V.2.6.  Mollusc fauna (Mollusca) of the Old Danube riverbed (1997-2005) and its perspectives

Vladimír Košel 

In connection with the construction and operation of the Gabčíkovo hydraulic structures we studied aquatic mollusc fauna since 1989. The results obtained in the period before 1997 were published in several papers [6, 7, 9]. In recent years, approximately since 2001, significant changes in the mollusc fauna of the Danube old riverbed occurred. So far these changes have only been published in the form of a lecture and an abstract [8]. 

Characteristics of profiles and methods 

Molluscs in the Old Danube riverbed were studied in two profiles, upstream from the study site Dobrohošť – Kriviny and at the study site Gabčíkovo – Istragov. Since the Danube damming in 1992, the discharge in the Old Danube has been considerably reduced, the dynamics of the water table level has been limited and shipping has been excluded. The banks of the studied sites are faced by rubble and up to the water table they are covered by dense willow overgrowth. The material was collected both qualitatively and quantitatively. 6-10 stones were taken from the bottom and fauna were taken from their whole surface into a hybrobiological net. The stones were measured according to geometric shapes and the projection of their surface was calculated. After the molluscs were identified and counted, their number was recalculated per 1 m². From all values (at least from three) we calculated the average annual abundance (Tab. 1-2) for each species in each year in both localities. 

In the beginning, the sampling was carried out three times a year, but 4-5 times in recent years, in spring, summer and autumn, and experimentally also in winter. The most suitable time for sampling was the period of minimum discharge or the period of long-term stabilised level corresponding to 90-120 cm on the water gauge at Gabčíkovo. However, it was not always possible to sample under such conditions, especially in the spring, which could distort the results. 

Results 

The Old Danube is poorer in the number of species than its arms. More regularly we recorded the presence of just nine species. Among the original rheophilous species only Ancylus fluviatilis occurred there, while Esperiana esperi and Theodoxus fluviatilis also appeared later. Other species have a broad ecological valence and occur in more habitats. These species are represented by the slugs Bithynia tentaculata, Lymnaea ovata, Potamopyrgus antipodarum, Lithoglyphus naticoides, Valvata piscinalis and by the lamelibranches Dreissena polymorpha and Sphaerium corneum. Irregular occurrence was recorded with the slugs Galba truncatula, Gyraulus albus, Lymnaea auricularia, Physella acuta, Pisidium henslowanum and the lamellibranches Sphaerium solidum and Corbicula fluminalis, having recently been introduced from the basin of the Rhine. The greatest changes were caused by the occurrence of the experimentally relocated Esperiana esperi and the introduced invasion species Theodoxus fluviatilis in the study site at Gabčíkovo. 

Study site Dobrohošť – Kriviny (Tab. 1)

From the viewpoint of the size of mollusc populations, the period 1997-2003 can be taken as relatively dynamic because the cumulative abundance of molluscs increased from 1997 to 2001, when it reached 8622 individuals per m². The largest portion was represented by two species - Ancylus fluviatilis and Lymnaea ovata. The abundance of A. fluviatilis culminated in 2000 to reach 5798 ind./ m², while that of L. ovata in 2001 reached 4050 ex./ m². In 2001 Bithynia tentaculata also reached a high abundance - 628 ex/m². 

From 2002 the cumulative abundance of molluscs gradually decreased. A regular decrease was recorded especially in A. fluviatilis and L. ovata also showed a similar trend. In 2005, mollusc abundance dropped to 17 individuals per m². This drop can be considered a total collapse of the mollusc community, affecting all species. 

The onset of this trend can be more precisely dated to the summer of 2004, when on 27 August I recorded only 40 individuals of A. fluviatilis, 41 individuals of L. ovata and 272 individuals of Bithynia tentaculata per m². The cumulative abundance of all molluscs was 380 ex./m² and in October 2004 it had decreased to just 45 ex./m². In 2005 three species, Lymnaea ovata, Lithoglyphus naticoides and Potamopyrgus antipodarum, absented all year round.

Study site Gabčíkovo – Istragov (Tab. 2)

In 1997-2002 the cumulative abundance of molluscs fluctuated in the range of 1712 ind./m² in 1999 to 3404 ind./m² in 2000. In 2001 and 2002 the abundance stabilised at the values of 2708 to 2522 ind./m². In 2003 the abundance suddenly increased to 15,819 ind./m² and then it decreased to 4545 ind./m² in 2004 and 4622 ind./m² in 2005. Until 2002 the greatest portion of individuals was represented by A. fluviatilis, reaching a maximum of 2005 ind./m² in 2000. Since 2001 its abundance decreased and in 2004 and 2005 we did not record it at all. 

On 6 September 1994, we relocated 60 individuals of E. esperi and 40 individuals of Theodoxus danubialis from the Danube at Štúrovo to this study site. The purpose of this relocation was to create several centres of occurrence and to reduce the degree of their endangering, and it was motivated by the fact that already in late 1980-s E. esperi was known to occur in Slovakia only in one locality, namely in the river bank zone of the Danube at Štúrovo, on a stretch of 200-300 m. The area of T. danubialis was larger and stretched from Štúrovo up to Zlatná na Ostrove, with a gap between Iža and Komárno. 

From 1997 I recorded the presence and successful reproduction of Esperiana esperi in this study site. On the contrary, up to the present I have not succeeded in confirming the occurrence here of T. danubialis. The abundance of E. esperi gradually increased up to 2003 when it reached an annual average of 653 ind./m², but a culmination of 1829 ind./m² was recorded in July 2003. Since 2004 its abundance has been falling and E. esperi is beginning to occur sporadically. 

Since 2003 the abundance of Theodoxus fluviatilis, a new species of fauna in Slovakia, has been rising strongly. I recorded it at this site for the first time in October 2001, but a strong increase in its abundance (to an average of 13288 ind./m²) was observed already in 2003 and a maximum of 34932 ind./m² in September 2003. In 2004-2005, its abundance slumped rapidly (3984 and 4521 ind./m²), which could indicate that this gastropod had reached its invasion maximum and its population size was stabilising on a lower level. It seems that competition pressure of the excessively large population density of T. fluviatilis has not affected the lamellibranches Dreissena polymorpha, which use other food resources (filtration of food from water), while it can seriously endanger the species scraping food from stones. 

Frank et al. [4] published the first data on the occurrence of T. fluviatilis in the middle Danube, found in Budapest. Later Csányi [1, 2] provided more precise circumstances of this finding. The first individuals were discovered upstream from Budapest in 1987, and in 1990 its occurrence was also confirmed farther downstream, while upstream it was recorded at Gönyű (1991) and farthest upstream near Rajka (1992). 

The first finding on the Slovak stretch of the Danube was made downstream of Bratislava, in the Čunovo reservoir (r. km 1863.5), on the left bank, downstream of the outlet of wastewaters from the Slovnaft refinery (27 August 1999). However, just one specimen was found there and it was the only finding over a three-year investigation of littoral zoobenthos of the Danube between Čunovo and Devín [5]. Further findings from 3 localities from a more downstream part of the Slovak-Hungarian stretch were published [3], downstream from Komárno and upstream from Iža in 2002. 

Discussion 

It is possible to state that the mollusc fauna in both sites near Dobrohošť studied in 1997-2002 and 2003, respectively, was qualitatively and quantitatively relatively stabilised. The year on year differences in abundance could be explained by dynamics in the development of populations of individual species due to a changing climate, food offer and hydrological regime. Changes observed in the site near Gabčíkovo after 2003 were unambiguously caused by the spread of the allochtonous species, Theodoxus fluviatilis, which satisfy all the criteria to be considered as invasive: allochtonous faunistic element quickly spreading and reaching high abundance to outbreak in the invaded territory. Its original area, from which it invaded into the Danube, is The Rhine basin, primarily the Main River, which is connected with the Danube by a shipping canal. [8]. 

The direct cause of reduced abundance or the disappearance of more gastropod species could be food competition on a stony substrate. A mass outbreak of a species cannot last without a response from other species, especially if they have similar food requirements. In this case the large growths on the stones of the littoral zone (periphyton). The stones were completely free of macroscopic growths of algae and mosses during the mass outbreak of T. fluviatilis. 

In the profile near Dobrohošť, the cause of the destruction of the mollusc community is unclear, but this destruction has other causes than near Gabčíkovo. The invasive T. fluviatilis has not still spread here and we have still not recorded it in the Danube near Bodíky either (November 2005). If we exclude saprobic or toxic pollution as a cause of the reduction or disappearance of several mollusc species, one of the possible causes of this decline could be the covering of the upper side of stones by a muddy sediment layer of 5-10 mm. In this layer or on its surface I did not find any molluscs. I also did not find them on the side or underside of the stones, which could supply a suitable food at least partially. The hypothesis that the covering of the stones by mud could be one of the causes of a reduction of molluscs is supported by regular, though not abundant, findings of Ancylus fluviatilis on stones of riffles in the submerged dam at r. km 1843, where no muddy sediments were formed (November 2005). 

We have recorded sediments on the stones since August 2004, but we do not know their origin and a more precise time when they started to be formed in a greater quantity. We observed turbidity always at increased water levels, similarly to as in preceding years. 

Differences between the mollusc fauna in Dobrohošť and Gabčíkovo were manifested first of all by quantity. The cumulative average abundance in Dobrohošť was always higher in 1997-2002, sometimes more than three times higher (in 2001). These differences can be explained by more suitable conditions created for development of algae as a food source by a more stabilised hydrological regime at Dobrohošť. According to hydrological data, the water level near Dobrohošť fluctuates, with the exception of flood levels, within the range of 1 m, while near Gabčíkovo it is within the range of 2 m. 

It will certainly be interesting to study how the mollusc fauna will develop in the Danube old riverbed in the coming years, if the old riverbed is not re-natured. I expect further spreading of Theodoxus fluviatilis in the upstream direction. This species will probably reach a highly predominant position among molluscs. The representation of autochtonous Danube species will be very low, in most species I expect total disappearance, the only regularly occurring species will be Dreissena polymorpha. The development of mollusc fauna on the stony substrate covered by mud is uncertain. It is possible that it will be also occupied by the invasion species Theodoxus fluviatilis, though only to a limited extent. 

References

V.2.7.  Monitoring Danube fish fauna and the influence of the Gabčíkovo project

Jaroslav Černý

Introduction – methods of monitoring 

The Danube deposed a large amount of sediments after the break through the Western Carpathians. Due to large accumulation of alluvial material, it often changed its riverbed and formed an arm system called inland delta. Thus, the present Slovak-Hungarian stretch of the Danube was characterised from ancient times by the great variability of hydrocoenoses, which was also reflected in high species diversity and the productivity of fish, which was directly dependent on the existence of the inland delta. The river branches served to fish not only as a refuge at times of high water levels, but foremost as a food and reproduction basis. 

Since ancient times humans tried to control the Danube, prevent floods, improve navigation, and use the Danube resources. They mostly included straightening the stream and cutting off the arms. Such regulations have negatively influenced the Danube floodplain. 

Water pollution, which was especially characteristic of the post war period, is among the strongest direct factors essentially influencing the fish fauna, foremost in the recent period. Danube water pollution culminated in the late 1970s when a continuous layer of oil pollutants and various remnants of communal wastes covered the riverbanks. In the 1980s the water quality started to improve thanks to the newly constructed wastewater treatment plants in Vienna and Bratislava (1985) and at the oil refineries in Schwechat and Slovnaft in Bratislava. 

At that time the inland delta partially lost its function. Apart from a few exceptions, upstream and downstream ends of the arms were cut off from the main stream. Communication of the main stream with the arms was already considerably broken and the Danube, riverbed was cut into the basement due to excessive erosion and gravel exploitation. Since 1966 the water level in the Danube on the stretch between Bratislava and Komárno declined by 1-2 m at a discharge of 1000 m³.s^-1. In the 1980s the main stream communicated with the arms only at above-average water levels by discharges above 2330 m³.s^-1 and the number of communication days declined continuously. In 1980-1990 the arms system was dry during a large part of the year and the arms were connected with the main stream about 20-30 days a year. This resulted in acceleration of terrestrialization of the river arms. 

