Biodiv. Res. Conserv. 28: 55-62, 2012 DOI 10.2478/vl0119-012-0029-y BRC www.brc.amu.edu.pl Differences in the ultrastructure of two selected taxa of phytoplankton in a thermally stratified Lake Holzmaar (Germany) Beata Messyasz1*, Joanna Czerwik-Marcinkowska2, Andreas Liicke3 & Bohuslav Uher4 'Department of Hydrobiology, Faculty of Biology, AdamMickiewicz University, Umultowska 89,61-614 Poznaň, Poland, *e-mail: messyasz @amu.edu.pl department of Botany, Institute of Biology, Jan Kochanowski University, Šwietokrzyska 15, 25-420 Kielce, Poland 'Institute of Chemistry and Dynamics of the Geosphere: Agrosphere (ICG-4), Forschungzentrum Jůlich, D-52425 Julich, Germany 4Department of Botany and Zoology, Masaryk University, Kotlářská 2, 61137 Brno, Czech Republic Abstract: This paper presents the results of ultrastractural studies and ecological aspects of some phytoplankton species belonging to the groups of cyanobacteria {Planktothrix rubescens, Synechocystis aquatilis) and green algae (Desmodesmus grahneisii). Specimens were collected during summertime from the mesotrophic and stratified Lake Holzmaar (Western Germany) as plank-tonic from the pelagic zone. The highest cyanobacterium P. rubescens concentration was detected in the metalimnion where the alkaline pH, low phosphorus and high nitrogen contents were recorded. It was characterized by straight filaments up to 1000 urn long and 5.4-8 urn wide and numerous aerotopes in cells. The accompanying algae were identified by ultrastractural analysis and photographic documentation was provided. In the case of D. grahneisii, chloroplast was concentrated in the parietal part of cell with one large, oval pyrenoid and, in addition, the granular and spiny cell wall clearly showed important taxonomical criteria for Desmodesmus genera. This is in contrast with cyanobacterium S. aquatilis where the presence of a homogeneous content with visible chromatoplasma was mostly distributed through the cell. This algal association was stable in the epilimnion characterized by the presence of high temperature, pH values (>8), nitrate nitrogen and oxygen concentrations. Key words: Planktothrix rubescens, Synechocystis aquatilis, Desmodesmus grahneisii, ultrastructure, autecology 1. Introduction Planktothrix rubescens (De Candolle ex Gomont) Anagnostidis et Komárek is a stenotherm species living in cold water. It is largely distributed in middle European (Reynolds 1984) southern sub-alpine lakes (Garibaldi et al. 2003), and is common in the lakes of North-Eastern Switzerland (Walsby et al. 1998; Kurmayer & Jeiittner 1999). Planktonic cyanobacteria were previously classified as Oscillatoria rubescens De Candolle, and later described as Planktothrix rubescens (Anagnostidis & Komárek 1988). This species has a very distinctive biology and high diversity of gas vesicle genes. During summer stratification, it is usually located within the metalimnion (Konopka 1980; Feuillade & Davies 1994; Scheffer etal. 1997; Omlin etal. 2001; Messyasz etal. 2003, 2005; Legnani et al. 2005; Lenard 2009) where it is photosynthetically active. In deep and very strongly stratified lakes, the species composition of phytoplankton is closely related to environmental conditions changing with season. Many species of phytoplankton are adapted to ecological changes by their specific growth strategy. The cyanobacterial Planktothrix blooms typically occur in deep lakes. Lake Holzmaar is characterised by intense development of small phytoplankton cells in the layer of 0-5 m depth, where blooms of P. rubescens during summer have been already studied (Messyasz et al. 2003, 2006). Cyanobacteria were dominant in the epilimnion zone of the lake except summer months, where green algae (Chlorophyta) were more abundant and represented 34.57% of all identified species. The ©Adam Mickiewicz University in Poznan (Poland), Department of Plant Taxonomy. All rights reserved. Beata Messyasz et al. Differences in the ultrastructure of two selected taxa of phytoplankton in a thermally. most frequent species