PART 7
Biological MonitoringThe biological monitoring of agreed groups of aquatic and terrestrial fauna and flora is carried out on six complex monitoring areas on the Slovak side. The same groups of fauna and flora, agreed in the frame of joint monitoring, are monitored at 31 monitoring sites on the Hungarian side (Fig. 7-1), however the evaluation in the Hungarian National report did not present the monitoring results from the same sites regularly every year. There are differences in some monitoring methods on both sides henceforth, however, further harmonization is ongoing. Some progress was reached in Trichoptera, Ephemeroptera and Odonata monitoring, where the Slovak Party in 2004 introduced an additional observation of flying imagines along with the larvae monitoring. In 2004 there were some changes in monitoring methods on the Hungarian side (e.g. sampling of aquatic and terrestrial molluscs, power output of the aggregate used in fish monitoring, expression of coverage values in phytocoenology). When monitoring or evaluation methods are changed the possibility of result comparison decreases, so it would be important to discuss first the intended changes. A short description of hydrological and climatic conditions in the year 2004, influencing the development of observed groups of fauna and flora on both sides of the Danube, is given above in Part 6. A short characteristic of moisture conditions, which are important for their development as well, in different parts of inundation area, is given as well. 7.1. PhytocoenologyThe left side river branch system The changes of biota on the monitoring area No. 2600 are caused by gradual long-term drying up of the locality, which was already observed since seventies of the twentieth century and after diversion of the Danube get more intensive. However, according to the results from phytocoenoses observation, the drying up of the locality is slowing down in last few years (2002-2004). The composition and coverage values of the tree and shrub layers are getting stable or slightly increase. In spite of that, that some of the poplar trees decline no premature defoliation was observed, not even after low amount of precipitation along with low flow rate in the Danube in July. These unfavourable conditions were not reflected by mosaic loosening of coverage in the herb layer as well. To the positive signs could be assigned the survival of hydrophytes, decreasing of invasive species number and the decrease of species diversity, while the high number of species is considered as atypical for the lowland forest. The phytocoenological monitoring on monitoring site No. 2603 is carried out in a an young, seven years old poplar stand (at cascade line D). Closing of the treetops of the young forest stand is reflected in significant yearly coverage increase, while the coverage of the shrub layer remains unchanged. The coverage of the plant layer reaches 100 % in the spring, and decreases to 50 % in the summer. However, the decrease of ruderal, nitrophilous and invasive species reflects the treetops closing process and the return of the plant community to the state before cut out in 1997. The plant communities on the monitoring area No. 2604 (situated between the cascade F and G) were well supplied by water in 2004. Both, the tree and the shrub layer are characteristic with stability, however, the absence of species typical for muddy and aquatic biotopes, along with decreasing number of flood-indicating species reflect changes in flooding regime after diversion of the Danube. The fluctuation of further ecological factors cannot be assigned to the power plant operation. Even the cut out of the neighbouring forest stand has not appeared in the phytocoenoses yet. Monitoring areas No. 2608 and 2609 are situated in a region where the groundwater level cannot be regulated by the water supply, however, the backwater effect upstream the confluence of the tailrace channel and the Danube old riverbed significantly decreases the impact of diversion. However, the results of last two years show, that the solution of the water regime is necessary. In case of long lasting low and average flow rates in the Danube the area becomes vulnerable. During the extremely dry year 2003 the tree layer, mainly willows were damaged on the monitoring area No. 2608. In the shrub and plant layers the shortage of moisture in last two years did not appeared significantly, though in long-term there is apparent decrease of flood-indicating species and significant fluctuation of soil moisture. Appearing of several graminaceous plants on this originally forest area is another adverse sign. The composition of the tree and shrub layers on monitoring area No. 2609 correspond to the community of frequently flooded lowland forest. However, in the plant layer the neophyte species Aster lanceolatus became dominant and overbore the original dominants – Galium palustre and Leucojum aestivum. Unsettling is the retreat of flood-indicating species and species typical for muddy and wet biotopes as well. The vegetation on the