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The population of an unknown naviculoid diatom from Lake Vrana in Croatia was identified as Navicula hedinii, a species described in 1922 from a small lake in eastern Turkestan (China). This species has some similarities with Navicula pseudocrassirostris, a marine species found in European coastal waters. Based on the ultrastructure of the two species, they can no longer be included within the taxonomical concept of Navicula sensu stricto. Following a comparative morphological analysis of both species with genera bearing similar characters (Adlafia, Veigaludwigia, Kobayasiella, Cavinula, Stenoneis, Climaconeis, Berkeleya, Sellaphora, Cosmioneis), a new genus, Envekadea is proposed for the two species. The new genus is characterized by a sigmoid raphe course with golfclub-like terminal fissures deflected in opposite directions, the areolae covered by external porous hymenes and the presence of one chloroplast, H-shaped in valve view. [ABSTRACT FROM AUTHOR]

Phytoplankton species composition and abundance of a shallow hypertrophic 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 stableenvironment 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 wasone 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 temporaland 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. [ABSTRACT FROM AUTHOR]

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. During 1994-1995, a total of 142 phytoplankton taxa was encountered in quantitative samples, most being common in deep, oligotrophic lakes. Seasonal development of phytoplankton is characterizedby a definite spring peak followed by a moderate summer peak. Autotrophic picophytoplankton made the largest contribution to the annual total biomass. This is probably true for other, temperate, non-acidic,oligotrophic lakes. Development of the spring assemblage (autotrophic picophytoplankton and centric diatoms) starts in February-March andis terminated by the onset of stratification when diatoms sink to the hypolimnion. Picophytoplankton, especially Synechococcus sp., assembles in a narrow deep-layer maximum in the upper hypolimnion. Our data show that neither deep circulations nor decreased incident radiation under winter ice and snow cover prevent the development of some specially adapted low-light--high-nutrient species. Our views about the length of vegetation period for phytoplankton need to be revised withrespect to winter and isothermal conditions. [ABSTRACT FROM AUTHOR]