With deep blue waters flanked by dramatic peaks, Guanabara Bay is the postcard view of Rio de Janeiro – but it is also one of Brazil’s most polluted coastal environments. Raw sewage and solid waste flow into the bay from surrounding cities, home to more than 8 million people. Cargo ships and oil platforms chug in and out of commercial ports, while dozens of abandoned vessels lie rotting in the water.

But at the head of the bay, between the cities of Itaboraí and Magé, the environment feels different. The air is purer, the waters are empty but for small fishing canoes, and flocks of birds soar overhead.

This is thanks to a thriving mangrove forest, protected as part of the Guapi-Mirim environmental protection area (APA Guapi-Mirim) and successfully restored by local fishers.

New mangrove trees thriving in an area of the APA Guapi-Mirim recently reforested by the Uça team. Photograph: Mariana Greif

“If the APA Guapi-Mirim hadn’t been created on 25 September 1984, Guanabara Bay would have died. All this area would have become an airport, logistics for trucking, housing estates,” says Alaildo Malafaia, a 63-year-old fisher turned environmentalist, as he steers a motorboat down the Macacu River in the protected area.

Solid waste affects the mangroves … crabs can no longer dig their burrows, and trees no longer find space to grow

Janaína Oliveira, Projeto Uçá

The APA Guapi-Mirim covers 14,000 hectares (35,000 acres), of which 6,000 hectares are mangrove swamps, home to a rich biodiversity of crustaceans, fish, mammals and birds. “My favourite is the cocoa heron,” says Malafaia as the boat passes great egrets and pink roseate spoonbills perched on tangled roots.

Although they make up less than 1% of tropical forests worldwide, mangroves play a vital socio-environmental role. They serve as nurseries for marine species that provide a livelihood for fishers. They filter pollution and help protect coastal areas from natural disasters such as storms and hurricanes. And they are an important carbon sink, absorbing two to four times more carbon than other forests in Brazil.

Mangroves growing in the APA Guapi-Mirim environmental protection area across Guanabara Bay from the city of Rio de Janeiro. Photograph: Mariana Greif

The country has the second-largest mangrove area in the world, covering about 1m to 1.4m hectares of its coastline from the Amazon in the north down to Santa Catarina state in the south. It recently joined the Mangrove Breakthrough, a global initiative to conserve 15m hectares of mangroves by 2030.

Brazil is estimated to have lost 25% of this fragile ecosystem since the start of the last century, mainly to urbanisation – but today, 87% of its mangroves are protected.

It all started with the APA Guapi-Mirim, the first conservation area specifically designated to protect mangroves.

Mangrove forests in Guanabara Bay – so far, 320 hectares have been restored by local people backed by the NGO Projeto Uçá. Photograph: Rodrigo Campanario/Projeto Uçá

In the late 1970s, Guanabara Bay’s remaining mangroves were being cleared on a large scale to fuel kilns in brick factories. Realising that the bay’s survival was at stake, researchers campaigned to conserve the area, prevailing over urban developers who planned to reclaim the land for construction.

For more than two decades, the mangroves were left to recover naturally. This was a success, but in some areas invasive species had taken hold, preventing natural regeneration.

“From 2008 onwards, once we realised that there were sizeable areas needing restoration, we implemented a community-based mangrove restoration project,” says Mauricio Barbosa Muniz, an environmental analyst at the government’s conservation agency ICMBio and former director of the APA Guapi-Mirim.

So far, 320 hectares have been restored with the help of NGOs that pay residents for their work. “Those mangroves over there, they were all reeds. They were replanted in 2013 or 2014. Look at the size of the trees,” says Eugênia Maria Santos, pointing to an area she helped restore on a bend in the Macacu River.

Santos, 60, is president of the Cooperativa Manguezal Fluminense, a group of fishers and crab pickers who help protect the mangroves that provide their livelihood.

Eugênia Maria Santos has worked on mangrove restoration since 2008. Photograph: Constance Malleret

The importance of involving local communities in environmental protection efforts is well documented. In this case, it led to the development of a more effective planting method.

Initially, fishers would grow mangrove seedlings at home. “But it didn’t work, because the plant was watered with fresh water. Then it would go to the mangroves, with seawater. It didn’t survive,” says Santos.

Mangrove seeds germinate on the tree, forming a pen-shaped structure called a propagule, which then falls into the mud or water. Fishers had the idea of collecting propagules from beneath parent trees, where they wouldn’t survive due to lack of sunlight, and replanting these seedling mangroves in the areas to be restored. “We called this a transplant,” says Malafaia.

Residents also work collecting rubbish that gets caught in the mangrove swamps, carried in from cities by the tide and rivers. Fishers’ boats usually carry sacks filled with driftwood, plastic bottles, and mud-encrusted toilet seats among the more unusual objects.

Janaína Oliveira, biologist and coordinator of the NGO Projeto Uça, with a mangrove crab found in a recently reforested area. Photograph: Mariana Greif

“Solid waste affects the mangroves, as it occupies an area where crabs can no longer dig their burrows and where trees can no longer find space for the seeds to grow,” says Janaína Oliveira, a marine biologist and coordinator of the conservation project Projeto Uçá.

It is named after the Ucides cordatus crab, one of these mangroves’ most notable species due to its ecological and economic importance.

Rita de Conceiçao Duarte at work. Photograph: Rodrigo Campanario/Projeto Uçá

Rita de Conceição Duarte, 68, earns 1,200 reais (about £170) a month picking rubbish two days a week with Projeto Uçá, which runs cleanup activities during the crab’s reproduction period, when catching it is prohibited.

Thanks to this and her work with another cleanup project, she has saved enough money to buy a plot of land where she grows squash, fruit trees and medicinal plants. More than anything, though, Duarte loves being out in the mangroves, wading through the swamp. She puts this down to coming from a long line of fishers. “It’s in my blood,” she says.

Like other fishers, Duarte also takes satisfaction from seeing her work bear fruit and the mangroves recovering. “There was a change for the worse. Even these crabs that we would see on the riverbank had disappeared. But after all this work, they started appearing again,” she says.

According to Oliveira, 60 animal species have returned thanks to the restoration of mangroves, while Santos says the trees now protect her neighbourhood from storm damage, cushioning the wind’s impact.

Guapi-Mirim’s mangroves remain at risk from climate breakdown and the activities of a nearby oil refinery owned by Petrobras, the state-controlled oil company. Petrobras funds Projeto Uçá as part of its ESG commitments.

A mangrove crab making its home in the mud of Guanabara Bay in an area recently reforested by the Uça team. Photograph: Mariana Greif

Yet the pollution of Guanabara Bay has improved, says Muniz, with more sewage being treated and stricter rules around industrial activity. And the work keeps going: Projeto Uçá has restored 18.2 hectares since 2013 and plans to more than double that by 2029.

“Even in the face of so much adversity, we were able to show that it is possible to not only conserve, but also recover this ecosystem,” says Muniz. “The mangroves of the APA Guapi-Mirim are a symbol of resistance.”