Why pristine lakes are filled with toxins
In 2016 a team of scientists scoured a dozen beaches around the shores of Lake Geneva in Switzerland – not for flora or fauna, but for litter. In particular, plastic litter.
It wasn’t hard for them to amass quite the collection of discarded common everyday objects. These included bottle tops, cotton buds, pens, toys and straws. They picked up lesser identified fragments too, such as blocks of polystyrene foam, the kind that keeps fragile goods soft in the post.
Collecting this debris wasn’t the team’s main goal, says Montserrat Filella from the University of Geneva. Instead, they wanted to assess whether chemicals emitted from these plastics were harmful.
Their analysis comes at a time when the world is uncomfortably waking up to the extent of human-caused plastic pollution – from islands of amalgamated plastic in our ocean to the smaller microplastics in riverbeds.
We have reached, as the UN has recently dubbed it, a “planetary crisis” that is ruining our ecosystem. Despite the increased awareness of its damage, plastic pollution is already everywhere. While potential solutions like a plastic-eating enzyme – announced in April 2018 – may someday help us cut down waste, there is no guarantee that it could tackle the millions of tonnes of waste already in nature.
But marine plastic pollution is much better studied and understood than that found in freshwater sources. “Freshwater systems are increasingly studied but still at a much smaller scale than oceans,” says Filella. This may simple be due to the fact that initial studies focused on the ocean – and so research proposals and grants followed suit.
It didn’t take long for the Geneva team to find what they were looking for. Filella and colleagues collected over 3,000 samples. They went on to analyse 670 of these, revealing some worrying results.
Many of these samples contained hazardous and toxic elements including cadmium, mercury and lead – in some cases in “very high concentrations”, as outlined in a 2018 paper in the journal Frontiers of Environmental Science. A large proportion of these toxic elements are now banned or restricted. This “reflected the age and residence time of the plastic stock in the lake,” says Filella: the plastic waste has been building up over several decades. And as we know, plastic can take hundreds of years to degrade.
据2018年《环境科学前沿》（Frontiers of Environmental Science）的一篇论文概述，许多样本含有有害及有毒元素，包括镉、汞和铅——这些元素在部分样本中的 "浓度非常高"。这些毒性元素中很大一部分现在已经被禁止使用，或者限制使用。这"反映了塑料物在淡水湖中产生的时间以及存留时间，"菲莱利亚说：这些塑料垃圾累积了数十年。众所周知，塑料需要数百年时间才能降解。
These types of plastics are in line with what washes up on many beaches. But there was one major difference. The hazardous elements the team found “appear to occur in higher abundance in the plastics retrieved from Lake Geneva” than in samples from the ocean, says Filella.
Its effect on wildlife therefore remains a major concern. Plastic’s prevalence in water and on the shorelines of beaches, lakes and rivers, means that it can, and often is, ingested by wildlife. If an animal swallows it, their stomach acids might speed up how quickly the plastic degrades – potentially thereby also releasing the hazardous elements faster. Then there is the effect on the animal itself. Due to the lack of studies, it’s not yet entirely clear how freshwater organisms cope when they ingest plastic waste.
Martin Wagner of the Norwegian University of Science and Technology spends much of his time looking at the effects of this. He was pleased to find that only when exposed to much higher concentrations of microplastics than are currently found in lakes and rivers did the species he examined appear to suffer.
He’s still concerned. His study was on one small invertebrate species – and we know that there are many other effects of ingesting plastic already documented in marine animals. Studies show that sea turtles routinely die when plastic blocks their digestive tracts, for instance. Plastics can also damage stomach linings, block digestive tracks or can cause entanglement. Filella suspects this is happening in fresh water too.
“You might need a lot of plastic to kill a water flea, but that does not tell you the long-term consequences and the ecological implications,” he says.
“Plastics will not go away. They are in the environment and will stay there for decades.”
Lake Geneva is not an outlier. Other lakes show similar levels of pollution. Italy’s Lake Garda, for example, also has high levels of plastic waste. A sample from the northern part of the lake contained 1,000 large plastic particles and 450 smaller particles (microplastics) per square metre.
Microplastics are particularly pernicious, as highlighted in a 2015 study. These are tiny fragments, often 5mm or smaller, often the broken-down pieces of larger plastics. They are widely found in lakes and riverbeds, particularly in the sediment, and are easily mistaken for food. Some come from plastic fibres from our clothing, others from cosmetic products that contain microbeads (now banned in the UK). One study even found microplastics in drinking water. Another discovered them in beer and honey.
But “the extent and relevance of their impacts on aquatic life” is not yet understood, the 2015 study reported. Nor is it clear how this might impact human health, something the authors say is “concerning”. The team’s leader Dafne Eerkes-Medrano explains that when it comes to freshwater, the more we look for plastic pollution, the more we will find. Even in the remote Mongolian Lake Hovsgol, microplastics are abundant – samples revealed a maximum of 44,435 microplastics discovered per square km, almost as much as in Lake Geneva, which had 48,146 per square km. Some of these are distributed by wind from the more populated parts of the lake, an analysis found.
It is now becoming clearer that much of the plastic that ends up in the ocean starts off in freshwater bodies in the first place – estimates suggest it could be as much as 70-80%. It can originate from industrial plants next to rivers, says Wagner.
There is no single solution for our plastic problem, particularly as plastic is so abundant in many everyday products we use. That’s why Wagner urges us all to “go back to the source of the problem” to think more about how we reduce our use of plastic, from the packaging on our food to single-use coffee cups. “We should abandon the logic of producing, using and throwing away, but try to create an economic system where it all goes back” into use, he says.
Reducing our use of plastic may slow down the waste that washes up on beaches – and plastics we use today are less toxic than they were in the past. But as the insights into Lake Geneva reveal, for many decades, at least, the plastic at the bottom of lakes will continue to release toxic elements.
“It might take decades to get rid of the problem,” says Filella.