PART 7

Biological Monitoring

The biological monitoring of agreed groups of aquatic and terrestrial fauna is carried out on six complex monitoring areas on the Slovak side. On these areas the phytocoenological observations were realised as well, which record the spring and the summer aspect of plant communities. The aquatic macrophytes observation was restarted again in 1999. The monitoring continued by the method of phytocoenological relevés of Braun-Blanquet till 2002, as it was introduced at start of the monitoring. In 2003, due to unification of observation methods, the Slovak side started to use the Kohler index method in the frame of aquatic macrophytes monitoring. 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). There are differences in some monitoring methods on both sides henceforth (e.g. on the Hungarian side the similarity index and the ecological analysis of terrestrial plant communities is not evaluated; reed communities on the Slovak side are not observed in such a detail like on the Hungarian side; the monitoring of Ephemeroptera, Trichoptera and Odonata on the Slovak side is based on aquatic larvae observation, while on the Hungarian side it reside in observation of imagines), however, further unification is ongoing.

Abiotic conditions, which influence the growth and development of plant and animal communities in the evaluated area on both sides of the Danube, in 2003 can be characterised as follows:

7.1. Phytocoenology

The 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. The observed phytocoenoses develops relatively favourable, which could be explained by increase of soil moisture after two strong floods in 2002. Similar phenomenon was registered at the beginning of vegetation period in 2003, owing to the high flow rates in 2003. The development of every three layers of phytocoenoses was relatively favourable. No premature defoliation or mosaic loosening of coverage in the herb layer occurred in the summer period. The extremely dry weather was visible on dry and declined individuals of nettle in August only. Only the increase of species diversity and rising number of invasive species reflect the tendency of gradual changing of community towards more xerophilous one.

Monitoring areas No. 2603 and 2604 are situated downstream of the intake structure to left side river branch system, thanks to what the moisture conditions have not changed significantly after damming the Danube and high water level can be kept in the river branches too. Some change was recorded in the flood regime of the region and in case of the monitoring area No. 2603 attention should be aimed also to changes caused by the drainage effect of the Danube.

Phytocoenological monitoring on the monitoring area No. 2603 (TMP 9A) was restarted in 2002 in the five-year-old cultivated poplar stand after an interruption in the period 1998-2001 due to cut out. The gradual development of the stand in the second year was shown in increase of the tree layer coverage and decrease of ruderal and nitrophilous species in the summer aspect. Decrease of species number and coverage of the herb layer in the summer period was caused by long-term drought.

The plant communities on monitoring area No. 2604 are stable. Life form changes after floods in 2002 and fluctuation of several factors of the ecological analysis, which are not caused by hydropower station operation, testify the dynamic development of the softwood phytocoenoses.

Monitoring areas No. 2608 and 2609 are situated in a region where the backwater effect upstream the confluence of the tailrace channel and the Danube old riverbed significantly decreases the impact of diversion. Although the situation in this area is relatively stable (particularly at monitoring area No. 2609), the solution of the water regime should not be underestimated. In case of long lasting low and average flow rates in the Danube the area becomes vulnerable. Premature defoliation of willows and fading and drying of plant species was registered during the extremely dry summer period in 2003, accompanied with high temperatures and low ground water level due to long lasting low flow rates. On the monitoring area No. 2609 well-preserved willow stand is present, typical for softwoods. Dry weather conditions did not affect the phytocoenoses in 2003. However, the spring dominance of Aster lenceolatus, accompanied with simultaneous retreat of protected species Leucojum aestivum is an atypical phenomenon.

The reference monitoring area No. 2612 is situated downstream the confluence of the tailrace channel and the Danube old riverbed, where the direct impact of the Gabčíkovo waterworks is insignificant. In spite of unfavourable moisture conditions in 2003 the development of transitive floodplain forest was relatively favourable. Strong spreading of nitrophilous species was registered in the evaluated year because of new sediment layer rich in organic matter, brought by two floods during previous year.

The right side river branch system

The evaluation of plant and reed communities monitoring on the Hungarian side is based on data of summer aspect. According to the obtained results it can be stated that changes of vegetation in the observed area are slow and not significant in last years. Interannual differences in meteorological conditions do not alter the vegetation to such an extent that could be detectable by field investigation methods used in the frame of joint monitoring. In 2003 according to the phytocoenological studies slight significant increase in species richness was recorded, however the rise from the point of view of coverage values was not significant. Average values of leaf area were lower in comparison to the previous year, while no clear tendency appeared in this variable for the reed stands. The vegetation of oxbow lakes had regenerated where water supply created sufficiently high water level. Since the water level in Mosoni Danube was artificially kept stable the ground water table depth stabilised, thus no vegetation change appeared that could be attributed to the diversion of the Danube. A number of seminatural willow stands grow at the banks of the Danube old riverbed, however, several willow forests has been replaced by hardwood plantations on the drying floodplain.

