Macrophytes are aquatic plants that are visible as individual specimens due to their size. Tall macrophytes grow in the Old Danube to the water surface, the plant parts easily get caught in the screws of boat engines, hinder the oars and also swimming becomes unpleasant.
In principle, the underwater plants ensure good water quality in the Old Danube because they filter pollutants and enrich the water with oxygen.
However, due to climate change, the floods that normally take place in spring and thus flush out the plant material are increasingly shifting into summer. Therefore, the algae, which are actually spring plants, can multiply unhindered. This prevents a balanced biodiversity in Vienna’s waters. Other aquatic plants cannot reproduce sufficiently and are increasingly displaced.
Therefore, from March to October, a total of 25 mowing boats are in use on the Old Danube to mow down the tall algae. Divers cut the underwater plants manually in shallow areas close to the shore.
In the previous record year of 2018, 3,350 tons of algae could be mowed down. In June 2022, 900 tons of algae were collected in two weeks.
But something good can also be achieved with it: The algae are collected and brought to the composting plant in the Lobau. There they are processed together with the Viennese organic waste for organic compost, which is then available at the Viennese waste sites.