Phytoplankton of the River Danube between Bratislava and Visegrád in 1990-1997

Jarmila MAKOVINSKÁ, František HINDÁK

CONCLUSIONS Species diversity of phytoplankton in the River Danube was found to be relatively rich and the Slovak section can be considered one of the best studied sections of the whole river. It was confirmed that water quality in the Slovak section has been improving. Updated lists phytoplankton contain 263 genera, 1063 species and 106 varieties and forms, which represents approximately a half of all genera and about a third of all species and infraspecific taxa ever recorded from Slovakia. The number of species was found to be the highest in diatoms (Bacillariophyceae, 44.54% of all taxa), followed by green algae (Chlorophyceae, 41.24% of all taxa). The remaining groups of blue-green algae and algae represented less than 15% of the taxa recorded. Mean annual numbers of algae cells in phytoplankton of the Danube varied from 4 936 to 28 589 per 1 ml. In the period 1990 – 1997, abundance decreased slightly in Bratislava, Rajka, Medveďov and Komárno, though the differences at the other sampling sites were not so apparent. Concerning the longitudinal profile of the Danube, abundance of phytoplankton increased. Diatoms were the most abundant group in autumn/winter period, whereas green algae predominated in summer, as well as during the periods of lower discharge.

Chlorophyll-a, as an indicator of biomass of phytoplankton, ranged from 1.2 to 104.54 mg.m-3, with the tendency to decrease. This tendency was not so important in the Bratislava-Hrušov section, more significant changes were observed in the Medveďov-Visegrád section. Net primary production of phytoplankton ranged from 0.08 to 25.61 mg.m-2.day-1 O2. Mean primary production ranged from 4.90 mg.m-2.day-1 O2 (Bratislava middle) to 10.67 mg.m-2.day-1 O2 (the Čunovo reservoir, Č6). In 1995, net primary production of phytoplankton peaked in August, whereas in 1996 it peaked in May. Summer increase in discharge resulted in a dramatic decrease in the primary production of phytoplankton.

Based on 8-years lasting investigations of phytoplankton in the Danube, it can be stated that the start of operation of the Gabčíkovo structures, inclusive of the division of discharge into two parts – one entering the by-pass canal, the other entering the old main channel of the Danube, has not impacted the development of phytoplankton significantly. Thus, the main factor affecting the development of phytoplankton in the Slovak/Hungarian section of the Danube can be considered the change in the character of the river from the lotic to the lenitic, inclusive of such factors as concentration of unsolved matters, transparency, current velocity, increasing euphotic zone, concentration of nutrients and other compounds affecting the water quality.

Introduction

The largest European river Danube flows through dense populated countries, with developed industry and intensive agriculture. As a result of human activities, high amounts of nutrients enter the river, supporting thus the undesirable development and growth of aquatic microorganisms. Therefore the problems of eutrophication have remained topical. Although the water quality in the Danube has been improving [43], possible effects of increased eutrophication, especially in association with the Gabčíkovo structures, continue to attract the attention of hydrobiologists and river authorities.

In 1960-s and 1970-s, both quantitative and qualitative aspects of phytoplankton of the Slovak section of the Danube were studied [21, 22, 23, 24, 7, 9, 10, 16,] and the first list of blue-green algae and algae was prepared and later updated (1995). A detailed prognosis of the development of phytoplankton in the prospective Hrušov reservoir (at present the Čunovo reservoir) has also been published [45].

Phytoplankton of the Danube in other countries have also been studied in Bavaria [53, 50], Austria [47, 3, 4, ], Hungary [54, 30, 2, 52, 49, 50, 46]and Bulgaria [5, 6, 28, ]. Preliminary results on phytoplankton of the River Danube from the Bratislava/Visegrád section have also been published [38, 33, 34, 35, 37, 37, 42, 14, 15, 44, 43].

Material and Methods

The development of phytoplankton in the Danube from the Austrian/Slovak boundary to Visegrád (Hungary) was studied in 1990-1997, especially with respect to the Gabčíkovo structures. During this period, two major events occurred: of the Danube damming in 1992, and the construction of the underwater weir in the Danube at Dunakiliti in 1995.

Samples of phytoplankton were taken in Bratislava (river km 1869.2), Rajka (river km 1848.0), Hrušov (river km 1842.0), Medveďov (river km 1806.2), Komárno (river km 1768.0), Szob (river km 1707.0) and Visegrád (river km 1690.0). In 1991, the samples in the section between Bratislava and Szob were taken from January to April, but in the other years from January to December. In 1990-1991, the samples were taken once a month, in 1992-1997 in two-week intervals. Since 1995, phytoplankton in the Čunovo Reservoir was also studied. Sampling sites in this reservoir were situated at the left and right sides of the reservoir at Kalinkovo, as well as the left and right sides at Šamorín. These sampling sites served as a basis for the prognosis of the phytoplankton development in the Čunovo reservoir (Fig. 1).