reference monitoring area No. 2612, situated downstream the confluence of the tailrace channel and the Danube old riverbed, was adequately supplied by water in the spring 2004. The individual layer of the transitive lowland forest developed similarly to the previous years. In spite of dry summer period the trees and shrubs in September had no signs of drying, however the plants were wilted. Ecological composition of vegetation is stable. Some of the parameters show a slight shift towards a more hygrophilous community, though the results of soil moisture monitoring do not support this. The right side river branch system The vegetation period 2004 was richer in precipitation in comparison to the previous year. This was reflected in increase of biomass of the plant layer on the forest monitoring areas. Simultaneously several of the meadow plants were higher than before in the whole monitoring period. Significant change in number species was not registered. The average leaf area values were lower in comparison to the previous years; decrease was registered in the height of reed stands as well. At the same time it can be said, that the naturalness of the willow stands and the hardwood lowland forest (oak-ash-elm) at Szigetköz exceeds the countrywide average. The reason is the higher naturalness of the shrub and plant layer composition. Since introducing the water supply in 1995 the water level in the Mosoni Danube is stabilised and the groundwater level is stabilised as well, thus there were no changes registered in the vegetation, which could be assigned to the diversion of the Danube. A number of semi-natural willow stands grow at the riverbanks. It can be stated again that the regenerative potential of the wetland vegetation is very high. Characteristic types of plants quickly and spontaneously colonize (or re-colonize) the artificially created wetlands. Banks of the Lipót marsh are a good example for this. This indicates a quick regeneration of the floodplain vegetation if necessary amount of water can be artificially provided for the oxbow lakes, river arms, or even for the whole river branch system. The influence of high water levels or temporary flooding of the area is much more effective, if they occur in the vegetation period (from April to September) than during the rest part of the year. 7.2. Terrestrial molluscsThe left side river branch system The process of drying out in the area of monitoring site No. 2600, which was recorded in previous decades, speeded up after diversion of the Danube. Since the biota is dependent on the amount of precipitation in long-term, the terrestrial mollusc’s community is represented by a typical community of dry softwoods forest. The gradually increasing shadow in the young poplar stand on the monitoring area No. 2603 in the year 2004 allowed the increase of number of more mezohygrophilous and hygrophilous species, decrease of which was registered in the extremely dry period in 2003. The structure of malakocoenose indicates suitable conditions for development of communities typical for wetland forest. The low number of polyhygrophilous species can be assigned to the absence of floods and lower groundwater level. Permanent presence of natural wetland community of terrestrial molluscs can be observed on the monitoring site No. 2604, thanks to positive influence of the water supply. Polyhygrophilous and hygrophilous forest species remain dominant. The malakocoenose of the monitoring area No. 2608 can be characterized as a medium degraded soft wetland forest community. Return towards the more hygrophilous community after floods in 2002 was expected, when the euryek species were washed out. But their occurrence in the extremely dry year 2003 was significant. During the year 2004, when the moisture conditions were more favourable they retreated, but the occurrence of original hygrophilous species was still significantly lower than before damming. Hydrotechnical solution of water supply for this part of inundation is necessary. Although the changes of water regime in the area of monitoring site No. 2609 after diverting the Danube are minimal thanks to the backwater effect, the extremely unfavourable moisture conditions in 2003 induced the retreat of several polyhygrophilous species and significant decrease of abundance of the typical wetland community. Regeneration of the community in the year 2004 was so considerable, however further regeneration is expected in the forthcoming years. Positive fact is the permanent absence of the xenocenous (alien) species. The hydrological regime of the area downstream the confluence of the Danube old riverbed and the tailrace channel was not changed. The malakocoenose of monitoring site No. 2612 rich in species consist of species with various ecological demands from polyhygrophilous to eurytopic species, mixed during regular floods. Decrease of species number and abundance during the summer was registered during last two years. The right side river branch system The monitoring of terrestrial molluscs community in 2004 was carried out by different method. Instead of the method of relative richness the presence and abundance of terrestrial species was determined by random sifting in 15 minutes time. The change of the method decreases the comparability of the obtained results in previous years. Generally no appreciable changes indicating some variation in the terrestrial molluscs community were recorded at the examined sites. In comparison to the previous years significantly lower species number was registered, however this fact reflects the shortcomings of the random sifting method, which detects rare species with significantly less probability. 