OBSERVATION OF THE QUALITY OF DANUBE WATER IN THE BELGRADE REGION BASED ON BENTHIC ANIMALS DURING PERIODS OF HIGH AND LOW WATER CONDITIONS IN 2002

The present paper states conclusions about the quality of Danube water in the Belgrade Region based on analyses of the invertebrate community. The investigation was performed during periods of high (May, 2002) and low (October, 2002) water conditions. Meioand macrozoobenthos were observed. Qualitative, quantitative, and saprobiological analyses were performed. The sampling area covered five stations along 66 km of the river. The community was represented by 26 species. Aquatic worms were the principal component of the benthos with respect to both species richness (six species) and abundance (58.39-99.47 % of the total community). Gastropods were also diverse (six species). Snails were found to be subdominant as far as participation in the total community density is concerned. Structure of the benthic community and the saprobity index (S= 2.78-3.43) indicated the presence of organic pollution. No notable differences of estimated environmental quality were observed between a station upstream from Belgrade and one situated below the exit from the broader territory of Belgrade. Since Belgrade is recognized as one of the main contaminants in regard to biodegradable pollutants in the Middle Danube, this finding points to an impressive self-purifying ability of this huge river. udc 574(497.11 danube)


INTRODUcTION
The present paper states conclusion about the quality of Danube water in the Belgrade Region in May (high water condition) and October (low water level) of 2002 based on analyses of the invertebrate community.Meio-and macrozoobenthos were observed.Results of qualitative and quantitative investigation as well as saprobiological analyses were included in the study.
Aquatic invertebrates were the target group, since they offer numerous advantages in biomonitoring, which explains why they are the most commonly used group in assessing water quality.They are the group most often recommended for use in aquatic ecosystem surveys because: 1) They are a generally well-known group; 2) aquatic invertebrates are basically sedentary organisms; 3) there is an array of widely distributed species among the group; 4) it is a diverse component of the aquatic environment, one which offers a spectrum of responses to stress; and 5) sampling can be done easily, using simple and inexpensive equipment (R o s e n b e r g and R e s h , 1993).
The investigated sector is situated in the middle part of the Danube basin -the largest segment of the river's watercourse from Bratislava to the Iron Gate dams (Serbia/Romania).It is an interesting area for hydrobiological investigations for the following reasons: 1) With its main tributaries, the Danube represents the most significant Serbian water resource, one which has been extensively used for the water supply, irrigation and land melioration, ship traffic, and hydroelectric energy production; 2) In this part the Danube flows through a densely populated area, and a permanent risk of pollution is present (M a r t i n o v i ć -V i t a n o v i ć et al. 1999); 3) A surface drinking water intake is situated in the sector near the settlement of Vinča, and this that necessitates regular biomonitoring; and 4) Due to constructions of the Iron Gate dam (943 km) on the Danube near Sip, hydrological changes were observed up to Slankamen (1215 km), and they too affected the ben-

