CONCLUSIONS The structure of plankton copepod taxocenoses in the by-passed Danube section and in the adjacent floodplain water bodies has changed since the Gabcikovo hydropower plant was put into operation. The greatest change occurred in the earlier river side arm upstream from the village of Dobrohost which had dried out. Essential changes were observed in the parapotamon-type side arms situated between 1840 and 1820 river km, fed artificially with water from the by-pass canal. The abundance of euplanktonic copepod species had substantially decreased and that of tychoplanktonic (benthic and phytophilous) species had increased. In addition, summer values for zooplankton biomass had significantly decreased. Certain, but less extreme changes were also exhibited in the composition of the copepod taxocenosis of the free-water zone in the Danube main channel (old river bed), and in the plesiopotamon-type side arms situated in the affected section.
In the framework of the monitoring co-ordinated by Slovak Institute of Hydrometeorology I have investigated the species composition, diversity, and dominance of copepods (Crustacea, Copepoda) in the medial and littoral plankton of particular aquatic habitats in the left-hand side of the inland Danube delta downstream from Bratislava, between the villages of Dobrohost and Klucovec (river km 1840-1804). Moreover, in the framework of two other research projects (GAV 331/2 and GAV 1079/1) I also had the opportunity to acquire knowledge about the composition, number, and biomass of copepod taxocenoses and also other zoplankton components from other habitats in the area influenced by the Gabcikovo barrage system. Results of monitoring of copepod taxocenoses collected in the period from 1991 to 1994 are briefly referred. In addition, some important relevant findings on zooplankton obtained in the framework of the two above mentioned projects are given.
In the years 1991 and 1992 (before damming of the Danube at Cunovo) euplanktonic species in copepod taxocenosis, mainly Acanthocyclops robustus and Eudiaptomus gracilis (1991), or A. robustus and Cyclops vicinus (1992) were dominant in the free water. In the years 1993 and 1994 (after damming of the Danube) the relative abundance of tychoplanktonic (benthic and phytophilous) forms, especially the species Nitocra hibernica and not so evidently of Eucyclops serrulatus had increased to such an extent, that the group had reached the same (1993) or higher (1994) shares than euplanktonic species. In individual years we found 8, 6, 7, and 5 species respectively. The mean index of the species diversity increased at the beginning from 0.46 (1991) up to 0.72 (1992) and 1.22 (1993), then it moderately decreased down to 0.84 (1994).
Investigation of the littoral copepod taxocenosis of this locality started only in 1992. In this year, the benthic or littoral species Nitocra hibernica and Eucyclops serrulatus f. typica prevailed modestly, then in the following two years highly. We found 3, 8, and 7 species in the respective three years and the diversity index had been summed to be in average 0.53, 0.86, and 0.71 (means within three samples) respectively.
Gabcikovo - Main stream/Old river bed of the Danube (r. km 1819.5)
Before damming a high prevalence of euplanktonic species, above all Acanthocyclops robustus and Cyclops vicinus (1991), or Eurytemora velox (1992), was characteristic of this profile. After damming their proportion initially fell considerably (to 30 %, 1993), and later reached about the level of tychoplanktonic species represented mainly by benthic harpacticoid Nitocra hibernica. The overall number of species found varied between 2 and 10 and the diversity index fluctuated in the range from 0.03 to 1.17, with the highest values in the years 1991 and 1994.
In the years 1991 and 1992 the copepod taxocenosis of the fre-water zone consisted exclusively of euplanktonic species. Among them, in both year-averages Thermocyclops oithonoides dominated. Numerous other species were also present to a smaller extent, e.g. Eudiaptomus gracilis, Eurytemora velox, Acanthocyclops robustus, and Cyclops vicinus. Though the five euplanktonic species mentioned above were present in the taxocenosis in 1993, the benthic Nitocra hibernica prevailed. The same benthic species dominated in 1994, but the number of euplanktonic species dropped to two. During the first three years 6 copepod species were found in samples, whereas only 4 species were found in the fourth year. The average diversity index varied from 0.72 in 1991, through 1.06 in 1992 and 0.55 in 1993, to 0.34 in 1994. Thus, distinct decrease was recorded after damming of the main stream.
