествените параметри на зоопланктона са установени във всички части на системата. Стойностите на индексите за видовото разнообразие (H) също са променени по начин, предполагащ напредване на процеса на еутрофизацията.