- Mangroves are known to trap plastic waste and prevent it from entering the ocean. However, a new study shows that this defense comes at a high cost for mangrove forests themselves.
- Researchers who worked in the Indonesian province of Central Java found plastic carpets on half of the mangrove floor in their study area, covering roots and sediment layers and starving the oxygen trees.
- The plastic build-up could also damage mollusks, crabs, and other bottom-dwelling organisms that form the foundations of the food web on the coast, causing cascading impacts in larger animals.
- The authors of the study called for a reduction in plastic waste through education and guidelines such as bans on single-use plastic packaging.
Celine van Bijsterveldt and her team had traveled to the mangrove forests of central Java to investigate efforts to restore them. But when the researchers saw plastic debris strewn in the swamps, rooting roots and shrouding branches, they were instead forced to investigate the toll on the trees.
“It’s an insane amount of plastic,” said van Bijsterveldt, a PhD student at Utrecht University and the Royal Netherlands Institute for Marine Research at NIOZ and lead author of the resulting paper in Science of The Total Environment. “You just couldn’t ignore it.”
Although mangroves adapt to plastic waste, too much can kill them. Scientists fear that its continued accumulation could endanger these forests and the ecological and human communities that depend on them. This concern is particularly pressing in Indonesia, which is home to nearly a quarter of the world’s mangrove ecosystems and is home to highly acute plastic waste and mangrove destruction.
Plastic lies on mangrove roots on the forest floor. Image by Celine van Bijsterveldt.
Van Bijsterveldt and her colleagues analyzed the distribution and influence of the plastic on the growth and survival of mangroves in Demak, a city on the north coast of Java. In addition to field observations, they conducted a six-week experiment to determine how the plastic cover affects root growth, stress, and tree survival.
Investigators counted an average of 27 plastic objects per square meter, mostly from the region that carpeted up to 50% of the mangrove floor. Ground core samples showed that plastic was often stuck in upper sediment layers, creating lengthy, deoxygenated conditions.
As expected, trees that were completely buried in plastic choked to death, mainly where such plastic and sediment layers appeared over roots. On the other hand, partially covered mangroves are “relatively resilient” and hold most of their leaves despite stress, said van Bijsterveldt.
“It was really amazing how they managed to survive some of the plastic,” she said, noting that they were starting to grow deformed roots to get away from it.
The abundance of plastic waste observed is consistent with previous evidence that Southeast Asian mangrove swamps capture significant amounts of plastic. While these ecosystems have long been touted as filters for reducing land-to-sea pollution, the new field research and asphyxiation trials show that this service “comes with a price,” said van Bijsterveldt, highlighting the possibility that escalating plastic buildup could cause the Could affect mangrove forests.
This mangrove root puckered as it tried unsuccessfully to grow out of the piece of plastic that covered it. Image by Celine van Bijsterveldt.
Possible consequences of plastic in mangroves
Plastic waste is adding to the increasing pressures Indonesia’s mangrove swamps are facing from erosion, subsidence, climate change, aquaculture and development, experts say.
However, according to Peter Harris, managing director of GRID-Arendal, a Norwegian non-profit working to combat marine plastic, other organisms in the mangrove ecosystem will have problems “long before mangroves show signs of stress”. Mangroves are “pretty tough compared to other species,” he said, “so they’ll be the last thing standing.”
Plastic intensification in mangrove habitats could damage the breeding and nurturing fish populations here, as individuals eat or get caught in the rubble. Locals are cutting off parts of the swamp to control the spreading waste, van Bijsterveldt said, but this could make things even more difficult for fish by cutting them off from the resources. And the buildup of plastic could damage mollusks, crabs, and other soil organisms that form the foundations of the coastal food web, causing cascading impacts in larger animals.
Plastic entanglements in boat engines also affect fishery productivity, added van Bijsterveldt. A plastic-induced decline in mangroves would affect the well-being of the villagers, who rely on the fisheries, timber, ecotourism and coastal protection to maintain these ecosystems.
