In this work influence of different concentrations of linear active substance (LAS-DBS) on activity of Serranus scriba CUV have been studied. Desynchronization of circadian rhythms is in direct correlation with LAS growing concentrations. The high disproportion was found between maximum permitted concentrations (3 mg/l) and our results which show the changes in fish behaviour at 0.2 mg/l LAS.

In 1996 (March and June) the mineral water "Rudovci" was sampled in its spring in the village Rudovci, Serbia (Yugoslavia). The physical-chemical analyses have shown that the temperature of the water was 180 deg C with sodium-carbonate content. It has been considered that the quality of water was variable in a short period from 3 months. Comparing the analyses of the mineral water from 1996, we can consider some remarkably distinctions in some chemical elements. The chemical analyses made in 1996, have shown changes in following contents: magnesium (47.6 and 64.5 mg/l); sodium (1313 and 1355 mg/l); iron (2.35 and 3.76 mg/l); boron (5.2 and 4.8 mg/l); arsenic (0.08 and 0.06 mg/l). In 1974, the chlorine content was 28.4 mg/l, but about 218 mg/l in 1996. The nitrate content varied from high (19.2 mg/l) in 1974 to low (about 0.75 mg/l) in 1996.

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 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.

Various physical, chemical and biological methods are used in complex investigations which are aimed at determining the quality of water. One of the oldest but also most widely accepted biological mathods is determination of saprobity on the basis of bioindicators. In application of these methods the existing lists of the indicators are most frequently used (Sladacek 1973, SEV 1977). In these lists only nine types of red algae (Rhodophyta) are specified as indicators of water quality. One of the main trends in development of algaeology in Serbia (Yugoslavia) is investigation of autoecological properties of algae and making a list of water quality indicator algae suited to the specific ecological conditions in water ecosystems of Serbia (Yugoslavia). The objective of this paper is to present information about apperance of red algae in the rivers of Serbia (Yugoslavia) and influence of physico-chemical factors upon their development, which represents a contribution to making of the above mentioned list of the water quality indicator algae.

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.

Within the comprehensive hydrobiological investigation of Grosnica water reservoir (Serbia, Yugoslavia) composition and dynamics of bacterioplankton, phytoplankton and zooplankton, as well as their seasonal successions were studied. The composition of plankton communities varies during the vegetation period in the function of the physicochemical features. Grosnica water reservoir with its microbial features belongs to the 1st-2nd class of bonity according to Kohl. The highest values pf bacterio- and zooplankton were recorded in August, while bruto primary production in September.

The paper presents one of the methods for calculation of temperature of a water course, downstream of the relase of a thermo-electric power-plant, taking into consideration the well-known temperatures and discharge values of the tributaries under different meteorological conditions. In view of the importance of temperature regime for the water eco-system, understanding the influence of the power plants on thermic capacity of the water course, is essential in design and use of the power plant. The author's intention is to help towards improving computer models for the computation of a water course thermic capacity. To this end a model developed at the Institute for Meteorology and Water Management in Poland and applied in many coutries has been presented.

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.

Mathematical modles and adequate investigation works enables successful water quality modeling. These models includes all relevant abiotic components (concentration of dissolved oxygen, temperature regimes and temperature stratification, concentration of organic and inorganic components of physical-chemical water conditions), while the biotic components are present only with phyto- and zooplankton. In new water reservoir, ichthyofauna mostly undergo mancontrolled successions, starting with planned stocking a water reservoir with fish. The paper presents analytical approach for mathematical modeling of ichthyofauna and application of Lotka - Volterr's equations.