THE MONITORING OF MOSQUITOES (DIPTERA, CULICIDAE) IN THE AREA AFFECTED BY THE GABCIKOVO HYDROELECTRIC POWER STRUCTURES

Jozef HALGOS

Institute of Zoology, Faculty of Natural Sciences, Comenius University, Mlynska dolina, B-2, 84215 Bratislava, SLOVAKIA

CONCLUSIONS Monitoring of the impact of the Gabcikovo hydroelectric power structures on mosquitoes (Diptera, Culicidae) in 8 areas and 16 stationary stands during 1990-1994 is presented. The following 9 species of mosquitoes were ascertained: Anopheles maculipennis s.l., Culiseta annulata, Aedes vexans, Aedes sticticus, Aedes dorsalis, Aedes cinereus, Culex pipiens, Culex territans a Culex modestus. Important changes in the quantity as well as the quality of mosquitoes, were observed in connection with the changes in the ground water level. In 1994 we recorded only 4 species of mosquitoes: Anopheles maculipennis s.l., Aedes vexans, Aedes sticticus, Culex territans. The quantity of mosquitoes decreased significantly at all monitored stands near the river Danube upstream of the confluence with the tail race canal. Cunovo reservoir acted positively on the adjacent monitoring localities Kopac Island and Cunovo-Ostrovne lucky.The recorded communities of mosquitoes Anopheles maculipennis s.l. and Culex territans, are, according to our observations, typical for stabilised water levels.

INTRODUCTION

The territory of the Zitny ostrov island, where the Gabcikovo hydroelectric power structures are situated, was for a long time well known for its quantitative and qualitative abundance of mosquitoes. Each year, a calamitous outbreak of some mosquito species, caused by summer floods, would occur. In 1962, Trpis M. [2], wrote about the occurrence of 28 mosquito species in this territory. Many of them have parasitic importance for the transmission of virus infections, for example Tahyna, Calovo or protozoan diseases, such as malaria. Since then, extensive changes and antrophic interference has happened in this territory. Probably the most important interference was the river training and the construction of the river Danube water structures in the pre(dam conditions and putting the Gabcikovo system of structures into operation. This caused extensive changes in flora and fauna, which also resulted in quantitative and qualitative changes in the fauna of mosquitoes. Other negative factors were the dredging of the sand and the gravel, large scale agricultural meliorations, disproportional cutting of the floodplain forests and the commercial forestry. We present the results of the monitoring of mosquitoes (Diptera, Culicidae) at pre(dam conditions and the conditions after the Gabcikovo structures were put into operation. This mosquito family is advantageous, because the mosquitoes are sensitive to all changes in the surface and the ground water regimes.

MATERIAL AND METHODS

The material of the preimaginal stages of mosquitoes, processed during monitoring, was obtained from 1990 to 1994 from 8 monitoring areas and 16 stationary monitoring stands. They have been chosen as they represented all types of stagnant water. Qualitative collections were made by using a plankton net, quantitative by using a wooden frame the size of 25 x 25 x 50 cm. A part of the obtained larvae were prepared as permanent microscopic slides using the phenol dehydratation method and putting them into Canadian balsam. The samples were collected at one month intervals during the vegetation season.

RESULTS

Kopac Island

We have only observed this area since 1993. Here we have chosen for monitoring two stationary stands: a dead meander of the river Danube, and a gravel pit created by gravel excavation. We observed a mosquito community typical for balanced stagnant water, such as the species Anopheles maculipennis s.l. and Culex territans. After filling the Cunovo reservoir in 1992, there was sufficient water in both stands. In spite of the high aridity, we also detected this mosquito community in 1994. The improved situation after the filling of the Hrusov reservoir is documented in Fig. 3. A three-peak development of of mosquitos attacking in an area near Bratislava in 1993, is illustrated, as it was, similarly on 1972 (Fig 1). Also the high aridity in 1994 influenced the mosquito attacks in anthe area near Bratislava (Bratislava-Ovsiste). This is expressed by a single-peak curve dependent on the decreasing of water level in the river Danube, downstream from Bratislava (Fig. 4).

Ostrovne lucky

In this locality, the material of preimaginal stages of mosquitoes was collected since 1993. We have chosen the margin of a small eutrophic lake as the monitoring site. Here we observed a mosquito community consisting of Anopheles maculipennis and Culex territans, which testifies to the stabilisation of this water biotope.