Before damming of the Danube, as in the medial zone, the copepod taxocenosis of littoral plankton consisted exclusively (1992) or almost exclusively (1991) of euplanktonic species with Eurytemora velox dominating. In the years 1993, and 1994, however, benthic Nitocra hibernica, prevailed in all samplings. The overall number of species found during the years of monitoring fluctuated between 4 and 10 and the average diversity index varied from 0.50 to 0.83.
Bodiky - Sulianske rameno (r. km 1830)
The copepod taxocenosis of the free-water zone consisted equally of the same 7 euplanktonic species, without extraneous tychoplanktonic forms during the two years before damming of the Danube. In the year 1993, however, a total of four taxa were found in the free-water zone, and among them two benthic/littoral species prevailed (predominantly Nitocra hibernica), not previously found here. In the three samples of the year 1994, only one adult specimen of Copepoda was found in medial plankton (N. hibernica). The number of copepod species decreased from 7 (1991 and 1992), down to 4 in 1993 and to 1 in 1994. The average diversity index decreased from 0.90 in 1991 and 1.08 in 1992 down to 0.58 in 1993 and 0.00 in 1994.
According to preliminary analyses, the copepod taxocenosis of the littoral plankton of the side arm had a similar composition to that in the Bodicke side arm; therefore we did not regularly monitor it.
Gabcikovo - Istragov side arm (r. km 1815.5)
Before damming of the Danube, euplanktonic species only occured in the free-water zone. Thermocyclops crassus and Eudiaptomus gracilis (1991), or Eurytemora velox and Acanthocyclops robustus (1992) usually dominated. The euplanktonic species also dominated in this side arm in 1993 and 1994. The prevailing taxon, however, changed to Cyclops vicinus. In all the samples together 5, 7, 8 and 8 species were found in respective years. The average diversity index increased from 0.41 in 1991 and 0.68 in 1992 to 1.12 in 1993 and 0.98 in 1994.
It is worth mentioning that euplanktonic species also dominated during all four years in the copepod taxocenosis of littoral plankton. Among these Acanthocyclops robustus, Eudiaptomus gracilis, Eurytemora velox and Thermocyclops oithonoides were the most constant, A. robustus being the most abundant in all four years. During the period of four years the copepod taxocenosis consisted of 6-9 species. The average diversity index was found to be very uniform: 0.87, 0.88, 0.88, and 0.92 in respective years.
This locality is the remainder of an old side arm of the Danube. At the end of 1980s it was nearly terrestrificated. Before damming it used to be flooded during periods of high discharges. After floods a shallow swamp remained which got dried during dry periods. Dynamics and periodicity of abiotic factors of environment caused the alternate occurrence of several ecological groups of copepods and this resulted in considerable species diversity. Since after the damming of the Danube this habitat lost water the following data concern only the years 1991 and 1992.
For some time after inundation of the area with water, generally euplanktonic species such as Cyclops vicinus and Thermocyclops crassus persisted in this side arm. During other periods species typical of the littoral zone or periodically drying pools overgrown by macrophytes, e.g. Eudiaptomus transylvanicus, Acanthocyclops vernalis, Microcyclops bicolor, Megacyclops viridis and Attheyella trispinosa occurred here. This habitat is different from the other regularly monitored habitats, because E. transylvanicus and A. vernalis species were found only in this former side arm. In samples a total of 5 (in 1991) and 7 (in 1992) copepod species were found. In both years a high average index of species diversity was recorded, 0.94 and 1.28 respectively.
It is worth mentioning that in the copepod taxocenosis of the littoral zone covered by macrophytes (in the 1991-1993 period) euplanktonic species predominated, of course not with such high prevalence as in the medial zone. In general, Acanthocyclops robustus had the highest percentage among recorded species, together with some other species, mainly Eurytemora velox or Cyclops vicinus. Littoral species reached higher share only in 1994. Among them, Eucyclops serrulatus or Eucyclops speratus predominated. Total numbers of species of 6, 10, 8, and 11, and average diversity indices of 1.19, 1,20, 0,64, and 1.09 were recorded in the respective years.