Field observations indicate high regeneration potential for wetland vegetation. Artificial wetlands are rapidly and spontaneously colonized (or recolonized) by characteristic wetland species. The bank of Lipót lake is a good example for this. This indicates a quick regeneration for floodplain vegetation if the water recharge for an oxbow lake, for a river branch or for even a complete tributary system could be provided artificially by distributing sufficient water in the area. The influence of high water table level and temporary inundation on vegetation is much pronounced if these occur from April to September than during the rest of the year.

7.2. Terrestrial molluscs

The 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 diverting the Danube. The terrestrial mollusc’s community had stabilized at medium degradation level of the original community of dryer softwoods type. Temporary decrease of further hygrophilous species was registered during the dry summer.

In the year 2002 the monitoring of terrestrial malakocoenose in the original monitoring site “9A” of the monitoring area No. 2603 was restarted. It was temporarily moved to the monitoring site “9B” due to cut out of forest stand. High degree of similarity was noted between the two sub-sites and the composition of malakocoenoses in 2002 was comparable with the state before the damming. During the dry year 2003 the malakocoenose got an atypical structure, with dominance of two ecologically different species (semisteppe and hygrophilous forest species). The gradual development of the tree layer will lead to decrease of microclimate fluctuations in next years, which can be reflected on the stabilisation of the malakocoenose.

Permanent presence of natural wetland community of terrestrial molluscs can be observed on the monitoring site No. 2604. Polyhygrophilous and hygrophilous forest species remain dominant. The drought in 2003 was reflected in the decrease of abundance only.

According to the monitoring results in 2003 the malakocoenoses on monitoring site No. 2608 is moving towards the xerophilous type. The expected decrease of non-original species after the flood in August 2002 was not confirmed, moreover further decrease of hygrophilous species was registered due to unfavourable moisture conditions in 2003. The high species diversity, atypical for wetland communities, maintains as well. Hydrotechnical solution of water supply for this part of inundation seems to be necessary.

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 unfavourable moisture conditions in 2003 were shown by irregular occurrence of several polyhygrophilous species and by significant decrease of abundance, mainly during the summer sampling. Penetration of xenocenous (alien) species was not registered.

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 consist of species wit various ecological demands from polyhygrophilous to eurytopic species. Wide species diversity was created by mixing of communities from diversified surroundings during regular floods. The lack of moisture in 2003 was shown in decrease of species number and their abundance during the summer.

The right side river branch system

The terrestrial molluscs community in the inundation area as well in the flood-protected area is intact. The decrease in the number of individuals of the terrestrial molluscs in the year 2003 is an unfavourable phenomenon. On the other hand, the proliferation of Lithoglyphus naticoides, the occurrence of Unio crassus and the regular presence of Palidilhia oshanovae should be regarded as a favourable trend of change. The settlement of the species Planorbis carinatus and Gyraulus riparius in the side branches was recorded.

7.3. Aquatic macrophytes

The 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 in the main riverbed downstream of the bottom weir, where the flow rate prevents creating of macrophyte communities. 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 the 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 not registered in 2003. Only one isolated submerged species was recorded along the line structure.

The results of macrophyte monitoring in the dead arm of monitoring area No. 2604, since restarting the macrophyte monitoring (1999-2002), are comparable with findings before damming the Danube. The marshy lake is bordered by a reed stand of the association Phragmitetum communis, 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.

The observed river arm on monitoring site No. 2612 was influenced by extremely low water level in 2003. Dense stands of species not directly depending on aquatic environment dominate due to very frequent occurrence of limose ecophase.

The right side river branch system

The discharges supplied into the active floodplain were similar to discharges in previous years. The species composition in last years is stable, however some quantitative changes were noticeable. In the deep slower flowing water of Csákanyi river arm (monitoring site No. 9) the macrophytes stand extended. The water of Schisler oxbow (monitoring site No. 4) is shallow and nearly stagnant. In the river arm algae growth occurred and rhizophytic macrophyte species were present in small groups in last years.