7.3. Aquatic macrophytesThe Danube In the Danube old riverbed two Hungarian monitoring sites are situated (No. 2 and 7). The old riverbed has not provided favourable conditions for development of macrophyte communities before damming the Danube. The monitoring area No. 2 is situated in the main riverbed downstream of the bottom weir, where the flow rate prevents developing of macrophyte communities. On the banks and cross weirs spreading of bush-type willow Salicetum triandrae is registered. The monitoring site No. 7 is separated from the Danube old riverbed by shoreline vegetation of Salicetum triandrae and Scirpo-Phragmitetum. Due to its intensive spreading the part of water area was isolated and it had a macrophyte community rich in species in last years. The left side river branch system The state of macrophyte community on the monitoring area No. 2603 is determined by the water supply through the intake structure since 1993. Macrophytes in deep arms with flowing water mainly occur along the line structures, where they are protected from the strong current. Their stands were wash out by strong floods in 2002, and their regeneration was registered in 2004. The results of macrophyte monitoring in the dead arm of monitoring area No. 2604, are comparable with findings before damming the Danube (especially when permanent of hydro-ecophase is present, as in 2004). The marshy lake is bordered by a reed stand of the association Phragmitetum communis; on the west side of the dead arm spreading of Phragmites australis was registered. The open water area is almost fully covered by Ceratophyllum demersum, in the littoral endangered aquatic and marshy species occur. The region of monitoring area No. 2608 is characterised by existence of depressions. Due to the regular flooding of the surface they are occupied by species of wet and very wet biotopes, which suffer water level fluctuation and flooding. An unfavourable fact is the presence of invasive species at the north side of the area, which was supported by occurrence of terrestrial ecophase in last two years. The extremely low water level in the river branch on monitoring area No. 2612 in 2003, which resulted in very frequent occurrence of limose ecophase allowed large abundance of species not directly dependent on aquatic environment. In 2004 the presence of hydro-ecophase was recorded on each of the three observed sub-areas, where the occurrence of species of aquatic vegetation were proved. However, on the sub-areas No. 2 and 3 the marshy species dominated. The right side river branch system The discharges supplied into the active floodplain were similar to discharges in previous years. Large part of the floodplain river branches is characteristic by higher water depth and moderate flow, which is reflected in the present stabilized aquatic plant communities. The year 2004 was characteristic by more intensive development of aquatic macrophytes on both observed sites in the floodplain (No. 4 and 9), which species compositions were rather rich. Along with the presence of several rooting species the spreading of the invasive species Elodea canadensis was registered again (observed on the flood-protected area as well). The hydrological conditions of the flood-protected area (monitoring sites No. 6 and 8), thanks to the continuous water supply, remained unchanged in last few years. The species compositions basically remain unchanged too. 7.4. Aquatic molluscsThe monitoring of aquatic molluscs on the Hungarian side in 2004 was carried out according to changed methodology – instead of the method of relative richness the presence and the abundance of aquatic species was examined in a sample of 3 l of riverbed sediments during autumn. By change of methodology the comparability with the previously obtained results decreases. The Danube The evaluation of the aquatic molluscs’ community in the Danube is based on the data provided by both the Slovak and Hungarian Party (Slovak observation sites are situated at rkm 1841, 1817 and 1804, from among the Hungarian observation sites in 2004 only the observation site at rkm 1831 is mentioned). The results from the observations of the Slovak Party indicate that the dominance of the original rheophile species decreases, while the alien rheophile species and species indifferent to the flow velocity spreads. Similar tendency can be recognized in results from the Hungarian side; samples were rich in species. The Hungarian