The Institute of Zoology of the Slovak Academy of Sciences has monitored the impact of the Gabčíkovo hydraulic structures on fish fauna since 1990, when we made a survey catch by means of seine nets in the monitoring plot (MP) Kráľovská lúka (rkm 1825-1826). At present we are monitoring fish fauna in eight localities. Two reference localities: MP Sporná Sihoť (at rkm 1802) and MP Starý les (rkm 1799.5, since 1998) are situated in the within-dike zone, downstream from mouthing of the tail-race canal into the Danube. These localities are not influenced directly by the construction of the Gabčíkovo hydraulic structures. Another two localities are situated in the former main stream of the Danube – MP Dunajské kriviny at Dobrohošť (rkm 1839-1841) and in the area of MP Istragov (rkm 1817.5). Other localities situated in the within-dike zone are: MP Bodícka brána (rkm 1830), the already mentioned MP Kráľovská lúka (rkm 1825 –1826), while two MP are on the downstream and upstream side of the “Foki” dam in the area of Istragov. This dam closes the upstream end of the Topoľovské rameno arm (rkm 1816). 

Since 1991 we have been catching the fish by electric aggregate, three times a year, usually in April-May, July-August and September-October. Due to limited financial sources we have chosen this method because the seine net catches and estimates by the “mark-recapture” method were not feasible in our localities. It is know that the efficacy of the electrical aggregate reaches approximately a distance of 1.5 m [7]. This aggregate therefore allows us to catch predominantly littoral fish or fish near the water surface. On the contrary, rheophilous fish occurring in the streamline at the bottom are sporadically recorded by this method. The author was aware of the selectivity of this method and he always interpreted the data from this viewpoint. 

Data on the impact of the Gabčíkovo hydraulic structures on fish were not obtained in addition also in the framework of international research projects carried out in collaboration with Dr. G. Copp [4, 5, 15] of the University of Hertfordshire (Great Britain) and with H. Persat of the University of Lyon (France) [29]. 

In October 2004, in addition to the usual three catches and usual method, we also carried out a fourth survey not only at the eight MP mentioned above, but also in the former main stream wherever we found a suitable place where it was possible to make catches by electrical aggregate and net. The catches were made in collaboration with the fishing group of G. Tóth of Štúrovo at the following localities: rkm 1850-1851 downstream from the Čunovo weir, at Dobrohošť, (rkm 1839-1841), in the arm system near Bodíky (rkm 1831-1833), at the level of the arms near Baka (rkm 1821) and on the stretch between Istragov and Sap (rkm 1811). The catching was completed by angling carried out in collaboration with the Slovak Anglers Union (E. Seemann) by six selected sport anglers. In addition to the usual monitoring plots, we also angled on stretches at Čunovo, at the submersed weir at Dunakiliti (rkm 1843) and in localities near Istragov [13]. 

Changes in fish fauna caused by putting the Gabčíkovo hydraulic structures into operation 

The construction of the Gabčíkovo hydraulic structures does not represent the first or only negative impact on Danube fish fauna. Species diversity, abundance and biomass of fish fauna in the last decades of 20^th century were reduced by many anthropogenous factors. Already in 1967 Balon [1] recorded a decline in the rheophilous and economically important fish in the Slovak stretch of the Danube. He explained this by the strong pressure of fishing, river regulation and pollution. Already during that period a high predominance of non-predaceous species over predaceous ones was recorded by Holčík and Bastl [20, 21]. Pollution, reduced discharge of the Danube water into river arms and shorter flood duration in the Danube floodplain essentially changed the conditions for development of hydrocoenoses, inclusive of fish, already before the Gabčíkovo hydraulic structures were put into operation [23, 8, 9]. 

Although many specialists gave their opinions about the Gabčíkovo project, fish were never considered to be an important enough animal group for the project to include measures moderating the impact of the construction of the Gabčíkovo hydraulic structures on these animals (e. g. [3]). Bypassing the Danube in 1992 need not have impacted the fish too dramatically. It was carried out in the autumn when the fish were already at wintering sites. However, the manner in which it was done was harmful. 

Rescue catches of fish stuck in depressions were not organized, but, on the contrary, poachers plundered the fish remaining in wintering sites on the arms. At that time the fishing zones were officially in the area of the building site. The Slovak Angler Union and nature protection organizations therefore had no possibility of preparing the rescue catches or of carrying them out. As a matter of fact, just in that period a further decline of predators fish started, as they had no possibility of natural reproduction in the following period. A low discharge in the abandoned main riverbed, as well as a low discharge in the arms immediately after the Danube damming, had caused the fish to withdraw into the stretch downstream from the Gabčíkovo structures, while the abandoned main riverbed and the arms remained without fish in that period. 

Although many new water surfaces (the Čunovo reservoir, material pits) arose during the construction of the Gabčíkovo project, the major part of the floodplain area lost the function of spawning places. Probably the liquidation of the former big spawning place and rearing place of juvenile phytophilous fish in the area of Istragov can be regarded as the greatest negative. 

The river arms were divided into separate sections (cassettes) and the arm system started to be supplied by water taken by the intake structure from the bypass canal in 1993. Although the size of fish populations was without fish stocking and natural reproduction considerably lower than in pre-dam conditions, the situation has partly stabilized in spite of the fact that the weirs inhibit the upstream migration of fish. Thus the fish fauna of individual cassettes considerably differed until the flood of 2002, as it corresponded to the momentary fish community in each section before the weirs were built. 

Building up weirs in the arms liquidated the chance for migrations to the possible relics of the spawning places. It made also impossible the migration of fish to former wintering places in the arm system. It is only logical that the liquidation of a functional inland delta and the loss of spawning places would result in a decline of the abundance and biomass of fish [10, 12]. Unfortunately, it is reasonable to assume that without any improvement measures the negative influence described above will become increasingly stronger [2]. 

Construction of the overflow weir at rkm 1843 near Dunakiliti in 1995 made possible the migration of fish from the former main stream into the right-side arm system. According to data of Hungarian colleagues, the species composition was extraordinarily poor both in the pre-dam conditions and before the supplying with water. There were only 3-4 species in one monitoring plot. After the start of supplying with water, the number of species in most monitoring plots increased twofold. This provides proof of the possibility of fish penetrating from the former main stream. The species occurring here in the pre-dam conditions or before supplying with water continued to occur here. 

The problem of poaching persists, especially downstream of the Čunovo and Gabčíkovo dams, in spite of measures undertaken to combat this negative phenomenon. 

Most important results of monitoring the impact of the Gabčíkovo hydraulic structures on fish fauna 

Since 1999 the results of monitoring the impact of the Gabčíkovo hydraulic structures on biota were regularly published in synthetic annual reports. Our data on fish fauna represents the only data available for commentary of the impact of the Gabčíkovo project on fish fauna, as no other institution or private party has monitored these impacts without collaboration with the Institute of Zoology of SAS. These data are used in all papers evaluating the impact of the Gabčíkovo structures on fish fauna even in cases where they do not cite us [24, 2]. 

Beside the synthetic annual reports, evaluation of the original state and of the initial impact of the construction of the Gabčíkovo structures on fish fauna has been published in different papers [6, 12, 14]. However, the impact of constructions on fish fauna was not studied only within the monitoring project. For example, the impact of the Gabčíkovo structures construction on juvenile stages of fish were studied in collaboration with foreign partners before construction of the Gabčíkovo project was finished and the results were published together with G. Guti [4]. After finishing the construction, we repeated the study, but unfortunately only on the Slovak side [15]. 

Since 1993, i.e. immediately after the Danube damming, we recorded striking changes in parameters (discharge, stream velocity, riverbed stratification) influencing the occurrence of fish in the Old Danube. The shallow littoral has widened in particular. It shifted more to the riverbed center. As a consequence many natural covers of fish were liquidated. The total abundance and species diversity of fish community declined in the littoral of the former main stream. The littoral of the abandoned riverbed started to be colonized especially by juvenile individuals. However, the permanent presence of alpine bullhead (Cottus gobio), which demands high oxygenation and clearness of water, documented a very good quality in that period after the Danube damming. 

Changes in the structure of fish communities also appeared in the arms. At individual monitoring plots, eurytopic, especially phytophilous and in the floodplain common species with wide ecological valence, became dominant. The relative abundance of rheophilous species, which created the essential component of fish communities of the arms in the past even out of the periods of communication of the arms with the main stream, became subdominant to receding [10, 11]. 

Decline of fish abundance, which was expected in the Slovak stretch of the Danube (for example Holčík et al. [22]) after finishing of the Gabčíkovo project, was not caused exclusively by these structures, but also by factors that caused this process already in the pre-dam period. In particular this was the deepening of the former Danube mainstream river bottom, which reduced the number of days of communication of the main stream with the floodplain, especially in those periods that in the past were important for the fish from the viewpoint of migrations for food or spawning. 

Illegal fishing has a considerable and very negative influence on the abundance and species diversity of fish communities. It reached gigantic proportions in the 1990s. It was fished by any available means, including by using explosives, gill nets and electroshocker, irrespective of the legal minimum size of fish, spawning period or economic significance of individual species. The over reproduction of cormorants (shag) and other birds that feed on fish is also far from negligible. 

The absence of floods after completion of the Gabčíkovo project reduced the number of habitats representing spawning places, which secondarily caused further changes in the structure of the fish community. This also explains the decline of fish community productivity [21]. The positive influence of natural floods on fish fauna in the Danube old riverbed and especially in the arms, can be documented by the increased abundance and diversity of fish recorded after the flood in 2002. 

When comparing the present average length of fish caught in the Danube [13] with that recorded upon marking of fish in the Old Danube in 1993 (Holčík et al. unpubl.), we can state that the average body length of individual fish species (common loach, vimba bream etc.) is increasing. This seemingly positive phenomenon signalizes that the populations is ageing and that younger fish are missing. Hence, there is the problem of population reproduction. Such data were obtained not only from the stretch influenced by the Gabčíkovo hydraulic structures, but also from the stretch between Komárno and Štúrovo, where the fishing group of G. Tóth catches generation starlets (Acipenser ruthenus) for artificial rearing. Recently (10-15 years ago) the average weight of the caught starlets was in the range of 500 g, whereas individuals of around 1000 g are frequently caught at present. 

The Gabčíkovo hydraulic structures do not influence individual species directly, but indirectly, through a reduction of their habitats, inhibition of migrations or spawning. A commentary on the impact on individual fish species was presented in the papers of [14, 13]. 

As follows from investigation and especially from monitoring carried out by means of slectroshocker, Alpine bullhead (Cottus gobio) disappeared from the area monitored in the recent period. Its disappearance was caused, apart from the influence of the Gabčíkovo hydraulic structures, by expansion of allochtonous species, in particular the Bighead goby (Neogobius kessleri) and Round goby (Neogobius  melanostomus), which have eliminated the gudgeon from its habitat. 

Very inconspicuous was extinction of Whitefin gudgeon ((Gobio albipinnatus), which commonly occurred in the past in the floodplain in the habitats which almost disappeared after the Danube damming. It habitat was represented by shallow muddy and overheated bays with standing, but sufficiently oxygenated water. 

Another species that disappeared from the Danube Slovak stretch is the Stone loach (Barbatula barbatulus). It was not too frequent in the Danube also in the past, occurring sporadically in the same habitats as Alpine bullhead (Cottus gobio), but it almost did not occur in the arms. Hence the Danube on the Slovak stretch did not offer this species optimal conditions, in spite of the fact that this species is common in tributaries of the Morava river (e.g. Močiarka brook) in the vicinity of its mouthing into the Danube [30]. At high water levels, these streams can serve as immigration sources for its re-appearance in the Danube. 

