## CHANGES IN ENVIRONMENT INDICATED BY MOLLUSCS OBSERVED IN THE BIOMONITORING PROGRAM OF THE HYDROELECTRIC POWER STRUCTURES GABCIKOVO

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Tomas CEJKA

Institute of Zoology and Ecosozology, Slovak Academy of Sciences, Dubravska cesta 9, 842 06 Bratislava, SLOVAKIA

CONCLUSIONS Some species or ecological groups of molluscs are relatively good bio-indicators, thanks to their low mobility, stenotopity and a relatively elastic reaction onto changes in monitoring stand conditions. To observe overland mollusc communities, we observe mainly reactions onto changes of soil moisture, and the resulting changes of the vegetation structure, which are connected also with forest management and miscellaneous hydrotechnical terrain adjustments or building arrangements in context with finishing the Hydroelectric power structures Gabcikovo.

#### INTRODUCTION

Since 1989 we perform monitoring using molluscs as a model group at 10 monitoring plots. Five monitoring plots are in the water level increase territory downstream Bratislava: 1 - Kopac, 2a, b - both Rusovske islands, 3 - Ostrovne lucky and 4 - Topolove hony. Four monitoring plots are in the area at by(pass canal: 6 - Dobrohost, 9 - Bodicka brana, 10 - Kralovska luka, 14 - Istragov. Monitoring plot No. 18, Sporna sihot, is outside the impact of Gabcikovo structures, downstream the confluence of the tail(race canal with the original Danube, at the village Sap.

For interpretation of the changes in environment at the stationary stands we used the Lisicky (1993) method of ecosozological and ecological evaluation of biotopes. We used the following five criteria in the concrete case of monitoring of the Danubian basin:

1. Occurrence and abundance of some demanding pretentious forest species important for bioindication, dependent on forest biotopes with relatively balanced ecological conditions.
2. Occurrence and abundance of bioindicators of the group of hygrophilous forest species.
3. Occurrence and abundance of strong hygrophilous, prevailingly river-bank species. The sum of these three parameters was taken after mathematical conversion as a parameter of the ecological importance of a concrete monitoring site. The result of the conversion is a non-dimensional index.
4. Occurrence and abundance of euryvalent agricolous species.
5. Occurrence and abundance of silviphobous species, or species preferring a thin vegetation cover.

The ratio of the first three groups to the last two groups is used as a parameter characterising the impact of the humidity decrease and the ground water level, clear cutting, and other human activities on molluscs of floodplain biotop.

#### MONITORING RESULTS

If we compare the conditions in malacocoenoses at individual monitoring plots during a five year period of monitoring, we can state that at the monitoring plots downstream Bratislava, in the territory of water level increase, the conditions for high humidity requiring species improved at some places, in coherence with the increase of the ground water level, after filling the Cunovo reservoir with water. This occurred mainly on the lower Rusovecky ostrov island (monitoring plot 2b) and on Ostrovne lucky (monitoring plot 3 - willow overgrow in a terrain depression). On Kopac ostrov island the conditions have stabilised at the level from 1993. We did not observe an increase of hygrophilous species in 1994, but we did not ascertain a continuation of penetration of mesohygrophilous euryvalent species into the community either. The same is valid for the upper Rusovsky ostrov island (monitoring plot 2a). At the monitoring plot Topolove hony the structure of malacocoenoses did not change significantly in the years 1989-1994. The abundance of xenocenous species is slightly increasing in the studied community. Since 1991 we have not ascertained the species Clausilia pumila (C. Pf.), which is an important forest species in the inundation area.

From the territories, which are situated in the inundation area, the most expressive changes occurred at the monitoring plot No. 6 (Dunajske kriviny), there the community is affected by the penetration of species from the near cultural environment, if we do not take the poplar monoculture into account. The monitoring plot is still out of the influence of the Dobrohost intake structure, thus the occurrence of high humidity requiring species, f.ex. Carychium minimum (Müll.), Succinea putris (L.) or Zonitoides nitidus (Müll.), is here restricted, in fact, only to humid terrain depressions, and the neighbourhood of a nearly draught small river arm, which is not yet supplied by water from the additionally built inlet canal. In this monitoring area the trend of penetration of patenticolous species into the malacocoenoses is significant too, which is also a consequence of diluting of tree heads, and the already above mentioned influence of the closely laying agriculture region.

At the monitoring plot Bodicka brana a retreat of demanding forest species, occurring since 1992, can be seen, prevailingly of forest and mesophilous species. In 1994 at the same monitoring plot the abundance of some agricolous species, mainly Cochlicopa lubrica (Müll.), Punctum pygmaeum (Drap.) and Trichia hispida (L.) increased. This state testifies the continuing degradation of the studied area, probably as a result of the application of the drainage effect of the original river bed of the Danube.

The monitoring plot 10 (Kralovska luka) represents a biotope with good conditions for the species dependent on water-logging-soil biotopes. In 1989-1994 the conditions for malacocoenoses did not change, although the structure of the herbal vegetation observes a drying trend of the stand-place in the last two years.

Out of the influence of the water supply structure is, for the present, also the monitoring plot No.14 (Istragov). This is manifested mainly in its central part, where adaptive succession changes occurred after filling the Cunovo reservoir. In the studied territory of the soft flood-plain forest (as. S-P) the conditions in malacocoenoses have not changed up to now.

At the monitoring plot 18 (Sporna sihot), which lies out of the impact of Gabcikovo structures, downstream the confluence of the hydropower station outlet canal with the main Danube, since 1993 we observed the increase of abundance of euryvalent agricolous species [(mainly Punctum pygmaeum (Drap.) and Trichia hispida (L.)] in the community, and a continuous degradation of the original malacocoenoses of the transitive flood-plain forest.

#### REFERENCES:

  [1] Lisicky, M.J., at.al., 1993: Expert group "biota" report on monitoring results of environment influenced by the Gabcikovo Hydropower structures, in 1992 (in Slovak). Ms. depon. in: UZE SAV Bratislava, 116 pp.

  [2] Lisicky, M.J., at.al., 1991: Report on starting point (zero stand( of environment SVD G(N, step Gabcikovo, from the point of biology and landscape ecology (in Slovak). Ms. depon. in: UZE SAV, Bratislava, 156 + 22 pp.

  [3] Matecny, at.al., 1994: Monitoring of environment influenced by construction and operation of Hydropower structures Gabcikovo - expert group "biota" (in Slovak, report for the year 1993). Ms. depon. in: PriFUK, Bratislava, 155 pp.

  [4] Rovny, B., at.al., 1992: Monitoring of environment influenced by construction and operation of Hydropower structures Gabcikovo - expert group "biota" (in Slovak, report for the year 1993). Ms. depon. in: UZE SAV, Bratislava, 64 pp.