Mangroves, typical of tropical and subtropical latitudes, have become veritable natural traps for land- and sea-based waste. The roots of these trees, known as mangroves, have a great capacity to trap litter — from both land and sea — which gradually breaks down until it is buried in the muddy bottom. Now, an article in the journal Environmental Pollution warns that the accumulation of rubbish threatens the environmental balance and biodiversity of these ecosystems, as well as the well-being of nearby local communities that depend on the resources provided by mangroves.

The study analyses, for the first time and on a large scale, the pressure and state of pollution from waste in Colombia’s mangroves. The first two authors of the article are Ostin Garcés-Ordóñez and Miquel Canals, from the UB Chair on Sustainable Blue Economy and the Consolidated Research Group in Marine Geosciences at the Faculty of Earth Sciences of the University of Barcelona. The study’s co-authors are Diana Romero-D’Achiardi, from the José Benito Vives de Andréis Institute of Marine and Coastal Research (INVEMAR, Colombia); and Martin Thiel, from the Smithsonian Environmental Research Center (SERC, United States) and the Catholic University of the North (Chile).

A major environmental problem

The mangroves most affected by rubbish are located near Colombian coastal cities such as Cartagena de Indias, Barranquilla and Riohacha (Caribbean coast), and Bahía Solano, Buenaventura and Tumaco (Pacific coast). “On the outskirts of coastal cities and in rural areas of the Pacific, the situation is exacerbated by the lack of adequate waste collection and treatment services. As a result, nearby mangroves are often used as uncontrolled dumps, although there are also accumulations of waste in the mangroves of marine protected areas, such as those on the island of San Andrés in the Caribbean,” says Ostin Garcés-Ordóñez, who is currently doing a postdoctoral research in the SERC’s MarineGEO programme under the supervision of Martin Thiel.

Much of the rubbish reaches the mangroves via rivers, rainwater and tidal currents. “In addition to natural factors, and beyond the distance from urban centres, the concentration of active sources of pollution in the mangrove environment is a determining factor. The largest amounts of rubbish — especially plastics — accumulate in marginal mangroves, a type of forest that grows on the margins of open bays and lagoons, where they are exposed to the action of the tides,” says Miquel Canals, professor at the UB’s Department of Earth and Ocean Dynamics and director of the UB Chair on Sustainable Blue Economy.

“This type of mangrove is exposed to the action of tides and coastal currents, which carry floating debris towards the coast. Marginal mangroves can be considered large sinks for marine debris, with around 2.5 objects accumulated per square metre,” says Canals.

In coastal mangroves, which occupy riverbanks, streams and floodplains, and in basin mangroves, which form around deep inlets and bays with little tidal influence, the amount of litter is less than 0.4 items per square metre.

Trapped rubbish accumulating on the mangrove floor

Over time, the trapped rubbish degrades and accumulates on the mangrove floor. “Plastics slowly fragment due to the action of the sun, water dynamics and interaction with mangrove organisms, such as crabs, generating increasingly smaller fragments that are progressively buried, further promoting their long-term persistence: this is ‘soil plastic carbon’,” warns Garcés-Ordóñez.

The structure of the roots of the trees that make up mangroves reinforces this process. The powerful stilt roots of the red mangrove (Rhizophora mangle) trap debris, which ends up becoming even more entangled in the mangrove root system due to the action of the tides. Similarly, the aerial roots (pneumatophores) of black mangroves (Avicennia germinans) and white mangroves (Laguncularia racemosa) also retain debris.

Some plastics can also be remobilized and transported by tidal action to nearby coastal bays or lagoons, or even to the open sea, where they tend to accumulate. “The smallest fragments, or microplastics, can enter marine food webs, a process that also poses a risk to wildlife and, ultimately, to the balance of coastal ecosystems,” says Thiel, executive director of SERC’s MarineGEO programme.

Protecting mangroves, an oasis of life and biodiversity

Rubbish that ends up in mangroves can cover much of the soil and roots, thereby affecting the delicate ecological balance, biodiversity and development of local communities. Fish, crustaceans and birds can become entangled in the litter or accidentally ingest fragments — such as microplastics — which can introduce toxins or pathogenic microorganisms into the food chain.
Avoiding single-use plastic packaging for mass-market products would be one of the most urgent measures to mitigate this environmental problem. In addition to promoting the use of reusable and returnable packaging, it is also essential to improve waste management and promote community education in public environmental protection policies.

“Access to basic sanitation and proper management of waste and other types of refuse are fundamental rights for a dignified life, and a key requirement for protecting coastal ecosystems for present and future generations in Colombia and other countries in the region and around the world,” the authors conclude.