Further expected influences of the Gabčíkovo hydraulic structure relevant to Danube fish fauna 

Decline of fish catches along whole Slovak stretch of the Danube

The data from statistics of fish catches made by the Slovak Angler Union have a guidance value only. They are unreliable, especially because of the huge spread of poaching and change in the reporting of catch data. The average annual catch of recreational angling in 1961-1979 was, according these statistics, 102.7 t of fish. In the period of construction of the Gabčíkovo project (1980-1992), but especially after 1984, the total catch decreased to as low as 79.4 t. In the 1993-1996 period, i.e. after the Danube bypassing and completion of the hydraulic structures, the annual average catch dropped to 26.8 t [18]. 

Problems with overgrown vegetation at individual cassettes

The discharging and dynamic regime has turned into a static regime; therefore individual cassettes are becoming increasingly overgrown by vegetation. We assume that the dying off of a large plant biomass in the autumn and its decomposition in winter, especially under ice, may cause a considerable deficit of oxygen, which will negatively influence the wintering fish. Up to present we have observed such an effect at the MP Kráľovská lúka, where in addition to the summer the oxygen deficit also occurs in winter and causes a drop in species diversity of fish fauna. 

Mass spread of expansive allochtonous species

Since 1996 we have recorded four new allochtonous fish species (Neogobius kessleri, N. melanostomus, N. fluviatilis, N. gymnotrachelus) of the Gobiidae family in the Slovak stretch of the Danube. They are spreading from the Pontocaspian area upstream into the Danube. The first two of them have already reached a eudominant position in various localities. In connection with the expansion of these species, we have recorded the extinction of populations of Barbatula barbatula and Cottus gobio and a decline of Gobio albipinnatus. In regard to the allochtonous origin of these species, further, until recently unknown, negative influences can be recorded in connection with their expansion. 

Possible development anomalies resulting from population isolation

Isolation of the fish population in individual cassettes, from the long-term viewpoint, could support different development anomalies, especially in species showing low abundance. These anomalies can result in a further decline in population size, as they most often manifest as increased mortality, reduced growth rate and problems in reproduction. 

Significance of monitoring the influence of the Gabčíkovo hydraulic structures on fish fauna 

Despite criticism [25, 26, 18], catching using the electroshocker and CPUE method (catch-per-unit-effort) produced results – within the limits of financial sources - which proved the adequacy of this method. If this method is used in just one locality, it enables the recording (in spite of its selectivity) of the majority of species occurring in the Danube, inclusive of those occurring in the stream line at bottom, for example also the large Danube perch (Zingel streber), which was caught at low water level directly in the weir at Dunakiliti [13]. 

Within the scope of monitoring it would be desirable to fish in habitats inhabited by the given fish species instead of fishing in monitoring plots strictly determined for the whole biota, which is probably why some species that may occur in the Old Danube are regularly omitted when catching by electroshocker. Such species prefer other types of habitats than the MP Dobrohošť and Istragov. They migrate in the Danube abandoned riverbed and in the arms according to water temperature and other climatic factors. Therefore three catches carried out in one locality in one year may not necessarily record even commonly occurring fish species. 

It is necessary to monitor endangered fish species over the whole area, i.e. catch in microhabitats characteristic of individual fish species, complementary on both the Slovak and Hungarian sides. In regard to the fact that in the past the reproduction abilities of the floodplain positively influenced not only the adjacent stretch of the Danube, but also its stretches upstream and downstream of the Gabčíkovo hydraulic structures, it would be ideal to monitor the Danube in collaboration with the Hungarian side along the whole stretch between the Čunovo reservoir and Štúrovo. Electoshocker, nets and sport angling should be used for catching. Although each of these methods is somewhat selective, our experience of 2004 [13] has shown that their combination can produce objective results. 

It is regrettable that the intended and proposed investigation of the fish fauna of the Čunovo reservoir by means of sonar technique and net catching has not yet been carried out, in spite of the fact that this reservoir is the least investigated among the water bodies in question. 

If financial means were to be found for a more expensive regular ichthyological investigation, which would have an overall character and would not be bound to just two localities, and would allow the search for individual species according to their typical habitats, more heterogeneous localities could be monitored. In this way, the occurrence of species that we suppose to be very rare (Zingel zingel, Rutilus pigus, Pelecus cultratus, Gobio albipinnatus etc.) would be probably confirmed. 

Proposals to mitigate the negative influence of the Gabčíkovo hydraulic structures on fish 

Creation of new eupotamal

One of the solutions could be also the proposal of Lisický [27], which presupposes the technically supported gradual abandoning of the Old Danube riverbed and the diversion of available discharges through the modified existing arm system on both the Slovak and Hungarian sides to create a new eupotamal. 

Thorough care to ensure respect for legislation and prevention of poaching

I consider the thorough respecting of applicable legal norms to be one of the most important proposed measures for mitigating the negative impacts of the Gabčíkovo project on the Danube fish fauna. Although controls have recently also started to be carried out along the Danube, they are not sufficiently effective because of terrain obstacles. Only if catching by explosives, different nets, including gill nets, or illegal angling is prevented, will it be possible to consider constructing devices to make fish migration possible. Otherwise, the construction of such devices will support illegal catches. 

Reconnecting the abandoned old river bed with arms

The aim of this measure is to make counter-current migrations of fish possible. It presupposes collaboration with the Hungarian side, as it will probably be necessary to increase the water level in the Danube abandoned riverbed that represents the national border. The area of Istragov, not supplied with water from the bypass canal, which has been dried out since the Danube damming, represents a special problem. Although there is a system of intake devices, intake structure, canal and pumping station with tubes for leading water over the dike into the within-dike zone, these devices are not in use. 

If the conditions for fish fauna in the within-dike zone are not changed and the fish cannot migrate between the former main stream and the arms, or within individual arm systems, and if the natural reproduction of fish is not possible (supplying Istragov with water, artificial spawning places), we could expect – in spite of systematic fish stocking – a further decline in the natural production of fish. 

Providing all barriers in the Gabčíkovo hydraulic structures with boulder-passes.

In order to make fish migrations possible, it would be necessary to build up boulder passes between the old riverbed and arms, between individual cassettes in the arms, and at the Čunovo dam. We support Holčík´s proposal [18], to connect the cassettes upstream and downstream from the weirs by boulder bypasses at the weirs separating individual cassettes. If it is not possible to substitute the Old Danube by a new eupotamal [27], we suggest, together with the Hungarian ichthyologists, constructing boulder passes everywhere where there are barriers to fish migration, hence also between the old riverbed and the arm systems. 

The Čunovo weir represents a barrier to migrating fish species. It is necessary to make the counter-current migration possible, especially of lowland fish species of the family Cyprinidae, like Barbus barbus, Chondrostoma nasus and Vimba vimba, which are dominant species on the Danube Slovak stretch. The constructed boulder passes would be also a suitable solution for fish species of the families Acipenseridae, Salmonidae, Coregonidae and Thymallidae. 

Simulated flooding of floodplain at periods of fish spawning

As presented by Holčík [18], two floods of different character occurred in the course of a year in the past: 1. spring flood beginning in February, culminating in March and dying away in April, and 2. a summer flood beginning in May, culminating in July and dying out in September. The migration of fish into the floodplain and arms and their spawning were linked to these floods. Artificial floods carried out to date did not simulate the natural state. 

According to Holčík [18], the flooding of the floodplain should simulate the original state, i.e. the floodplain should be simulated twice a year – in spring and summer. The optimum flood is one that covers the whole within-dike zone. 

Construction of artificial spawning places for phytophilous species in the arm system

As the downstream migration or drifting of fish in the arm system in the within-dike zone is possible at present, a further improvement, above all for the phytophilous component of the fish community, would be to modify the arm system littoral. Bays could be created at certain places of the riverbank line, which are currently mostly on dry. Their banks would be made lower by several decimeters so that these places are on dry and covered by vegetation at the average water level, but permanently submerged at high water levels. 

At times of spawning of the majority of phytophilous species, only moderately above-average discharges in the arms would simulate the period of floods and would create suitable conditions for spawning of the phytophilous species, which were essentially influenced by the Gabčíkovo hydraulic structures construction. 

Updating fish-stocking in collaboration with the Slovak Angler Union and monitoring fish survival

Strong pressure of anglers during the spawning period or concentrated in places where the fish come up against a barrier inhibiting migration can vitally endanger the existence of many fish species. This concerns both mass catching devices as well as sport angling, which is currently on a high level. I believe that the legal norms regulating the possibility of finishing under barriers where fish concentrate should be reconsidered and modified so that they exclude the possibility to essentially influence fish abundance by excessive fishing in such places and wintering places. 

Fish reproduction considerably influenced by all human interventions should be compensated by artificial reproduction. This means breeding and stocking not only of economically important species, but also those species that were most influenced by human interventions. The Slovak Angler Union conducts fish stocking in the Old Danube and the arm system by some fish species, especially predaceous, which are preferred by sport anglers. However, no organisation pays attention to those species which were most affected by the changes in the floodplains, i.e. small economically insignificant species like Gobio kessleri, Gobio uranoscopus, Gobio albipinnatus and Cottus gobio. These species and endangered species should be reared for the Danube floodplain and re-introduced there. The Gabčíkovo hydropower station should provide the financial support. 

Reconsideration of protection status of cormorant in collaboration with Hungary

Some decades ago the nature reserve Ostrov kormoránov (Cormorants´ Island) was founded in the Danube as this species was very rare in this area. Today the cormorants rest calmly at a distance of 20-30 m from the observer. The cormorant flocks are huge. Although one cormorant does not consume more than half a kilogram of fish and is able to gulp only relatively small fish of about 10-15 cm, groups of cormorant expel the fish flights to shallow places or to shores and are able to seriously damage larger fish, though they are unable to gulp them. They wound them with their sharp bill so that they can die. At present the cormorants are so abundant in the Danube around Gabčíkovo that it would be desirable to consider regulating their populations or reconsider the status of its protection. 

Modifying and expanding monitoring of fish communities in collaboration with Hungary

Apart from the hydrological regime, fish fauna is subjected to very dynamic seasonal changes in dependence on food occurrences, reproduction period and wintering. This means that very large differences in structure and abundance of fish community can occur in the same place in the Danube and its arms in each season. 

With joint monitoring we suggest expanding the monitoring on the stretch between Sap and Štúrovo and to unify monitoring methods. An overall monitoring carried out by the point method according to Persat and Copp [28] would be suitable. Using this approach we have obtained more objective data [4, 15] than by the approach used to date. This method should be differentiated for individual fish species. 

We have least data available on fish fauna of the Čunovo reservoir. Once in a 3-5-year period it would be desirable to carry out a survey of the reservoir that could be realized in collaboration of Institute of Zoology of Slovak Academy of Sciences by the working group of Doc. Kubečka from the Hydrobiological Institute of the Academy of Science of the Czech Republic in České Budějovice, which possesses the necessary equipment for such a complex ichthyological investigation. 

Conclusion 

Cutting off the arms, deepening the river bottom in the main stream, a drop in the water level in the Danube and shortened duration of floods, all considerably changed the conditions for development of hydrocoenoses, including fish of the Danube floodplain, already before the Gabčíkovo hydraulic structures were put into operation. These factors gradually limited the natural functions of the originally broad inland delta, the remnants of which still exist between the flood protective dikes. 

The fish fauna in the area was fundamentally changed as the flood plain surface was considerably reduced, the spawning places were limited and the fish migrations were inhibited. Without measures to improve this state, a further decline in the diversity and abundance of fish in the area can be expected. Concrete measures and proposals for improvement of the present state and its more effective monitoring have been presented. 

Some species that were common in the Danube in the past, have been irreversibly disappearing (Cottus gobio, Gobio albipinnatus, Barbatula barbatulus) in recent decades or have possibly already become extinct (Gobio uranoscopus, Gobio kessleri). Irrespective of the immediate causes (Gabčíkovo hydraulic structures, climatic changes, and others) of why the expansion of allochtonous species is eliminating some autochtonous species, the primary cause is human activity. 

