During the period 1990-1997, the content of 11 trace metals (Fe, Cr, Co, Al, Mn, Cd, Cu, Ni, Pb, Zn, Sr) was studied in muscle and other organs of fish from DTD (Danube-Tisza-Danube) channel system (Yugoslavia). According to obtained and analysed data, the trace metal bioaccumulation can possibly act as enviromental indicator in biomonitoring. DTD (Danube-Tisza-Danube) Channel System can, therefore, be classified as the 3rd class, concerning analysed trace metal content.

This work is based on biotests being experimented at sea organisms - fish Serranus scriba. We were studying the activity of the mentioned fish in sublethal concentrations of organochloric pesticide - lindane. Besides the classic biotests, we used some new methods for neurotoxic testing that include the influence of this pesticide on circadian activity of fish as well as the variations in obtaining the positive conditioned reflexive responses. The results of these experiments show that this kind of fish reacts at the subtile changes of toxic concentration with the change of quality and rhythm in its conditioned reflexive reactions, that appeared earlier than other global signs of intoxication.

The determination of main and some biogenic and trace elements in the sea water of the Boka Kotorska Bay (Adriatic sea, Montenegro, Yugoslavia) was carried out by the atomic apsorption and emission spectrometric method (FAAS) and by the hydride technique AAS (HGAAS).

Biosystematic research of fish population from the river Nisava (Serbia, Yugoslavia) is placed from the mouth of Jerma river and Nisava river to the mouth of Nisava river and Juzna Morava river (Serbia, Yugoslavia). The next part of the research is about determining the methodology for counting of particular fish species in the river Nisava. Hematological analyses of some species were done at the same time in order to establish the relation between water quality and number of erythrocytes, leucocytes and quantity of haemoglobin. Two autochthonous and allochthonous species were analyzed. The method used in this research are explained as well.

The paper presents some procedures for identification of low flow required for taking preventive measures in the rivers with no comprehensive hydrological observations. Besides using time - space parameters, temparery or simultaneous hydrometric observations in the observed rivers are also recommended. The paper concludes with the presentation of relevant low flow calculation results for one river in Zlatibor region (Serbia, Yugoslavia), which has not been examined so far.

During the 1981-1996 period, qualitative and quantitative structure of oligachaeta communities as indicator of the eutrophication process was investigated in the lake Palic (the 2nd oxidation lake), Serbia (Yugoslavia). The mean annual number of oligochaetes was related to nutrient concentration (total nitrogen and total phosphorus). The analysed data for both oligochaete community and nutrients, leads to the conclusion that the second oxidation lake Palic can be classified as an highly eutrophic environment, with tendency towards dystrophic, particularly in past two years.

The Mostonga, once the tributary of the Danube river, now a water-stream with a conducted regime it represents an important water-streams in West Backa (Serbia, Yugoslavia). The most wide-spread hydrophytes (i.e. plant species with the biggest abundance, covering values and occurrence degree) are: Wolffia arrhiza, Lemna gibba, Lemna minor, Lemna trisulca, Spirodela polyrrhiza, Ceratophyllum demersum, Elodea canadensis, Vallisneria spiralis, Stratiotes aloides, Nuphar lutea, Bolboschoenus maritimus, Typha angustifolia, Typha latifolia and Phragmites communis. Based on the analysis of bioindicating values of mentioned hydrophytes of the Mostonga it can be concluded that the present plant species are significant indicators of eutrophication (considerable organic pollution) as well as the increased contents of nitrogen and nitrogen compounds (nutrients), presence of salt in the soil and weakly aerated and mud base.

The kinetics of phytoplankton is complicated function of phytoplankton present and their differing reactions to solar radiation, temperature and balance between nutrient availabilities and phytoplankton requirements. Computers program for water quality problems helps users interpret and predict water quality responses to natural phenomena. But model parameters and initial condition needs hydrodynamic and hydrologic measures in long time interval, with combination of laboratory studies. Lake Gracanka (Serbia, Yugoslavia) is good example for description of the processes kinetics of phytoplankton. But, simulation model can not be used to guide future monitoring efforts without needing measuring and developing network of measuring station.

On the basis of the enzymatic (phosphatase) activity and microbiological analysis as well, the influence of the oil refinery "Vital" - Vrbas (Serbia, Yugoslavia) wastewater to the recipient DTD (Danube-Tisza-Danube) canal water from the microbiological point of view was determined. In addition the efficiency of the factory's waste water purification plant was estimated. The obtained results of the complex microbiological-enzymological analyses indicated a good water quality at the point where the refinery's water supplies plant is situated (site 1). Very low water quality was determined downstream (site 4) and upstream (site 2) from the inflow of the refinery's wastewater's inflow. The later results might indicated the influence of an unknown pollutant in the region. However, the obtained O/H index value suggests that the self-purification ability of the canal waters is still preserved.

Processes of transformation of organic matter are slowed down so that cellulose which originates from cell walls of Sphagnum is retained long in unchanged form. The highest valises of cellulolytic bacteria (0.33-126.67 bact/ml) were recorded in October. In most cases slightly active species were found, Cellvibrio fulvus was dominant. The participation of fungi in cellulolytic community was significant 23-31.5.