CONCLUSIONS The Saprobic values of the river Danube did not change essentially in the observed period. An expressive lowering of the discharge of the river Danube was manifested by a partial or entire elimination of the more the rheophilous, stenoecic species of aquatic insects, which are not tolerant to a decisive decrease of discharge as well as the temperature oscillation of the river Danube. An essential change is occurring in the trophic guilds, in favour of scrapers. The watering of stagnant water by surface or ground water significantly increases the richness of aquatic insect species.
The shutting of the arms and the decrease of the river bottom (erosion, excavation) essentially changed the discharge condition of the arm system in the past, but in the opposite direction as in the main river-bed. Unceasingly an increa in of sedimentation, the development of macrophytes, eutrophisation, and regular desiccation and land setting of the river arms occurred. The ecological relation between the arm system and the river Danube was gradually disappearing. In the 20th century strong organic and toxic pollution of the river Danube followed these changes, culminating in the seventies. The construction of dams in the upper part of the river Danube as well as intensive navigation (irregular waves) had a negative influence on the benthic settlement. This has caused the essential reduction of the original Danubian fauna. For example, the reduction of Plecoptera before the damming reached 90 - 100 % [15].
Since November 1992 the derivation of an essential amount of the Danubian water influenced the structure of mayflies in the old river-bed of the Danube, in comparison with the results of other authors [5, 15, 16]. In the mayfly taxocoenoses of the river Danube (Fig. 1) the more asserted species Caenis luctuosa and C. macrura can be observed, instead of the originally dominant Danubian taxocoenose, Caenis pseudorivolorum. The above mentioned two species are flying to this area from their original hatching areas, the Danubian river arms. In the summer period there is an abundant spread of the summer generation of Baetis fuscatus in the river Danube, sometimes accompanied by the species Heptagenia sulphurea. However, other species, regularly occurring in the past, are not record anymore.
Very surprising also, is a gradual discontinuing succession of the caddisfly taxocoenoses of the river Danube. Psychomyia pusilla has prevailed since 1993 (Figure 2). Hydropsyche modesta has began to prevail among filtrates, because this species does not as such pretend to rheophility as the other species, and in addition, is tolerant to a large dispersion of the temperature amplitude of flowing water. It prevails, for example, in less watered plain flows of the river Morava [17]. This situation very closely approaches the results of long-past hydrobiological observations in the river Rhone [3]. In 1994 another two species with similar characteristics - Hydropsyche pellucidula and H. exocellata - were attached to this species. These three species prevail above the former dominant Danubian forms of Hydropsyche contubernalis and H. bulgaroromanum, of which the total share increased in 1994.
We can summarise that a significant lowering of the discharge in the old river Danube was manifested by a partial or entire elimination of the more rheophilous, stenoecic species, not tolerant to a significant drop in the Danube water and its significant temperature oscillations (Tab. 1). A similar lowering was recorded also in the composition of Simuliidae, Diptera. The originally dominant Danubian forms of Simulium colombashense and S. voilense [12, 13] were very rare in 1994 too. But with regard to the quantitative point of view, the situation in the upper part of the old river Danube-bed (Dunajske Kriviny - Bodiky) improved significantly - see Fig. 3. After a significant decrease in the discharge and flow velocity and an increase of the transparency of the water, scrapers began to prevail in the river Danube to the detriment of filtrators (Fig. 3). This indicates a lower supply of bioseston to this ecosystem. However, in 1993 we also recorded a similar situation in the river Danube at Sporna Sihot (a stretch with its original, not influenced discharge). A relatively low biomass of phytoplankton was observed in the Hrusov reservoir in 1993 [11]. Similarly Kiss [14] and Bothar [4] observed more significant changes of phyto - zooplankton in the river Danube (to the benefit of smaller forms) in this period.
Different development of changes of quantitative and qualitative parameters of macrozoobenthos trophic guilds in the river Danube can be illustrated in two profiles of the old river-bed. The subjects are stretches with different streaming and speed of sedimentation of inorganic material to the river bottom. In the profile of Dunajske kriviny (a rheophile stretch with a rocky bottom) a significant increase of scrapers was represented by the species Psychomyia pusilla and Baetis fuscatus (essentially in this period biomass of periphyton in the old river-bed bottom increased) occurred after the marked depression at the end of 1992 (Fig. 3), and in the first half of 1993. In 1994 a certain stabilisation of filtrators occurred, but on a different quantitative and mainly qualitative base. The abundance of macrozoobenthos (mayflies and caddisflies) increased significantly in comparison with previous periods, and in comparison with other Danubian localities (Table 2). It is connected with a high trophic level (high periphyton biomass) of this stretch of the river Danube in 1994 (stabilisation of the bottom, increased transparency and a decreased average depth in the water table). A similar result was obtained by Tirjakova and Matis [20] at the study of ciliated protozoa.
However, the exchange of a higher number of related species of potamal to rhitrhophilous (Caenis pseudorivolorum - C. luctuosa, Rhyacophila pascoei - R. dorsalis, Hydropsyhe contubernalis - H. pellucidula) indicates a definitive shift of the river continuum up the river stream [5, 8, 15]. In the Danube - Istragov profile (a stagnant stretch with high sedimentation of Danubian sands as a consequence of the river back swelling of waters above the estuary from the tail race canal) we did not record any mayflies and caddisflies, with the exception of a single occurrence of the species Ephemera vulgata. Here it originally concerned the richest settled ma
