To combat the rising sea levels in Bangladesh, a farmed shellfish came to help

Bangladesh is a low-lying South Asian country, located in the north of the Bay of Bengal. Most of the country's land is the Brahmaputra alluvial plain, with many small islands and sandbars along the coast. Water gave birth to rice and civilization, but it also brought suffering. Seasonal floods and regular waterlogging often inundate a quarter of Bangladesh’s land, while the coast is threatened by storms and hurricanes, and rising sea levels associated with climate change exacerbate the crisis.

It is estimated that by 2100, the sea level on the coast of Bangladesh will rise by 0.4 to 1.5 meters . By 2050, as many as 13.3 million Bangladeshis may be displaced by climate change.

▲ An artificial island in Bangladesh. Picture from: Science

But on Kutubdia Island on the southern coast of Bangladesh, the oyster reef that has surfaced offers a glimmer of hope. These oyster reefs are no longer just a sweet home for marine life and a source of income for locals, but may also serve as a barrier to protect islands from rising sea levels.

▲ Oyster Reef. Picture from: M. Shah Nawaz Chowdhury

Oyster reefs are aggregates composed of live oysters, dead oyster shells, and other reef organisms. It has a long history as a "fortification".

In the US state of Louisiana , wetlands occupies one-third of the state’s area. Among them is an artificial seawall with a total length of about 0.8 kilometers and composed entirely of oyster shells. For this purpose, 26 local restaurants provided approximately 1.7 million pounds of oyster shells. . They were placed in the sun for six months to remove food residues and bacteria, and then "weaved" into 434 "baskets" of 1m x 1m x 2m. After being inoculated with oyster larvae, they reached their new home in the swamp by barge.

▲ Oyster Reef. Picture from: Amanda Ruggeri

The advantage of the oyster shell is that its corners and crevices provide extra surface area, which can absorb wave energy and buffer the coastline better than a flat structure; it also allows bacteria and algae to flow through it, providing protection for smaller marine life, and Provide tidal currents for the swamp to "breath". One of the benefits of cultivating live oysters is to make oyster reefs grow vertically, and the growth rate is faster than expected sea level rise.

Although the environmental conditions in Bangladesh are quite different from those in Louisiana, scientists are aware of one thing-building a living defense system may be more effective than traditional structural interventions such as concrete dams.

▲ Oyster shell. Picture from: Getty Images

In the sea, oyster reefs can not only alleviate the impact of strong waves on the coast, but also provide a wider foreshore and calmer waters. In addition, its function is similar to that of being in a swamp—as a water filter to retain nutrients in the water, provide habitat for fish, etc., promote biodiversity, improve water quality, and make seagrass flourish.

Although concrete is not as superior as natural oyster shells, it is also one of the best alternative substrates. On Kutudia Island, large groups of oysters grow naturally on concrete pillars and eventually slowly accumulate into oyster reefs. Concrete is also a locally available and reasonably priced product.

▲ Oysters grow on concrete pillars. Picture from: M. Shah Nawaz Chowdhury

It is especially important to understand the natural forces that shape Kutudia Island. It is the local fishermen who help researchers find the best location on the oyster reef. Living here for so long, they have adapted to the rhythm of seasonal tides. Under the right location and conditions, the oyster reef will grow upwards and keep up with the sea level. The slight damage can also repair itself, and new populations will continue to grow in the damaged area.

▲ Oysters. Picture from: Getty Images

Researchers also hope that oysters can become a nutritious food for local communities, but this is a bit contrary to local tastes. Although fishermen do not eat oysters directly, they know the fertile habitat and fishing grounds provided by oyster reefs. Mud crabs attracted by oyster reefs have a huge export value. 1 kg can be sold for $10.

Local fishermen believe that oyster reefs will make their livelihood less burdensome:

If we create a larger oyster reef, it will increase our chances of getting more fish. If you take care of the oyster reef, it will take care of you.

Oysters are only the first line of defense, and they can also promote the growth of mangroves on the second line of defense. Mangroves are an effective way to resist storm surges. Roots, tree trunks and leaves block the water flow, reducing the water flow speed by 29-92%.

▲ Mangrove. Picture from: unsplash

But the problem in Bangladesh is not just about nature. Human factors such as groundwater extraction, oil and gas extraction, etc., have caused an imminent problem-the soil is sinking rapidly. This is fatal to a low-lying coastal country.

Therefore, when Kutudiya's oyster reef protects this island as a gift of nature, the participation of humans becomes even more necessary. Aad Smaal, Professor Emeritus of Sustainable Shellfish Farming at Wageningen University, is still hopeful about this:

Oysters are robust and will become part of the paradigm that changes coastal defense. The starting point of this idea is that humans use natural resources to build together with nature.

▲ References:
https://www.bbc.com/future/article/20210827-the-unlikely-protector-against-rising-seas-in-bangladesh
https://www.science.org/news/2018/03/sea-levels-rise-bangladeshi-islanders-must-decide-between-keeping-water-out-or-letting
https://www.bbc.com/future/article/20180822-recycled-shells-of-louisiana-oyster-reef-protect-new-orleans

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