Results

Species diversity

According to the updated lists, 263 genera, 1063 species and 106 varieties and/or forms have been found in the Slovak section of the Danube (Tab. 1). These represent about a third of all species and infraspecific taxa of blue-green algae and algae of Slovakia [12]. However, this list also includes data collected in the mouth of tributaries of the Danube, side arms of all types, and gravel and sand pits situated in the within-dike area.

Table 1. The number of genera, species, subspecies, varieties and forms of blue-green algae and algae, which have been recorded from the Slovak section of the Danube. 

The richest group in number of species were the diatoms (Bacillariophyceae) representing 44.45% of all taxa, followed by green-algae (Chlorophyceae) with 41.24% of all taxa. The other groups of blue-green algae and algae represented less than 15% of all taxa.

In the first years of observations (1990-1993), Euglenophyta occurred relatively frequently in phytoplankton of the Danube in the sampling sites affected by its tributaries, especially by the rivers Morava, Mosoni Danube, Ipeľ and the Kenyérmezei Patak creek, etc. They were represented by the species indicating an increased organic pollution, i.e. the Euglena species (E. anabaena var. minor, E. caudata, E. clara, E. ehrenbergii, E. intermedia, E. oxyuris, E. smitzii, E. spirogyra, E. texta, E. tripteris), Phacus species (P. triqueter) and Trachelomonas species (T. armata, T. conica, T. lefevrei, T. scabra, T. similis, T. stokesii, T. superba). However, in the next period, these species occurred only very rarely in the Danube.

In 1994, a flos-aquae of blue-green algae occurred at the left bank of the Danube in Bratislava. It originated from the Nové Mlýny reservoirs (the Dyja river, a tributary of the Morava river). It consisted mainly of Aphanizomenon flos-aquae, Microcystis aeruginosa and M. incerta.

In 1995, the diatom Aulacoseira granulata appeared in spiral forms in the Szob and Visegrád. These forms were not recorded in such amounts either in the previous or later period.

In 1996, increased numbers of Pseudanabaena limnetica (filamentous blue-green algae) were recorded in all sampling sites except for the right bank in Bratislava. P. catenata was also very common.

In general, during the eight years of investigation, species indicating a greater pollution (Euglenophyta and Cyanophyta) tended to retreat, whereas species more sensitive to pollution and/or associated with the lotic environment tended to spread.

The species composition of phytoplankton in the Čunovo reservoir did not differ significantly from that in the Danube main channel. Thus, the character of this reservoir (turbulent currents, short period of retention, high content of unsolved matters) probably does not provide preconditions for the development of a characteristic community structure of the phytoplankton.

Abundance of phytoplankton

Mean annual numbers of cells of algae in the phytoplankton of the Danube varied between 4 936 and 28 589 cells per 1 ml (Fig. 2). During our investigations, the mean values of abundance in Bratislava, Rajka, Medveďov and Komárno were slightly decreasing while in other sampling sites no significant differences were recorded. Concerning the longitudinal profile of the Danube, abundance of phytoplankton was increasing. The highest increase was observed in 1995, when the abundance in Visegrád was 2.86-times higher than in Bratislava. Another relatively high increase (2.34-times) was found in 1997. The time series of the abundance values in selected sampling sites and their linear trends are presented in Fig. 3. In general, the numbers of cells of algae tended to decrease. In Bratislava, this tendency was rather weak, but in the other sampling sites along the studied section, the number of cells was decreasing more significantly.

In autumn/winter period, diatoms (Bacillariophyceae) were the most numerous group of phytoplankton, however, in summer (June-August), as well as during the periods of low discharge, green algae (Chlorophyceae) predominated (Fig. 4). Abundance of these two groups reached several tens of thousands of cells per 1 ml, whereas the quantity of most of other groups of phytoplankton was low. Only blue-green algae (Cyanophyceae) and yellow-brown algae (Chrysophyceae) occurred in tens and/or hundreds of cells per 1 ml.

Seasonal changes of phytoplankton abundance were characterized by the spring proliferation of diatoms, especially of the centric species, which represented 60-70 percent of total amount of algae in April and May. The second maximum occurred in July and August, during massive proliferation of green algae. For example, the green algae (Chlorococcales) represented in that period 50-60 percent of total amount of algae. In individual years of our investigation, the phytoplankton abundance varied, especially due to the variable discharge at the time when our samples were taken. In 1992, 1993, 1994 and 1996, course of seasonal changes in abundance and dominance of each group of phytoplankton was similar, though in 1995, the numbers were lower than in the other years. In 1997, the second maximum was shifted to the autumn (Fig. 4).