Party points out the possible infest of the strange mollusc species Corbicula fluminea, which is spreading from the Rumanian stretch of the Danube (Bij de Vaate, Hulea, 2000), in the next years. The left side river branch system The aquatic molluscs’ community in the river branch system on the Slovak side is monitored on areas No. 2603 and 2604. As a consequence of connection of the river branch at monitoring site No. 2603 with the neighbouring river arms, high species diversity of the mollusc’s community can be continuously observed, which consist mainly of stagnicolous and indifferent species. The long-term isolation of the mollusc’s community in the dead arm on the monitoring area No. 2604 (since last big flood in 1991) resulted in continuous decrease of diversity and abundance. The isolated dead arm had been temporarily reconnected with the neighbouring river arms during floods in 2002. Subsequently in 2003 increase of species richness and abundance was registered, however this state have not been maintained in 2004. Regarding the wide food offer the community of aquatic molluscs is not adequate. The right side river branch system No significant changes in aquatic mollusc’s community of the right side river arm system were observed in last years. The stabilisation of biotopes on the flood-protected area, which are not antropically disturbed, as well as survival of rare species on isolated biotopes can be presumed. The stable occurrence of endemic species Paladilhia oshanovae is not endangered, the occurrence of live examples near the surface proves the continuous presence of clean flowing groundwater in the Szigetköz region. 7.5. Dragonflies (Odonata)The species composition of dragonflies’ community on the Slovak side, based on the larvae observed, rather poor during the whole observed period. Since the larvae of several species live encrypted, buried in the substrate, or are not registered by other reasons, observation of flying imagines were introduced on the Slovak side. These data were not included in the evaluation till now. The evaluation started in 2004 to get more realistic picture of this community. The left side river branch system In 2002 the monitoring of dragonflies on the monitoring area No. 2600 was restored in a newly created gravel pit, which was already rather abundantly overgrown by macrophytes in 2004. Corresponding to this the number of present dragonflies’ larvae as well as imagines increased. The species composition was mainly composed by stagnicolous species and species tolerant to drying up, but semi rheophilous dragonfly species were registered as well. The abiotic conditions of aquatic biotopes on the monitoring area No. 2603 were not changed since introducing the water supply into the river branch system (in 1993). Since 1995 a stable odonatocoenose, living in moderately flowing lowland water, is registered. The dragonfly community consist of rheophilous and semirheophilous species of rather high abundance. While the period 2001-2003 was characteristic by continuous increase of species diversity of dragonfly community of the dead arm on monitoring area No. 2604, the occurrence of individual species in the spring and summer period in 2004 became more stable and more abundant. The dragonfly community consisted of eurytopic species, with one exception of species typical for warm, drying up water bodies. Missing of all groups of macrozoobenthos (including dragonflies, caddisflies and mayflies) in the littoral during the autumn sampling can indicate the oxygen shortage in the water. The sampling site on monitoring area No. 2608 was moved to the next river branch due to unfavourable biotope conditions for dragonflies’ communities. Although the abiotic conditions of the newly chosen site were favourable in the years 2003 and 2004, irregular occurrence of one or two dragonfly species was registered, typical for warm, drying up water bodies. One imagine of vulnerable species was observed in the neighbourhoods of the previously monitored site. Since 2002 the monitoring of odonatocoenose on the monitoring area No. 2612 was moved to neighbouring river arm (Abbot’s river branch) with more favourable conditions for dragonflies’ occurrence. The dragonflies’ community in 2004 consisted of flying imagines, typical for warm, eutrophic water bodies, along with observation of one semirheophilous species registered in the spring period. Additional reconnaissance of the neighbouring shallow depressions along the previously observed river arm showed an irregular presence of dragonflies’ larvae, while several species of flying imagines were registered in the neighbourhoods of the river arm during the whole year. The right side river branch system The species composition of odonatocoenoses of regularly monitored sites has stabilised during last years. The fauna of the previously slowly flowing side arms and channels had significantly changed as a consequence of the continuous water supply. In last period permanent presence of rheophilous and eurytopic species is registered. 