among the summer phytoplankton in the pelagic zone were: Cosmarium spp., Desmo-desmus spp., Scenedesmus spp., Golenkinia radiata, Lagerheimia subsalsa, Tetraedron minimum, Pandorina morum and Chlamydomonas passiva (Messyasz et al. 2003, 2005). The green algae Desmodesmus grah-neisii, D. brasiliensis and D. serratus which are the R type of strategy (fast reproducing) were compared with cyanobacterial species Synechocystis aquatilis representing also the R type strategy, preferring more or less turbulent waters (Reynolds 2006). Planktothrix rubescens, forming little changing density peaks in the metalimnion layer during summer in deep mesotrophic lakes, is classified according to Reynolds (1996) in the R functional group of phytoplankton. The main aim of our study was to examine some phytoplankton species (e.g. Desmodesmus grahneisii, Synechocystis aquatilis) accompanying Planktothrix rubescens using light and transmission electron microscopy to show differences in their cells ultrastructure with a special focus on the ecological requirements of these phytoplankton species. 2. Material and methods 2.1. Study area Lake Holzmaar is a small, water-filled, crater situated in the volcanic field of the West Eifel region in western Germany, 95 km south of Cologne (Fig. 1). The studied lake is an example of a deep crater lake (maximum depth 20 m) and with almost round shape. Lake Holzmaar is stratified thermally from April to October with the metalimnion zone between 6 and 8 m. Some relevant physical and chemical factors (temperature, pH, oxygen and nutrients concentrations) are presented in Table 1. The rim of the crater and the near shore areas are forested with stands of beech, oak and spruce (Raubitschek et al. 1999). 2.2. Sample preparations The phycological material, including phytoplankton, was collected every two weeks from July to September (2002-2004) from a single station situated above the deepest point of the lake and preserved with Lugol's Fig. 1. Map of investigated area of Lake Holzmaar in the West Eifel Volcanic Field: A - location, B - catchment area, C - bathymetric map of the lake (changed after Scharf & Oehms 1992) Biodiv. Res. Conserv. 28: 55-62, 2012 solution. The samples were studied both with the light and electron microscope. For the transmission electron microscopy (TEM), the cultures were maintained on the standard Bristol agar medium at 20°C under a 16/8 h light/dark cycle at 3000 uEnrV1 provided by 40 W cool fluorescent tubes. Cells were fixed in 2% glutar-aldehyde in 0.1 M phosphate buffer (pH=7.2) for two hours, washed several times in buffer and postfixed in 1% osmium tetroxide in the same buffer. The cells were dehydrated by a graded acetone series and embedded in Spurr's resin. Ultra-thin sections were stained with uranyl acetate and lead citrate (Reynolds 1963). Observations and photographs were carried out on a TESLA BS 500 transmission electron microscope. Specific identification was done with the light microscope, according the cited monographs (Komárek & Fort 1983, Anagnos-tidis & Komárek 1988; Komárek & Anagnostidis 1989, 1999; John et al. 2011). Taxonomical classification of algae was adopted after Van Den Hoek et al. (1995). 3. Results 3.1. Morphology and ultrastructure In our study we found Planktothrix rubescens (De Candolle ex Gomont) Anagnostidis et Komárek (after Starmach 1966; Suda et al. 2002) which is the plank-tonic cyanobacterium, forming single trichomes or distinct bundles. Its trichomes were straight, gradually tapering at ends. Filaments were generally from 470-1000 um long. Cells were 5.4-8 um wide and 2-4 um long, in the shape of one-third to one-half as long as wide, and possessing granules and gas vesicles inside. In light microscope, they usually paled purple-pink color. Another characteristic feature was that cells at the end of trichomes slightly capitates (Fig. 2), with convex calyptra at the top. Thylakoids were arranged perpendicularly to the longitudinal side of cells. The even distribution of gas vesicles in the protoplast was