Magic of Mangroves

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Mangroves aren’t much to look at. Clustered around the brackish waters of lagoons and estuaries, a mop of messy green foliage is all you see from afar. It’s only when the tide goes out, and the tangle of knotted roots appear, that you really ‘see the whole thing’.

One of the first western records of this plant appeared in the works of Pliny the Elder, the Roman author and naturalist who lived between AD23 and AD79. He had never seen plants that could grow on water and wrote of his discovery: “on the Red Sea, the trees are of a remarkable nature”.

Almost 2,000 years later, scientists are still discovering remarkable things about these plants, like the fact that they can sequester more CO2 than rainforests, says Professor Carlos Duarte, a leading marine ecologist at Saudi Arabia’s King Abdullah University of Science and Technology (KAUST).

As many Sri Lankans know from personal experience, mangroves also act as a first line of defence against extreme weather phenomenon, such as cyclones and tsunamis, something that’s happening with increasing regularity as a result of climate change.

Professor Duarte was among a team of experts working with the UN to assess the damage to coastal areas in South-East Asia, following the devastating Indian Ocean tsunami in 2004, where nearly a quarter of a million people lost their lives.

One surprising discovery they made was that those who lived in areas with mangroves suffered considerably less damage. “Those villages where pockets of mangrove remain between the village and the ocean actually had almost no loss of human lives, and the losses of infrastructure were much less,” he explained. “So, the mangroves are also the first line of defence for shorelines against cyclones, tsunamis and sea-level rise.”

But in Sri Lanka, one-third of the mangroves have been uprooted since 1990 to make way for expanding cities, coastal development and shrimp farms, says Anuradha Wickramasinghe,  Chair of Sudeesa, a non-profit organisation that oversees mangrove conservation and replanting projects. “People were only looking at short-term profits,” he says. “They destroyed the environment.”

Where mangroves were destroyed, fishing yields plummeted from around 20 kilogrammes to around four kilogrammes a day, says Wickramasinghe. However, the Sri Lankan government is now partnering with Sudeesa and a US-based conservation group Seacology to implement a programme of mangrove preservation and replanting that covers 35,000 hectares.

Thanks to their density and network of gnarly roots, mangroves are known for reducing beach erosion by holding the soil in place.

Any sediment that washes in with the tide, including the microplastics that are now so pervasive in the ocean, get trapped in this ever-expanding network of filters. Over time, as the clarity of the water improves, this sheltered environment becomes a nursery ground for fish and shellfish, which in turn help to support a rich ecosystem of marine life.

As well as helping to clean up polluted water, mangroves also play an important role in fighting climate change.  One study from 2011 estimated that a single hectare of mangrove forest stored an average of 1,025Mg C (1,025 tonnes of carbon). This is around four times that of tropical rainforests and, based on the World Bank’s estimates of CO2 emissions per capita, is enough to offset a year’s worth of emissions for 228 people.

The mangrove has several ways of capturing carbon dioxide, the first being the CO2 stored in the plant itself. The second is through its network of roots, which allow microbial mats to grow in their sheltered crevices. These mats are constantly sucking in the CO2 from the atmosphere to create oxygen, so much so that, you can actually see clusters of bubbles.

Just a few years ago, scientists discovered a third way that mangroves can eliminate carbon emissions: by using acids produced by their roots to dissolve carbonate rock such as limestone. This reaction turns carbon dioxide into a carbonate, a salt, and stops it from being released into the atmosphere.

In terms of climate change, growing mangroves on a large scale is actually much more sustainable than other types of forests, as they don’t need any fertilisers or additional water resources. And of course, mangroves’ coastal habitat means there’s never danger of fires, unlike its counterpart on land.

By Michael Gregson