Due to permanent water supply the hydrological conditions of the flood-protected area (monitoring sites No. 6, 8) remained unchanged in last years. The number of observed macrophytes species remained unchanged as well.

7.4. Aquatic molluscs

The Danube

The evaluation of the aquatic molluscs’ community in the Danube in 2003 is based on the data provided by both the Slovak and Hungarian Party (Slovak observation sites are situated at rkm 1841, 1817 and 1804, Hungarian observation sites are situated at rkm 1831 and 1824). Based on observations it can be stated that the molluscs’ community in the Danube is stable. The malakocoenose is characterised by dominance of rheophilous species during the whole observed period, although a slight increase of species number indifferent to the flow velocity was observed after damming the Danube.

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.

Because of connection of the river branch at monitoring site No. 2603 with the neighbouring river arms and the Danube, high species diversity of community can be continuously observed, which consist mainly of stagnicolous and indifferent species. Changes in the community after floods in the previous year were not registered.

The connection of isolated community of aquatic molluscs on the monitoring area No. 2604 (since last big flood in 1991) was temporarily restored during the floods in 2002. Increase of species richness and abundance was registered in the molluscs community. However, regarding the wide food offer, the community of aquatic molluscs could be richer in species and number of individuals.

The right side river branch system

No significant changes in malakofauna of the right side river arm system were recorded in 2003. Stable occurrence of some species in the river branch system should be regarded positively.

7.5. Dragonflies (Odonata)

The left side river branch system

The species composition of dragonflies’ community on the Slovak side during the whole observed period is rather poor.

Monitoring of dragonflies in the monitoring area No. 2600 was in 2002 restored in a gravel pit, which is filled up with Danube water and had muddy-gravel bottom. The newly formed biotope was not overgrew by macrophytes yet and the dragonflies’ community was poor. More significant settling of this biotope by dragonflies can be expected in the following years.

The abiotic conditions of aquatic biotopes on the monitoring area No. 2603 was not changed since introducing the water supply to the river branch system (in 1993). Stabilisation of odonatocoenose mainly consisting of rheophilous and semirheophilous species was registered since then. The raised values of species number and abundance in 2003 probably reflect the reaction of the community to antropic influences (forest cultivation).

Moderately rich dragonfly community on monitoring site No. 2604 remains on similar level as in 2002, in spite of the dry vegetation period in 2003. Flooding and washing out of the dead arm in previous year slowed down its natural gradual degradation.

The sampling site on monitoring area No. 2608 was moved due to unfavourable biotope conditions for dragonflies’ communities. Although the abiotic conditions of the newly chosen site were favourable, irregular occurrence of one rare species was registered only in 2003.

Monitoring of odonatocoenose on the monitoring area No. 2612 was moved to neighbouring river arm with more favourable conditions for dragonflies’ occurrence in 2002. The dragonflies’ community in 2003 had relatively high species richness and consisted of species of eutrophic water bodies and semireophilous species, however with low abundance.

The right side river branch system

The species composition of odonatocoenoses of regularly monitored sites has stabilised during last years. The recorded number of species in 2003 was 36.

7.6. Crustaceans (Cladocera, Copepoda)

The Danube

Evaluation of development of Cladoceras and Copepods communities is based on the results of the Slovak side (monitoring site No. 2600 and 2608). The stabilised conditions of the Danube old riverbed allow formation of the stable communities of Cladoceras and Copepods. Actual spreading of the macrovegetation in the river influences their development in the respective year. Their species diversity was rather high in the evaluated year; even the proportion of euplanktonic and tychoplanktonic species was comparable with the pre-dam state.

The left side river branch system

The state of Cladoceras community 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 this community before the damming, became insignificant. Similar changes were observed in the Copepods community as well. The euplanktonic species were displaced by gradual spreading of macrophytes, however their repeated dominance was recorded in 2003 (probably due to wash out of macrophytes in previous year).

The gradual overgrowing of the dead river arm on the monitoring area No. 2604 causes changes in Cladoceras community, where loss of dominance of euplanktonic species going on. The rate of phytophilous species and species settling biotopes rich in organic detritus increases. The temporary interconnection of the river arm during floods in previous years did not stop this tendency. Similar changes in Copepods community have significantly lower intensity. Only fluctuation of abundance of present species within the dominant euplanktonic group of species is registered.