In 1995-1997, mean abundance of phytoplankton in the Čunovo Reservoir varied from 5 781 to 8 065 cells per 1 ml. In Bratislava (BS), the abundance means at the sampling sites Č3 and Č5 in the Čunovo reservoir were very similar (6527, 6587 and 6523 cells per 1 ml, respectively). However, in the sampling site Č4 the abundance was lower (5781 cells per 1 ml), and in the sampling points Č2 and Č6 higher (7 213 and 8 065 cell per 1 ml, respectively) (Fig. 5).

In general, the mean number of cells during the three-year period varied significantly (from 6 cells per 1 ml at Č6 in October 1996 to 32 100 cells per 1 ml at BS in May 1997). Similarly as in the Danube, diatoms and green algae formed the main portion of phytoplankton in the Čunovo reservoir, whereas the numbers in the other groups did not exceed several tens of cells per 1 ml.

Biomass of phytoplankton

The biomass of phytoplankton was expressed as the level of chlorophyll-a. During our observations, the chlorophyll-a levels ranged from 1.2 to 104.54 mg.m-3 (Fig. 7). In general, the biomass of phytoplankton tended to decline. This trend was much stronger in the section between Medveďov and Visegrád than between Bratislava and Hrušov ( Fig. 7).

In the longitudinal profile of the Danube, mean annual levels of chlorophyll-a increased, ranging from 12.7 mg.m-3 (Bratislava, 1996) to 42.9 mg.m-3 (Komárno, 1993, Visegrád, 1992). The levels recorded in Visegrád were always higher (1.9-times in 1992, 1.7-times in 1993, 1.8-times in 1994, 2.4-times in 1995, 1.9-times in 1996, and 1.6-times in 1997) than in Bratislava in 1992 (Fig. 6).

In the Čunovo reservoir (Č2-Č6), the chlorophyll- a level ranged from 0.21 to 69.38 mg.m-3 . An extremely high level (106.80 mg.m-3 ) was found only once, in the sampling site Č6, in May 1996. In the sampling sites Č2 and Č3, the mean levels of chlorophyll-a were similar to each other (14.02 and 14.56 mg.m-3 , respectively) and also to those from Bratislava (BS). In the latter, the mean biomass of phytoplankton during our observations (1995-1997) reached 12.66 mg .m -3 . Relatively higher mean levels of chlorophyll-a were found in the sampling sites Č4 (17.52 mg.m -3) and Č5 (16.35 mg.m-3 ). The highest level of chlorophyll-a (20.92 mg.m-3 ) was recorded in the sampling site Č6, which corroborated the prognoses. In general, mean levels of chlorophyll- a were slightly decreasing in the Čunovo reservoir during our observations (Fig. 8 ). 

Primary production of phytoplankton

Net primary production of phytoplankton ranged from 0.08 to 25.61 mg.m^-2 .day^-1 O₂. During the observations, it ranged from 4.90 mg.m^-2 .day^-1 O₂ (Bratislava – middle) to 10.67 mg.m^-2 .day^-1 O₂ (the Čunovo reservoir, Č6).

In 1995, the highest net primary production was recorded in August, whereas in 1996 in May. In 1997, when most data were obtained, the highest production for all three-years of investigations was recorded. The summer increase in discharge in the Danube (mid-June to late July) resulted in significant decline of net primary production of phytoplankton.

A comparison of mean values of net primary production of phytoplankton, recorded at each of the sampling sites during individual years, revealed that the lowest net primary production was in 1996 (Fig. 9). In the lower part of the Čunovo reservoir (sampling sites Č5 and Č6), the net primary production of phytoplankton was approximately twice higher than in the Danube in Bratislava). The difference between these two areas was especially large during the last two years (1996 and 1997).

Mean primary production of phytoplankton in the Danube in Bratislava and in the Čunovo reservoir for the whole investigation period was 4.90 mg.m^-2 .day^-1 O₂, and 8.56 mg.m^-2 .day^-1 O₂ respectively.

Discussion

Species diversity of phytoplankton in the Danube is relatively rich. This has already been documented in the first list of blue-green algae and algae from the Slovak section of the Danube during 1926-1981 [16], which includes 191 genera and 590 species and infraspecific taxa. The second list of blue-green algae and algae from the Danube included 150 genera and 405 species found during 1982-1994 [6]. Among these species, 145 species were recorded for the first time in the Danube. Since 1996, several papers on the Danube, containing records of new species for this river, have been published. In total, 218 genera, 693 species, and 62 varieties and forms have been recorded from the Slovak section of the Danube. The complete list of blue-green algae and algae from Slovakia includes 529 genera with 2 507 species and 482 infraspecific taxa [12].