Fig. 2. Microscopic observations of cyanobacterial filaments of Planktothrix rubescens in the studied lake Lake Holzmaar Explanations: photographs were performed using a LM, a-c - single trichomes and a TEM, d - longitudinal ultra-thin section of cyanobacterium, a part of a filament with terminal cell; C - calyptra, T - tylakoids, GV - gas vesicles characteristic in young cells, while in the older cells, they were more densely packed. Small cyanobacterium Synechocystis aquatilis Sauvageau was associated with P. rubescens filaments in both the epi- and metalimnion zones. This species had spherical or widely oval cells which were solitary or agglomerated. Its widely oval cells from 2.9 to 6.4 um in diameter, after division, were took hemispherical form with more or less visible and colorless mucilaginous envelopes (Fig. 3). Transmission electron microscopy studies revealed the interior of the cell in a pale blue-green color containing homogeneous content or with several distinct granules, sometimes with visible chro-matoplasma. Single Planktothrix trichomes were observed in the epilimnion where representatives of small green algae of genus Desmodesmus formed quantitatively large populations during summer. Morphology of Desmodesmus grahneisii (Heyning) Hegewald (Fig. 4) was taken into account that cells are ovoid to ellipsoid (cell length, 5-13 um; width, 3-7 um), with rounded apices (usually 2, 4 or 8-celled). The cell wall was granular, spiny or toothed. Electron microscopically investigations have shown the presence of parietal chloroplast with a single pyrenoid in each cell. Another green alga that was found as an accompanying species occurring in the epilimnion of Lake Holzmaar was Desmodesmus serratus (Corda) An, Friedl & Hegewald. Desmodesmus serratus was a taxon with coenobia of 2,4 or 8 linearly arranged cells, which were 2.5-7 um wide and 7.8-20 um long. Moreover, oval-elongated cells possessed a row of short spines on the lateral walls, with 1-4 teeth on rounded or sometimes almost truncate apices. The cell wall was smooth except for four teeth in longitudinal rows or occasionally scattered. Also together with Desmodesmus spp., the development of numerous large forms of green algae such as Pandorina morum (O.F. Müller) Bory was recorded in the epilimnion at the end of the summer. Spherical coenobia of Pandorina morum consisted of 8 -16 cells compressed into dense spherical aggregates. The cup-shaped chloroplast contained a pyrenoid up to 2.2 um in diameter. The most characteristic feature of this species was the eyespot occurring in the anterior cell end. Biodiv. Res. Conserv. 28: 55-62, 2012 Fig. 4. Transmission electron microscopy (TEM) of vegetative and reproductive stages of Desmodesmus grahneisii in the studied Lake Holzmaar Explanations: a - four-celled colony (spines up to 10 urn), b-d - vegetative cells with nucleus, pyrenoids, chloroplast and spines, e-f - reproductive stages; ch - chloroplast, n - nucleus, p - pyrenoid, v - vacuoles 3.2. Ecology of selected species Filamentous cyanobacterium Planktothrix rubescens was forming dense water bloom in the metalimnion of Lake Holzmaar during summer (Table 2) characterized by a sharp peak of its biomass as well as the considerable value of chlorophyll a and the oxygen contents (Table 1). In the matalimnion, orthophosphate concentration in water was very low (> 10 ug-1"1) in contrast with the nitrate nitrogen whose value exceeded 10 mg-1"1. The pH range was found between 7.6 and 9.1. In the begin- ning of September, P. rubescens concentration started gradually to increase, also in the epilimnion. In the case of the epilimnion, cyanobacterium Syne-chocystis aquatilis development was related to low light intensity (SD<1.5 m) and alkaline pH (8-10.5) during July and August (Tables 1-2). Only small differences were observed between epilimnion and metalimnion zones with regard to the orthophosphate concentrations. An exception was the epilimnion where the maximum concentrations of nitrate nitrogen (18 mg-1"1) were Table 1. Basic