Declining shallow water fish numbers would force small-scale fishermen to invade the open ocean, creating greater risks and fuel costs, said Novi Susetyo Adi, a marine ecology researcher with Indonesia’s Ministry of Maritime Affairs and Fisheries.
Since the swamps act as important carbon sinks, their loss would hinder climate protection efforts.
“We will lose all of these ecosystem services if we lose mangroves,” said Novi.
Even so, he said, the study’s conclusion that mangrove mortality could worsen when plastic litter builds up might not be as severe as the impact on the coast as a whole. He argues that the presence of the plastic shifts with the tides, currents, and seasons because mangrove forests are in the rapidly changing intertidal zone.
Garbage pollutes a mangrove swamp near a village and uses it as a landfill. Picture by: Celine van Bijsterveldt.
A vision for a holistic solution
Deforestation has claimed 75% of Java’s historic mangroves, according to Novi, but thanks to a better understanding of its benefits, effective replanting and rehabilitation projects have taken root in Indonesia, led by locals, governments, corporations and nonprofits.
However, these initiatives overlook the plastic challenge. “It is important that it gets more attention,” said van Bijsterveldt. Because it is “wrong in so many places,” she said, solving the problem will not only depend on improving local and national management, but will also mean addressing the plastics market itself.
Four fifths of the country’s trash is mistreated, she said. Villagers currently disposing of litter through incineration or landfilling from waterways need better training and infrastructure for waste disposal. The government should offer rural and community programs to collect, process, and recycle garbage.
Noting that the public is not aware of the wider implications of throwing plastic away, van Bijsterveldt said “It would be really helpful if they are just starting school” and taught the children that the non-biodegradable material the environment has become increasingly contaminated over time.
In a nation where shampoo is even available in single-use plastic bags, laws should restrict climbing consumption, as bans in Bali and Jakarta have already done. She recommends using traditional eco-friendly packaging alternatives like banana leaves.
An old diaper hangs from a mangrove root on an eroded cliff overlooking the sea. Locals throw away diapers, a common sight in the mangroves, due to a superstition that burning a dirty diaper could harm the baby. Image by Celine van Bijsterveldt.
In his own work, Harris found that more than half of the planet’s mangrove forests exist within 20 kilometers (12 miles) of an estuary that releases more than a ton of plastic annually. “You’re pretty exposed to this pollution problem,” he said.
But it’s most extreme in Indonesia. Given the high mountains, heavy rainfall, short catchment areas, and low wave and tidal energy, huge amounts of plastic pour onto the coast and collect in the mangroves.
“A natural process is working against the Indonesian government,” said Harris.
Since the plastic is the same material that comes from upstream regions and floods coastal settlements, Novi calls for collaboration between state, municipal and private stakeholders across the watershed. “We need to take an integrated approach to this issue,” he said.
Van Bijsterveldt, C. EJ, Van Wesenbeeck, BK, Ramadhani, S., Raven, OV, Van Gool, FE, Pribadi, R. & Bouma, TJ (2021). Is plastic waste killing mangroves? A field trial to evaluate the impact of macroplastics on mangrove growth, stress response and survival. Science of The Total Environment, 756, 143826. doi: 10.1016 / j.scitotenv.2020.143826
Harris, PT, Westerveld, L., Nyberg, B., Maes, T., Macmillan-Lawler, M. & Appelquist, LR (2021). Exposure of coastal areas to plastic pollution from rivers. Science of The Total Environment, 769, 145222. doi: 10.1016 / j.scitotenv.2021.145222
Smith, S. DA (2012). Marine Debris: An Immediate Threat to Ocean Sustainability in Bootless Bay, Papua New Guinea. Marine Pollution Bulletin, 64 (9), 1880-1883. doi: 10.1016 / j.marpolbul.2012.06.013
Banner image: The researchers cut through sediments and underlying plastic layers to quantify the plastic objects in the topsoil. Photo by Kuswantoro / Wetlands International.
Conservation, deforestation, environment, forestry, forests, Indonesia, mangroves, plastic, pollution, rainforests, research, saving rainforests, trees, tropical forests, water pollution