Although no complete list of blue-green algae and algae of the whole Danube from its springs to the mouthing into the Black Sea have been published yet, the Slovak section of the Danube can be considered one of the best studied sections of the whole river. This assumption is also supported by the fact that several new taxa, for example new genera of green algae Neodesmus Hindák, Marvania Hindák, Danubia Hindák, flagellates Bitrichia danubiensis Juriš, Stenocalyx welshii Juriš, Chromulina slovaca Juriš, Chlamydomonas rapa Ettl, green algae Dactylosphaerium jurisii Hindák, Micractinum parvum Hindák, Fottea minuta Hindák, and others have been described on the basis of the material from the Slovak section of the Danube [16, 6].

Euglenophyceae are among those groups of algae in which the largest number of new species have been recorded from the Slovak section of the Danube. Most of these species come from tributaries, especially from the rivers Morava and Ipeľ. These rivers suffer from organic pollution, which provides good conditions for phyto-flagellates. In the first years of our investigations of phytoplankton in the Danube, the samples contained relatively high number of species of Euglenophyceae , however, as the water quality was gradually improving, especially in the Ipeľ river [43], the level of organic substances and the number of these organisms were decreasing. Results of physic-chemical analyses, as well as other data have confirmed that water quality in the Slovak section of the Danube has been improving [43, 38, 39, 1, 55]. Therefore, cumulative abundance and dominance of Euglenophyceae in the phytoplankton of the Danube are expected to decline.

In the past, the Danube was known as a river of diatoms, which predominated both qualitatively and quantitatively in the phytoplankton throughout the year [7, 21, 22, 23, 24]. Nevertheless, such conclusions could have been biased by the methods used in individual studies, i.e. by the fact that material conserved in formaldehyde was used for both qualitative and quantitative analyses. In the samples conserved in this way many species of phytoplankton were not identifiable. However, investigations carried out in vivo revealed that after thee spring period of dominance of diatoms can occur a period of dominance of green algae and/or blue-green algae. In our study, green algae, especially celled algae, predominated during the periods of lower discharge (from June to August, sometimes also in autumn).

The occurrence of the spiral diatom Aulacoseira granulata in Szob and Visegrád is considered interesting. This species was drifted into the Danube from the Váh river mouth in Komárno [43, 12].

In 1996, an increased occurrence of filamentous blue-green algae Pseudoanabaena limnetica and P. catenata was recorded. Filaments of these algae were found in all sampling sites except for the Danube right bank in Bratislava. This suggests that these algae have been drifted from the Nové Mlýny reservoirs (southern Moravia), where a bloom of blue-green algae occurs in mass in summer.

In the examined section of the Danube, no typical littoral zone was formed (perhaps except for the Čunovo reservoir). Therefore, no significant occurrence of tychoplanktonic species of algae was observed. However, the situation in this reservoir has not stabilized yet due to the continuing construction.

Nonetheless, many species of typical compact sessile algae were found in the phytoplankton of the Danube. Many of them, like Melosira varians, Cladophora glomerata, Oscillatoria, Phormidium, Cocconeis, Cymbella, Gomphonema and others, have been drifted downstream by strong currents. This is evident from the construction of their shells, certain type of colony, and/or their ability to attach the substrate, e.g. using slimy disc or stalk [17, 11].

In comparison with left-sided tributaries, the phytoplankton in the Danube the whole section examined was much richer in taxa number. This concerns all groups of blue-green algae and algae, but in diatoms, more taxa were found in the Morava river [17, 11, 12]. A lower number of taxa of blue-green algae and algae has been recorded in the Váh and Ipeľ rivers (384 species of blue-green algae and algae [13] and 200 taxa respectively [56]).

On the other hand, the species composition of phytoplankton from the Danube upstream (Bavaria, Austria) appears very similar to that from the section examined. This also concerns the numbers of algae, which reached several tens of thousands of cells per 1 ml [53, 49, 47]. In 1983-1990, decreasing tendency in the chlorophyll-a level was found in Austrian section of the Danube. Along the section examined, biomass was found to increase by several mg.m-3 from one sampling site to another. A similar tendency was observed in the section between Bratislava and Visegrád during 1990-1997.

In the Hungarian section of the Danube, the numbers of cells increased gradually, being 10- to 15-times higher in Baja (close to Hungarian/Yugoslavian border) than in Rajka, close (downstream) to Bratislava [51]. In contrast, species composition of phytoplankton in the Hungarian section is similar to that in the Slovak section.

Kiss [30] compared the composition of phytoplankton at Dunakiliti before and after the damming and stated that the Gabčíkovo structures were the main cause of all changes in the phytoplankton development. However, our results do not seem to support this statement. Both, the species composition, and the numbers of cells of phytoplankton in the Slovak section of the Danube appear to be very similar to those in the German and Austrian sections of the Danube. More than 20 hydropower stations with reservoirs have been constructed at the Danube upstream. All these structures also affect the development of phytoplankton in the Danube [53, 47, 49]. Moreover, in the Slovak/Hungarian section, the Danube changes its character from lotic to lenitic, which is associated with changes in various factors, such as unsolved substances, transparency, current velocity, increasing euphotic zone, concentration of nitrate and compounds affecting the water quality [43].

