As was the case with many other lakes in Europe, the lake Müggelsee had undergone a steady decline in underwater vegetation since the 1970s, due to high turbidity, culminating in the almost complete loss of such vegetation. It was not until after a significant reduction in nutrient inputs that aquatic plants slowly reappeared from the 1990s onwards. Initially only sparsely, with a few species populating the shallow edges, and then going on to settle in the deeper areas of the lake. Since around 2011, turbidity in the Müggelsee has declined even further due to the influence of the quagga mussel, which has invaded the lake. Underwater flora is now present down to a depth of 3-4 metres, in some cases forming very dense stands, and there has also been a significant increase in species richness.
Low-growing species such as stoneworts were last sighted in the Müggelsee over a century ago. Now, after 20 years of intensive mapping and diving surveys, not just one but three of those desirable species of underwater vegetation have been found: the Fragile Stonewort (Chara globularis), the Starry Stonewort (Nitellopsis obtusa) and another Nitella species (Nitella spec.). Stoneworts are not quite algae and not quite plants, but they are certainly a clear indicator of lower nutrient concentrations and cleaner waters. It remains to be seen whether they can establish larger populations over the next few years – we truly hope so.
Given the many positive effects of underwater plants, this is a very welcome trend from an ecological perspective. However, if plants grow right up to the water surface, conflicts soon arise with recreational uses such as swimming and boating, which would not be the case with the low-growing stoneworts. In the latest IGB Fact Sheet researchers explain the phenomenon of the mass development of aquatic plants – and why their management necessitates a rethink among the public, authorities and water management.