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Shallow aquatic ecosystem Lake Vrana, with special attention to phytoplankton was studied over one year period. Phytoplankton was investigated monthly with respect to temporal variability of selected environmental factors. Regular annual development was observed not as seasonal changes of species compositions but as species contribution to the total abundance. Assemblage was dominated by Cosmarium tenue Arch. and Synedra sp.. Winter, same as vernal assemblage was clearly dominated by Cosmarium tenue. The assemblage was determined by the high contribution of Synedra sp. during the summer and autumn. Nutrient availability as external control and competition between species were assessed as forces which drive species diversity and changes in predominance.

Algal flora of 12 bog lakes was investigated during the period of March 1995 to August 1999 in Hungary. Of the 129 samples, 624 taxa of algae were identified. Species richness of individual samples ranged between 8 and 107 except the extraordinarily species rich Balata-to where 533 algal taxa were observed. Ordination of the samples resulted in five groups: (1) assemblages dominated by chlorococcalean algae and planktonic Cyanoprokaryota; (2) assemblages dominated by flagellates (Dinophyta, Cryptophyta, Euglenophyta, Chrysophyceae, Raphidophyceae); (3) chlorococcalean algae and cyanoprokaryotic assemblages with desmids, cryptophytes, dinoflagellates or euglenophytes as subdominants; (4) assemblages dominated by diatoms and (5) a group of samples where other taxa belonging to Xanthophyta and filamentous green algae dominated. The bogs were typically rich in inorganic N and P, moreover, their water chemical characteristics (including pH and conductivity) were rather uniform. Therefore, other factors than chemical properties were responsible for different flora. The above groups were characteristic to certain types of habitats. In the first group, plankton samples from relatively large pools with considerable open water can be found. The second group included samples taken from small bog pools. The third group contained the periphyton samples from macrophytes, living in bogs with constantly reliable water supply. Samples of group four and five comprised small bogs that occasionally dry up. Periphyton of lakes with Sphagnum belonged exclusively to the fifth group. This study has shown that small bog-pools are often inhabited by different species of flagellates and desmids are not as important as it has been widely believed. Hydrological properties and habitat diversity are the major factors influencing species richness of Hungarian bog-lakes.

Peridinium lomnickii Woloszynska was investigated by scanning electron microscopy with special attention to the importance of development of thecal plate during the life cycle. Different life cycle stages (gymnodinoid-, glenodinoid-, peridinioid) are described on the basis of development of cell wall, presence and development of sutures, appearance of pore and the change of the cell shape. Differences and possible relationships between the three existing varieties of the species are discussed. We suggest that the three varieties of P. lomnickii, P. lomnickii var.lomnickii, P. lomnickii var. wierzejskii and P. lomnickii var. splendida, represent the different life cycle stages of the species. These results and the known ontogenic cycle of dinophyta taxa should be taken into consideration, when a phylogenic tree of the dinophytes is constructed.

Phytoplankton species composition and abundance of a shallow hypertrophie fishpond (Mezeshegyi-to, East Hungary) was studied for the period 1992–1995. The pond showed a pronounced algal periodicity. High-diversity phytoplankton assemblages occurred in spring and autumn. During the winter period, low diversity values were related either to stable community states, when K-strategist species dominated the plankton, or to a large bloom of r-strategist species. In summer, the stable environment led to low-diversity, high-biomass phytoplankton assemblages, dominated by Cylindrospermopsis raciborskii. At this time, the growth conditions for Cylindrospermopsis were akin to those prevailing in a continuous fermentor. The overwhelming dominance of this species lasted for more than four months, during which time, the phytoplankton resembled that of one in the tropics. In August, 1993, an unsuccessful chemical treatment for reducing the algal bloom succeeded in killing the pond’s entire population of fish. The large fish-stock comprised the planktivorous silver carp. Although the summer of 1994 was one of the warmest summers of this century, the expected Cylindrospermopsis bloom failed to develop probably because of a higher grazing pressure by large zooplankton. In spite of the fact that the temporal and spatial pattern of the phytoplankton is influenced principally by bottom-up effects, changes in cascading trophic interactions may also considerably influence the species composition and biomass.

Phytoplankton samples were taken weekly from January to December of 1994 (epilimnion) and 1995 (0–25 m, euphotic zone) from the deep, stratified, alkaline, oligotrophic Lake Stechlin, Baltic Lake District, Germany. The purpose of the study was to gain detailed information about phytoplankton changes including those of picophytoplankton, to relate these changes to stratification patterns and nutrient chemistry of the lake and to compare them to results from other lakes of similar character.