7.6. Crustaceans (Cladocera, Copepoda)The Danube Evaluation of development of Cladocera and Copepoda communities is based on the results of the Slovak side (monitoring site No. 2600 and 2608). In the stabilised conditions of the Danube old riverbed stabilised Cladocera and Copepoda communities were registered, compositions of which were often close to their composition before damming the Danube. However, according to the results obtained in the year 2004 significant decrease of abundance and species number in both communities was registered. In several samplings fluctuations of ratio between euplanktonic and tychoplanktonic species was registered as well. In general, in the Copepoda community smaller fluctuations in above-mentioned characteristics were observed in the whole monitoring period. The left side river branch system The state of Cladocera and Copepoda communities in the river arm on monitoring area No. 2603 was relatively stable during last years, however this state is not comparable with the pre-dam period. The difference is in lower species diversity and the occurrence of euplanktonic species, which formed the main part of these communities before the damming, becomes insignificant. The euplanktonic species were displaced by gradual spreading of macrophytes (mainly in the last 2-3 years after cut out of the neighbouring forest) and a subsequent spreading of tychoplanktonic species. The gradual overgrowing of the isolated dead river arm on the monitoring area No. 2604 causes changes in Cladoceras community; loss of the dominance of euplanktonic species going on. The rate of phytophilous species and species settling biotopes rich in organic detritus increases. The temporary reconnection with the neighbouring river arms during floods in 2002 did not stop this tendency. Similar changes in Copepods community have significantly lower intensity, although during macrophytes growing in the summer period the proportion of phytophilous species increases. Cladocera and Copepoda communities rich in species settled the river arm on monitoring area No. 2608 during the whole observation period. Changes indicating the process of isolation of the river branch (the connection with main river branch occurs only at higher discharges) can be seen in increasing proportion of tychoplanktonic Cladocera species in several samples of last years, in spite of macrophytes absence in littoral of the river branch. In the Copepoda community the euplanktonic species remained dominant. The Cladocera community reacts to the unfavourable hydrological conditions on the monitoring area No. 2612 (low water level in the river branch) in last two years more significantly than the Copepoda community, which have lower ecological demands, especially towards the oxygen content in the water and pH value. The occurrence of Cladocera species in the summer and autumn 2004 was insignificant and tychoplanktonic species were recorded. The Copepoda community was abundant, but the tychoplanktonic species were dominant. The tychoplanktonic species in both communities were in the minority before damming the Danube. This can indicate the terrestrification of the area. It should be mentioned that the observed area is a reference site and is situated downstream of hydropower station. The right side river branch system The planktonic crustaceans communities in the Szigetköz in 2004 can be characterized in general by increase of species diversity caused by spreading of the macro-vegetation in the observed river arms (sites No. 4, 5, 6, 9). Increase of phytophilous species is connected to this (Pleuroxus truncatus, Sida crystalina). The maximal abundance was registered during the springtime. One exception to this is the monitoring site No. 9 (Csákányi Danube), where the species diversity remained on the same level as in the previous year and the species in general, except two phytophilous species, had a low abundance. On the monitoring site No. 5 (Zátonyi Danube) the occurrence of several species – Disparaloma rostrata, Graptoleberis testudinaria, species of Pleuroxus family, Simocephalus vetulus, etc. – indicates stable conditions of the biotope, with appearance of macro-vegetation. The stabile species number of planktonic crustaceans in individual years (22-43) indicates a good ecological state of the monitoring site No. 6 (Lipót oxbow lake), although some changes in composition of the community were registered. 