DEScRIPTION OF THE SAMPLING STATIONS AND THE METHODS USED
The investigation was performed during periods of high (May, 2002) and low (October, 2002) water conditions.The sampling area covered five stations along 66 km of the river (Fig. 1 Station No. 4 -the vicinity of a surface drinking water intake near the village of Vinča, downstream from the mouth of the River Tamiš (a left-hand tributary).
Station No. 5 -the village of Orešac, at the exit from the broader territory of Belgrade, downstream from the town of Grocka, where the river enters a region characterized by more intensive agricultural activities.
Benthic samples were collected from soft substrates using a Van Veen dredge with a grab area of 270 cm 2 in the shore region.Animals were separated from sediment with a 200-μm sieve.The samples were preserved with 4% formaldehyde.Sorting and identification were carried out using a binocular magnifier (5-50 x) and a stereomicroscope (10 x 10 and 10 x 40).
The study included qualitative and quantitative analysis of the benthic community.The number of observed taxa and community density (number of individuals per square meter -ind m -2 ) are presented in order to describe the distribution of invertebrates along the sector.correspondence (reciprocal averaging) analysis or cA  1995).The saprobic level was estimated applying the P a n t l e and B u c k (1955) saprobic index S. Water quality was evaluated according to national standards (YUFROW, 1985).
During the period of a high water level, 15 taxa were recorded, while the number of taxa observed during the period of a low water level period was 22 (Tab.1).
Aquatic worms (Oligochaeta), which were represented by species belonging to the Tubificidae family, were the principal component of the community with respect to species richness and abundance.The participation of aquatic worms in the benthic community varied between 58.39 % (station 2, October) and 99.47 % (station 2, May).Snails (Mollusca: Gastropoda) and non-biting midges (Diptera: chironomidae) also made up a significant part of the benthic community.Dense populations of snails were observed at stations 5 in May (17.31 %) and 5 and 2 in October (27.40 % and 18.98 %, respectively).chironomids were abundant in May at stations 3 (19.88% of the total community) and 1 (5.75 %).Mussels and clams (Bivalvia) were represented with 18.25 % of the total community density at station 2 in October.At the other stations, representatives of Bivalvia were observed with considerably lower densities of up to 2.12 % (loc. 1, May and October; loc. 4 and 5, October) or they were not Within the principal benthic group (Oligochaeta), the following species, adapted to a high organic load, were observed: Tubifex tubifex, Limnodrilus hoffmeisteri, Limnodrilus claparedeanus, and Branchyura sowerbyi (Tab.1).Among snails, Litoglyphus naticoides was the most abundant (up to 96.15 % of the gastropods and up to 18.25 % of the total invertebrate community at loc. 2, October; up to 40.00 % of the gastropods and up to 10.96 % of the total invertebrate community at loc. 5 in October).Among other species of gastropods, worth mentioning in connection with population density is Acroloxus lacustris, which was found to represent 12.33 % of the total community at station 5 in October.
The cA ordination diagram (Fig. 2) shows the relationships between invertebrates (mean abundance of taxa per station) and sampling stations.The position of stations 2, 4, and 1 on the ordination diagram is determined mainly by the presence of species that are common to the majority of sampling stations -SG 1 (Fig. 2).Moreover, station 2 (in both periods) and station 4 (in May) were characterized by higher total community density in relation to the other stations.Station 1 was characterized by the presence of Branchiura sowerbyi (Oligochaeta).This aquatic worm was also observed at station 5, but it was less abundant.At the other stations, B. sowerbyi was not recorded.Also, a high population density of Dikerogammarus vilosus was observed, at the station 1, which is in keeping with the station position on the ordination diagram in regard to dimension 1 (Fig. 2).The position of stations 1 and 3 is affected as well by high density of Diptera larvae belonging to the family chironomidae.Station 5 was distinguished from the others by the presence of the snails Theodoxus danubialis and Acroloxus lacustris, along with the species Pontogammarus obesus (Amphipoda) and Asellus aquaticus (Isopoda) (Fig. 2, species group "SG" 2).The total density pattern and presence of the mentioned animals most strongly affected the position of station 5 on ordination diagram in relation to the others (Fig. 2).
The results of saprobiological analysis (Tab.2) indicated that water quality varied within the limits of category III according to national standards (YUFROW, 1985).The Saprobic index (P a n t l e and B u c k , 1955) varied between S=2.78 (station 5, October) and S=3.43 (station 4, May).