In order to preserve the initial high level of biodiversity of the Danube we are obliged not only to state the negative influence of changes caused by human activities, but also to endeavour to correct them. Further efforts to improve the conditions of the unique ecosystem of the inland delta should focus on the question of communication of the main riverbed (new eupotamal) and the river branches. 

References

V.2.8.  Monitoring the vegetation of aquatic habitats influenced by the Gabčíkovo project

Silvia Kubalová

Monitoring aquatic and wetland vegetation represented part of the area influenced by the construction and operation of the Gabčíkovo project since establishment of the permanent monitoring plots (MP) in 1989. Vegetation of water bodies, marshes and the denuded bottom was analysed at 14 of 24 localities in the vegetation periods 1989 and 1990 in order to record the so called zero state (initial state). In addition, the vegetation at 4 further MP was analysed [13]. In the subsequent year, the selection of MP was considerably reduced and aquatic and wetland vegetation was then monitored only at 5 MP [14]. Particular attention was paid to littoral vegetation at 2 MP [20]. The stands were analysed by methods of phytocoenologic sampling in the sense of the Zürich-Montpellier school [1] and data were stored in the DBASE III environment. Monitoring of aquatic macrophytes was stopped in 1992. Problems with vegetation monitoring were shown already at the beginning of the monitoring project. It was not always possible to analyse the whole monitoring plot in complexity at the first inventory of flora in 1990-1991. Only certain types of habitats were analysed (only the forest or only aquatic habitat), according to the availability of specialists. The flora inventory, however, was not repeated in the first years of putting the Gabčíkovo project into operation [18]. 

Monitoring of aquatic macrophytes was renewed as late as in 1999 in accordance with the specification of data exchange on the base of the Agreement [21]. Indirect and sporadic information about aquatic and wetland vegetation from 1992-1998 can be found in some papers [17, 16, 19]. 

Monitoring and monitoring plots 

Since 1999 we have conducted continual monitoring of the time and spatial changes in the vegetation of aquatic habitats. Monitoring Plots are: MP 2B Rusovské ostrovy; MP 3 Čunovo – Ostrovné lúčky; MP 9 Bodíky – Bodícka brána; MP 10 – Kráľovská lúka; MP 14 – Istragov; MP 18 Kľúčovec – Sporná sihoť; MP 23 Číčov – Starý les. At present we can present a complex evaluation of data obtained during seven vegetation periods. In the case of MP 3, 9, 10, 14, 18 and 23, which had been analysed already during the inventory of 1990-1991, we also compare the current state of aquatic and wetland vegetation with the pre-dam conditions. 

After the monitoring of water macrophytes was renewed in 1999, we methodically resumed the earlier approaches – the vegetation was sampled in the sense of the Zürich-Montpellier school [1] and stored in the DBASE III environment. Since 2003 we have been applying another approach when analysing the macrophytic vegetation, namely the Kohler´s method of mapping macrophytes [4]. This change resulted from the fact that this method is also applied in analysis of macrophytes on the Hungarian side. Internationale Arbeitsgemeinschaft Donauforschung – Societas Internationalis Limnologiae (IAD SIL) also applies it when investigating the Danube in Germany, Austria, Hungary and more recently is also applied in Slovakia. The application of a unified method makes comparisons of monitoring results possible. Phytocoenologic evaluation need not necessarily record species occurrence in the whole monitored stretch of the streams or water body, but only individual coenoses. Accurate mapping of vegetation enables the recording of changes in the character of species distribution, which arise from habitat changes, and, in this way, provides important information about the succession course. 

The vegetation development of aquatic habitats of the Danube floodplain is strongly influenced foremost by the dynamics of the water regime, manifested in the fluctuation of water levels during the course of the year, and the character of water streaming and climatic conditions in the given years. Therefore different ecophases – from hydrophasis to terrestrial ecophasis - can alternate in one locality in the course of a vegetation period. This is also reflected by species composition of growth in such habitats. There are also year-on-year differences in the values of abiotic factors, which can influence the vegetation state in given years. That’s why not only the aquatic vegetation, but also the vegetation of marshes and the denuded bottom is sampled. 

The monitoring plots MP 2 (Rusovské ostrovy) and MP 3 (Čunovo – Ostrovné lúčky) are situated in the area of the Čunovo reservoir downstream from Bratislava, where an increase in the ground water level by even several meters was recorded. The plots MP 9 (Bodíky – Bodícka brána), MP 10 (Kráľovská lúka) and MP 14 (Istragov) are situated in the within-dike zone between the bypass canal and the Danube old riverbed, where the influence of the Gabčíkovo hydraulic structures is usually manifested by a moderate decline in the ground water level and changes in the flood regime. The regime of surface and ground waters is seen in a large part of the within-dike zone; hence also these influences can be modified by existing or planned water-management structures. The monitoring plots MP 18 (Kľúčovec – Sporná sihoť) and MP 23 (Číčov – Starý les) are also situated in the within-dike zone, downstream of the confluence of the tailrace canal with the Old Danube, and serve as control reference plots [12]. Changes in the hydrological regime have also reflected in the state of aquatic vegetation. 

Description of monitoring results of aquatic and wetland vegetation 

In the MP 2B (Rusovské ostrovy) we monitor the arm at the periphery of the Dolný rusovský ostrov Island. The initial state of vegetation was not recorded here. In 1989-1991 the former island was bordered by the drying up remnant of the Rusovské rameno arm. Along the Danube margin an artificial arm was hollowed up as part of the technical resolution of polder. Some marches were situated in the central part of the island, but they dried up becouse of groung water level decrease and construction activity [8]. At present the arm around the whole island is deep, with permanent hydroecophasis and discharges especially during floods. Aquatic macrophytes occur in it only sporadically in places with slower streaming or in the bays with standing water. Above all submerged and natant pleustophytes (Ceratophyllum demersum, Lemna minor, Spirodela polyrhiza) were recorded here, i.e. species not rooting in the substrate and therefore easily spreading with the streaming water. Low values of their biomass or their absence in some vegetation period indicate that water streaming during the flood is relatively intensive and populations of individual species can develop only at low discharges, like in 2001. During the whole monitored period (1999-2005) we did not record significant changes in the water level, with fluctuations moving within the range of 10-20 cm, except the flood events. A determining factor for the occurrence of aquatic macrophytes in this plot is water streaming. 

At the MP 3 (Čunovo – Ostrovné lúčky) we monitor the vegetation of the arm situated in its eastern part. Under pre-dam conditions, the arm was dried up and covered by a softwood floodplain forest of the association Salici-Populetum (R. Tx. 1931) Meijer Drees 1936. A vanishing marsh habitat was situated in its lowest part [8]. Monitoring aquatic macrophytes could not be carried out at that time, but a representative investigation plot in the forest stand was regularly monitored every year since 1990. In 1990-1992, the tree layer consisted of Salix fragilis with admixed S. alba, which reached coverage of 65-70%. The shrub layer absented. The herbage layer of nitrophilous species with total coverage of 70-100% was dominated by Urtica dioica, and in the spring aspect also by Galium aparine and Stellaria media. After the Gabčíkovo project was put into operation, the ground water level increased considerably (by 2-4 m) in comparison with the pre-dam state. Since the spring of 1996, stagnant water remained in the willow stand in the former arm bottom. Up to 1995, the tree layer coverage was 70%, but in the course of the following two years it suddenly dropped to 30%. The biggest changes appeared in the herbage layer during the period 1993-1997. Since July 1997, the water level has stabilized at the height of 30-50 cm. Due to this, the original herbage layer of the forest stand vanished and Lemna minor was recorded there for the first time [19]. Hence, since 1999 the monitoring of macrophytic vegetation is also being carried out in this arm. The vegetation developed in 1999-2005 in an aquatic environment and reflected the dynamics of the water regime. The water level moved in the range of 20-50 cm. According to hydrological conditions in the respective year, the water level decline even to below the bottom surface (the bottom is denuded predominantly on the arm margins at the shore, and even sporadically denuded over the whole bottom, like in 2001) or it even exceeded 50 cm, like during the flood waves of the respective year. The surviving individuals of willows are gradually dying out (in 2005 their coverage declined to 25%). In 1999, we recorded the mass occurrence of liverwort Riccia fluitans (coverage 100%) and lemna minor (coverage 100%) on the water surface. Riccia fluitans survived on the denuded bottom in terrestrial form. In the following years, the species richness of macrophytes increased (2 species of hydrophytes in 1999, 8 species in 2005). Without direct human intervention, the community of floodplain forest headed to the initial phase of terestrialization with the occurrence of aquatic communities [7]. Riccia fluitans is a rare and endangered liverwort species, which found optimal conditions for its existence in the arm. Its occurrence is evaluated by Kubalová [6]. As a complement to the inventory of flora and vegetation made before the Gabčíkovo project was put into operation, aquatic macrophytes were also evaluated, namely the growths in the gravel pit situated outside the monitoring plot [13]. We regularly monitored this gravel pit in 1999-2002. The species composition of aquatic macrophytes did not change essentially in comparison with the initial state. Stabilization of the water level due to the Čunovo reservoir and suitable climatic conditions also allowed the regular occurrence of the greater bladderwort Utricularia vulgaris since 1994, which was also recorded by Králik [5]. 

A part of the MP 9 (Bodíky – Bodícka brána) is represented by the stretch of the Šulianske rameno arm, a deep discharging arm, situated at the northern edge of this plot. From time to time the arm was discharging in 1989-1991 and the water level strongly fluctuated. The bottom was sometimes denuded. Aquatic macrophytes occurred only sporadically and with a very low coverage. Denuded sites overgrew with typical vegetation [13, 14, 20]. Since 1993, after the Gabčíkovo project was put into operation, the water level in the arm has considerably increased and has been permanently maintained on a relatively stable level during the whole year. The fast streaming and considerable depth do not create suitable conditions for the occurrence of aquatic macrophytes. We found them only in the littoral, along the banks, where they are protected against drifting. Ceratophyllum demersum, Elodea nuttallii and Myriophyllum spicatum are the most frequent and possess the largest biomass values, being accompanied by some other taxa, especially rhizophytes, i.e. species rooting in the substrate. When compared with the initial state, the species composition completely changed [13. 14, 20]. During flood the permanently discharging arms are among the most exposed sites. Therefore the macrophytic populations in the monitored stretch were almost eliminated, e.g. during the extraordinary flood in August 2002. Only a small growth of M. spicatum has preserved here. However, all earlier recorded taxa re-appeared here in 2004, which results from the fact that the aquatic and wetland communities have adapted to such conditions, and within a short space of time they are able to regenerate even after strong devastation caused by a flood wave. 

In the MP 10 (Kráľovská lúka) we focus on aquatic vegetation in the relic of the Danube arm with the status of a protected territory in the category of natural monuments. It is situated immediately at the foot of the dike. The growths in the arm were monitored already in 1989-1991. Its eastern part was shallower than the western part. Its shores were bordered by reed stand (Phragmites australis), except from the side along the dike. The arm has preserved this character up to the present. Under pre-dam conditions, Nymphaea alba and Trapa conocarpa with a coverage up to 100% dominated in the shallower part. Vegetation in the western part showed a smaller coverage, Nuphar lutea and partly also Ceratophyllum demersum and Potamogeton perfoliatus dominated here [13, 14]. Apart from them, we also recorded other species of water macrophytes were recorded. The monitoring plot was flooded every year [8]. The results of monitoring from 1999-2005 show that species composition has not substantially changed, but more competitive taxa spread here, especially Ceratophyllum demersum, which continuously overgrows the whole water body. In the eastern shallower part it is abundantly accompanied by Nymphaea alba. The occurrence of Nuphar lutea is limited only to a small stable population on the southern shore, while water chestnut (Trapa natans s.l.) occurs only sporadically. The vegetation development is strongly influenced by the regulated water regime. At low water levels, the bottom denudes at the margins around the whole arm, especially in the eastern part. The central part maintains permanent hydroecophasis. The largest decline in the water level was recorded in 2001, when the bottom of the whole shallower part denuded already at the beginning of the vegetation season. Ranunculus sceleratus appeared in the limose ecophasis in the spring aspect, and later Bolboschoenus maritimus and Sparganium erectum appeared. Nymphaea alba filled mosaic-like only depression in which water remained. In the autumn the denuded part was flooded once more, but this had no influence on the marsh plants. In subsequent periods such a decline in the water level did not occur and the growths gradually returned to the state before 2001. This fluctuation can characterise the expected course of succession if the water level decline were to have a more long-term character. 

