Biodiversity, succession and the functional role of macrophytes in the New Danube (Vienna, Austria)
Janauer, G. A.; Wychera, U.
In the flood-relief channel New Danube, waterplants are the visually dominant life-form, supporting a varied aquatic biota. Their positive ecological value is partly counteracted in public opinion by the restraint on recreational activities exerted by macrophytes. Formulation of a balanced management plan taking into consideration ecological values and the nutrient buffering ability of the waterplants, as well as the comfort of swimmers and surfers, was a major objective for research. Although it is a man-made channel, the aquatic macrophyte vegetation of the New Danube resembles closely that of other, more natural water bodies in the floodplains of the Danube in Austria. Species number and composition are briefly considered; more than 75% of the species occurred regularly throughout eleven years of investigation (1987- 1997). Macrophytes are a key factor in the phosphorus regime of the New Danube, competing with the phytoplankton for a limited supply of the nutrient and thus supporting a high water quality. Two severe floods resulted in considerable biomass reduction, but the aquatic plant stands recovered quickly each time, evidence of the high resilience potential for macrophyte growth in the New Danube. Estimated biomass for preceding years and initial biomass values for following years correlated well when inter-annual relationships were studied. Therefore, avoidance of overcontrolling macrophytes by weed-cutting must be a major management objective, as intensive reduction could trigger a shift into an alternative state dominated by phytoplankton. The result would be a substantially decreased water quality with numerous negative features, including algal blooms and possible loss of animals such as fish.