Klucovec - Side arm at Sporna sihot (r. km 1804)
From this shallow remnant of an earlier side arm, as in the above mentioned side arm at Dobrohost, we collected mixed samples from medial and littoral zone. It is worth mentioning that, despite the small depth (mostly up to several tens of cm) and submerged macrophytes throughout the whole surface of the arm, in general, euplanktonic species, mostly Cyclops vicinus, Eudiaptomus gracilis, Thermocyclops oithonoides, Th. crassus, or Mesocyclops leuckarti prevailed in the mixed samples. In respective years we recorded 6, 10, 13, and 15 species and a gradually increasing average index of diversity, 0.49, 0.98, 1.02, and 1.39. Fifteen copepod species and an average diversity index of 1.39, recorded in the year 1994, are the maximal values recorded throughout all the monitored localities.
The most significant changes, though, have been recorded in the side arms of parapotamon type (according to the classification of [2]) in the river branch system between Dobrohost and Baka villages. Whereas before damming of the Danube the local copepod taxocenoses of medial zone consisted exclusively or almost exclusively of authentic plankton species, after damming, in 1993 and 1994, euplanktonic species formed only a small proportion. In the zooplankton samples mostly benthic and phytophilous littoral species occurred, but usually only in small numbers. (Tab. 1.B). This is also a sign of the fact that the zooplankton assemblages, typical of these habitats in the previous two years and the preceding decades [4, 5], were practically eliminated from them after damming of the Danube.These findings demonstrate that the substitution of the dynamic mutual interconnection between the main stream of the river and branch systems with a water supply only from the head-race canal has a negative impact on the zooplankton development in the river side arms. As well as these responses, the changed hydrological regime also caused a drop in the summer amounts of zooplankton biomass in the river branches (Vranovsky, unpublished data), which supports the conclusion stated above.
The stage in Istragov side arm, which is located between Gabcikovo and Palkovicovo, i.e. in the part of left-hand side floodplain which is not artificially supplied with water till now, is something different. There pelagophilous species remained dominant in the copepod taxocenosis of medial plankton, although in 1993 and 1994 a small proportion of littoral forms also appeared. Penetration of littoral species into the medial zone is obviously caused by another reason: elimination of surface connection of the side arm with the former main stream and its shallowing.
In the case of the plesiopotamon-type side arms, putting the Gabcikovo river barrage system into operation affected each of the three monitored side arms in a different way (Tab. 1.C):
Tab. 1. Average dominance (percentage) of euplanktonic and tychoplanktonic copepod species in plankton of the medial zone of the inland Danube delta habitats (r. km 1840.5-1804.0).
| A. Eupotamal | ||||||||
|---|---|---|---|---|---|---|---|---|
| Locality | Dobroho | Gabčíkovo | ||||||
| main stream / old river bed | main stream / old river bed | |||||||
| Year | 91 | 92 | 93 | 94 | 91 | 92 | 93 | 94 |
| Euplanktonic species | 68 | 65 | 52 | 41 | 91 | 100 | 30 | 56 |
| Tychoplanktonic species | 32 | 35 | 48 | 59 | 9 | 0 | 70 | 44 |
| B. Parapotamal | ||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Locality | Bodíky | Bodíky | Gabčíkovo | |||||||||
| Bodícke arm | ulianske arm | Istragov arm | ||||||||||
| Year | 91 | 92 | 93 | 94 | 91 | 92 | 93 | 94 | 91 | 92 | 93 | 94 |
| Euplanktonic species | 100 | 100 | 24 | 33 | 100 | 100 | 22 | 0 | 100 | 100 | 86 | 89 |
| Tychoplanktonic species | 0 | 0 | 76 | 67 | 0 | 0 | 78 | 100 | 0 | 0 | 14 | 11 |
| C. Plesiopotamal | ||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Locality | Dobroho | Trstená na Ostrove | Kžúčovec | |||||||||
| Dun. kriviny arm* | Krážovská lúka arm | Sporná siho arm* | ||||||||||
| Year | 91 | 92 | 93 | 94 | 91 | 92 | 93 | 94 | 91 | 92 | 93 | 94 |
| Euplanktonic | 50 | 4 | ** | ** | 100 | 100 | 100 | 99 | 98 | 86 | 83 | 86 |
| Tychoplanktonic species | 50 | 96 | ** | ** | 0 | 0 | 0 | 1 | 2 | 14 | 17 | 14 |