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 Vienna. Increasing nutrient concentrations and massive blooms of cyanobacteria mainly caused by Limnothrix redekei Van Goer and Cylindrospermopsis raciborskii (Wolsz.) Seenayya et Subba Raju were recently registered. 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 initiated in 1993 and extended to metazooplankton, ciliates and bacteria in 1994. The first half of the year 1994 was characterised by relatively clear water and a high diversity of the phytoplankton composition due to 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 macrophytes enlarged the nutrient supply during summer 1994 and favoured the development of cyanobacteria. The high water temperatures which excluded certain zooplankton species, and the inedibility of the filaments further increased the dominance of cyanobacteria. In November, when the algal bloom finally ceased, the highest bacterial numbers of the investigation period occurred. Thereafter, other algal groups, bacteria and metazooplankton gained more importance. Interactions are possible because of close overlap in space and time due to the turbulent mixed conditions of the water body and the change from the macrophyte dominated to the algal dominated stable state. Planned restoration measures must aim to re-establish the previous macrophyte dominated clear-water stage.