To evaluate water quality of the Danube, in collaboration with our Hungarian colleagues, the chlorophyll-a level was used [43]. During 1990-1997, water quality in the whole section examined was classified into the II and III classes (Tab. 2).

Table 2. Classification of water quality in the Danube based on the chlorophyll-a level used during the joint Slovak/Hungarian observation of water quality in the Danube (C_90% is a statistical value; minimum number of measurements for the calculation is 24-25).

In connection with the Čunovo reservoir construction, several prognoses of water quality have been elaborated [32, 20, 29, 19, 45]. Holobradá and Marvan [19] also elaborated a prognosis for the “variant C” of the Čunovo reservoir, which has, eventually, been realised. The authors applied a mathematical model to calculate chlorophyll-a levels which was applicable for various combinations of input data. With regard to a short stay of water in the Čunovo reservoir (1 to 3 days), conditions for the development of phytoplankton are very heterogeneous. The shallow lateral parts of the reservoir provide better conditions for phytoplankton development than the deeper central parts. Therefore, in the lateral parts with the reduced current velocities, a higher biomass of phytoplankton was expected, than in the mainstream. A strong development of zooplankton and epipelic communities was also expected to occur in littoral parts of the reservoir. Indeed, our results have been confirmed that biomass increases in the reservoir lateral parts, especially in the sampling sites Č5 and Č6, though the increase in the chlorophyll- a level was not found to be as dramatic as expected (4 to 8 mg.m -3 in the average). In the littoral zone, stands of algae grew on stones and solid blocks at the reservoir shores, which corroborated the prognosis.

Concerning the chemical composition of Danubian water, the phosphorus level was decreasing [1, 38, 39, 43]. As a result of this, phosphorus has become the limiting factor for the development of biomass of phytoplankton in the Danube, inclusive the Čunovo reservoir. This has been supported by the calculations of ratios of nutrients levels and their critical values in the Čunovo reservoir [48]. 

Conclusions

Species diversity of phytoplankton in the Danube was relatively high. the updated lists include 263 genera, 1 063 species and 106 varieties and forms, which represent approximately a half of all genera and about a third of all species and infraspecific taxa ever recorded in Slovakia [12]. The highest number of species was exhibited by diatoms (Bacillariophyceae, 44.54% of all taxa), followed by green algae (Chlorophyceae, 41.24% of all taxa). The other groups of blue-green algae and algae represented less than 15% of the taxa recorded.

Mean annual numbers of algae cells in phytoplankton of the Danube ranged from 4 936 to 28 589 per 1 ml. In the period 1990 – 1997, abundance decreased slightly in Bratislava, Rajka, Medveďov and Komárno, though the differences at the other sampling sites were not so apparent. Concerning the longitudinal profile of the Danube, abundance of phytoplankton increased. Diatoms were the most abundant group in autumn/winter period, whereas green algae predominated in summer, as well as during the periods of lower discharge.

Chlorophyll-a, as an indicator of biomass of phytoplankton, ranged from 1.2 to 104.54 mg.m^-3 , with the tendency to decrease. This tendency was not so important in the Bratislava-Hrušov section, more significant changes were observed in the Medveďov-Visegrád section.

Net primary production of phytoplankton ranged from 0.08 to 25.61 mg.m^-2 .day^-1 O₂. Mean primary production ranged from 4.90 mg.m ^-2 .day^-1 O₂ (Bratislava middle) to 10.67 mg.m ^-2 .day^-1 O₂ (the Čunovo reservoir, Č6). In 1995, net primary production of phytoplankton peaked in August, whereas in 1996 it peaked in May. Summer increase in discharge resulted in a dramatic decrease in the primary production of phytoplankton.

Based on the 8-year investigations of phytoplankton in the Danube, it can be stated that the start of operation of the Gabčíkovo structures, inclusive of the division of discharge into two parts – one entering the by-pass canal, the other entering the old main channel of the Danube, has not impacted the development of phytoplankton significantly. Thus, the main factor affecting the development of phytoplankton in the Slovak/Hungarian section of the Danube can be considered the change in the character of the river from the lotic to the lenitic, inclusive of such factors as concentration of unsolved matters, transparency, current velocity, increasing euphotic zone, concentration of nutrients and other compounds affecting the water quality.

REFERENCES

[1] Ardó, J. 1995. Vývoj kvality vody Dunaja pred a po prehradení koryta Dunaja v Čunove. Medzinár. konfer. Ekológia Dunaja, Stupava, pp. 140-144.