The occurrence of macrophytes in the MP 14 (Istragov) was not monitored in 1989-1990. At that time the island at the concave bank of the Old Danube represented a plot that was flooded every year. Monocoenoses of Phragmites australis predominated in the island’s most humid parts; growths of littoral vegetation occurred in the moist and denuded depressions [8]. The littoral vegetation was analysed in detail by Zaliberová [20]. Later the results were also published by Hodálová and Zaliberová [2]. Among the hydrophytes an unidentified species of the genus Batrachium was recorded here. The peripheral arm of the island divides into two branches at the northern part of the monitoring plot. At present we are monitoring the southern branch. At present this arm branch is at a considerably advanced stage of terrestrialization, it is rarely flooded and hydrophytes do not occur there. In 1999, the water level reached 50 cm, species composition still indicated frequent flooding and denuding of the bottom (dominant species were Alopecurus aequalis, Galium palustre, Oenanthe aquatica, Rorippa amphibia). In the following years the floods were not so intensive and the water level declined under the substrate surface or maintained just above its surface. Therefore the vegetation in individual years developed in limose to terrestrial ecophasis. Phalaris arundinacea, Agrostis stolonifera and species of the genera Polygonum and Carex were the dominant species. A unique fluctuation of hydrological conditions occurred in 2004, when the water surface once more reached the level of 1999. The long-term trend, however, indicates the gradual drying off of the arms. Several wooden plant species are penetrating into the shrub layer. According to the latest observations from 2005, a large number of wildings of Acer negundo have reached or are reaching the height of shrub layer. 

The MP 18 (Kľúčovec – Sporná sihoť) includes one of the less cut-off branches of the Opátske (Kľúčovské) rameno arm and a small side arm. Already in 1989-1990 this locality was found to be very valuable from the viewpoint of aquatic and wetland vegetation, as a broad scale of communities occurred there. Especially extensive growths with Trapa conocarpa and sporadic occurence of Hippuris vulgaris and Najas minor were of great value. Of extraordinary value was the discovery of several specimens of Schoenoplectus triqueter [3, 13]. Until 1990 this species was taken as a missing taxon in the Slovak territory [3]. The occurrence of the community Scirpetum radicantis (Hejný in Hejný et Husák 1978) is also significant. A detailed analysis of aquatic and wetland vegetation is included in [15]. During the course of regular observations, we have not recorded any particularly substantial changes in the species composition of vegetation of these arms since 1993, except for the more abundant species of Elodea nuttallii, which did not occur here previously. Development of the growths is influenced by water level fluctuations during the vegetation period. These fluctuations directly depend on the water level in the Danube. In some years the fluctuations can be stronger. For example, during the August flood of 2002 the growths were considerably devastated, but populations of all recorded species were gradually restored. On the contrary, the following year the habitats of water vegetations remained almost completely without water during the whole vegetation period. This was the largest water level decline since 1999. During that time Hippuris vulgaris, Oenanthe aquatica, Rorippa amphibia and species of genera Polygonum dominated here. 

The locality of MP 23 (Číčov – Starý les) lies on the Danube stretch not directly influenced by the Gabčíkovo structures. Here we regularly monitor the relic of the river oxbow in an advanced degree of terrestrialization, situated immediately at the foot of the dike. In the course of the first inventory of growths in 1990, the oxbow was without water and the denuded bottom was overgrown with communities bound to corresponding habitats [13]. In 1991 the oxbow was flooded, and so it was not possible to repeat completely the floristic and phytocoenologic survey [14]. Data on the occurrence of hydrophytes are missing for this reason, except for Batrachium circinatum and Ricciocarpos natans, but they are also able to survive in terrestrial form in limose ecophasis. That is why it is not possible to evaluate objectively the changes in species composition and the coenotic structure of vegetation and to compare the results of monitoring with the pre-dam state. In the past and at present the water level in the arm with the same relation directly depends on the water levels in the Danube. The maximum water level reaches 90 cm. Under such conditions many aquatic macrophytes survive there. The most abundant are Potamogeton lucens, P. pectinatus, P. pussillus agg., Salvinia natans, Lemna minor, Spirodela polyrhiza, Batrachium trichophyllum and Nymphoides peltata. The opposite state is the complete denuding of the bottom, which creates conditions for such species as Carex gracilis, Rorippa amphibia, Oenanthe aquatica and Glyceria maxima, which are characteristic dominants of marshy growths. Glyceria maxima also forms a littoral border around the whole arm. In the 1999-2005 period we recorded such a low water level as in 1990 only once (2003), when vegetation on the denuded bottom developed almost during the whole vegetation period (from June to October), similarly to MP 18. Species of the genus Polygonum (especially Polygonum lapathifolium) reaching a height of 150 cm predominated in the growths, with Cyperus fuscus, O. aquatica and R. amphibia also occurring here. Even in May the water retained in the central depression, which was overgrown by one regularly occurring dominant of the hydroecophasis - Potamogeton lucens (this species usually starts to grow as late as summer, but the extraordinarily high temperatures in May overheated the water and accelerated the development by one month). The influence of the 2002 August flood on growths was not too strong. Hydrophytes in this locality usually die out in the autumn. The locality is also significant from the viewpoint of the occurrence of rare and endangered species. 

Summary 

The increased water level in the area of the Čunovo reservoir favourably influenced the occurrence of aquatic macrophytes in the localities, which are not intensively influenced by water streams. The aquatic habitats that vanished in the past are once more being supplied with water. In the within-dike zone the vegetations of aquatic habitats are influenced by the manipulation of water levels, and so they do not depend only on water levels in the Danube as it was in the past. The aquatic macrophytes find optimal conditions for their existence in habitats flooded by stagnant or slowly flowing water over a large part of vegetation period (especially in the summer). Observations at two reference localities situated downstream from the confluence of the tailrace canal with the Old Danube indicate, as supposed, that in this area the habitats of aquatic and wetland plants are not influenced by the Gabčíkovo project. They are developing according to the Danube water levels. Changes in macrophytic vegetation after the Danube damming are not strong and are subject to regular fluctuations of water levels. The fluctuations are characteristic of the Danube floodplain. 

As the habitats of aquatic vegetations on the Slovak side of the Danube are monitored three times a year – in the spring, summer and autumn aspect, and they are not always flooded and overgrown by hydrophytes, it would be desirable from the methodical viewpoint to use, in addition, the traditional sampling according to the Zürich-Montpellier school [1], concurrent to Kohler´s method of mapping the macrophytes [4]. 

The significance of monitoring macrophytes does not lie merely in identifying the impacts of the construction and operation of the Gabčíkovo project, but also in studying the succession of river arms stages at different degrees of development. This can also have practical implications for nature protection in the form of recording the occurrence of rare and endangered species and communities, factors endangering them, recommendations for providing territorial protection and management. What’s more, it is also necessary to monitor invasive species, their spread and behaviour in relation to the autochtonous vegetation. The obtained results can be used in revitalizing activities and as a base for evaluating the impact of such measures on flora and vegetation in the affected area. 

In our climatic region, the recorded taxa and coenoses of aquatic and wetland vegetation are typical of the floodplains of the Danube as a river of lowland character. The exception is Elodea nuttallii, which was introduced from North America by aquarists. At present its spread has an invasive character. 

However, the monitoring has been running for a relatively short time. It cannot therefore record changes of a long-term nature (for example, the influence of the gradual silting of the Čunovo reservoir, climatic changes etc.). The significance of monitoring could increase especially in terms of the interconnection of the arms system with the Old Danube and/or the development of new eupotamal [9, 10, 11]. 

The interconnection of the arms system with the Old Danube seems to be a suitable solution for ensuring a naturally functioning river phenomenon, i.e. the origin, development and disappearance of river meanders, hence also of a naturally functioning river ecosystem. Aquatic macrophytes find optimal conditions foremost in standing and slowly flowing waters; therefore, the restoration of discharge in the arms will result in a considerable decrease in their species diversity in the main riverbed (eupotamal). The new eupotamal is interconnected with revitalization of other types of arms - parapotamal, plesiopotamal and paleopotamal, and various water level fluctuations in the within-dike zone, which supports the natural and mosaic like ecosystem, so typical for the Danube inundation. In this way it is possible to create natural fluctuations of water levels, flow velocities, and thus the diversity of habitat conditions of aquatic and wetland vegetation. 

For better interpretation of the monitoring data I propose the following:

• Prior to realization of proposed revitalization measures, to revise aquatic and wetland vegetation at all existing monitoring plots that are not monitored at present. Further plots for monitoring the influence of applied measures on wetland habitats should be selected during field reconnaissance. At least one plot should be in each current cascade line (A-J).

• More reference plots should be selected outside the directly influenced area, further downstream of the confluence of the tailrace canal with the Danube.

• In selected monitoring plots it would also be desirable to record the chemical and physical parameters of the abiotic environment, which are determining for wetland communities, i.e. surface and ground water level (the use of existing monitoring wells should be satisfactory for this), water temperature, pH, nutrients level.

• It would be wise to ensure long-term and continual monitoring (not only of aquatic and wetland vegetation) in order to obtain a sufficient amount of data for assessment of the influence of applied measures, for proposing further measures, for estimating the impact of climatic changes (years 2002, 2003 winter 2005/6 could be examples of various climatic impacts and response), and for general planning of the implementation and fulfilment of EU Directives related to nature and environmental protection, water quality, and the sustainable use and development of the area. 

References:

 V.2.9.  Flora and forest vegetation in the area of the Gabčíkovo project

Eva Uherčíková

Development of forest vegetation in the monitoring plots 

Monitoring plot 2B Rusovské ostrovy – Rusovce

The community Fraxino–Populetum from the union Ulmenion represents the forest stand, situated on the bank of the Rusovce arm polder. The monitoring plot and soil horizon is well supplied with water and is flooded during the flood discharges in the Danube. 

The total species diversity during the monitored period was relatively high, in spring 20-30 species (in 1985 and 2005, respectively), in summer 12-28 (minimum in 2002, maximum in 2005) with a typical altering of the spring and summer aspect. The tree layer consisted of 5 tree species. Its coverage was stabilized, except for 2004, when some ashes were damaged (broken stems, probably by wind). The shrub layer consists of 3-8 species of trees and shrubs, and its coverage of 7% in spring and 20% in summer is stable. The coverage of the herb layer in spring (95-100%) is higher than in summer (45-95%) due to dying of spring species. There is a considerable fluctuation of species number: in spring minimum 16 (1998), maximum 26 (2003 and 2005), in summer 6 (2002) to 24 (1995 and 2005). The between years similarity expressed by Jaccard´s index [30] fluctuated from 60.0% (1996/1997) to 81.2% (2004/2005) in spring, but the dispersion was large in summer – from 47,6% (2001/2002) to 83.3% (1999/2000). This means that in summer more species alternated than in spring, as a consequence of selective effect of summer floods. 

The dominant life form in the whole monitored period was trees – phanerophytes with a proportion exceeding 30%. Out of hydrophytes there were also represented all other life forms and mutual proportion of geophytes and therophytes fluctuated in dependence on the occurrence of floods in individual years. The ecological conditions of the stand evaluated by Ellenberg´s method [4] were characterized by semiskiophilous to skiophilous, moderately to medium termophilous, moderately oceanic to suboceanic species on the freshly humid to humid neutral soils, rich in soil nitrogen. Values of ecoindices changed during the monitored period only insignificantly (less than 0.5 scale degree). 

