The investigations of dynamics and composition of plankton, bottom fauna and fish community have been done at three localities (1-inflow, 2-the middle, 3-dam) at the Barje water reservoir (Serbia, Yugoslavia) during 1997. During winter period diatomea Asterionella formosa was dominant at all the localities. At the same time among zooplankton species Rotatoria group was dominant, with the emphasis of Polyalthra dolicoptera at the locality 2, while Keratella cochlearis was dominant at the localities 1 and 3. The present species are oligo- and betamezosaprobionts. Among the investigated bottom fauna species Oligochaeta and Chironomidae were present. The number of oligochaeta was between 1665-7878 ind./square m. By qualitative analyses the domination of Tubificidae family was present. It is known that the presence of this group of organisms and it's high number indicate the organic pollution of ecosystem. 30 individuals of the bighead, (Hypophthalmichthys nobilis) with the average weight of 5.6 kg was captured by net.

During the 1994-1995, seasonal investigations of the fountainhead and spring region of the river Susica, near Uzice in West Serbia (Yugoslavia) were carried out. The bottom fauna composition, termic properties of air and water as well as chemical parameters were also determined. By the study of bottom composition 26 taxa were determined. The most uniform fauna was found in the Susica fountainhead while its diversity was increasing farther downstream the spring region.

During the period from 11th May, 1991 to 5th March, 1994, seasonal study of the river Jablanica (Serbia, Yugoslavia) macrozoobenthos was carried out. Investigations were done of three sectors: in the upper, middle and down reach of the river, with the aim to estimate water quality changes by the changes in bottom fauna composition. Bottom fauna community was composed of 17 invertebrate groups with 100 taxa determined. The most diverse Oligochaeta (21 taxa) and Ephemeroptera (19 taxa). Macroinvertebrates diversity decreased downstream the river as a result of the pollutional degree increase. Saprobic indices were from 1.51 (sector 2) to 1.8 (sector 3).

In Yugoslavia, application of macrozoobenthos for the purpose of bioindication has been relatively limited. This communities has been used in conjugation with saprobiological methods, while biotic indexes have been unjustifiable used to a small extent. This paper shows the most significant biotic indexes used in the World and possibility of their application in Yugoslavia, as well as the necessity for their modification and elaboration of new ones specific to Yugoslav conditions.

During the 1996-1997 period, dynamics and structure of zooplankton and benthos in sugar mill canal in Crvenka (Serbia, Yugoslavia) were studied. Two representative sampling sites were chosen, while the third one served as a control site, providing it has no impact on cage culture. The canal section where the cages were established, had previously been drainage and dredged. Immediately after the refilling, the zooplankton qualitative structure was scarce, with dominating Rotatoria. Two months later, the situation improved and Cladocera and Copepoda emerged. Those two groups of organisms have the major role in enhancing the commercial feed consumption Soon after the refilling, the benthos was reestablished. Sample analysis shows the presence of Oligochaeta and Chironomidae. Qualitative and quantitative analysis of benthos points out no significant difference between the canal sections upstream and downstream the cage culture.

The river Timok with its watershed area is one of very imperilled running water on the territory of Serbia (Yugoslavia). By several years investigations of colonies bottoms (phytobenthos, microzoobenthos and macrozoobenthos) and plankton we have found the following: great diversity and specific phytobenthos and bottom fauna (frequency of certain taxa is considerably larger in relation to the other areas, and new taxa have been recorded). Stated specific structure of phytobenthos and bottom fauna have considerably reflected to right estimation of water quality of tested area.

There were estimated of water quality of the river Banja and its trubutary Pocibrava, Serbia (Yugoslavia), during the period 1991-1994 by seasonal investigations of benthos fauna composition as bioindicators. The results have shown higher diversity of the Pocibrava bottom fauna community (66 taxa) than in the river Banja (53 taxa) where it was more uniform. Both water flows were receiving some organic loading and by the use of macrobenthos indicator organisms it was concluded that they belong to beta-mesosaprobic water quality class.

In the period from May to August in 1996, the samples for hydro-biological and inchthyological from the following localities were collected: the Klaicka river, the Radevacka river, the Sumanka river and the Lipovacka river (Serbia, Yugoslavia). The water quality was determined n the basis of bioindicator values and the abundance of particular species of phytoplankton, zooplankton, bottom fauna and fish community. According to the portion of plankton organisms, the saprobic index for the Klaicka river was between 2.01 and 2.57, for the Radevacka river it was between 2.83 and 2.93, for the Sumanka river between 2.59-2.78 and for the Lipovacka river this index had a value between 2.72 and 2.83. Among the botton fauna, 10 zoobenthic groups were determined. The highest frequency and abundance was registered in the groups of Ephemeroptera and Trichoptera, and the water quality is between oligo- o-beta-mesosaprobic (the Lipovacka river), -beta-mesosaprobic (the Klaicka river), o-beta to alfa-mesosaprobic (the Radevacka river) and beta to alfa-mesosaprobic (the Sumanka river). The ichthyofauna of the Klaicka and Sumanka rivers include the species like southern barbel, chub and spirlin. Basing on the fact, the water quality was determined as beta-mesosaprobic (1.65). The majority of results point to the satisfactory biological quality of water in this hydroecosystem, which is, therefore, considered to be one of the potential sources of water supply.

During hydrobiological investigations of the river Dulenska (Serbia, Yugoslavia) in June 1996, algological samples were taken at this river. In the algae community are found 34 taxa from two divisio: Bacillariophyta (28) and Chlorophyta (6). While qualitative composition of the algae colony was relativelly uniformed quantitative one was changeable along the course of the Dulenska river. By saprobiological analysis, it was found that the quality of water was changing along the course of the river. At the upper and middle course of river the water belonging to the second class. At lower course of the river (below Rekovac) water quality was getfing worse and it belonging to the third class.