limnological data for Lake Holzmaar observed during July-August in 2002-2004 Parameter Pelagic zone (epilimnion) Thermocline (metalimnion) Stratified dimictic lake 0-5.0 m 6.0-8.0 m Transparency (SD) 0.9-4.0 m - Chlorophyll-a ugT1 4.0-24.0 32.0-80.0 pH 8.0-10.5 7.6-9.1 Nitrate nitrogen mg-1"1 10.7-18.0 10.7-22.4 orthophosphate mg-11 0.001-0.010 0.001-0.006 Oxygen concentration mg-11 7.0-22.3 0.1-35.2 Beáta Messyasz et al. Differences in the ultrastructure of two selected taxa of phytoplankton in a thermally... Table 2. The average biomass (mgT1) of particular phytoplankton species occurring from July to September in 2002-2004 Taxon July July August August September September 0-4 m 6-8 m 0-4 m 6-8 m 0-4 m 6-8 m (n=18) (n=12) (n=18) (n=12) (n=18) (n=12) Planktothrix rubescens 0.782 10.716 1.635 8.512 3.990 8.131 Synechocystis aquatilis 0.035 0.001 0.023 0.001 0.012 0.001 Desmodesmus brasiliensis 0.138 0.002 0.139 0.005 0.006 0.001 Desmodesmus grahneisii 1.446 0.295 3.242 0.090 0.541 0.567 Desmodesmus serratus 0.007 0.0001 0.016 0.0001 0.003 0.001 Pandořina morum 0.068 0.041 0.289 0.258 0.836 0.169 observed only in the beginning of July, while later this value was much lower. Green alga Desmodesmus grahneisii and species from the Desmodesmus spp. formed typical aggregates at the depth of 0-4 m (Table 2) and contributed to a great value of chlorophyll a, even though still smaller than in the metalimnion zone (Table 1). These depths were also characterized by the presence of high pH values (>8) and oxygen concentrations. The aggregation phenomenon was observed each year during almost the same period, i.e. from late July to beginning of September, and within a narrow temperature range, 19-25°C. Every single time, intense green algae development in the epilimnion zone was positively correlated with the decrease in the transparency of water. The quantitative analyses showed that alga Pandorina morum constituted a considerable part of the green algae biomass during the summer. However, this alga was growing under stable water conditions and was mainly noted in the epilimnion zone of Lake Holzmaar in August and September (Table 2). 4. Discussion Planktothrix rubescens is one of the most studied organisms among the filamentous cyanoprokaryotes. This is because of its early seasonal occurrence and the striking visual appearance of blooms. It is plank-tonic species in standing waters, occasionally forming blooms as purplish floating masses and there are only a few modern records (John et al. 2011). The main morphological features of P. rubescens include solitary trichomes or filaments that are able to move by gliding forwards and backwards (trembling), thylakoids arranged perpendicularly to the longitudinal side of cells, even distribution of gas vesicles in the protoplast and lack of false branching of trichomes. This alga is a phycoerythrin-rich cyanobacterium and reaches its maximum growth rate at light intensities typical of the metalimnion (Skulberg etal. 1994). High water transparency in Lake Holzmaar was the factor which generated P. rubescens concentrating in the depth between 6 and 8 m. Its adaptations such as length of filaments and graz- ing resistance contributed to its supremacy over other algal species. Reynolds (1984, 2006) screened P. rubescens domination to assess the diversity of accompanying algal communities. He designated these epilimnion assemblages of algae, and inferred that higher water temperature and the concentration of nitrogen compounds (especially nitrates) constitute niches for specific species with small cells. Such a system, an example of which is Lake Holzmaar, contained only one species of cyanobac-teria as a dominant one in the metalimnion, but several species of algae in the epilimnion. The results indicate that the concentration of chlorophyll a was higher in the metalimnion than in the epilimion zone. As a general principle, the presence of few P. rubescens filaments and its better survival than other algae in the epilimnion were associated with the presence