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AMPHIBIANS IN SELECTED DANUBIAN FLOODPLAIN HABITATS IN THE GABCIKOVO HYDROELECTRIC POWER STRUCTURES SURROUNDING

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Milan KMINIAK

Department of Landscape Ecology, Faculty of Natural Sciences, Comenius University, Mlynska dolina, 84215 Bratislava, SLOVAKIA

CONCLUSIONS The study of amphibians has evaluated the stand and development of their population in the floodplain of the Danube river (near the Gabcikovo hydroelectric power structures). For the inventarisation survey a method of field monitoring was used during 1991-1994. Eleven species of amphibians were found in the floodplain area (eight monitoring localities). Quantitative and qualitative data and the changes of amphibian habitats are recorded and described.

#### INTRODUCTION

In the last pre-dam period the amphibians were under a strong pressure of changes in the natural as well as urbanised environment. In the surrounding of the Hydroelectric power structure Gabcikovo downstream Bratislava the changes in composition of the amphibians were manifested first of all as a result of changes in the quality of aquatic habitats as a consequence of technical measures and water management in the Danubian floodplain environment.

The aim of the monitoring was the evaluation of quantitative and qualitative changes in the amphibian populations in connection with the changes in alluvium of the river Danube and changes in the groundwater level. We have continued in the research of amphibians performed on the territory of Zitny ostrov island in the past, namely in the surrounding of Bratislava, Samorin and Cicov [1, 3, 4, 6, 7].

#### METHOD

Monitoring research of amphibians was performed at eight localities, during 1991-1994. The selected localities were:

MP 1 - Ostrov Kopac | MP 3 - Cunovo-Ostrovne lucky
MP 6 - Dunajske kriviny (Dobrohost) | MP 9 - Bodicka brana (Bodiky)
MP 10 - Kralovska luka | MP 14 - Istragov
MP 18 - Sporna Sihot | MP 23 - Stary les

The numbering of localities is in accordance with the numbering of biota monitoring sites. The knowledge of the quantitative and qualitative parameters of amphibian populations were obtained by a field research (at each locality several sampling stands were chosen). Eleven amphibian species were found: Triturus vulgaris, Triturus cristatus dobrogicus, Bombina bombina, Salamandra salamandra, Bufo bufo, Pelobates fuscus, Hyla arborea, Rana ridibunda, Rana esculenta, Rana lessonae, Rana arvalis.

For the purpose of documentation of adult, and juvenile exemplars, as well as the development stages of the majority of the observed species material was collected. The determination of the material (adult and larvae) was done using the determination key published in [2, 8, 9].

The abundance wwas estimated by counting of adult and juvenile individuals by the water bodies. The abundance was calculated to a 1 m of sampling line, and the abundance of larvae was calculated to one sweep by a dimensioned net (volume 0.706 m3, length of sweep 2 m) in the sites of their concentration [6]. For the evaluation of the environmental changes the species diversity and abundance dynamics were evaluated regularly (as a sum along the sampling line and the volume of sweep) at individual localities, in four years of the research.

#### EVALUATION OF LOCALITIES

#### Ostrov Kopac

Six sampling stands were situated along the Biskupicke rameno arm in the three typical aquatic habitats (bog, patch of water, gravel-pit). The area was monitored since 1992. The river arm around the locality started to be filled with ground water after putting the Gabcikovo structures and the reservoir into operation. Species are now regularly reproducting here.

We have recorded the occurrence of 10 species of amphibians in this locality. The species diversity varied from 5 to 9 species in individual years (1992-1994) - Fig. 1. The largest number of species was observed in 1992-1993. The average abundance of all species was between 0.2 - 0.35 individuals/m/s (s - sweep), at the minimal and maximal range of 0.1-0.7 individuals per unit. The amphibian taxocoenosis is stable in this locality now.