DIScUSSION
During the investigation, 26 benthic taxa were recorded.Dominance of four species -[Tubifex tubifex, Limnodrilus hoffmeisteri, and Limnodrilus claparedeanus (Tubificidae: Oligochaeta) and Litoglyphus naticoides (Gastropoda)] was observed.The sampling stations were distinguished by variation in total density and principal components of the community, as well as by the presence and distribution of taxa with minor participation in the total invertebrate association, present affected the cA ordination diagram (Fig 2).
Relations between the community present and saprobic conditions have been extensively discussed in the literature (K o v a č e v and U z u n o v , 1986; R o s e nb e r g and R e s h , 1993).Thus, a small number of taxa recorded, together with dominance of one a or few species, indicates the presence of stress (c h a p m a n , 1996).Mass development of oligochaetes, accompanied by reduction of other benthic species, points to the occurrence of organic matter in both water and the substratum (S l e p u k h i n a , 1984; T i m m , 1987).
The dense populations of aquatic worms of the family Tubificidae (adapted to high organic loads) which were observed during the investigation indicate the presence of organic pollution.The number of animals indicating lower saprobity levels was considerably smaller.comparable results of investigating the zoobenthos community along the Serbian reach of the Danube River were reported previously.A high density of aquatic worms was observed along the Serbian part of the river (D j u k i ć et al. 1987, 1994; S i m i ć et al.1997; M a r t i n o v i ć -V i t a n o v i ć et al. 1999).A tendency toward increase in the density of eutrophic species of oligochaetes after damming of the Danube was also emphasized (N e d e l j k o v i ć 1979; D j u k i ć et al. 1987, 1994).The results presented here corroborate earlier drawn similar conclusions concerning zoobenthos and water quality of the Danube in the Belgrade Region (J a k o v č e v , 1987, 1988; M a r t i n o v i ć and V i t a n o v i ć et al. 1999) -all studies reported the same community structure (dominance of aquatic worms and mollusks) and confirmed that water quality was within limits of the third category according to national standards (YUFROW, 1985).This means that the Danube is exposed to the constant inflow of a high organic load.On the other hand, in spite of the high pollution level, the significant self-purifying ability of this huge river was confirmed.Our investigations, as well as previous studies (J a k o v č e v , 1987; 1988; J a n k o v i ć and J o v i č i ć , 1994; M a r t i n o v i ć -V i t a n o v i ć et al. 1999), showed that there is no considerable difference of water quality between stations upstream and downstream from the narrow city area.(58,39-99,47 % укупне заједнице).Пужеви (Gas-tropoda) су заступљени са шест врста и они су субдоминантна група у односу на учешће у укупној густини заједнице.Структура бентосне заједнице, као и вредности индекса сапробности (S = 2,78-3,43) указује на органско загађење реке.Битне разлике у процењеном стању окружења на локалитетима узводно и низводно од ужег подручја Београда нису уочене.Како је Београд препознатљив као један од главних загађивача биодеградабилним материјама у средњем току Дунава, овај налаз истиче импресивну способност самопречишћавања ове моћне реке.

АНАЛИЗА КВАЛИТЕТА ВОДЕ
thic fauna in the Belgrade Region (M a r t i n o v i ć and V i t a n o v i ć et al. 1999).Those alterations affected both water quality and the biota (N e d e l j k o v i ć , 1979; J a n k o v i ć and J o v i č i ć , 1994; S i m i ć et al. 1997).
): Station No. 1 -the village of Stari Banovci, downstream from the Tisa's confluence, upstream from the boundary of the Belgrade Region; Station No. 2 -Zemun, inside the narrow city area; Station No. 3 -Višnjica, on the periphery of Belgrade, situated below the downtown and downstream from the Sava River's inflow.Ten municipal sewage outlets are located on the right bank upstream from the site.Effluents from the nearby port and shipyard as well as from several upstream industrial facilities also affect the river at this station.

Fig. 1 .
Fig. 1.Sampling stations -the Danube, Belgrade Region, 2002 (sampling stations correspond to the explanation in the text)

Fig. 2 .
Fig. 2. Ordination diagram based on correspondence (reciprocal averaging) Analysis or cA (P i e l o u , 1984) -input table 26 rows (taxa) x 5 columns (mean abundance of taxa per station); Sampling stations correspond to Fig. 1 and explanation in the text.

Tab. 2 .
Saprobity index [S] (P a n t l e and B u c k , 1955) and category of the watercourse evaluated according to Yugoslav standards (YUFROW, 1985) -the Danube, Belgrade Region, 2002.