Integrated management to restore macrophyte domination

Recent changes which have been observed at Alte Donau, a shallow urban lake within the city of Vienna, have been interpreted as a shift to a new stable state. The former macrophyte-dominated state changed to a turbid state dominated by high biomass of filamentous cyanobacteria, associated with a significant reduction in Secchi-depth. Phytoplankton was dominated by the filamentous cyanobacterial species Cylindrospermopsis raciborskii (Wolosz.), Seenayya et Subba Raju and Limnothrix redekei (Van Goor) Meffert. Integrated restoration plans included internal and external measures. Improvements in the catchment aim to minimize the input of nutrients from contaminated groundwater and from storm water and to reduce large numbers of water fowl. Internal restoration measures included water exchange, chemical flocculation and nitrate oxidation of the sediments. Additionally, macrophyte re-colonisation was enhanced through planting. A pelagic predator (Aspius aspius L.) was stocked to reduce bleak (Alburnus alburnus L.), the dominant cyprinid planktivore. Results from the period after water exchange and chemical treatment, showed significant reduction of nutrient and chlorophyll a concentrations. A shift in the phytoplankton species from cyanobacteria towards diatoms and greens was observed. Secchi depth greatly increased. Macrophyte growth became apparent both through re-colonisation, as well as from the planting.