CHANGES IN THE COMPOSITION OF CLADOCERA TAXOCOENOSES AFTER SETTING HYDROELECTRIC POWER STRUCTURES GABCIKOVO INTO OPERATION.

Marta ILLYOVA

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

CONCLUSIONS In the free-water of monitoring profiles in the main Danube stream (Dunajske Kriviny, Gabcikovo), a side arm of parapotamal type (Bodicka Brana), and a side arm of plesiopotamal type (Kralovska luka), dominance of pelagic species had decreased. Before damming of the Danube river and setting the Barrage System at Gabcikovo into operation the pelagic species were dominant. After damming the Danube, littoral species of Cladocera in plankton had begun to prevail. Differences in species spectrum and in dominance cannot be attributed exclusively to the Gabcikovo hydropower structures because of extreme climate circumstances during the year 1993 and 1994. Two-years after damming the Danube does not allow unambiguous conclusions.

INTRODUCTION.

Over four years of monitoring selected habitats of inland Danube delta two profiles of the main stream of Danube river (eupotamal), two river arms of parapotamal types and two river arms of plesiopotamal type had been monitored. There are six monitoring places situated between Dobrohost (rkm 1840) and Klucovec (rkm 1804). The objective of this work was find out how the changes initiated by the operation of Gabcikovo structures affected quantitative and qualitative composition of Cladocera.

MATERIAL AND METHODS.

Samples of Cladocera taxocoenoses were collected during 1991 and 1992 before damming, and in 1993 and 1994 after damming the Danube river in each locality three times a year. The average samples of Cladocera taxocoenoses were taken from the pelagic zone and from littoral zone by several draws of plankton net (60-70 (m) from the bottom up to the water surface. Concentrated samples were fixed by 4% formaldehyde.

Quantitative and qualitative analysis was performed on fixed material. In case of the weaker samples, the dominance was established from whole the sample volume. In case of higher number of Cladocera, individuals of five divided samples were counted in calibrated cell of the Sedwick-Rafter type. Relative values of species dominance (percentage) are given in Fig.1. In Tab. 1 individual six steps 1 to 6 represent the following set ranges of percentage : 0-1, 1-3, 10-20, 20-40, high as 40 respectfully.

The classification of water in the inland delta according to Roux [6] is used. The method was modified for Danube river conditions by Sporka [7].

RESULTS AND DISCUSSION.

Dobrohost (rkm 1840.5) - main stream (eupotamal), old Danube river bed, profile Dunajske Kriviny.

Seven species of Cladocera were observed at this profile during 1994 and 1992. Bosmina longirostris and Daphnia cucullata dominated in this taxocoenoses. During 1993 and 1994 the number of detected species increased up to 13. In the pelagic zone the tychoplanktonic Cladocera Macrothrix hirsuticornis, Alona quadrangularis, Alona affinis, Pleuroxus uncinatus, Disparalona rostrata and Simocephalus vetulus were dominant. Bosmina longirostris almost extinguished (Tab.1). In early spring 1994 explosive increase of the benthic species Macrothrix hirsuticornis (100%) was found and its high dominance (40%) was observed still in June.

These changes may have several reasons. The one of them was the decrease of the Danube main stream water level in 1994. After the change of the hydrologic conditions the trophic conditions probably changed too. Transparency of water increased, amount of periphyton increased too and appropriate conditions for the development of benthic species, trofically belonging to scrapers, had been established.

Another reason might be the time of sampling. Macrothrix hirsuticornis belongs to forms occurring in water temperature up to 10° C [2]. Its possible spring appearance was not recorded.

Gabcikovo (rkm 1819,5) - main stream (eupotamal), old Danube river bed, profile at harbour.

In first two years, in the Gabcikovo profile 11 Cladocera species were observed. Significant changes after damming the Danube (1993-1994) were not found: ten taxa. During first two years Bosmina longirostris and Daphnia cucullata prevailed. In 1993-1994 dominance of Bosmina longirostris decreased by 30% (Fig.1) and proportion of benthic species increased by 16%. In 1993 Alona quadrangula, Alona rectangula, Chydorus sphaericus and in 1994 Macrothrix hirsuticornis (80% in march) were most dominating.

In both monitoring profiles Dobrohost and Gabcikovo interesting gradual decrease of relative occurrence of Bosmina longirostris in the potamoplankton of the Danube was observed. Bosmina longirostris usually occurs in rheopelagial of big river (see i.e. [5]), as a part of plankton of small waters [8], and it may be dominant also in littoral zone [4]. It was dominating in Danube eupotamal in 1970-1980 [9, 11].

Relative high occurrence of this species was monitored at both profiles also in 1991-1992 (Fig.1). In 1993 and 1994 Bosmina longirostris has almost disappeared. Our findings correspond with those of [1], who finds abundance decrease of this species in the Danube main stream at rkm 1669 and explains this extreme reduction in 1993 as the consequence of the damming of the Danube river. The results decreased above were probably influenced also by the climatic conditions. Temperature optimum for the occurrence of Bosmina longirostris is the interval 8-18 C, and its spring and autumn maximum occurrence is separated by summer minimum [2]. The main copulation period is autumn [8]. Because the samples were collected in autumn, its dominance was high. On the other hand, hot and dry summers in 1993 and 1994 could have affected our results.

