The aquatic plant Groenlandia densa, which is widespread in Europe, was long thought to have disappeared in Lake Constance and was extinct due to poor water quality. But at the beginning of this year, a research diver accidentally rediscovered the “fishweed” in a shallow part of the lake. Six months later, an entire underwater lawn of Groenlandia densa was already thriving there. Where this population came from is still unclear. But their rapid return indicates that the water quality and condition of the lake have improved significantly. It remains to be seen whether it will spread across the entire lake again in the future.
The aquatic plant Groenlandia densa, commonly known as “fishweed” or “fishweed”, is found in flowing waters and lakes and is widespread in Europe. Until around 60 years ago, the plant was also abundant in Lake Constance, as historical records show. But over the past few decades, fishweed has gradually disappeared from Lake Constance and has even been considered lost since 1993. Since then, no one has seen a specimen of this aquatic plant there. Scientists suspect that Groenlandia densa was unable to cope with the deteriorating environmental conditions in Lake Constance. Water pollution as a whole and in particular the ammonium and phosphorus pollution in the lake probably killed it.
The fishweed has returned
But now research diver Almut Hanselmann from the University of Konstanz has rediscovered the long-lost aquatic plant during a routine dive in Lake Constance. The researcher regularly examines the plant population at various locations in the lake, actually with a view to climatic changes. However, in January 2025, during a dive near Immenstaad, she surprisingly found an unusually green lake bottom in the middle of winter.
The biologist then examined more closely the dense lawn of aquatic plants that grew there at a depth of four meters and determined that they were specimens of Groenlandia densa. This plant is wintergreen and survives the cold season when many other aquatic plants wither. Finding them on the lake bed in winter is actually no surprise. However, data from the past decades show that fishweed has not grown at this exact site for a long time. Hanselmann’s team concludes that Groenlandia densa only recently found its way back into the lake.
It is still unclear where Groenlandia densa returned to Lake Constance from. “Spread via the tributaries could have promoted or accelerated the recolonization in Lake Constance,” explains Hanselmann. For example, the plant could have come from the tributaries Argen, Rotach or Schussen to Immenstaad. Another possibility is that she never really left, but was just in some kind of Sleeping Beauty sleep. Theoretically, the seeds of Groenlandia densa could have survived for a long time in the sediment layers of the lake and “slumbered” waiting for better times. “The moment the right conditions prevail again, the seeds that have been stored in the sediment for decades and sometimes centuries can sprout,” says Hanselmann. This may have been exactly the case with Groenlandia densa.
Is the return permanent?
But will the reappeared plant stay and settle again in other parts of the lake in the future? To find out, the research team will keep an eye on the Groenlandia densa population in the coming years. The fact that the water quality of Lake Constance has improved over the last 40 years and the lake bed as a whole has become green again speaks in favor of a permanent return throughout the lake, as long-term excursion and sampling data show. Both ammonium and phosphorus pollution in the lake have decreased. The biologists hope that this would allow original species such as Groenlandia densa to colonize again. In fact, further dives at the site in June 2025, only around six months after the first discovery, showed that the “old new” aquatic plant had in the meantime spread further there and was already occurring in abundance again.
This is good news for the environment in and around Lake Constance: “The reappearance of the Groenlandia densa plant is a good sign that the lake is returning to its natural state,” explains Hanselmann. “Aquatic plants always mean an improvement in the ecosystem.” The plants produce oxygen for the lake and attach its sediments with their roots. In addition, aquatic plants such as fishweed serve as food, habitat and shelter for numerous underwater animal species, from microorganisms to snails and fish. In this way, the plants promote biodiversity in the lake, which has recently actually improved measurably. “The return of biodiversity is evidence of the positive changes in Lake Constance – and a clear indication that the renaturation measures are paying off,” says Hanselmann.
Source: University of Konstanz