MONITORING WATER QUALITY USING ZOOPLANKTON ORGANISMS AS BIOINDICATORS AT THE DUBICA FISH FARM , SERBIA

The quality of water at the Dubica Fish Farm was evaluated by the saprobiological method using the PantleBuck saprobity index, during one carp production cycle in the year 2000. By qualitative and quantitative analysis of the zooplankton community, bioindicator species were selected for evaluation of water quality. During the whole period of research, the saprobity index indicated class II water quality, which is suitable for rearing carp in a semi-intensive system as far as natural food is concerned. There was a general trend of decrease in the saprobity index from the beginning to end of the season, its values ranging from 2.05 do 1.77, and a gradual improvement of water quality towards the end of the season. This is partly a result of natural processes (terminated degradation of introduced organic matter as well as settlement of suspended matter introduced with the supply water) and partly a consequence of the application of ichthyological, agrotechnical, and hydrotechnical measures.


INTRODUCTION
Fish nutrition a semi-intensive system of rearing carp is based on natural food (zooplankton and bottom fauna) as the main source of proteins and additional feed as the main source of carbohydrates (M a r k o v i ć, 2003; Marković et al., 1998Marković et al., , 2004Marković et al., , 2005)).For this reason, it is more appropriate to supply carp farms water of quality class II, since it has higher productivity than class I water (V i d m a n i ć, 1993; M i t r o v i ć-Tutundžić and B r k o v i ć-P o p o v i ć, 1995).
In this type of fish farm, organic production depends on several factors: productivity of the water body that supplies the fish farm; the type of fish pond bottom soil; total ichthyomass; appropriately prescribed quantity and quality of additional feed; and application of ichthyological, agrotechnical, and hydrotechnical measures (L i v o j e v i ć et al., 1967).
When fish farms are supplied with water of quality class II, eutrophic water with a high content of dissolved nutrients (P and N) and appropriate levels of dissolved gases (O 2 , CO 2 ) and favorable temperature is introduced, which improves secondary production and keeps the ecosystem in a state of dynamic balance (Mitrović-Tutundžićet al., 1988).
If the fish farm is supplied with water of bad quality, several negative processes will occur such as sedimentation and biodegradation of organic matter, which will increase the biological oxygen demand (J o n e s, 1990).Organic matter degradation can provoke oxygen depletion and appearance of intermediate products of degradation like NO 2 , NH 3 , and H 2 S ( Mitrović-Tutundžićet al., 1988).
However, considering the global problem of water pollution, the semi-intensive type of fish production recommends itself as the least polluting (B a u e r and Turk, 1980;Bauer, 1982;Mitrović-Tutundžić and V i d m a n i ć, 1995) Abstract -The quality of water at the Dubica Fish Farm was evaluated by the saprobiological method using the Pantle-Buck saprobity index, during one carp production cycle in the year 2000.By qualitative and quantitative analysis of the zooplankton community, bioindicator species were selected for evaluation of water quality.During the whole period of research, the saprobity index indicated class II water quality, which is suitable for rearing carp in a semi-intensive system as far as natural food is concerned.There was a general trend of decrease in the saprobity index from the beginning to end of the season, its values ranging from 2.05 do 1.77, and a gradual improvement of water quality towards the end of the season.This is partly a result of natural processes (terminated degradation of introduced organic matter as well as settlement of suspended matter introduced with the supply water) and partly a consequence of the application of ichthyological, agrotechnical, and hydrotechnical measures.
Key words: Water quality, monitoring, zooplankton, bioindicators, fish farm UDC574.583.016.2 : 639.3 so that mud with most of the waste discharge either stays in the pond or else flows into the receiving water in only small amounts.By this, the environment is protected against pollution from fish ponds during most of the year (P i l l a y, 2004).