[2] Dobler, E. & Schmidt, A. 1980. Weitere vergleichende Beiträge zur Kenntnis limnologischer Verhältnisse der Donau und Theiss.Tiscia, Szeged, 25: 45-51.

[3] Dokulil, M. T. & Janauer, G. A. 1990. Nutrient input and trophic status of the "Neue Donau" a high-water control system along the River Danube in Vienna, Austria. Wat. Sci. Tech. 22/5: 137-144.

[4] Dokulil, M. T., Janauer, G. A., Eisl, L. & Skolaut, C. 1988. Phosphoreintrag und Eutrophierung in der Neuen Donau. Perspektiven, Der Aufbau, 9/10: 73-76.

[5] Draganov, S. J. & Stoyneva M. P. 1988. Algal flora of the Danube River and adjoined water basins. I. Composition and distribution of the phytoplankton of the River Danube. Ann. Univ. Sof., 79/2: 38-62.

[6] Draganov, S. J., Stoyneva M. P. & Georgiev, B. 1987. Algoflora bolgarskogo sektora Dunaja i primikajustchih basseinov. pp. 23-38. In: Nedjalkov, S., Kochev, C., Michev, T., Damjanova, A. & Velev, V. (eds) Int. Symp. The role of wetlands in preserving the genetic material, Srebarna, Bulgaria, BAS, Sofia.

[7] Hanzlíková, G. 1973. Dynamik der Veränderungen des Phytoplanktons im tschechoslowakischen Donauabschnitt. Ac. Rer. Natur. Mus. Nat. Slov., Bratislava, 19/1: 57-76.

[6] Hindák, F. 1995. Súpis siníc a rias slovenského úseku Dunaja (1982-1994). Zborník z konf. Výsledky a skúsenosti z monitorovania bioty územia ovplyvneného vodným dielom Gabčíkovo, Bratislava, pp. 207-225.

[9] Hindák, F. & Ďurkovičová, O. 1977a. Das Phytoplankton des Donaunebenarmes bei der Wasserwerkinsel in Bratislava. Biológia, Bratislava, 32: 517-531.

[10] Hindák, F. & Ďurkovičová, O. 1977b. Biologische Analysen des infiltrierten Wassers auf der Wasserwerkinsel Sihoť in Bratislava. Biológia, 32 : 787-803.

[11] Hindák, F. & Hindáková, A. 1997: Druhové zloženie fytoplanktónu slovenského úseku rieky Moravy. Bull.Slov. Bot. Spoločn., Bratislava, 19 : 89-95.

[12] Hindák, F. & Hindáková, A. 1998: Zoznam siníc a rias Slovenska, pp. 11-100.- In: Marhold, K. & Hindák, F. (eds) Zoznam nižších a vyšších rastlín Slovenska. Veda, Bratislava, 688 pp.

[13] Hindák, F., Hindáková, A. Makovinská, J. & Tóthová, L. 1998. Druhové zloženie a biomasa fytoplanktónu rieky Váh. Bull.Slov. Bot. Spoločn., Bratislava, 20 : 7-14.

[14] Hindák, F. & Makovinská, J. 1996. Use of algae for monitoring rivers in Slovakia. In: Whitton, B. A. & Rott, E. (eds) Use of algae for monitoring rivers II. Proceedings of an International Symposium, Innsbruck, Austria, pp. 133-136.

[15] Hindák, F. & Makovinská, J. 1999. Phytoplankton of the Danube River from Bratislava (Slovakia) to Budapest (Hungary). In: Whitton, B. A. & Prygiel, J. (eds) Use of algae for monitoring rivers III. Proceedings of an International Symposium, Douae; v tlači.

[16] Hindák , F. & Záhumenský, L. 1983. Algenverzeichnis im tschechoslowakischen Donauabschnitt. Arch. Hydrobiol. Suppl., Stuttgart, 68/1: 114-133.

[17] Hindáková, A. 1994. Planktic diatoms of the River Morava at Bratislava-Devín, Slovakia. Ekológia, Bratislava, Suppl.1, pp. 37-42.

[18] Hindáková, A. 1995. Planktónové rozsievky stojatých a tečúcich vôd v intraviláne Bratislavy a v rieke Dunaj. Doktorská dizertačná práca, Bratislava, pp. 1-123.

[19] Holobradá, M. & Marvan, P. 1993. Prognosis of the phytoplankton development in the Hrušov Reservoir. Arch. Hydrobiol. Beih., 40: 57-67.

[20] Holubec, M., Kelnárová, Z., Lindtner, J. & Holobradá, M. 1991. Prognóza kvality podzemných vôd po uvedení zdrže Hrušov - variant "C" do prevádzky. Záv. správa VÚVH, Bratislava, 66 pp.