Following the temporary interruption of the permanent dominance of euplanktonic Cladocera species on monitoring sites No. 2608 in 2001 and 2002, the proportion of euplanktonic and tychoplanktonic species reaches the pre-dam state again. The Copepods community did not show significant fluctuation during the whole observed period.

The water level of the isolated river arm on monitoring site No. 2612 was very low in 2003 due to unfavourable hydrological and meteorological conditions. Therefore significant dominance of tychoplanktonic species was registered in Cladoceras and Copepods communities, which occurrence in the pre-dam period was very poor. Due to the extreme hydrological conditions the species richness of Cladoceras community was poor, while the species diversity of Copepodas community remained relatively rich.

The right side river branch system

The development of planktonic crustaceans in the Schisler arm (monitoring site No. 4) and in the Zátonyi Danube (monitoring site No. 5) was similar to the previous years. In 2003 significant decrease of species number was observed in the Lipót oxbow (monitoring site No. 6), where several species dependent on macrophytes was not observed. Increase of species diversity was registered in the Csákányi river arm (monitoring site No. 9) in the inundation area, but the species density was low, except one tychoplanktonic phytophilous Cladocera species.

The Cladoceras and Copepods communities were characteristic by maximum abundance during the spring period in 2003. Probably because of the extremely unfavourable hydrological condition the species densities in summer and autumn were low, while the maximal density in the previous year was registered in autumn. The most frequent species were Bosmina longirostris and Chydorus sphaericus, like in the previous year.

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. On the Slovak side the larvae of mayflies and caddisflies living in the water are observed, while on the Hungarian side the flying imagines are monitored. The Hungarian Party did not carry out sampling of caddisflies communities in 2003; evaluation of the amassed biological material was performed only.

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 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. The mayflies community was represented by semirheophilous and rheophilous species, while only one species in the caddisflies community, belonging to the filtrator group, was registered in last years. The occurrence of the species indicate the gradual decrease scrapers, spread in the Danube due to changes in food offer after diversion of the Danube.

The differences in the mayflies community registered by the Hungarian side during the last year in the upper and middle part of the Danube old riverbed were balanced in 2003. Rich appearance of eutrophic water mayfly species was recorded on both sampling sites (Rajka and Szőgye). The monitoring of mayflies based on flying imagines shows the existence of richer community, than is represented by results obtained by the Slovak Party, which are based on insect larvae. While the Slovak Party indicate the presence of rheophilous and semirheophilous species, imagines observed on the Hungarian side give evidence of several stagnant water species, which settle the Danube old riverbed.

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 every monitored river arm in the left side inundation area (monitoring sites No. 2603, 2604, 2608 and 2612).

The mayflies and caddisflies communities in the river arm of monitoring area No. 2603 in 2003 consisted of irregularly present species from semirheophilous to stagnicolous type. The forest cut out in river arm surroundings did not influenced the development of the community for the present.

After a moderate increase of species diversity in the mayflies community on the monitoring site No. 2604 in 2002, only larvaes of two stagnicolous species were registered in 2003. Regeneration of caddisflies community, after its absence in 2000 and 2001) did not occurred in 2002 and 2003, not even after favourable hydrological (floods) in 2002. In 2003 only one species was registered, which was bound to macrophytes.

The presence of mayflies and caddisflies communities on the monitoring site No. 2608 was irregular in the last period. No larvae were found in 2002 and 2003. Moving of the monitoring site within the complex monitoring area seems to be necessary.

The mayflies’ community in the monitoring area No. 2612 is represented by single stagnicolous species during last years, which was accompanied by further species of eutrophic water bodies in 2003. The presence of caddisflies community in individual years and individual samples was irregular. Occurrence of one species of filtrator type was registered in the summer sample only in 2003.

The right side river branch system

Different results in monitoring of mayflies and caddisflies on the Slovak and Hungarian side have been reached in the river branch system too.

The species diversity and the abundance of mayflies’ species on the Hungarian side are significantly higher in long-term. High species diversity was observed on the monitoring site at Mosoni Danube near Feketeerdő again. The Ephoron virgo, liking river branches with silted bottom, was the dominant species in 2003. The mayflies community is characterised by high stability. Mass occurrence of mayfly of eutrophic water bodies (Caenis horaria) persist in the Lipót marsh, along with lower number of other species. Species composition changes were registered in the individual years as well.