Dunajske kriviny

We have chosen two types of the monitoring stands in this locality. The first one is an open meadow biotope, overgrown by a compact Phragmites communis. During a high water level in the river Danube the stationary monitoring site was flooded by the ground water. The second biotope we have chosen is a reminder of a dead river arm in the flood-plain forest. Up until the Danubian water structures were set into operation, there was sufficient ground water in both biotopes. This was also manifested by the quantity and as well as the structure of the mosquito fauna during the year (Tab. 1). Regular three-peak activity curves, which were normal in the past [2], changed to single-peak curves under the influence of intensive antrophic activities, mainly by a substantial deepening of the main river bed of the river Danube. We refer also to a seasonal dynamic of mosquito attacks in the area near Bratislava in 1972 (Fig. 1) and 1992 (Fig. 2). Before putting the hydropower structures into operation, the monitoring area was characterised by several thousands of mosquitoes, which could be found in an area of 25x25 cm (Tab. 1). The meadow biotope had the richest spectrum of the the calamitous mosquito species Aedes vexans and Aedes sticticus. The quantity of mosquitoes in the forest biotope was significantly lower (only few tens of larvae and pupae). Since 1993 the monitoring area has been dry, and we can notice a transformation of the vegetation requiring a higher humidity to plant species tolerant to less humid habitats. This locality has an increasing trend to a xerophilous character.

Bodicka brana

This monitoring area is characterised by a number of through-flowing as well as stagnant river arms with fluctuating water level. At present the river arms are filled from the Cunovo reservoir, and this is the merit of having a sufficient amount of water in this locality. Because the preimaginal stages of mosquitoes raerly occur in river arms, we have chosen three stationary monitoring stands for the observation of changes. The first two were inundation pits, one of them was in a shaded overgrowth of the flood-plain forest, and the second in forest shrub vegetation with a gravelled bottom. Both are placed only some tens of meters from the main stream of the river Danube. Before setting the Danubian hydropower plant into operation, the most advantageous monitoring stand for the monitoring of mosquitoes seemed to be the monitoring stand No. 2 (an inundation pit in a shrub vegetation), where the water depth was approximately 1 m in 1990-1992. In these years the species spectrum of mosquitoes was significantly different. In 1991 the species of the genus Aedes (Aedes vexans, Ae. sticticus and Ae. cinereus) prevailed. In 1992 we did not find these species and we observed only the occurrence of the species Culex pipiens and in the summer months the species Anopheles maculipennis s.l. and Culex territans (Tab. 1). In the following years the biotopes started to dry up under the influence of a water level decrease in the main stream of the river Danube. At present, both studied biotopes are dry, and covered by shrub vegetation. In 1994 we included into observation an another biotope, a stagnant river arm (stationary stand No. 3(, having arisen from the water supply at the Cunovo reservoir, situated approximately 100 m from the main stream of the river Danube. During the whole year, in spite of, according to our opinion, good conditions for the development of mosquitoes, we did not observe any preimaginal stages of mosquitoes. A probable reason of this can also be a rich fish community.

Kralovska luka

We have chosen the western side of a dead river arm with a compact overgrowth of Phragmites communis, changing into a flood-plain forest, as the stationary monitoring stand. Until 1992 this area was heavily inundated during an increased water level in the river Danube, and especially the part placed in the floodplain forest was a typical breeding site of calamitous mosquito species. In May and June the quantity of larva and pupae in the inundation depressions in the forest approached 10,000 of individuals an the area of 25x25 cm. After the Gabcikovo structures were set into operation, we observed a significant decrease of the water level in 1993, and the calamitous outbreak of the mosquitoes stopped. At present, there are only few individuals of preimaginal stages of mosquitoes per 25x25 cm, which are the species Aedes vexans (7 pieces), Anopheles maculipennis s.l. (3 pieces), and Culex territans (2 pieces). In 1994 the monitoring area improved significantly because of the water supply in the river arms. A stable character of stagnant water in some places, with a typical community of preimaginal stages of the mosquito species Anopheles maculipennis s.l. and Culex territans, was attained.

Istragov

In Istragov we have chosen four monitoring stationary stands; two depressions a few meters deep (monitoring stand 1 and 4); a surface water area in an open space (monitoring stand 2), which was originally a part of a narrow dead river arm; and a periodical shallow water hole (monitoring stand 3). Monitoring site 1 (at the margin of the flood-plain forest) was the highest from the point of the preimaginal stages of mosquitoes. In 1991 and 1992 the quantity of mosquitoes reached 500 individuals per 25x25 cm. The other monitoring stands were relatively poor (5-7 larvae). We observed a calamitous outbreak of mosquitoes in the periodical water hole only once, in 1991. From 1993 a strong degradation of the studied monitoring stands began, when all of them dried up except the inundation pit at the margin of the flood-plain forest (monitoring stand 4). Here we found 5 mosquito species, Anopheles maculipennis s.l. and Culex territans (in 1993 and 1994), and ascertained the occurrence of the species Aedes, Aedes vexans, Ae. sticticus and Ae. dorsalis in the previous years. From 1992 this monitoring stand began to continuously dry up. At present all observed monitoring stands are dry. The monitoring area Istragov shows an unfavourable situation from the point of view of the ground water level. An intensive cutting of the flood-plain forest significantly contributes to this trend.