[21] Juriš, Š. 1971. Príspevok k poznaniu fytoplanktónu československej časti Dunaja. Ac. Rer. Natur. Mus. Nat. Slov., Bratislava, 17 /2: 1-16.

[22] Juriš, Š. 1972a. Príspevok k poznaniu fytoplanktónu československého úseku Dunaja a ústia hlavných prítokov z nášho územia. Ac. Rer. Natur. Mus. Nat. Slov., Bratislava, 18/1: 19-27.

[23] Juriš, Š. 1972b. K poznaniu fytoplanktónu Dunaja v riečnom km 1819,5. Ac. Rer. Natur. Mus. Nat. Slov., Bratislava, 18/ 2: 3-13.

[24] Juriš, Š. 1974. Niektoré poznatky o zmenách kvantity fytoplanktónu Dunaja. Ac. Rer. Natur. Mus. Nat. Slov., Bratislava, 19/2: 103-110.

[25] Juriš, Š. 1977. Príspevok k poznaniu rias Slovenska I. Ac. Rer. Natur. Mus. Nat. Slov., Bratislava, 23: 91-102.

[26] Juriš, Š. 1979a. Príspevok k poznaniu rias Slovenska II. Ac. Rer. Natur. Mus. Nat. Slov., Bratislava, 25: 65-72.

[27] Juriš, Š. 1980. Príspevok k poznaniu rias Slovenska III. Ac. Rer. Natur. Mus. Nat. Slov., Bratislava, 26: 3-13.

[28] Kalchev, R. K., Kurejschevich, A.V. & Saiz, D. 1996. Photosynthetische Pigmente und Biomasse des Phytoplanktons in der bulgarischen Donaustrecke - Verteilung und Zusammenhänge. 31. Konf. IAD, Baja - Ungarn, 1996, Wissenschaftliche Referate, pp. 137-142.

[29] Kelnárová, Z., Holobradá, M. & Marvan, P. 1992. Prognóza kvality vody v zdrži Hrušov. Zbor. konf.” Hodnotenie rôznych variantov projektu Vodného diela Gabčíkovo”, VÚVH, Bratislava, 15 pp.

[30] Kiss, K. T. 1997. The main results of phytoplankton studies on the River Danube and its side arm system at the Szigetköz area during the nineties. 32.Konf. IAD, Wien - Österreich, 1997, Wissenschaftliche Referate, pp. 153-158.

[31] Kiss, K.T. & Genkal, S. I. 1996. Phytoplankton of the Danube´s reservoirs in September 1995 from Germany to Hungary. 31.Konf. IAD, Baja - Ungarn, 1996, Wissenschaftliche Referate, pp 143-148.

[32] Lehotský, J. & Rothschein, J. 1987. Vplyv Vodného diela na kvalitu vody. Záv. správa VÚVH, Bratislava, 76 pp.

[33] Makovinská, J. 1994a. Primárna produkcia fytoplanktónu dvoch ramien v inundačnom území Dunaja. Zborník X. Limnolog. konf., Stará Turá, pp. 130-134.

[34] Makovinská, J. 1994b. Die Primärproduktion des Phytoplanktons der Donau. 30. Arbeitstagung der IAD, Zuoz, Schweiz, Wissensch. Kurzreferate, pp. 120-122.

[35] Makovinská, J. 1994c. Planktonic green algae of the River Danube from Bratislava Slovakia) to Szob (Hungary). Biologia, Bratislava, 49: 539-545.

[36] Makovinská, J. 1995. Primárna produkcia fytoplanktónu Dunaja, dvoch ramien inundačného územia a zdrže Hrušov. Zborník z konf. Výsledky a skúsenosti z monitorovania bioty územia ovplyvneného vodným dielom Gabčíkovo, Bratislava, pp. 227-235.

[37] Makovinská, J. 1998. Phytoplankton development of the Čunovo (Hrušov) Reservoir in the Danube river (Slovakia) during 1995-1997. Biologia, Bratislava, 53: 499-502.

[38] Makovinská, J. & Ardó, J. 1994. Water quality of the Danube section Bratislava - Visegrád. In: Proceeding XVIIth Conference of the Danube Countries on Hydrological Forecasting and Hydrological Bases of Water Management, pp. 707-712.

[39] Makovinská, J. & Ardó, J. 1996. Tendencie a dynamika zmien v kvalite vody Dunaja v rokoch 1989-1995. Vodohospodársky spravodajca, Bratislava, 10: 5-7.

[40] Makovinská, J. & Hindák, F. 1995. To the taxonomy of genus Lobomonas Dang. (Volvocales, Chlorophyceae). Biologia, Bratislava, 50: 315-324.

[41] Makovinská, J. & Hindák, F. 1998. Use od algae for monitoring rivers in Slovakia: recent development. in press.

