Polluted rivers and lakes worldwide are hosting a new and evolving population of microorganisms and bacteria that have settled on the surface of plastics. According to research published in the August 2024 issue of the journal Water Research, this new waste-associated ecosystem, dubbed the "plastisphere," is having multiple consequences: from oxygen depletion in water to the potential introduction of diseases, and it is altering the overall health of major river systems.
"Rivers offer a wide range of ecosystem services, from providing drinking water to irrigating crops and supporting inland fisheries that hundreds of millions of people rely on as a food resource," says Veronica Nava, a researcher at the University of Milano-Bicocca and the study's lead author. “Our study is one of the first to go beyond describing the microorganisms that grow on different plastic materials polluting waterways on our planet and shows that they are changing nutrient cycles and water quality in the river, causing a dramatic reduction of oxygen in the river system. These changes impact a river's health and its ability to support biodiversity within its ecosystems.”
A consortium of research institutes analysed the plastisphere of the lower Mekong River system in Cambodia, one of the world's most diverse and productive rivers. Monitoring the various impacts on river health, researchers found that the thriving populations of bacteria living on the surface of plastic debris significantly altered the overall water quality and impacted ecosystem services, especially in areas with poorly managed waste. Additionally, they observed the presence of potentially pathogenic organisms that could have implications for human health, although further analyses are needed.
The consortium, part of the Wonders of the Mekong project funded by USAID, included researchers from the University of Milano-Bicocca, the University of Nevada, Reno, the Institute of Technology of Cambodia, the Desert Research Institute, and the Royal University of Phnom Penh.
Veronica Nava's work was made possible by funding from the Italian Society of Ecology and the University of Milano-Bicocca.
The study examined the accumulation of microorganisms on plastic debris found in rivers and assessed in the laboratory the impact on water quality related to the presence of these microorganisms on the plastic itself. Specifically, the group investigated the selection of microorganisms to better understand if the type of waste impacts their growth and productivity.
“The combination of studying four different plastics in three different rivers with various types of measurement makes this study unique,” says Monica Arienzo, an associate professor in the Division of Hydrologic Sciences at the Desert Research Institute. “Bringing together these data is important for understanding the potential impacts of plastic on aquatic ecosystems.”
Although the Mekong River supports over 60 million people, it is also one of the planet's most stressed rivers due to hydrological changes from dam construction and land use impacts, including deforestation, overfishing, and the illegal trade of giant fish coveted throughout the region. The river is home to the world's largest recently discovered freshwater fish, a giant stingray, as well as other rare endangered species. Plastic waste discarded into the river is becoming increasingly common, potentially impacting the rich riverine diversity and fishery productivity.
“The Mekong River and its tributaries are rich in biodiversity, but plastic pollution is a growing problem in the Mekong basin, as well as in freshwater bodies around the world,” says Professor Sudeep Chandra, one of the study's authors and director of the Global Water Center at the University of Nevada, Reno. “If this work is expanded, it is possible that due to the microorganisms populating the floating plastic islands that are reducing oxygen in the river, we will start to find ‘dead zones’ where fish and other animals cannot survive, especially during the dry season.”
It is hypothesised that oxygen depletion also contributes to the production of greenhouse gases such as carbon dioxide and methane.
“High levels of plastic pollution could create biogeochemical hotspots that produce greenhouse gases within rivers,” continues Chandra. “We simply cannot take the load of plastic in freshwater lightly as it could trigger many changes in rivers and impact what we are trying to conserve.”
Waterways have often been studied as sources of plastic debris in the ocean, with the world's rivers transporting up to 265,000 tonnes of plastic waste to the sea each year. This research, the first of its kind, brings to light a new set of challenges that must be addressed to protect rivers and the ecosystem services they provide.
According to researchers, this new and growing freshwater plastisphere could have far-reaching impacts:
- Water quality, oxygen depletion, and nutrient loss could impact the health of fish and other river inhabitants. This is particularly concerning in rivers like the Mekong, one of the most biodiverse and functionally important in the world.
- The colonisation of bacteria and tiny algae forming the biofilm on plastic can drive larger organisms to ingest "flavoured" plastic waste.
- Potentially pathogenic microorganisms living on plastic can compromise access to drinking water for humans.
- Plastic debris coated with biofilm can potentially travel long distances, expanding the geographical impact of the plastisphere as it moves along the river with the currents.
In 2023, Nava and Chandra, along with other co-authors, published research in Nature on the high concentrations of microplastics in freshwater lakes worldwide. This new research provides the first evidence of the broader ecological impacts of plastic debris in river systems and is a first step towards better understanding the impacts of the plastisphere on the world's freshwater ecosystems.
“This study highlights the surprising interconnections between plastic pollution and aquatic ecosystems and underscores the need to develop solutions to reduce plastic waste,” says Zeb Hogan, research associate professor in the Department of Biology at the University of Nevada, Reno, and Director of the Wonders of the Mekong Project. “Policies that reduce plastic waste will benefit people through better ecosystem functionality, better water, and more fish.”