Based on monitoring data we evaluate the monitoring plots (MP) as stabile, the changes in the species correspond to natural development of floodplain forest supplied with floodwater from the Danube at higher water levels. 

MP 3A Ostrovné lúčky – Čunovo

In the depression of a former arm, we monitored a remnant softwood floodplain forest of association Salici – Populetum var. with Urtica dioica from union Salicion albae. In the early 1990s, the arm was dry due to a declined ground water level and the soil profile was only rarely moistened by ground water [15]. Therefore stinging nettle, Urtica dioica, a strongly nitrophilous species not tolerating soil water-logging and flooding dominated in the homogeneous stand of willow Salix fragilis [34]. Since spring 1996, after an increase of groundwater level surrounding the Čunovo reservoir, ground water appeared on the ground surface and a fluctuating water level (19-50 cm) is maintained there. This situation was reflected in the parameters of the phytocoenosis as follows:

• In spite of a luxurious supplying of willow with water, the tree layer coverage has decreased from the original 60-70% to the present 25-30%. Individuals died off (the twigs, branches and parts of stems break) and fell out of the stand. This was a manifestation of the intolerance of willow, which grew earlier in other conditions, to the long-term water logging and small aeration of soil horizon [12], as well as a consequence of infestation by wood-destroying fungi and a long-term disturbance of habitat conditions [36].

• The shrub layer still present in 1995-1997 gradually ceased to exist, as the originally dominant elder Sambucus nigra does not tolerate water logging and permanent flooding.

• Presence of water surface conditioned significant changes in herb layer. They were manifested by change in composition of species and life forms and in the overall physiognomy and diversity. The water surface is overgrown by emerged and submersed species. The total species number dropped from an original 25 species (spring 1996) to 8 (summer 1998) and again increased to 22 (summer 2002) and has fluctuated considerably. While among the life forms the hemicryptophytes dominated in 1995-1997, since 1998/1999 the predominant hydrophytes are represented by the dominant Lemna minor and rare bryophyte Riccia fluitans.

• Changes at the species level were also reflected in values of ecological indices: humidity significantly increased toward the humid environment, content of soil nitrogen decreased (representation of nitrophilous species declined), the stand has opened. All these changes were illustrated by very low values of between-year similarity with a minimum of 12.5% in 1995/1996 (indicates a very intensive change of species) and maximum of 52.9% in 2003/2004. In the last few years the situation hasn’t been changing so dynamically. 

After restoration of water level in the former arm, the geobiocoen started to change from a softwood floodplain forest to a wetland-type permanent marsh, marsh with emerged vegetation water-logged during a major part of the vegetation season in the sense of the Ramsar classification [31]. 

MP 4A Topoľové hony – Podunajské Biskupice

We monitored a hardwood floodplain forest of subassociation Ulmo – Quercetum convallarietosum Jurko 1958 at the age of about 85 years. In the frame of the stands monitored it was the driest type [34]. The area had remained without surface floods for a long time, and the ground water level had declined deeply, to 4 m [15], in spite of the fact that after 5-years of operation of the Gabčíkovo project the ground water level downstream from Bratislava had increased by about 100 cm [11]. However, this increase was not reflected in the parameters of the phytocoenosis monitored, as the ground water level does not reach to rhizosphere of herbs and shrubs and the habitat was supplied with rain water only [36]. Over the period 1995-2005, monitoring was not carried out in this MP in the years 1998-2001. 

In the mid 1990s the tree layer coverage was only 15% (in spring) to 30% (in summer) and this trend also continued after monitoring was renewed in 2002. The stand of pedunculate oak and hedge maple tended to crown the opening and disintegration of tree layer. The dominant shrub layer consisted of cornelian cherry Cornus mas and hawthorn Crataegus monogyna. Coverage in spring 1995-1997 was 20% (spring) to 80% (summer) and later increased to 70-95% (max. in 2003 and 2004). 

The shrub layer, characterized by a changing of spring and summer aspects, had coverage of 50-65% in spring, while in summer the coverage declined to 40-45%. This trend also repeated in 2002-2005 as a consequence of string shadowing by layer E3 and E2 and the dying off by spring species (geophytes). Similarly, the total average diversity was higher in spring than in summer- 27.6 / 25.5 species as in the first monitoring period (1995-1997), as in the second period (2002-2005) – 25.3 / 18.0 species. 

The dominant life forms in the stand was hemicryptophytes, with a proportion of 21-31%, nanophanerophytes (shrubs) were subdominant. Representation of geophytes was significant in spring; hygrophilous species and hydrophytes were absent. According to ecological requirements, there occurred semiskiophilous species of dry to freshly humid, neutral soils, poor to medium rich in soil nitrogen. Fluctuations of ecoindex values were insignificant during the period monitored. Relatively high between-year similarity – minimum 60.6% (summer 1996/1997) to maximum 85.2% (spring 2003/2004) indicate relatively stable development in this MP, without larger changes. 

MP 6 Dunajské kriviny - Dobrohošť

The forest community consisted of a stand of white poplars and willows on the margin of a former marsh. It was classified into association Salici-Populetum, variety with Cornus sanguinea, union Salicion albae. After the Danube damming, the dynamics of this MP ecosystem decreased with the absence of floods and a decline of ground water level by approximately 1.5 m (measurements of Dept. of Ecosozology of Institute of Zoology SAS). The plot is situated upstream of the intake structure supplying the arms system with water. In addition the soil profile contains interposed layers of pit-run gravels and the locality is also influenced by the drain effect of the old Danube [35, 37]. 

The tree layer coverage shows a gradually declining trend with certain stabilization features in the last year, however on a very low level – 20% in summer. The crown canopy has been opened, as a consequence of the drying of part of crowns, premature defoliation, and the decreased surface of the foliage. The original number of willow had decreased already in 1995 to 45% [26] and the last individuals died off in the following years. The shrub layer shows an opposite trend of development. Its coverage increased from 50 to 85%, and diversity from 3 species in 1995 to 10 species in 2002. The herb layer coverage is dense, reaching 80-100%, except for the dry summer periods (65% in 1998 and 2001). There ran, however, dynamic changes at the diversity level, the total number of species fluctuates according to aspect. Until 1999 there occured an average of 33 species, after 2000 their number increased to 41. In summer such a trend was not observable. The fluctuation of species occurrence exceeded 50% [35]. Change of species was manifested by low between-year similarity with a minimum of 53.2% in 1997/1998. In the two last years the stand relatively stabilized, showing between-year similarities of 70.7% in summer 2004/2005, or 83.3% in spring 2003/2004. The dominant life form during the whole monitored period was hemicryptophytes, but all other life forms are still present. The changes were manifested in the stand ecological constitution by values of light, humidity and soil nitrogen. The largest significant opening of the stand occurred in 2001, in the following years the situation returned. An increased proportion of nitratophilous species was recorded in spring 2001 and in summer 2004-2005. A trend exists of a general increase in nitrophilous species, requiring soil rich to very rich in soil nitrogen. 

In the monitored period 1995-2004, the forest stand is subjected to dynamic internal changes manifested by fluctuating species diversity, ecologic structure of plant community, and the languishing and gradual dying off of the tree level. 

MP 9A Bodícka brána – Bodíky

The monitored stand of poplar cultivars (Populus x canadensis) was negatively influenced by the draining effect of the nearby old Danube. The simulated flood did not compensate for unfavourable humidity conditions, a declined ground water level, and the absence of flood in 1995 and 1998. In 1995 the stand reached its mature age, in autumn 1997 it was clear-cut and the area afforested again. It was an essential, from the monitoring viewpoint, negative intervention, as after the stand clear-cutting there began other types of succession changes (humidity aspect). Simultaneously with the clear-cutting of tress, the shrub layer was also destroyed, in the herb stratum the expected advent of heliophilous, synanthropic and weed species was confirmed [36]. We recorded changes in all parameters monitored – coverage, species composition and diversity, in decreased between-year similarity of the stand. The cause of this was the dynamic development of the edificatory woody plant. The poplar cultivars, which were again planted here, belong to the most quickly growing woody plants. In connection with an increasing crown canopy, the total shadowing of the undergrowth, competition in the aboveground and underground production space increases. In addition, the site was permanently influenced by a declined ground water level, which caused earlier incidences of humidity deficit in cases of insufficient precipitation. Ecological analysis has shown that in the last vegetation season, four among six ecological factors have strongly changed (index of light, humidity, temperature and continentality). The dynamic changes in species composition of the herb stratum were also illustrated by similarity values – in summer more than one half of all species changed. More weed species and species with a wider ecological amplitude, which established themselves on the clearing after its reforestation and in the first years of existence of the new ligniculture, gradually reduce their coverage and retreat [39]. 

MP 9B Bodícka brána – Bodíky

The willow stand of the union Salicion albae on a sandy deposit, about 100 m from the old Danube is also negatively influenced by its drain effect. In addition it was damaged by the cutting of the neighbouring stand of poplar cultivars on the MP 9A. After 1995, the willow vitality decreased, the tree layer has a declining coverage (from 40-45% to 10-15%), and visually it has disintegrated, the willows now grow only on the plot periphery. The stand opening was manifested by the ecoindex “light” and advent of heliophilous species. The shrub layer, after a temporary decline of coverage and diversity, which was caused by tree cutting on MP 9A, shows an increasing trend of these parameters. The herb layer is characterized by a stable high coverage, predominance of nitrophilous and synathropic species, as well as by the presence of the invasion species Aster lanceolatus. Between-year similarity of the stand showed large fluctuation in late 1990s, when an intensive species change ran. It is also documented by the eco-index humidity: - up to 1998 the proportion of hygrophilous species increase, but since that time their proportion has continuously declined. In the last two years the between-year similarity was high (88-91%). At present, the willow stand bound on the precipitation water shows features of relative stabilization. 

MP 10 Kráľovská lúka

In this MP we monitor two willow stands situated by the side: MP 10A is situated in a moderate terrain depression – it represents a waterlogged type with occurrence of species requiring large humidity and presence of the indicative, rare, protected species summer snowflake (Leucojum aestivum). MP 10B is situated on a relatively elevated part covered by a monoculture of tall-trunk willows with undergrowth of nitrophilous, little hygrophilous species and neophytic Aster lanceolatus, [36]. While in 1995-1996 there were unfavourable humidity conditions and the soil dried in summer, after 1997 the situation improved. Development in both localities is influenced by ground water level, which contacts rhizosphere of herbs and woody plants. We recorded an alternation of drier years (e.g. 2000 and 2001) and humid year with flooding or water logging of the soil (e.g. 1998 and 2003). 

MP 10A

The tree layer consisting only of willow Salix alba did not show significant differences in coverage in the 10-year period: in spring 1995 15%, in summer 25%, in 2005 20%. The shrub layer did not exist in late 1990. It started forming in 1999 – to 2005 its coverage reached 5-10% (spring – summer) and consisted of 3 species. The herb layer significantly reacts to changes in humidity: after a flood its coverage (30-40%) and diversity (6-8 species) declines, while in years without floods and stagnant water in the stand it is dense, with 100% coverage and a higher species diversity (14 in 1995, up to 22 in 2005). 

The dominant life form was hemicryptophytes, geophytes were subdominant (especially the presence of summer snowflake, which have constantly good conditions). This situation persisted during the whole period monitored. The between year similarity shows considerable difference: the lowest between-year similarity in spring aspect was recorded in 2002/2003 (36%), the highest in 2000/2001 (64.5%). The summer floods influence the stand more strongly, destructively, the between year spring similarity is lower then (31.6% in 1996/1997 to 39.3% in 1999/2000). In the last three years the between-year similarity exceeded 50%, with a maximum of 76.9% in 2004/2005 – this indicates a lower change of species. The values of ecological indices also varied significantly, especially the soil humidity and soil nitrogen. Since 1995 the humidity has increased to the humid type, but with a decline in 2000, 2001 and 2005. According to representation of hydrophytes we can classify this stand, in relation to other MP, as the third most humid MP. Rich to very rich occurrence of species characterise high values of soil nitrogen (above 8 degree of Ellenberg scale). At present they have a declining tendency (with exception of 2000 and 2001). These trends, however, must be still statistically verified. 