[42] Makovinská, J., Holobradá, M. & Hucko, P. 1995. Fytoplanktón Dunaja v úseku Bratislava - Szob v rokoch 1991-1994 a otázka vplyvu zdrže Hrušov na eutrofizáciu Dunaja. Medzinár. konfer. Ekológia Dunaja, Stupava, pp. 176-181.

[43] Makovinská, J. & László, F. (eds) 1997. Tendency and dynamics of water quality changes of the Danube and its tributaries (1989-1995). Práce a štúdie VÚVH, Bratislava, 134, 115 pp.

[44] Makovinská, J. & Topoľská, J. 1996. Development and modelling of Eutrophication of the Čunovo Reservoir. In Proceeding of the XVIIIth Conference of the Danube countries, Graz, Austria, pp. 26-30.

[45] Marvan, P. & Holobradá, M. 1994. Phytoplankton of the Hrušov Reservoir on the River Danube. Biologia, Bratislava, 49: 547-552.

[46] Németh, J. 1996. Qualitative and quantitative phytoplankton investigation of the River Danube between Rajka and Budapest (1848,4 - 1659,0 r.km) during 1994 - 1996. 31. Konf. IAD, Baja - Ungarn, 1996, Wissenschaftliche Referate, pp. 149-154.

[47] Rodinger, W. 1991. Chlorophyllgehalte des Phytoplanktons der Donau in den Jahren 1983 bis 1990. Wasser und Abwasser, 35: 251-264.

[48] Rončák, P., Adamková, J. & Borovičková, A. 1995. Sledovanie vplyvu VDG na kvalitu povrchových vôd. Medzinár. konfer. Ekológia Dunaja, Stupava, pp. 145-151.

[49] Schmid, R. 1994. Limnologie und Gewässergutezustand der bayerischen Donau und ihrer wichtigsten Zubringer. Donaubasisuntersuchung 1985 - 1992. Ergebnisse der Donau-Forschung , IAD, 3, 291 pp.

[50] Schmidt, A. 1994. Main characteristics of the phytoplankton of the Southern Hungarian section of the River Danube. Hydrobiologia, 289: 97-108.

[51] Schmidt, A., Kiss, K. T. & T. - Bartalis, É. 1994. Chloroccoccal algae in the phytoplankton of the Hungarian section of the River Danube in the early nineties. Biologia, Bratislava, 49: 553-562.

[52] Schmidt, A. & Vörös, L. 1981. A Duna magyarországi alsó szakaszának fitoplanktonja az 1970-es években. Hidrológiai Közlom., 7: 322-330.

[53] Steinberg, C., Heindel, B., Tille-Backhaus, R. & Klee, R. 1987. Phytoplanktonstudien an langsamfliessenden Gewässern: Donau und Vils. Arch. Hydrobiol. Suppl., Stuttgart, 68/3: 37-456.

[54] Uherkovich, G., Schmidt, A. & Voros, L. 1975. Adatok a Duna magyarországi szakasza algáinak ismeretéhez. Bot. Közlem., 62: 165 -175.

[55] Valúchová, M. & Kanala, A. 1995. Vývoj akosti vody v Dunaji od roku 1975. Medzinár. konfer. Ekológia Dunaja, Stupava, pp.152-165.

[56] Vaško, D. 1995. Fytoplanktón rieky Ipeľ v Salke. Bull. Slov. Bot. Spoločn., Bratislava, 17: 134-138.

Figures

Fig. 1. Sampling sites for the investigation of phytoplankton in the Slovak section of the Danube during 1990-1997		Fig. 2 Mean phytoplankton abundancy in 1992-1997 in Bratislava (BS), Rajka (R), Hrušov (H), Medveďov (M), Komárno (K), Szob middle (SS), and Visegrád (V)		Fig. 3 Time series of the phytoplancton abundance in selected sampling sites
Fig. 4 Time series of the abundance of diatoms (BAC), green algae(CHLO), and other groups of algae (OST) in 1992 - 1997 in Bratislava and Szob		Fig. 5 Mean phytoplankton abundance in the Danube in Bratislava (BS) and Čunovo reservoir (Č2, Č3, Č4, Č5, Č6) in 1995-1997		Fig. 6. Mean levels of the chlorophyll a in 1990-1997 in Bratislava (BS), Rajka (R), Hrušov (H), Medveďov (M), Komárno (K), Szob middle (SS), and Visegrád (V), (*January – April)
Fig. 7.  Time series of the chlorophyll-a levels in the Danube sampling sites		Fig. 8. Mean levels of the chlorophyll a in the Danube in Bratislava (BS) and Čunovo reservoir (Č2, Č3, Č4, Č5, Č6) in 1995-1997		Fig. 9. Mean primary production of phytoplankton in the Danube in Bratislaba (BS) and Čunovo reservoir (Č2, Č3, Č4, Č5, Č6) in 1995-1997