Arms of parapotamal type

Bodicka Brana (rkm 1830) - flowing arm, before damming an arm of parapotamal type of the IInd range, downstream the 3rd transversal dike.

In this habitat 18 species of Cladocera were found before the damming. In 1993-1994 no significant differences were observed: 16 taxa (Tab.1). In the first two years Bosmina longirostris, Daphnia longispina and Alona affinis prevailed (Tab.1). In 1993 in pelagic zone only one euplanktonic species, Diaphanosoma orghidani org.,had been recorded. Decrease of dominance of limnetic species continued also in 1994. During this year quantitative and qualitative dominance attained tychoplanktonic (littoral) species Disparalona rostrata, Macrothrix laticornis, Chydorus sphaericus, Sida crystallina and Alona rectangula (Tab.1)

Istragov (rkm 1930) - flowing arm, before damming an arm of parapotamal I-St range.

Before setting the hydropower structures into operation (1991-1992) 13 Cladocera species were found in this arm. No significant changes were recorded in the following years (1993-1994): 14 taxa.

During the first two years Daphnia cucullata, Diaphanosoma brachyurum, Moina brachiata, and dominant Bosmina longirostris prevailed (Tab.1). In 1993 pelagic species Daphnia cucullata (25%) and Bosmina longirostris(63%) were dominant.

In summer 1994 the dominance of Bosmina longirostris significantly decreased (2%) and increased of dominance of Daphnia cucullata (50%) in comparison with preceding year. Dominance of Bosmina longirostris increased again in September (94%) with high proportion also in littoral (99%). In 1994 the dominance of tychoplankton species Alona affinis, Alona quadrangularis, Disparalona rostrata, Alonella nana, and Pleuroxus aduncus increased.

During 1993 and 1994 the dominance of Bosmina longirostris decreased the pelagic in both monitored habitats. The decrease was observed either during the whale year (Bodicka Brana) or only during the summer (Istragov). In such side arms Bosmina longirostris dominated in 1970-1980 [11], which corresponds with our observation over 1991 and 1992. These changes in following 2 years (1993,1994) could have been caused by the same reasons as it was with the main stream profiles.

After damming the Danube (1993) we have not recorded any Cladocera in the pelagic zone of the side arm of Bodicka Brana. They were found only in the littoral zone. This fact together with the significant prevalence of tychoplanktonic species in 1994 could be explained by the transformation of fluvio-lacustrine system to permanent fluvial one [12].

Side arms of plesiopotamal types

Kralovska luka (rkm 1825) - mostly un-flowing river arm, before damming the Danube river plesiopotamal of the I-st range, the rest of the through arm of the Baka river branches system.

During 1991-1992 17 species of Cladocera were found in this habitat. In the medial zone species Bosmina longirostris, Diaphanosoma brachyurum, and Chydorus sphaericus dominated. In the littoral zone, inside submersion and natant macrovegetation, phytophilous species occurred: Simocephalus vetulus, Sida crystallina, benthic species Chydorus sphaericus, and species of genus Alona: A. affinis, A. guttata and A. costata.

In 1993 and 1994 15 species of Cladocera was found. In medial zone dominance of Bosmina longirostris and Diaphanosoma brachyurum decreased. In September 1994 Moina micrura dominated among pelagic species (50%).

In summer 1993 and 1994 the dominance of tychoplankton species increased. The compositions of Cladocera species of the medial and littoral zones were almost the same. The higher dominance than 20% had Sida crystallina, Chydorus sphaericus, Diaphanosoma orghidani org, and Scapholeberis mucronata. (Tab.1)

Sporna sihot (rkm 1804) - mostly un-flowing river arm, plesiopotamal of the II-nd range, out of the influence of the Gabcikovo structures.

During 1991-1992 14 species of Cladocera were found. Bosmina longirostris, Chydorus sphaericus, and Scapholeberis mucronata dominated. In 1993-1994 their number increased up to 21. In 1993 significant superiority (100%) attained littoral species: Chydorus sphaericus, Eurycercus lamellatus, and Simocephalus vetulus dominated. They occurred in 1994 too, when also pelagic species Moina micrura and Ceriodaphnia pulchella appeared, despite of well-developed submerged macrovegetation (Tab.1).

The dominance of tychoplankton species of Cladocera in the pelagic zone of the Kralovska luka side arm occurred in 1994 probably as a consequence of the water level decrease. Moreover, annual average of water temperature increased and submersed macrovegetation spread from the littoral to the medial zone of the arm. The consequently pelagic species of Cladocera almost extinguished and in the taxocoenoses of Cladocera prevailed tychoplanktonic, mainly phytophilous, species. Such changes, as a consequence of water level decrease, had been presupposed by Vranovsky [10] for Cicov dead side arm.

REFERENCES

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Tab. 1 List of Cladocera species, their dominance (given in 6 degrees scale 1-6) at selected habitats in years 1991-1994 (1, 2, 3, 4). Dk - Dobrohost (Dunajske Kriviny), Ga - Gabcikovo, Bb - Bodicka Brana, Is - Istragov, Kl - Kralovska luka, Sp - Sporna Sihot. Cladocera found in: L - littoral zone, + - qualitative