SEASONAL SUCCESSIONS AND TROPHIC RELATIONS BETWEEN PHYTOPLANKTON, ZOOPLANKTON, CILIATE AND BACTERIA IN A HYPERTROPHIC SHALLOW LAKE IN VIENNA, AUSTRIA

Alte Donau nowadays is an eutrophic urban lake within the city of Vien na. Increasing nutrient concentrations and massive blooms of cyanobact eria mainly caused by Limnothrix redekei Van Goer and Cylindrospermops is raciborskii (Wolsz.) Seenayya et Subba Raju were recently registere d. As a consequence Secchi depth was significantly reduced especially during the summer season (minimum: 0.25 m). An investigation including water chemistry, phytoplankton, macrophytes, and sediment was initiat ed in 1993 and extended to metazooplankton, ciliates and bacteria in 1 994. The first half of the year 1994 was characterised by relatively c lear water and a high diversity of the phytoplankton composition due t o flushing of the lake with water of better quality by the end of 1993 . Ciliates and metazooplankton held about 10% of the total biomass of all the investigated trophic levels. The vanishing of the remaining ma crophytes enlarged the nutrient supply during summer 1994 and favoured the development of cyanobacteria. The high water temperatures which e xcluded certain zooplankton species, and the inedibility of the filame nts further increased the dominance of cyanobacteria. In November, whe n the algal bloom finally ceased, the highest bacterial numbers of the investigation period occurred. Thereafter, other algal groups, bacter ia and metazooplankton gained more importance. Interactions are possib le because of close overlap in space and time due to the turbulent mix ed conditions of the water body and the change from the macrophyte dom inated to the algal dominated stable state. Planned restoration measur es must aim to re-establish the previous macrophyte dominated clear-wa ter stage.