CONCLUSIONS The Danube ecosystems of floodplain forests were affected by serious anthropogenic changes during the last few decades. There is no doubt, that the changes to the ecosystem were occurring even without the impact of the Hydroelectric power structure Gabcikovo. However, the hydroelectric power structure has created a new situation that requires consideration and than a decision made as to how shall the former inundation area look under these new conditions. Should, for numerous purposes, a consensus be found on the demands for the preservation of ecological conditions, it would seem, on the basis of the requirements that follows the available information, that the main problem would be the appointment of the time and duration of flooding the area.
In the frame of the soil monitoring only forest areas with original soil profile structure (spontaneously developed and anthropologically unaffected) were examined. Our objective was the monitoring of soil as an acceptor, a source and a medium of changes in the ecosystem. For this purpose monitoring areas and soil probes were selected and examined. Soil characteristics and methods used for soil analysis were chosen. Some methods and results have been published elsewhere [2 - 7]. This report concentrates on some of the conclusions and surveys concerning the consequences of the ground water level and the inundation regime for the monitored ecosystems.
The key characteristic that was analysed with respect to the potential impact of the Gabcikovo Hydroelectric power structure (GHPS) on the ecosystem is the quantitative humidity of the soil. Since 1990 it has been monitored at approximately 10 day intervals, using the neutron logging method (1, 2 or 3 times monthly except the vegetation period). Since the quantitative humidity is measured in 10 cm intervals to a depth below the ground water level, relatively objective and complex information on the moisture distribution in a vertical profile as well as in a time course can be obtained.
Down to the depth of approximately 0.7 m the localities (6, 9, 14, and 18) have usually a balanced course of quantitative soil humidity in the period from the beginning of the year till May, with a small increase during April (or March). From mid May till the end of August there is a typical decline with a different intensity depending on the hydrophysical soil characteristics and microclimate conditions. In the summer of 1994, for example, in all localities an approximately 30-70% decline was caused by extremely dry and hot weather as compared to the situation at the beginning of May.
During the period September to November quantitative soil moisture increase begins which continues till December. Such a course is typical for arenic fluvisoils, that means soils with relaxed granularity. In other soils the course is less noticeable.
At depths below 0.7 m the course was atypical, when comparing annual courses in individual localities and also between localities. Quantitative soil moisture corresponded sensitively to alterations in the ground water level (especially at depths of 1.5 - 2 m and deeper).
It is important to realize that the course of soil moisture (or substrate humidity) was notably similar in all localities, with no regard to differences in the ground water level regime alterations after opening the Gabcikovo Hydroelectric power structure, and furthermore after the intake structure at Dobrohost was opened, which confirms the importance of flooding the inundation area to the ecosystems of soft floodplain forests. Localities 6 and 14 have significantly lower ground water levels than at original pre-dam conditions, locality 10 is higher and localities 9 and 18 have stabilised ground water levels.
The different development of quantitative soil humidity at depths above 0.7 m and below 0.7 m points to the fact that soil moisture in the monitored areas is affected by various factors or by their different intensity. Since the ground water level is currently migrating deeper, permanently with no contact to layers affecting vegetation, the predominant impact is the climatic conditions and the inundation of the area down to 0.7 m depth, typical for floodplain forests.
Understanding the granularity conditions and existing ground water level dynamics in the monitored localities, we can confirm that even before setting the Hydroelectric power Structure Gabcikovo into operation the ground water level had no significant impact on the genesis of most ecosystems. However, the strongest and typical causes seem to be regular and irregular inundation. Some of the previous analyses have confirmed that even a permanent 1 - 2 m ground water level increase would not mean a significant change for the majority of ecosystems in the former Danube inundation area. On the other hand, the recent absence of inundation is indicative for changes that were found in the monitored biota characteristics. Technical possibilities for inundation of the area exist.
A map and the situation of the monitoring areas is included in the previous article.
Fig. 1. Soil moisture profile, locality Dobrohost
Fig. 2. Soil moisture profile, locality Bodiky
Fig. 3. Soil moisture profile, locality Istragov
Fig. 4. Soil moisture profile, locality Klucovec