MATERIAL AND METHODS
Research was carried out at the Dubica Fish Farm in Banatska Dubica during one rearing cycle of two-year old carp in the year 2000.Five points for sample collection were chosen on fish pond No. 4, with surface area of 112.25 ha.These points were chosen in such a way as to represent the whole pond (A p h a, 1998).Sampling transparent was carried our every two weeks, from May to October, using a narrow each plastic transparent 2 m long and 1 L in volume.Two samples of 2 L were taken at stored point.Water samples were filtered through No. 20 plankton mesh and stored with 4% formalin in glass bot-tles.Single samples were combined and analyzed as a summarized sample of the whole pond.
Zooplankton identification was performed using appropriate identification keys (Š r a m e k -H r u s e k et al., 1962; D u s s a r t, 1969; K o s t e, 1978).Samples were analyzed in the Faculty of Agriculture's Laboratory of Zoology and Fisheries, using a Sedgwick-Rafter cell and a Carl Zeiss (Jena) compound microscope with 16x10 magnification.After qualitative analyses of zooplankton and estimation of their abundance, the Pantle-Buck saprobity index was determined (P a n t l e and Buck, 1955).Ortendorfer and Hofrat's list of indicator species was used for determination of water quality (Ortendorfer and H o f r a t, 1983).

RESULTS AND DISCUSSION
Twenty-five species of zooplankton were identified Table 1.Qualitative and relative quantitative composition of zooplankton species with saprobic values («S») and saprobity index.
It can be concluded that the pond's diversity is rather low.
In any event, it is known that zooplankton diversity in aquatic ecosystems (including carp ponds) is lowered by increase of eutrophication (L a z z a r o, 1987).
Saprobiological analysis showed that the investigated fish pond had class II water quality, which is essential for good functioning of a carp pond (M i t r o v i ć-T u t u n d ž i ć and V i d m a n i ć, 1991; V i d m a n i ć, 1993).There were limited variations of saprobity index values during the investigation, but a general trend of decrease from the beginning to end of the season was noted.The highest index value (2.05) was at the beginning of the season (Fig. 1).During most of the season, from May to the end of August, saprobity index values where rather stable, varying from 1.88 to 1.82, except at the beginning of July, when the saprobity index was 1.58 (Fig. 1).During September, saprobity index values were lower than at the beginning and during most of the season of carp rearing (1.78 and 1.77).This trend (Fig. 1) can be attributed to a great quantity of suspended solids, which usually contain an increased amount of organic matter, the indicated solids having introduced into the fish pond from the supply water body (in this case the DTD Bečej-Banatska Palanka Canal) during filling of the pond.
The increased amount of degradable organic matter introduced at the beginning of the season (when the saprobity index was the highest), provokes increase of oxy-gen demand in the water and general aggravation of the environment for the zooplankton community.Species that are more tolerant of environmental changes, ones which have a higher saprobity value, become the most frequent, and this results in greater total value of the saprobity index.As a consequence of sedimentation organic matter in still aquatic ecosystems such as carp ponds and correct application of ichthyological, agrotechnical and hydrotechnical measures, gradual decrease of saprobity index values can be expected.A greater decrease of the index (1.58) was noted at the beginning of July, when the hydrotechnical measure of pond water refreshment was carried out by introducing fresh water from the water supply (the DTD Canal), whose quality has improved of late (Fig 1).

CONCLUSION
The quality of water at the the Dubica Fish Pond in Banatska Dubica (112.25 ha of water surface) was monitored using zooplankton as bioindicator organisms during one production cycle of two-year old carp in the year 2000.
The obtained results showed a general trend of decrease of saprobity index values from the beginning to end of the season.Gradual improvement of water quality is partly a result of natural processes in aquatic ecosystems (sedimentation of suspended solids and terminated degradation of introduced organic matter) and partly an effect of proper application of ichthyological, agrotechnical, and hydrotechnical measures.
The determined beta mesosaprobic water quality is optimal for carp ponds with the semi-intensive type of rearing because such water contains appropriate amounts of dissolved nutrients (phosphorus and nitrogen) and dissolved gases (oxygen, carbon dioxide, etc.), which improve productivity of the pond and lead to a better rate of growth of the reared fish.5.31.5.20.6. 4.7. 14.7. 1.8. 18.8. 5.9. 18