7.7. Caddisflies and Mayflies (Trichoptera, Ephemeroptera)There are differences in the mayflies (Ephemeroptera) and caddisflies (Trichoptera) taxocoenoses monitoring methods used by the Slovak and Hungarian Party. This can be one of the reasons that the monitoring results of both sides differ. On the Slovak side the larvae of Caddisflies and Mayflies living in the water are observed and since 2004 (based on the Recommendations of Joint Annual Report on the environment monitoring in 2003) additional data of flying imagines, casually registered during sampling in river arms, are evaluated as well. These additional data are used to get more realistic picture of these communities. On the Hungarian side the flying imagines of both communities are observed (light traps are used). The Danube Changes of ecological conditions in the Danube old riverbed resulted in changes in mayflies’ and caddisflies’ communities. The monitoring results on the Slovak side (monitoring sites No. 2600, 2603 and 2608) indicate the presence of very poor mayflies community. However, the presence of further species can be expected in the deeper part of the river, where gravel bottom bed occurs. Sampling of this part of the riverbed is planned during the year 2005. The occurrence of caddisflies and mayflies communities was irregular in samples taken from the Danube during last years. The presence of no mayflies’ larvae had been proved, no flying imagines had been observed in 2004. The caddisflies’ community was represented by 1-2 species of the filtrator group, which indicate the lasting retreat of scrapers, spread in the Danube after damming the Danube. The species present indicate conditions typical for slowly flowing river branches. Abovementioned statement is presented also by the Hungarian Party, pointing out the gradual nearing of the mayflies’ community composition in the Danube at Rajka and in the river branch system in the Lipóti branch. The monitoring results of the Hungarian Party repeatedly prove the presence of rich mayflies’ community, with dominance of the (slightly) potamophilous species of the Caenis family in the main riverbed. The caddisflies’ community in the Danube according to the Hungarian data is rich in species and abundant as well. The left side river branch system The mayflies and caddisflies communities in the left side river branch system are generally characterised by low species diversity and low number of individuals of present species. This statement is valid for each monitored river arm in the left side Danube inundation area (monitoring sites No. 2603, 2604, 2608 and 2612). The mayflies and caddisflies communities in river arm on the monitoring area No. 2603 in 2004 consisted of irregularly present species of semirheophilous and stagnicolous type. Several rheophilous autochthonous Danube species were recorded on old trunks caught in culverts. The influence of cut out of the forest in river arm neighbourhoods is constantly insignificant. After a temporary increase of species diversity in the mayflies and caddisflies communities, as a consequence of favourable moisture conditions in 2002, their presence on the monitoring site No. 2604 in 2004 is sporadic again. Stagnicolous species and species bounded to the spreading macrophytes were registered. Changes in the mayflies and caddisflies communities can reflect the changes in flooding regime of this part of inundation area. The presence of larvae of mayflies and caddisflies communities on the monitoring site No. 2608 was not proved since 2002. No flying imagines during the complementary observations in 2004 were registered as well. The settlement of the monitoring area No. 2612 by mayflies’ and caddisflies’ communities is in long term very low and poor. In the springtime 2004 there were several species, including rheophilous species, of mayflies and caddisflies registered thanks to favourable hydrological conditions. The only one stagnicolous mayfly remained present during the following months; caddisflies were not registered during summer and autumn. The right side river branch system The mayflies’ community is represented by community of the river arm at Lipót only. This community consisted of five species, living mainly in eutrophic water bodies (mostly Caenis family). Based on the flying imagines observations of caddisflies’ community (six monitoring sites) it can be stated that the caddisflies’ community in Szigetköz is very rich. This is reflected in the high species number and high abundance, with the dominance of common species, typical for the Szigetköz region. In 2004 there were found two species, new for the Szigetköz fauna. 7.8. Fish (Osteichtyes)The ichtyofauna monitoring on both sides is carried out by electro-fishing, but the power-output of the apparatus changed several times. This can decrease the comparability of results of the individual years. Moreover it have to be mentioned, that the methods of electro-fishing have limited range and they are species-selective, therefore they cannot give complete results of the ichtyofauna composition However, the results of the complex Danube monitoring, carried out by three different types of sampling by the Slovak Party (electro-fishing, net-catching and fishhook-catching), obtained in 2004, prove that the electro-fishing, regardless of its selectivity, is the most suitable method for the monitoring of the hydro-power structure’s influence on the ichtyofauna. It records most of the present fish species (Černý J., 2005 in the Slovak National Annual Report for 2005). The Danube The evaluation is based on Slovak observation results on monitoring areas No. 2600 and 2608 and Hungarian observation results on monitoring sides No. 10 and 11. In general it can be