No caddisflies observation was performed in 2003, however according to the evaluation of amassed biological material of several years it can be stated that composition of caddisfly communities basically did not changed. According to the observation of flying imagines the caddisfly communities in the Szigetköz region are very rich, which is proved by high species number and high number of individuals (51 species was determined). Rheophilous species preserves its dominance; population of several other characteristic species are stabilised as well. Based on the caddisflies communities’ development and composition the water bodies of Szigetköz region are in a good state.

7.8. Fish (Osteichtyes)

The ichtyofauna monitoring on both sides is carried out by electro-fishing (with moderate high power-output apparatus – 600 W). In 2002 very high power-output electro-fishing boat was alternatively used in the Danube by the Hungarian Party (10000 W). The mentioned apparatus has higher range and therefore allows observation of deeper parts of water. Due to the different species-selectivity of the mentioned methods, the obtained results are not directly comparable with the data of previous years, therefore the Hungarian Party in 2003 used the medium power-output apparatus again. The methods of electro-fishing have limited range and they are species-selective, therefore they cannot give complete results.

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 is stabilised. After diversion of the Danube decrease of the number of rheophilous species was recorded; eurytopic species replaced them. Significant changes of the relatively rich species composition were not registered during last years. Fluctuation of species number and abundance in individual samples could by assigned to different hydrological conditions at the time of sampling (different flow rates). Results of last years are influenced by difficult access to the shoreline, due to spreading of littoral willow stands.

The left side river branch system

The ecological conditions in the river branch system at Bodíky (monitoring area No. 2603) have changed 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. The positive impact of floods in previous year was observed in immediate significant increase of species diversity, which remained on the same level in 2003 as well.

The long-term isolation of the monitoring area No. 2604 resulted in decrease of species diversity of the dead river arm. However, moderate increase of species number and abundance as well was registered in last years, what is explained by the absence of overwarming of the water body and by floods in 2002. However, in 2003, due to unfavourable meteorological and hydrological conditions, low species diversity and abundance was registered in the ichtyocoenose.

The ichtyocoenose 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 permanently connected with the Danube, particularly during higher flow rates. The fish community is stable, with high species diversity and abundance as well as, with dominance of eurytopic species. While in 2002 occurrence of rheophilous species was characteristic, significant spreading of limnophilous species was registered in 2003 due to extremely low flow rates. The connection with the main riverbed was lost in the river arm downwards from Foki weir since damming the Danube. Only temporary reconnection occurs during big floods. After floods in 2002 temporary increase of species diversity and abundance was registered, however the ichtyocoenose in 2003 consisted of several mainly eurytopic species. The occurrence of rheophilous species was insignificant.

The development of ichtyocoenose on monitoring area No. 2612 depends on the hydrological regime of the Danube. The area is situated downstream the confluence of the tailrace channel and the Danube old riverbed and is not influenced by the flow rate discharged into the old riverbed. The connection with the Danube is restored only during big floods on the Danube (1991, 2002). During the ten years long isolation it almost lost its spawning function, which was accompanied by significant decrease of fish species and number of individuals (to 3-4 limnophilous species). The connection with the Danube was temporarily restored during floods in 2002. Due to this significant increase of species diversity and abundance was registered. Besides limnophilous species the eurytopic ones became dominant. In consequence of the unfavourable hydrologic conditions in 2003 the species diversity and abundance of individuals returned to the level recorded before floods in 2002. However, the dominance of eurytopic species remained.

The right side river branch system

The ichtyocoenose 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 ichtyofauna of the Schisler river arm restored after its artificial reconnection with Csákányi Danube in 1997. The species number radically increased, the species richness of the fish community is moderately rich at present. Eurytopic fish species were dominant in 2003. Limnophilous species occurred in the middle part of the river arm, however rheophilous fish species was found in the connection channel as well.

The state of ichtyofauna in Csákányi Danube (monitoring site No. 9) in last years (since introduction the water supply in 1995) was comparable with the community found before the damming. The macrophytes vegetation retreated due to continuous water supply and the limnophilous fish species were replaced by rheophilous ones. A slight decrease of species diversity can indicate the tendency of silting of the river bottom at the sampling site, however this cannot be generalised to the whole river arm system.

After introduction of the water supply to the monitoring site No. 5 – Lipót marsh regeneration of ichtyofauna is observed. Eurytopic species were 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 of the flood-protected area. Moderate species richness characterised the stabilised ichtyofauna, which consisted of eurytopic and limnophilous species. The eurytopic species Rutilus rutilus remain dominant.

	Fig. 7-1 Monitoring network of observation objects agreed in the frame of the Joint monitoring