Sporna sihot

This monitoring area is placed below the tail race canal, and therefore it is not influenced by the Gabcikovo structures. We have chosen two monitoring stationary stands in this area. First is a margin of a dead river arm and the second is an inundation hole flooded by the ground water during an increased water level in the river Danube. On the monitoring stand we observed preimaginal stages of mosquitoes only 3 times, and only in small amounts, approximately 2-3 larvae of the species Aedes vexans, because of a strong fluctuation of the water level. The studied inundation hole is typical for the breeding of calamitous species of mosquitoes. During flooding by the ground water a mass development of calamitous species Aedes vexans and Ae. sticticus took place. During 1990-1994 such a mass development took place from May to June, when the quantity approached 500 to 600 individuals per measured area. The biotope dries up during the summer months. Here the curve of emergence has a single-peak character too, during the year. The research of microzoobenthos confirmed too that this locality is not affected by the Danubian Water Structure [1].

Stary les

This monitoring stand is represented by a dead river arm, in the shape of a small lake, that arose during the flood period of 1965. It is compactly overgrown by water plants, and the water level varies in relation to the water level in the main stream of the river Danube. We only started to study this area in 1993. It is a well-preserved area, with a water regime typical for a Danubian inundation forest. We observed here the occurrence of the species Anopheles maculipennis s.l. and very rarely, the occurrence of the species Culex territans.

SUMMARY

The filling of the Cunovo reservoir had a significant influence in the eight monitored areas. The decreasing ground water level had a negative influence on monitoring areas of Dunajske kriviny and Istragov. We observed partially negative changes also in areas Bodicka brana and Kralovska luka. In these areas the monitoring stands either totally dried up or a reduction of the spectrum of mosquito species (from 6 species to 4) as well as a change in the mosquito communities occurred. In 1994 we ascertained an improvement in the areas of Bodicka brana and Kralovska luka, under the influence of water supply into the Bodiky river branch system. The decreasing quantity as well as quality of mosquito species in the given areas (in 1991-1992 we detected 9 species and a mass occurrence of calamitous species, at present we ascertained only 4 species) testifies to the significant hydrological changes in the monitoring stationary stands. The drying process had a remarkable influence on the disappearance of calamitous mosquito species in the floodplain forest, and mainly on the reduction of their quantity. An improvement can be expected only if the ground water level is increased and extreme anthropic activities are prevented. The cutting of flood-plain forest should be prevented. Up to now well preserved complexes of the flood-plain forest, which is the living place of original animals and presently some rare communities, should be protected.

REFERENCES:

[1] MATIS, D., TIRJAKOVA, E., 1995: Changes in communities of Ciliophora of the Danube river system in 1989-1994. Proceedings of the Results and the experience of monitoring of biota in the territory influenced by the waterworks at Gabcikovo : 94-103

[2] TRPIS, M., 1962: Ökologische Analyse der Stechmückenpopulationen in der Donautiefebene in der Tshechoslovakei. Biologicke prace, Bratislava, 3: 5-129.

Tab. 1 Quantity of preimaginal stages of the more abundant mosquito species during the vegetation period in individual monitoring areas at the sampling places in 1991-1994.
speciesyearI.II.III.IV.V.VI.VII.VIII.
Anopheles
maculipennis Meigen, 1830 s.l.		1991
1992
1993
1994		7
8
-
-		-
-
1
-		-
-
7
3		-
2
5
-		-
-
1
-

37
17

-
3

4
2
Aedes
vexans Meigen, 1830 		1991
1992
1993
1994		8400
14
-
-		38
4
-
-		10000
10500
7
-		500
7
6
130*		230
230
500
420

-
-

-
-

-
-
Aedes
sticticus Meigen, 1838 		1991
1992
1993
1994		2100
2
-
-		14
-
-
-		2300
420
2
-		-
-
4
-		180
180
200
420

-
-

-
-

-
-
Aedes
cinereus Meigen,1818 		1991
1992
1993
1994		-
-
-
-		7
28
-
-		-
-
-
-		37
327
-
-		-
-
-
-

-
-

-
-

-
-
Culex
territans Walker, 1856 		1991
1992
1993
1994		-
-
-
-		-
6
-
-		-
3
3
2		-
7
5
-		-
-
-
-

-
-

-
2

3
-
Culex
pipiens Linnaeus, 1758		1991
1992
1993
1994		-
-
-
-		-
8
-
-		-
-
4
-		-
-
-
-		-
-
-
-

-
-

-
-

-
-
* data was taken only from the fourth monitoring stand, the other water bodies on this monitoring area were dry
Mosquito species in which preimaginal phases were observed only once: Culiseta annulata (Schrank, 1776), Kralovska luka, 1992 - 37 ex., Aedes dorsalis (Meigen, 1830), Istragov, 1991 - 87 ex., Culex modestus (Ficalbi, 1889), Kralovska luka, 1993 - 4 ex.
Explanation:
Locality: I. - Dunajske kriviny, II. - Bodicka brana, III. - Kralovska luka, IV. - Istragov, V. - Sporna sihot, VI. - Stary les, VII. - Kopac Island, VIII. - Cunovo-Ostrovne lucky