stated that the fish community in the Danube old riverbed has stabilised under changed condition caused by diverting the river. After diversion of the Danube decrease of the number of rheophilous species was recorded; eurytopic species replaced them. In the last year the expansionary rheophilous species Neogobius kessleri became dominant. The left side river branch system Ecological conditions in the river branch system at Bodíky (monitoring area No. 2603) have changed and stabilised after introducing the water supply in 1993. The ichtyocoenose of the area is characteristic by stability in last years, with the dominance of eurytopic species along with lower occurrence of indifferent species. The fish community is characteristic by high species diversity and stabilised abundance. The long-term isolation of the dead arm on monitoring area No. 2604 resulted in decrease of species diversity of the ichtyofauna. Slight increase of species number and abundance in certain years is explained by the absence of over warming of the water body and by floods in 2002. Ageing of the dead arm continued in 2004. The fish community was poor in species number as well as in abundance. Only two limnophilous species occurred more abundantly. The ichtyofauna on the monitoring area No. 2608 is observed on two partial monitoring sites in the river branch system (upwards and downwards from the Foki weir), which differs in their connection with the Danube. The monitoring site upwards from Foki weir is frequently connected with the Danube, particularly during higher flow rates. The fish community is stable, with high species diversity and abundance, with the dominance of eurytopic species. In last years increase of abundance of one limnophilous species is observed, while from among the rheophilous species only one expansionary species was recorded in 2004. Fluctuation of the registered parameters is dependent on the actual flow rate in the Danube at sampling. Floods in 2002 were an exception, when the gradual decrease of the species number and abundance of the present species had been interrupted. The species number and the abundance of the present species (mostly eurytopic species) in 2003 and 2004 were very low again. The development of the ichtyofauna on monitoring area No. 2612, situated downstream of the confluence of the tail-race canal and the Danube old riverbed, depends on the hydrological regime of the Danube, which is not influenced remarkably by the power-station operation. The connection with the Danube is restored only during big floods on the Danube (1991, 2002). The river arm during ten years long isolation almost lost its spawning function, which was accompanied by significant decrease of fish species (3-4 limnophilous species) and number of individuals. During floods in 2002 a temporary increase of species number and abundance were registered, however as a consequence of unfavourable hydrologic conditions in 2003 and 2004 the species diversity and abundance fell back to the level recorded before floods in 2002. The dominance of eurytopic species remained, along with the occurrence of limnophilous species and one specialized species, surviving the extreme over-warming of the river branch and oxygen deficiency. The right side river branch system The ichtyofauna on the Hungarian side is monitored on two monitoring sites in the inundation area (No. 4 and 9) and on two monitoring sites in the flood protected area (No. 5 and 12). The sampling in 2004 was carried out by electro-fishing apparatus with power output of 4500 W (in the Danube as well). The ichtyofauna of the Schisler river arm (monitoring site No. 4) was restored after its artificial reconnection with Csákányi Danube in 1997. The species number radically increased, the species richness of the fish community is significant in last years. Similarly to previous years the eurytopic fish species were dominant in 2004. The state of ichtyofauna in Csákányi Danube (monitoring site No. 9) since introduction the water supply in 1995 is comparable with the community found before diversion of the Danube. In 2004 minor occurrence of rheophilous species was registered repeatedly, along with abundant occurrence of eurytopic species. A slight decrease of species diversity may indicate a tendency of silting of the river bottom at the sampling site. After introducing the water supply to the monitoring site No. 5 – Lipóti marsh (in 1995) regeneration of ichtyofauna is observed. Eurytopic species are dominant. The species richness of this site is significant and balanced at present. The hydrological conditions of Gazfűi Danube (monitoring site No. 12) are equal since introducing the water supply to the flood-protected area. Moderate species richness characterizes the stabilised ichtyofauna, which consisting of eurytopic and limnophilous species. However, the presence of foreign rheophilous species Neogobius kessleri was proved again. The occurrence of protected limnophilous species Misgurnus fossilis is considered valuable.
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