of gas vesicles. Samples from 0-4 m were considered to have many species with a similar number of cells, based on the phytoplankton analysis. Our results showed that the largest amount of Chlorophyta biomass and much smaller in the case of chroococcal cyanobacteria was observed in the epilimnion during summer in Lake Holzmaar. Electron microscopic investigations of S. aquatilis showed the presence of cell aggregation in the process of division indicating favorable habitat conditions for the growth of this cyanobacterium population. These results are in accordance with the ecological description of this planktonic species as inhabiting mildly eutrophic waters (John et al. 2011). However, the ability to form numerous natural populations in aquatic ecosystems is rather rare for S. aquatilis and, thus, detailed studies are carried out, primarily, under laboratory conditions. The observation of Komarek & Anagnostidis (1999) that in several strains of this cyanobacterial genera the cell walls contain a special "S-layer" with a characteristic hexagonal (p6) substructure is also valid to Synechocystis aquatilis. The function and taxonomic significance of this layer is not clear. Fundamental differences in the internal cell structure between Synechocystis and Desmodesmus resulted from characteristics typical for prokaryotic cells, such as the nucleus and lack of pyrenoid. Biodiv. Res. Conserv. 28: 55-62, 2012 The genus Desmodesmus (formerly Scenedesmus p.p.) has been known and investigated for nearly 200 years (Schubert & Wilk-Wozniak 2003). Meyen described in 1829 Scenedesmus which included non-spiny and spiny colonies. Hegewald and Silva (1988) described three subgenera: Scenedesmus, Desmodesmus and Acutodesmus. However, in 1999, using molecular analysis based on ITS-2 rDNA sequence, the non-spiny and spiny forms were found to be separate genera. The spiny forms are now called Desmodesmus sp. (An et al. 1999) and the non-spiny forms retain the original name, Scenedesmus sp. Hegewald (2000) transferred 32 species and 22 varieties to Desmodesmus from the subgenus Scenedesmus. Desmodesmus grahneisii is known from Southern Hungary (Schmidt et al. 2003), Slovakia (Hindak & Hindakova 2000) and recently was found in Spain (Fanes etal. 2009). This species was also found in Lake Holzmaar and the low water transparency in August corresponds to a massive development of small green algae with it as a dominant. There was a significant correlation between Desmodesmus spp. development and phosphate concentration. When the cell density was the highest, the phosphate content was considerably decreased. Besides, green alga Desmodesmus serratus is a common small species whose strains under the light microscope differ in the formation of a mucilage envelope (Fawley et al. 2011). This species is known from many locations in the plankton of various water bodies and is classified as cosmopolitan and eutrophic (John et al. 2011). No habitat differences are immediately apparent for our Desmodesmus species from Lake Holzmaar. All new species are known from eutrophic water bodies, and some of the species have also been isolated from meso-trophic and dystrophic habitats. Fawley et al. (2011) suggested that variability present in one lineage of the genus Desmodesmus and the importance of analyzing this variability result from using sequence data from multiple loci and morphology. Cells in the colony of D. grahneisii and D. brasiliensis occurred in coenobia of two cells and with four cells in the case of D. serratus. The species differed in the number and type of spines on the cells and the texture of the wall. It may be added that the volvocalean green alga Pan-dorina morum with large colonies was also observed in all investigated summer seasons when it was associated with Staurastrum paradoxum and Peridiniopsis elpa-tiewskyi. Colony forming Pandorina morum is generally known as a widely distributed cosmopolitan species, often found in stratified, shallow lakes rich in nitrate. Furthermore, it occurs in mesotrophic