The relation with global warming - and heatwaves - is evident.
A major ecological shift has been quietly taking place in the turbid rivers of The Thames, London. In the last six decades, almost all of the freshwater mussels have died off, leaving only a few living ones within cemeteries of shells.
Throughout Europe and North America, studies have shown that the situation for the mussels is dire, and the lack of evidence from other continents is most likely due to lack of research, and not the vitality of the local mussel species.
The freshwater mussels, of which there are about a thousand different species, have been studied only relatively little, so it is difficult to tell the true extent of the damage.
Analyses
But these molluscs are believed to be one of the most imperilled groups of animals in the world. Mass mortality events have been reported worldwide with varying causes such as algal blooms, drought, pollution and different diseases.
Tadeusz Zając, an associate professor at the Polish Institute of Nature Conservation (PAS) believes most of these causes can be linked by a single factor: "The relation with global warming - and heatwaves - is evident. It is characteristic that all of them happened during the summer."
Zając leads the FACEMUSSEL-project, which is investigating the causes behind the mussels' decline and their abilities to adapt to the Anthropocene conditions.
The team has been studying and monitoring several different populations of the endangered thick shelled river mussels in Poland and Croatia. Even within the same species, the die-offs happening in Poland didn't seem to happen in Croatia, though there has been an observed decline in the numbers there as well.
"We collected samples for genetic analyses, some basic info on demography and rough data about the environment – morphology of the channel, vegetation, basic chemistry. No special differences were found, thus, it seems that genetic or biotic factors might be at play," Zając told The Ecologist.
Benefits
Like most bivalves, freshwater mussels are filter feeders. As they feed, they are simultaneously purifying the water of sediments, bacteria, herbicides and other chemicals.
A healthy population of mussels can have thousands of individuals in a single mussel bed, providing nourishment for a multitude of species in the ecosystem.
Humans, especially indigenous North Americans, have historically filled their stomachs with freshwater mussels as well, although their saltwater cousins have always been the more popular choice to serve at the dinner table.
If the mussel populations disappear, the waters they inhabit will turn murky and pollution will increase. The increased nitrogen in the water can cause algal blooms, which can make the water anoxic killing large amounts of life.
Although Zając, unlike some of his colleagues, hesitates to call the mussels a keystone species, he does remind us of their importance in the ecosystem. "In the network, some threads can be broken but the network can survive, only when many threads are broken, the network fails."
The relation with global warming - and heatwaves - is evident.
Restoration
Mussels clean the water from pollutants but all of those pollutants pass through their body. The threshold the mussels can tolerate is passed in the most polluted rivers. Because of regulations, the water quality in Western countries has increased in the last few decades, and it has opened up room for mussel reintroduction.
The Nida River mussels in Poland were eradicated in the eighties but after sewage treatment plants removed a lot of the pollution, the thick shelled river mussel has been reintroduced with success.
In the United States, the Anacostia River running through the nations’ capital, Washington DC, is being cleaned, not for but by mussels. The reintroduction programme tries to clean the river of sewage, E. Coli, and microplastics among other pollutants.
Freshwater mussels' lack of a cute face makes it harder for environmentalists to gather funding for their conservation.
So, like in the case of The Anacostia River, many of them are turning the other direction. Instead of trying to procure funding and goodwill for the restoration of the mussels, they are pitching the idea of using the mussels to save whole ecosystems.
Future
While pollution has decreased and reintroductions have succeeded, the populations are still vulnerable to the die-offs that are ultimately caused by climate change. The mussels living in deeper, colder waters are in less of a danger but the situation is still critical.
In North America, which is considered a hot spot for freshwater mussel biodiversity with close to 300 species, more than 70 per cent of the mussels are considered at risk of endangerment or extinction, and many have already gone extinct.
Out of the sixteen European species, twelve of them are at risk and three critically endangered. And with climate warming more the mass mortality events are not likely to end anytime soon.
Yet there is some hope for the mussels. They have a great range of variety within species, which can help them adapt and survive. While the Polish thick shelled river mussels had mass casualties, the Croatian ones in the warmer climate didn't.
"The only hope is in evolution – in each of the die-off events, some survive, those selected will create new populations", Zając believes.
This Author
Tommi Rinne is a writer from Finland interested in nature, especially in the hidden world beneath the waves.