MP 10B

Development in this stand has a similar character to MP 10A, but with a more moderate impact of floods, as this site is situated higher and the vegetation is less hygrophilous. The dominant species in spring are stinging nettle (Urtica dioica), bedstraw (Galium aparine), and the neophytic Aster lanceolatus. There were also changes in species level ran here; the similarity index in spring moved around 50%, in summer its dispersion was wider, from a minimum of 29.4% (2002/2003) to a maximum of 72.2% (2003/2004). 

MP 14 Istragov – Gabčíkovo

The forest stand was represented by softwood lowland forests classified earlier as the subassociation Salici – Populetum myosotidetosum [13], with the specific ecological condition of regular floods. On its margin it turned into a plantation of poplar cultivars. During the period monitored the ecological conditions worsened. As a consequence of unrealised hydrotechnical measures, the hydrological regime in the area of tailrace canal changed and the ground water level declined in this MP by 1.5-4 m [36]. The monitoring plot was flooded in 1997 (a strong flood in July, up to a height of 1.5-2 m) and the ground water ascended to the surface in 1999. The last time this site was flooded was in 2002 (in March and the beginning of August). In recent years the stand has depended only on rainwater, therefore a humidity deficit occurs in summer. This resulted in the dying off of all willows in the plot. Thus, the edificatory wooden plant disappeared from the stand [38]. The assimilation apparatus of poplar cultivars is also worsened, with a decline of leaf surface and defoliation up to 25%, the tree layer coverage declined to 35%. The development of the shrub layer has an increasing trend – its coverage and diversity moderately increased. Dynamic changes also ran in the herb layer. The floods in 1997 and 1998, and the extreme drought in 2003, had a selective effect on the decline of coverage and diversity. In recent years the herb layer had a high coverage and consisted of 19-26 species. There occurred all life forms, with a predominance of hemicraptophytes (20-36%), although their proportions strongly vary. In values of ecological indices, significant changes occurred in humidity – a gradual decline from 1997. Similarly the light index values signaled the opening of the stand. Due to a higher proportion of sub-continental species, the continentality increased. Levels of soil nitrogen, after a temporary decline in 1997 and 2002, remain on a high level (above 8), which is illustrated by the presence of species characteristic of soils rich to very rich in nitrogen. The large proportion of the nitrophilous species is a feature of ruderalization and the gradual degeneration of the original plant community. An intensive change occurs on the species level, values of between-year similarity strongly fluctuate in the range of 42.8% (1996/1997) and 78.5% (2000/2001). A change of about one third of the species in the between-year comparison probably represents the trend. 

MP 15 Erčed – Palkovičovo

This MP lies in the area of confluence of the tailrace canal and the Danube old channel. It is situated on a moderate bank slope of a shallow depression. We monitored a willow stand of subassociation Salici-Populetum myosotidetosum [13], with occurrence of predominantly original species of this subassotiation, among them indicative, rare and protected summer snowflake (Leucojum aestivum). Ground water level ascends into the soil upper horizons during a major part of the year, floods occur regularly once or twice a year, except for 2003 and 2004, when no flood occurred. Especially unfavourable was the year 2003, with extremely low discharges in the Danube and high temperatures, in which the ground water never ascended on the soils surface, but it even moved down to a depth of 2 m [24]. 

The tree layer consists of only one willow species - Salix fragilis, which showed stable coverage during the period monitored with a maximum in 2000 and 2001 (70%). Since 2003 it has started to open as a consequence of breaking of branching, the shrub layer has not been developed, the result of selection during regular flooding in the past. Coverage of the herb layer reflects the regular occurrence of floods or stagnation of water in the stand. From this viewpoint the occurrence of March flooding is more favourable than a later term (May). In March the soil is sufficiently saturated by water and water does not have such a destructive effect, as the vegetation is not still sufficiently developed. The average species diversity in spring is only by 1 species higher (18) than in summer (17), and in the period monitored it moved in the range of 11-22 species, hence is not too high. Besides the missing nanophanerophytes (shrubs), all other life forms are represented there with hemicryptophytes being dominant. Strong fluctuations were recorded in the proportions of geophytes, terophytes and hydrophytes. Although from the viewpoint of occurrence of hydrophites, this site is the second most humid type of the monitored localities, their proportion shows a declined trend (it must be verified statistically). This trend is also documented by decreasing average values of the ecoindex of soil humidity (even by 0.85 degree of the scale). The stand opening and increase of the soil nitrogen (especially in the summer relevés) is also significant. Values of the between-year similarity vary considerably: the largest change of even 60% species appeared in 2000/2001 when the Jaccard´s index was only 43%. In the last few years the situation stabilized, the species change has not been so intensive. In spite of this, about one third of the species are subject to change. The elimination of original wetland species by the neophyte – Aster lanceolatus (in summer 2005 it dominated here) is striking. 

In spite of relatively more favourable hydrological conditions than in MP Istragov, there were changes of the species level, in the ecological constitution of the stand, and already also a weakening of the edificatory wooden plant Salix fragilis. 

MP 18 Sporná sihoť - Kľúčovec

This monitoring plot belongs among the reference localities (originally represented a site out of influence of the Gabčíkovo project) and the hydrological regime depends here on the Danube water level. It is flooded almost every year, sometimes even twice a year. We monitor a homogeneous, even-aged stand of grey poplar (Populus x canescens) corresponding to the transitional floodplain forests of association Fraxino –Populetum Jurko 1968. A relative low level of ground water characterizes the habitat conditions. The dynamics of soil humidity are relatively independent on the dynamics of ground water level, except for flood periods [29]. 

The tree layer coverage, in a stratified three-layer stand, reaches 40-70% in spring (in 1995 and 2001), in summer it was higher – 70-75% (in 1998-2001). In 2002 it decreased to 50% as a consequence of thinning of the suppressed trees, the diversity decreased by 1 species. Simultaneously the shrub layer was thinned and its coverage decreased from 45% to 25%, while its diversity decreased to one half (4 species). The herb layer was most influenced by floods. In years with an earlier flood (in March), the growth is well developed in May and reaches coverage of 70-80%. If the first flood doesn’t occur until end of April, then the growth coverage in May is only 35% (1999). The summer floods reduced coverage from the maximum of 100% (1995 and 1996) to 40-10% (1997, 2002). Thus the species number strongly fluctuated: in spring 13 (2002) to 26 species (1995), in summer a min. of 17 (1997, 2005) to a max. of 29 species (2003). Among the life forms, the hemicryptophytes dominate. Values of ecological indices significantly changed only in the soil nitrogen, with a maximum of nitrophilous species in 1995, and a decline in 2002. In the last few years the values oscillated, but in general there was a high representation of nitrophilous species. The plot was covered every year by muddy sediments after the floods. As to relation to humidity, the species of fresh humid to humid soils (average value 6,58) predominated with insignificant fluctuations. The habitat conditions, especially soil conditions, do not allow the occurrence of more hygrophilous species, in spite of regular floods. The highest between-year similarity of the stand was 81.8% in 2002/2003, the lowest was 57.1% in 2001/2002, when almost one half of the species changed. In summer, the similarity was relatively balanced through 1995-2000, but since 2001 it strongly fluctuates, with the minimum of 32.5% in 2002/2003. At that time the most intensive change (largest absence) occurred in the sequence of two extreme years – the year 2002 with two floods, then 2003 without floods and with an extremely dry summer. The extremely low discharges in the Danube, together with the precipitation deficit, directly influenced the ground waters in the area between Gabčíkovo and Medveďov [24]. On the contrary, in two last years the highest similarity was reached (78.3%), which indicates a relative stabilizing of species composition. 

MP 23 Starý les - Čičov

This Mp represents the second reference locality lying out of the direct influence of the Gabčíkovo hydraulic structures. Its hydropedological regime is influenced by discharges and the Danube water level. The monitored stand of softwood floodplain forest belongs to sub-association Salici-Populetum phragmito-caricetosum [13] and is situated in the vicinity of the dead arm, at the foot of the protective dike. It is flooded every year and water stagnates in the stand [35]. 

The tree layer consists of 3 wooden plants. At the beginning of the monitored period the tree coverage was 40-65%, which in recent years increased to 75%. The larger coverage is also reflected in the eco-index of light by a shift to more skiophilous species. This development is favourable. The shrub layer consists of rejuvenated individuals of willows and white polar. Its average coverage is continuously only 15%, in spite of a diversity increase to 5-6 species. The dynamics of the hydrological regime is reflected in the dynamics of the herb layer, similarly as in MP 18. The high coverage of 90-100% is usually reduced by floodwater or by water stagnating in the stand. Therefore the diversity fluctuates from 9 (2003) to 19 species (1997, 2000, 2004) in spring, in summer from 11 (1997) to 21 species (2002). As to ecological requirements, the species of humid to moist soils predominate (with values 7.56 to 8.2 of the Ellenberg´s scale). The overall average value of soil nitrogen in this Mp is lower (6.68 mg/l) than in the reference MP 18 (7.10 mg/l); a significant increase was recorded in 2001. Among the life forms, phanerophytes and perennial hemicryptophytes are dominant; the proportion of geophytes is also high, especially due to the presence of summer snowflake (Leucojum aestivum), a rare and protected species. Hygrophilous hydrophytes represent 4-8%. Between-years similarity in spring shows considerable fluctuation from 56% (1995/1996) to 88.2% (2001/2002). In summer the fluctuation is a little smaller, from 57.1% (2001/2002) to 85% (2000/2001). These values indicate changes at the species level, according to the natural rhythm of floods. 

Inventory of flora in monitoring plots 

In the period 1995-2005, the flora was evaluated only in 2002 (Institute of Botany Slovak Academy of Sciences, Bratislava) in 10 monitoring plots [6]. This 2002 state can only be compared with the initial state from the year 1990-1991, before putting the Gabčíkovo project in operation, when flora inventory was made in 15 MPs (Institute of Botany SAS, Bratislava [1,2,9,10,14,25,32,40].). The inventory from the 1990s was not complete in all MPs. Only some habitats were examined (only forest or aquatic), according to the availability of specialists [34]. In the years of operating the Gabčíkovo structures, no inventory was carried out. The aim of the 2002 inventory was to establish the present state of occurrence and endangering of individual species of vascular plants in the whole monitoring plots, as well as noting possible invasions of undesirable taxa. They used the same methods and working procedures as were applied at the previous inventory of the initial state of flora in 1990-1991 [6]. 

General evaluation of changes

In MP Rusovské ostrovy 168 taxa of vascular plants were recorded in 1990-s [9], in 2002 only 116, less by 52 species (30,9%). There were recorded 5 rare or endangered species in 1990, while in 2002 only 1. The number of invasive species remained at the same level – 10 (1990) / 9 (2002). Invasive species spread along paths and forest roads, however they did not penetrate in the monitoring plot interior. From the viewpoint of preserving natural habitats of aquatic and wetland vegetation, it is a valuable locality, from the floristic viewpoint relatively well preserved, which should be protected [6]. 

In MP Ostrovné lúčky two different types of habitats occur: softwood floodplain forest and a xerothermic habitat. In 1990, 113 taxa were recorded, among which 18 are rare and endangered species and 4 are invasive species [19, 7]. In 2202 there were 351 taxa, among which 15 were rare and endangered and 9 invasive species. The “increase” by 238 taxa is caused by the incompleteness of the inventory from 1990s, especially in the xerothermic habitat. However, the number of invasives increased twice. There spread allochtonous invasive species, but also more ruderal and weed species, especially along the roads and paths, they penetrate into disturbed opened microhabitats. In this way they endanger the original floristic composition of the territory. The xerothermic habitat in some places loses its original xerothermic meadow character and obtains a ruderal character [6]. Strong changes have also been observed in succession of trees and shrubs. The territory is exposed to negative impact of recreation. The numerous visitors violate limitation of the nature protection (it is a protected area and protective zone of water resource). 