water ecosystems as well as in eutrophic water bodies (John et al. 2011). High peaks of the phytoplankton biomass are a distinctive feature of Lake Holzmmar in metalimnetic waters in the summer period (Messyasz et al. 2003, 2005). It was related to Planktothrix rubescens dominating in phytoplankton community. We confirmed that representatives of cyanobacterium Planktothrix rubescens, which avoids the intensive light and prefers deeper layers of water, occurs also in mesotrophic reservoirs such as Lake Holzmaar. The strong thermal water stratification in deep Lake Holzmmar additionally enables the intense development of small cyanobacteria (S. aquatilis) and green algae species {Desmodesmus ssp.) during the entire summer period and shows their availability to rapid colonization of free space in the euphotic zone of lake. So numerous populations of both species are rarely found in lakes and thus the results of our studies of Lake Holzmaar enrich the information on their ecological and ultrastructural characteristics. In conclusion, it can be stated that specimens of P. rubescens, S. aquatilis and D. grahneisii collected from Lake Holzmaar exhibited cells to demonstrate intense divisions allowing their populations to increase in size rapidly. Further studies are needed to accurately determine the factors influencing the repeated occurrence of these species at different depths in the water column during summer stratification. Acknowledgements. The authors thank the anonymous reviewers for valuable remarks on the manuscript. References An S., Friedl T. & Hegewald E. 1999. Phylogenetic relationships of Scenedesmus and Scenedesmus-like coccoid green algae as inferred from ITS-2 rDNA sequence comparisons. Plant Biol. 1: 418-428. Doi: 10.1111/j. 1438-8677.1999.tb00724.x Anagnostidis K. & Komarek J. 1988. Modern approach to the classification system of Cyanophytes, 3-Oscillatoriales. ArchHydrobiol Suppl Algol Stud. 80(1-4): 327-472. Fanes T., Comas G. & Sanchez C. 2009. Catälogo de las algas verdes cocales de las aquas continentales de Andalucia. Acta Botanica Malacitana 34: 1-22. Fawley M., Fawley K. & Hegewald E. 2011. Taxonomy of Desmodesmus serratus (Chlorophyceae, Chlorophyta) and related taxa on the basis of morphological and DNA sequence data. Phycologia 50(1): 23-56. Doi: 10.2216/10-16.1 Feuillade M. & Davies A. 1994. Seasonal variation and longterm trends in phytoplankton pigments. Arch Hydrobiol Beih. 41: 95-111. Garibaldi L., Anzani A., Marieni A., Leoni B. & Mosello R. 2003. Studies on the phytoplankton of the deep subalpine Lake Iseo. J. Limnol. 62: 177-189. 62 Beata Messyasz et al. Differences in the ultrastructure of two selected taxa of phytoplankton in a thermally.. Hegewald E. 2000. New combinations in the genus Desmo-desmus (Chlorophyceae, Scenedesmaceae). Algologi-cal Studies 96: 1-18. Hegewald E. & Silva P. 1988. Annotated catalogue of Scenedesmus and nomenclaturally related genera including original descriptions and figures. Bibl. Phycol. 80: 1-587. Hindak F. & Hindäkovä A. 2000. Checklist of the cyano-phytes/cyanobacteria and algae of the Slovak stretch of the Danube river (1926-1999). Biologia Bratislava 55^=34:—---- k J. & Fott B. 1983. Chlorophyceae (Grünalgen), Ojilnung Chlorococcales. In: G. Huber-Pestalozxi (ed.TUDas Phytoplankton des Süßwassers. Di^ Binnengewässer 16 (7. Teil, 1. Hälfte), pp. 1-lOJ K tgart, Schweizerbart'sche Verlags Tue27^hch.l985013ežŤ9" ilune O Doi: 1( JärnefeltH. 1952. Plankton als indikátor der trophiegruppen der seen. Annales Academiae Scientiarum Fennicae 18:1-29. John D.M., Whitton B. A. & Brook A. J. 2011. The freshwater algal flora of the British Isles. An identification guide to freshwater and terrestrial algae. II, pp. 1-878. Natural History Museum, London, Cambridge University Press. Komárek J. & Anagnostidis K. 1989. Modern approach to the classification system of Cyanophytes. 4 - Nostocales. ArchHydrobiol Suppl Algol Stud. 82(3): 247-345. Komárek J. & Anagnostidis K. 1999. Cyanoprokaryota. 