Long-lost plant species rediscovered in Lake Constance
Have you ever seen Groenlandia densa? The aquatic plant is also commonly known as opposite-leaved pondweed. Its main habitat is flowing water, but it can also be found in lakes. This aquatic plant is actually not uncommon in Europe. But it had disappeared from Lake Constance. We know from historical records that it still existed in Lake Constance around 60 years ago. By 1990, however, it had become extremely rare there, and after 1993 not a single plant had been seen, even though the plant population is regularly monitored at around 70 locations across the lake. Groenlandia densa? No such luck. Only in the Bay of Constance, a few individual specimens grew. In the rest of the lake, however, the plant was considered to have been displaced and disappeared. Researchers assume that water pollution as well as ammonium and phosphorus contamination of the lake had wiped it out.
Now, however, the lost plant has been rediscovered. Almut Hanselmann is a postdoctoral researcher and research diver at the Limnological Institute of the University of Konstanz. In the Interreg project "SeeWandel-Klima" of the European Union, she regularly monitors the plant population in Lake Constance. When she dived down near Immenstaad in January 2025 and viewed the pondweed at a depth of four metres, she wondered why the lakebed was so unusually green in the middle of winter. A dense "lawn" covered the sediments where in winter usually the bare lakebed is visible. The solution to the riddle was a real surprise: Hanselmann realized that the unexpected greening was a returning plant species. Wintergreen Groenlandia densa survives the cold season, when many other aquatic plants wither away.
© Almut HanselmannGroenlandia densa, photographed in January 2025 near Immenstaad (in the foreground of the picture)
Reliable data showed that the plant had not occurred on precisely that site for decades – nor anywhere else in the lake. Only recently Groenlandia densa must have found its way back into the lake and is spreading well: Just under six months later, at the end of June 2025, Haselmann documented a lush occurrence of the "old new plant" on that spot.
A good sign for the lake
"The reappearance of the plant is a good sign that the lake is returning to its natural state", says Almut Hanselmann. Long-term water samples and analyses show that the ammonium and phosphorus levels in the lake have decreased over the last 40 years. This paves the way for native species such as Groenlandia densa to re-establish themselves. And indeed, there is evidence of increasing greening of the lake bed over the same period.
"Aquatic plants always enhance the ecosystem", emphasizes Hanselmann: The plants provide a food source and habitat for numerous animal species, from microorganisms to snails and fish. They offer protection for fish larvae and a safe haven for many underwater creatures. In addition, they produce oxygen for the lake and their roots stabilize the lake sediments.
"The return of biodiversity is proof of the positive changes in Lake Constance – and a clear indication that the renaturalization measures are paying off."
"The reappearance of the plant is a good sign that the lake is returning to its natural state."
Almut Hanselmann
Awakened from its slumber
The return of Groenlandia densa has of course not come out of nowhere. Probably the plant was never really gone, it rather was in a kind of long slumber. The sediment layers of the lake contain seeds and permanent stages of countless plant species, including those of Groenlandia densa. This dormant, germinable material just didn't have the right conditions to spread. "As soon as the conditions are right again, the seeds that had been buried in the sediment for decades or even centuries can sprout", explains Hanselmann. This is probably the case with Groenlandia densa, too.
Almut Hanselmann describes a possible alternative: "We cannot rule out the possibility that Groenlandia densa re-entered the lake via the tributaries of Lake Constance, for example via the Argen, Rotach or Schussen. These rivers flow into the lake on the Immenstaad side, where we found the plant. A spread via the tributaries could have driven or accelerated the recolonization in Lake Constance". But it does not matter how the plant returned – Groenlandia densa is highly welcome. The research team will keep a good eye on the plant population in the coming years.
Going down: Almut Hanselmann gets ready to examine the plant population at the bottom of the lake.
Samples are collected under water.
Groenlandia densa, photographed in June 2025 near Immenstaad. A dense lawn of the once lost plant has formed.
Research on Lake Constance: The team behind the Interreg project “SeeWandel-Klima” is investigating changes in and under the water.
Research diver Almut Hanselmann
About the project SeeWandel-Klima
The research project SeeWandel-Klima investigates the impacts of climate change and invasive species (neobiota) on Lake Constance. Research institutes from three countries, including the Limnological Institute at the University of Konstanz, are working together across borders. SeeWandel-Klima is a "lighthouse project" as part of the European Union's Interreg funding programme. The project is managed by the Swiss Federal Institute of Aquatic Science and Technology (Eawag).