#### Ostrovne lucky-Cunovo

Four sampling stands in neighbourhood of the village Cunovo with three types of habitats (periodical patches, bog and gravel-pit) were monitored. The research started informatively in 1992. In the second half of 1993, after filling the reservoir with water, the water level in the seepage canal increased as well as in the gravel-pit near Cunovo and in the whole surrounding of the Rusovecke rameno arm in the bogs of the natural alluvial depressions. The reproduction conditions for more species were recovered.

In the locality the occurrence of eight amphibian species was been recorded (Fig. 2). The average number of observed species was 3-8 in the period 1992-1994. The largest number of species was observed in 1992 and 1993 (7-8). The average abundance varried between 0.15 - 0.28 ind/m/s, within the range of 0.01-0.8 per unit. The abundance has an increasing character since 1992. The abundance of dominant species is becoming stable due to the recovery and preservation of the water habitats for the reproduction. But the reproduction will take place only in scattered water bodies. Five from the occurring species are protected and an increase of the species composition is assumed. Namely the species, Salamandra salamandra, Triturus cristatus dobrogicus, Bufo bufo, Bufo viridis and Pelobates fuscus are expected to occur.

#### Dunajske kriviny (Dobrohost)

Three sampling sites were placed in the old dead river arm (in two of its meanders), as well as at places of eroded terrain depression, with periodical occurrence of water.

The occurrence of eight amphibian species was recorded, in the range of 4-8 species in a season (Fig. 3). From the diversity point of view this locality belonged to the most suitable one. In the 1991 there were observed 6-8 species. The average abundance varied from 0.71- 0.79 ind/m/s, at the tange of 0.01-4.2 individuals per unit in 1991 and 1992. The population was stable. In 1994 the mean value of the abundance was ascertained in a substitution locality in the vicinity - 0.56 individuals.

After the diversion of the water to the power canal and after long-lasting drought all three sampling water sites disappeared. During 1993 they were not supplied with water, and this in spite of the fact, that the river arm system was supplied with the water from the by-pass canal. In 1993 the amphibians did not occur anymore at previous sites. They remained only at gradually disappearing periodical patches behind the flood protective dike, and they partially migrated to places (in the direction of the irrigation canal), where the water level rose to the original level after watering of the river arm system. At present the amphibian taxocoenosis has an unstable character at this locality, its habitat has changed prevailingly to a steppe type and it became unsuitable for the reproduction of amphibians.

#### Bodicka brana

There were five sampling sites in this locality. During 1991 and 1992 the surrounding river arm system turned to one with stagnant water. After a drought in 1992 only the deepest water bodies in the old river arm preserved some stagnant water (3 sampling sites). The conditions for the occurrence of amphibians were, however, less influenced by the long-lasting drought, and the amphibians occurred in this locality also in the following years.

We observed the occurrence of six species, in average 2.1-4.0 species in a season, at the range of 1-6 species (Fig. 4). The average values of the abundance varied from 0.21 to 1.26 ind/m/s, at the range of 0.01-7.33 individuals per year. The highest quantity per unit was recorded in 1992. The data in 1991 and 1993 were at the same quantity level, meanwhile in 1994 a decrease was observed. After the increase of the water level and discharge in the river arms (and after the increase of the groundwater level) the taxocoenosis has become stabilised be stable (five species in 1994). There are good conditions for the amphibians in this locality, enabling them to survive also in the case of unsuitable conditions (drought or floods).

#### Kralovska luka

The three sampling sites were situated along the shore of the old river arms and in its bends. The water level in the open river arm enabled a reproduction of all dominant species during the monitoring years.

We recorded the occurrence of maximally five amphibian species (3-5 species during a season). The average occurrence varied from 2.0-2.3 species per one year the range of 1-5 species(, (Fig. 5). It can be seen, that species diversity changed only partially during the four years. The average values of abundance per unit were at the range of 0.4 - 0.78 ind/m/s, at the min.-max. value of 0.01-0.21 individuals. The taxocoenosis was only partially threatened by the drop of the ground water level during the season. In this locality the amphibians completed the metamorphosis each year. The monitoring quantity indicated, that the population of observed dominant species will be preserved in this locality.