1. Chroococcales. In: H. Ettl, G. Gärtner, H. Heyning & D. Mollenhauer (eds.). Süßwasserflora von Mitteleuropa, pp.(í^548) Spektrum, Akad Verl. Berlin. Komárek J. & Komárkova J. 2004. Taxonomie review of the cyanoprokaryotic genera Planktothrix and Plankto-thricoides. Czech Phycology, Olomouc 4: 1-18. Konopka A. 1980. Physiological changes within a metalim-netic layer of Oscillatoria rubescens. Applied and Enviromental Microbiology 40(3): 681-684. KurmayerR. & JeuttnerF. 1999. Strategies for the coexistence of zooplankton with the toxic cyanobaterium Planktothrix rubescens in Lake Züric, J. Plankton Res. 21: 659-683. Legnani E., Copetti D., Oggioni A., Tartari G, Palumbo M.T & Moraito G. 2005. Planktothrix rubescens seasonal dynamics and vertical distribution in Lake Pusiano (North Italy). J. Limnol. 64(1): 61-73. Lenard T. 2009. Metalimnetic bloom of Planktothrix rubescens in relation to environmental conditions. Oceanol. and Hydrobiol. Studies 2: 45-53. Messyasz B., Czerwik-Marcinkowska J. & Lücke A. 2006. Ultrastructural observations on some species of cyanobacteria and green alga in the Lake Holzmaar. Proceedings 2nd Croatian Congress on Microscopy, pp. 108-110. Messyasz B., Lücke A. & Schleser G. H. 2003. Dominance of cyanobacteria Planktothrix rubescens (D.C. ex Gom.) Anagn. et Kom. in Lake Holzmaar, Germany - an indication of the trophic status ?(^nTAlga£^^ Cbjojogical State of Watgt>Acta Botanica Warmiae et Masuriae 3: 21-31. Messyasz B., Lucke A. & Schleser G. H. 2005. Comparison of the spring and summer phytoplankton in stratified lakes: Kociolek (Poland) and Holzmaar (Germany). Phycologia 36(4): 68-69. Omlin M., Reichert P. & Forster R. 2001. Biogeochemical model of Lake Zurich: model equations and results, Ecological Modelling 141: 77-103. Raubitschek S., Lucke A. & Schleser G. H. 1999. Sedimentation patterns of diatoms in Lake Holzmaar, Germany - (on the transfer of climate signals to biogenic silic oxygen isotope proxies). J. Paleolimnology 21(4): 437-448. Doi: 1023/A: 1008022532458 Reynolds©S. 1984. The ecology of freshwater phytoplank- (^^) ton. 384 pp. Cambridge University Press, Cambridge. Reynolds C. S. 1996. The plant life of the pelagic. Verhan-dlungen International Verein Limnology 26: 97-113. Reynolds C. S. 2006. The Ecology of Phytoplankton. 535 pp. Cambridge University Press, Cambridge. Reynold/e) S. 1963. The use of lead citrate at high pH as \___) an electron-opaque stain in electron microscopy. J. Cell Biol. 17(1): 208-212. Doi: 10.1083/jcb.l7.1.208 Scharf B. W. & Oehms M. 1992. Physical and chemical characteristics. In B. W. Scharf & S. Bjork (eds.). Limnology of Eifel maar lakes. Advances in Limnology 38: 63-83. scheffer M. S., RlNALDI A., gragnani L. R., MUR E. & VAN Nes E. H. 1997. On the dominance of filamentous cyanobacteria in shallow, turbid lakes. Ecology 78(1): 272-282. Schmidt A., Feher G. & Padisak J. 2003. Some rare green algae occurring in the Danube river and its dead-and side-branches in Southern Hungary. Biologia Bratislava. 58(4): 475-481. SchubertL. E. & Wilk-WozniakE. 2003. SEMinvestigation of several non-motile coccoid green algae occurring in different types of water bodies. Biologia 58(4): 459-466. Skulberg O. M., Undertal B. & Utkilen H. 1994. Toxic waterblooms with cyanophytes in Norway - current knowledge. Algol. Stud. 75: 279-289. STARMACHK. 1966. Cyanophyta-sinice, Glaucophyta-glau- kofity. In: K. STARMACH (ed.). Flora slodkowodna Polski, 2, 753 pp. PAN, PWN, Warszawa. Suda S., Watanabe M., Otsuka S., Mahakahant A., Yong- manitchai W., nopartnaraporn N, LlU Y. & day J. G. 2002. Taxonomic revision of water-bloom forming species of osillatorioid cyanobacteria International Journal of Systematic and Evolutionary Microbiology 52: 1577-1595. Walsby A. E., Avery A. & Schanz F. 1998. The critical pressures of gas vesicles in Planktothrix rubescens in relation to the depth of winter mixing in Lake Zurich (Switzerland). J. Plankton Res. 20(7): 1357-1375. Van Hoek C, Mann D. G. &Jahns H. M. 1995. Algae. An introduction to phycology. 623 pp. Cambridge University Press, Cambridge.