MP Topoľové hony are a nature reserve which has obtained its status as a set of preserved habitats ranging from willow-poplar floodplain forests to the mezohygrophilous forests, with a transition to the shrub stands and xerotermophilous stands. A large number of taxa has been recorded there, even 408 in 1990 [2], while 329 in 2002. However, whereas 35 rare and endangered taxa were recorded here in 1990, in 2002 occurrence of only 3 taxa was confirmed! Number of invasive species endangering the original species composition remained on the same level [8, 7, respectively, 6]. 

The originally softwood floodplain forests with local transition into the hardwood floodplain forest in MP Dunajské kriviny – Dobrohošť shows the drying of old willows and poplars and a decline of hydrophytic species in a large area. When compared with the state of 1990, there appeared new species, especially in the shrub and herb layer [6]. The total number of taxa of vascular plants is 130, while in 1990 there were recorded 190 taxa. This difference can be explained by a change of species composition and the disappearance of hydrophytes (e. g. Potamogeton, Chara, Eleocharis, Potamogeton, but also of the species Polygonum hydropiper, P. lapathifolium, Zannichelia palustris etc.). 

In MP Bodícka brána 146 taxa were recorded in the 1990s, among which 19 were endangered and rare and 7 were invasive [1]. In 2002 altogether 100 taxa of vascular plants were recorded, among which 2 were endangered and rare and 6 invasive. Floristic structure is poorer at present, but we cannot do unambiguous conclusions from this statement, as the inventory in 1990-s was carried in transects, which were not precisely localized in the map [6]. 

Floristically interesting and valuable parts of MP Kráľovská lúka are the oxbow and the reed stand, with the occurrence of 11 rare and endangered taxa. The total number of taxa is 91, among which 5 taxa are invasive [6]. In 1990, the number of taxa was lower – 85 [25, 32] with the same number of rare and endangered species, the number of invasive species was lower (3). 

In 1990-1991, 275 taxa were recorded in MP Istragov, among which 12 were rare and endangered and 13 invasive. The floristic data from this area have been published [10]. In 2002, by a repeated inventory, only 109 taxa were recorded, among them 8 were rare and endangered and 5 invasive. In this area, communities of softwood floodplain forest degraded in comparison with the years 1987-1992 due to the changed ecological conditions (especially unfavourable hydrological regime) [10]. The Danube arms and their denuded bottoms are relatively preserved and floristically rich. A great part of the Istragov Island is taken by a reed stand, which is in a relatively good state from the floristic viewpoint. A strong synathropisation and penetrating of invasive species affected especially the “drier” forest habitats [6]. 

In MP Erčed, which is covered by softwood floodplain forest and growths of hygrophilous and wetland species on its bottom of the shallow Danube arm, 101 taxa of vascular plants were recorded in 2002. Among them 6 were rare and endangered. The stands are penetrated by invasive species (5), penetration of two of them - Aster lanceolatus and Solidago gigantean is especially intensive. Abundance of some nitrophilous species (Urtica dioica, Galium aparine, Glechoma hederacea) also increased. In 1991, there were recorded 86 taxa. One among them was rare and endangered and 5 were invasive [14]. 

MP Sporná sihoť, with a rich variety of plant communities (ranging from communities of open water surfaces, ephemeral terrestrial communities, through stands of high sedges, reed, shrubs, meadows to the floodplain forest is floristically represented by 105 taxa, among which 6 are rare and endangered and 7 are invasive [40]. In comparison with the initial state of 133 taxa recorded, there were not recorded 4 endangered taxa. On the contrary, there appeared some new species of trees, shrubs and herbs. There is also a striking growth of allochtonous species (especially of the invasive Aster lanceolatus). The natural floodplain forests are damaged by economic activities. The degree of their synathropization is strongly influenced by regular timber exploitation [6]. 

MP Starý les (Čičov) belongs among the very valuable localities of natural habitats, and aquatic and wetland vegetation. In the repeated inventory in 2002, 122 taxa of vascular plants were recorded, among them 9 were rare and endangered, 7 were invasive [6]. When comparing with the initial state from 1991, 2 rare aquatic species were not recorded (however in 2002 the hydro-ecophasis lasted only a short time). On the contrary, there appeared other species in the tree, shrub and herb layer of forest habitats; there are new data on 3 protected and endangered species. 

Conclusion 

Monitoring plots in the upstream part of the territory are strongly influenced by the Čunovo reservoir, resulting especially in ascending ground water levels. After its filling there began a sudden fluctuation of ground water levels, which stabilized in the following years on a higher level than before putting the Gabčíkovo project in operation. This had a favourable influence on the localities Čunovo, Ostrovné lúčky and Rusovské ostrovy. MP 3A Čunovo is characterized by the most dynamic development. The degraded willow stand, with a dominance of nitrophilous and mezophilous species, has changed into the waterlogged type. Dynamic changes of species composition have occurred. The water level has stabilized. However, willows staying in water for a long time gradually die off, infested by wood destroying fungi. The further transformation of a forest community into a wetland community can be expected. The stand in MP Rusovské ostrovy can be considered as generally stabilized. A similarly stabilized situation is in MP 4 Topoľové hony, however its vegetation depends on precipitation water. 

In the central part of the territory – in the stretch between Dobrohošť and Gabčíkovo – the negative impact of a decrease in ground water level due to of the Gabčíkovo hydraulic structures was confirmed, especially in MP Dunajské kriviny and Istragov. The forest stand in Dunajské kriviny was subject to dynamic internal changes in the monitored period 1995-2005. These were manifested on the level of fluctuating diversity, in the ecological requirements of present species, and in the low between-year similarity of the stand, as well as by a languishing and gradual disintegration of the tree layer. 

The hydrological conditions in MP Istragov considerably worsened in the monitored period. In the last few years the stand has depended on precipitation water and a humidity deficit appeared in summer. This caused the gradual dying off of all willows in the monitoring plot. In this way, the edificatory tree has disappeared from the stand, but the state of assimilation apparatus of poplar cultivars has also worsened. 

The monitored stand of poplar cultivars in MP Bodícka brána 9A was clear-cut in 1997, and the plot was reforested. There began a type of succession other than what we had monitored earlier (humidity aspect). The expected advent of heliophilous, synanthropic and weed species was confirmed. We recorded changes in all parameters monitored – coverage, species composition, and diversity, with a decreased between-year similarity of the stand. Six years after clear-cutting, the situation returns to the state from 1990: there has been formed a tree layer, an herb layer develops in mosaic patches, heliophilous, synatropic, and strongly nitrophilous species disappear. The key formation in MP Bodícka brána 9B is the dense and high herb layer, which effectively inhibits the secondary succession of woody plants. High values of between-year similarity of the stand in recent years indicate the stability of this substitute community, which stabilized after the cutting of the original willow stand in the habitat permanently influenced by a declined ground water level. 

While in 1995-1996 the humidity in MP Kráľovská lúka was unfavourable, after 1997 the situation improved. Succession in the two monitored stands is influenced by the water supply from the simulated floods. The ground water level contacts the rhizosphere of trees and herbs. An alternation of drier years and humid years with flood and water logging of soil is characteristic. The development can be considered as stabile, not exceeding the natural fluctuations. A good indicator of adequate habitat conditions is also the vitality of the local population of the rare summer snowflake. 

Although MP Erčed represents the second most humid habitat among the localities monitored from the viewpoint of humidity demanding species (hydrophytes) it shows a declining trend of their representation. In spite of more favourable hydrological conditions than in MP Istragov, the species composition and ecological constitution of the stand changed and the edificatory tree has languished. In 2005 there arose a completely new situation – the adjacent economic stand was clear-cut. This resulted in a reduction of resistance potential of the monitored stand, the increased possibility of penetration of xenocoenous species and weeds, and stress for the original semiskiophilous plants. The opening of the stand will support the development of xenocoenous, heliophilous species. 

MP 18 Sporná sihoť and MP 23 Starý les are reference localities where the humidity regime depends on water levels in the Danube. The stands are flooded every year, in rich water years even twice a vegetation season. The course of monitored parameters reflects this natural development by oscillations in coverage of herb layer, diversity, and the between-year similarity of stands. 

Proposals of changes and completing in monitoring, changes in monitoring areas, localities and monitoring methods 

Monitoring of flora and vegetation in the area of interest has continued in Slovakia since 1990, and the initial state before the putting of the Gabčíkovo project in operation was recorded. The elaborated methods of the botanical part [33] were reviewed by specialists. The original extent, divided in monitoring of macro-, mezo- and microstructure, was appropriately focused in the course of monitoring on the mezzo-structure. For an interpretation of the spring and summer aspects, the basic key is the annual stationary phytocoenological monitoring on permanent plots. It has been confirmed that the selected methods of data collection and evaluation have sufficient informative value. Quantification of plant populations and cartographic evaluation of special relationships has ceased due to different, especially capacity, reasons. It was a mistake that the flora inventory was repeated only after 12 years, instead of the planned 3-5-year periodicity. In the future this part of the monitoring should be carried out regularly in 5-year intervals. Besides this we recommend:

• taking inventory of all types of phytocoenosis in the frame of Mp in 5-year intervals (all forest types, meadows types, fringe, shrub, aquatic and littoral and xerothermic communities). Monitoring of aquatic and wetland vegetation should be done in one-year intervals.

• completing the monitoring localities: in the within-dike zone, Vojka and one locality in the surrounding of Gabčíkovo, between Istragov and Erčed. On the left side of the Gabčíkovo hydraulic structures no plot is monitored (MP5 Hamuliakovo – Horná vrbina was monitored in the early 1990s).

• monitoring the occurrence of synanthropic vegetation and invasive species in the whole area of interest or in each MP,

• renewing the monitoring of vegetation in new artificial habitats in the area of interest (new dikes, plates, seepage canals etc.).

• monitoring vegetation of the Danube old channel, the dynamics of its overgrowing, in regard to possible technical solutions in the future.

• Improving analysis of changes in biological and abiotical data, regularly supply biologists with abiotical data and provide synthetic processing of both data types by modern methods (gradient analysis, ordination techniques etc).

• organizing conferences and seminars for researchers and experts in Slovakia in 5-10-year intervals. 

Significance of monitoring the natural environment for its cognition and recognition of human impacts on the natural environment 

The monitoring of biota as an integral part the monitoring of environment can be taken as a relatively reliable feedback between human society and its environment. It should give information on how the ecosystems work, how they react to disturbances and what is the result of ecosozological measures [16]. It uses bioindicative properties of living organisms for measuring pollution effects and the quality of the environment. Selection of biomonitors (plant and animal species and their features) depends on the monitoring aims. The objects of biota monitoring are the proper living systems, biological communities (individuals, populations, species and their assemblages) and ecosystems. Monitoring of biota is repeated, and regular (systematic) observation of biota in large- or small-size areas is carried out in order to document changes in living systems without respect to a concrete unfavourable environmental factor. This contributes considerably to cognition of long-term ecological processes. It gives necessary information for the maintenance of biota and possible restoration of biota in concrete territories [3]. 

Changes in vegetation have a certain rhythm and run differently, even in very similar phytocoenosis, in dependence on various endo- and exogenous factors. Only a few species in phytocoenosis of a “stable community” have a tendency towards little quantitative variability [8, 28]. A hierarchy of periodical processes and changes exists in vegetation, which are mutually interconnected. All our vascular plants have one-year development periodicity, which is connected first of all with changing temperature, but also with the species genome [7] Changes in species composition run in regular or irregular periods. Knowledge of them is important not only from the theoretical viewpoint, but also for prognosis of development in the future. Possible future changes can be predicted and the recently running changes can be described only on the basis of a large amount of information of different quality about state of vegetation [7]. 

References