#### Istragov

Four sampling sites were situated at the terrain depressions with seepage water and at the side river arm. In spring months the arms are filled by seepage water, and reproduction of the amphibians occurs here. During the research suitable water areas were unequally settled by the amphibians. Consequences of floods in 1991 and the influence of a drought in 1992 were manifested on the species diversity and its abundance. In this way many sampling sites disappeared. More stable conditions remained preserved only in two sites.

In this locality we recorded the occurrence of 6 amphibian species, 1-6 species, at collection time of the year. A partial decrease of the number of species was observed in 1993 and 1994 (Fig. 6). The average abundance varied at the range of 0.15 - 2.81 ind/m/s, at the min.-max. values of 0.01-8.0 individuals per unit in individual years. The abundance is influenced by the accessibility of water habitats for the spring reproduction as well as by rapidity of desiccation of some water bodies, where the amphibians occurred in the spring. The taxocoenosis of amphibians has an unstable character and a scattered distribution in this locality, which is affected by sporadic watering of the river arm system.

#### Sporna Sihot

Two sampling sites and lines were situated in the surrounding of an old river arm, connected to the main flow of the Danube by its lower part. This locality is situated downstream the Hydroelectric power structure Gabcikovo and it is not influenced by their structures and operation. There are regularly scathed meadows near the river arm. Seasonal natural fluctuation of the surface and ground water level, in connection with changes of discharge in the Danube river, are manifested on amphibians only to a minor degree.

There we recorded the occurrence of five species, with a range of 2.2-4 species in individual years, with the min.-max. values of 2-3 species, according to the sampling periods (Fig. 7). The average abundance varied between 0.15 - 0.53 ind/m/s, and the min.-max. values were 0.15-1.5 individuals per unit in individual years. The taxocoenoses are well preserved, and they are not destroyed. It also shows also stable species diversity and constant abundance to. The influence of the technical measures onto the Danube river has not been manifested in this habitat. The river arm system in this locality and its surrounding will remain a suitable biotope for the amphibians also in the future.

#### Stary les - Cicov

The sampling areas are situated outside the inter-dike area, close to the state nature reservation Lyon (in the direction towards the Danube river). In this territory a boggy habitat, regularly filled with ground water is dominant. During the observation of the seasonal changes in 1993 the character of the aquatic habitay changed only a little in dependency on the water level in the Danube river-bed.

We recorded a permanent occurrence of seven amphibian species (Fig. 8). The average species diversity was between 2.7-5 species, at min.-max. values between 2-7 species during a season. In 1993 and 1994 the average abundance varied between 1.6-3.08 ind/m/s, with min.-max. values of 0.01-0.7 individuals per unit. The amphibian taxocoenosis is preserved at this locality, where the inundation character of territory is manifested in the highest degree (more frequent floods and increased sedimentation processes). The boggy territory will enable a good migration of metamorphosed individuals to the surrounding ecosystems also in the future.

#### SUMMARY

There were observed 11 amphibian species in eight monitoring localities and 33 sampling sites, which occurred in four types of aquatic biotopes. In 1991-1994 the seasonal dynamic and number of species was influenced by three main factors: by floods (1991(, by long-lasting drought (1992 and 1993) and by the decrease of the ground water levels in the locality Dunajske kriviny (No. 6). The increase of the groundwater level after putting the Gabcikovo structures into operation is manifested positively in the localities No. 1, 3, and 9. A stepping of environment have occurred in the locality No. 6. A decrease of the water level manifested partially in the localities No. 10 and 14 during the summer drought. Localities No. 18 and 23 were not influenced by technical measures performed in the surrounding of the Danube river. The taxocoenoses of amphibians in the Danubian alluvium exhibit yearly differences in the species diversity, at some stages of the seasonal dynamic and in quantitative parameters (monitored abundance in the localities during the sampling periods).

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