OVER TIME, THE OYSTERS CLUSTERED THICKLY ON THE CONCRETE RINGS TO FORM A RICH REEF
(CREDIT: M. SHAH NAWAZ CHOWDHURY)
The humble oyster has proven remarkably effective in protecting one of Bangladesh's most vulnerable islands from fast-rising seas.
Nothing could have prepared Mohammed Shah Nawaz Chowdhury for the sobering reality of Kutubdia Island. The island, off the coast of southern Bangladesh, was rapidly eroding into the sea, causing both land and life to retreat. Many packed up and moved. Those who couldn't, stayed – inching back, as the island they had called home for generations changed around them. "These were families I had gotten to know, trust and respect," he says. "It regularly moved me to tears."
Chowdhury was witnessing Bangladesh's climate migration, caused by sea level rise linked to climate change. By 2050, up to 13.3 million Bangladeshis may become displaced due to climate change.
Yet, just off Kutubdia Island's shore, a glimmer of hope is visible amid the waves. Jutting out of the water, oyster-encrusted reefs glisten in the sun. These reefs are vibrant homes for marine life, a potential source of income for local people and, Chowdhury hopes, they could become a formidable force to defend Kutubdia Island from sea level rise.
Kutubdia Island, with its fast receding shoreline, is a microcosm of these challenges at their fiercest
The idea of Kutubdia Island's oyster reefs was born in 2012, when Chowdhury was a research associate at the Institute of Marine Sciences, at the University of Chittagong, in Bangladesh. The premise was simple: oyster reefs could serve as a defence mechanism against coastal erosion by calming waves before they reached the shore. The idea had worked before in the Netherlands, and had also had success in Louisiana in the US. Now, researchers at Wageningen University would work together with Chowdhury and his colleagues to see if they could improve the situation at Kutubdia Island.
The catch, of course, was that Bangladesh and the Netherlands, and indeed Louisiana, are thousands of miles apart and have wildly different environmental contexts. "We were facing a lot of challenges like natural sedimentation due to river run-off, storm surges and other monsoonal effects that make our coast so dynamic," says Chowdhury, "I wasn't sceptical, but we had to plan. A lot."
It was a tantalising prospect. Could ecological engineering through oysters save the dynamic coastline of Chowdhury's homeland? He spent the next six years, including more than 600 days with 27 students living on Kutubdia Island, finding out.
(Credit: Getty Images)
Regularly pelted by storms, cyclones and sea level rise, Bangladesh's coast is susceptible to many climate stresses. Kutubdia Island, with its fast receding shoreline, is a microcosm of these challenges at their fiercest. "Potentially, it has been estimated that by 2050, one in every seven Bangladeshis will be displaced by climate change," says Chowdhury. "We're getting more ferocious waves due to global warming and heating waters."
Traditional structural interventions, such as concrete embankments or dykes, have been a common response to protect the shoreline. According to Rezaul Karim Chowdhury, executive director of the Bangladeshi NGO Coast, 60% of the country's coast is protected by embankments. But some argue that building a living defence, rather than one made of concrete, could be more effective.
Oyster breakwaters
Ecological engineering, or eco-engineering, involves designing sustainable ecosystems with both natural and human value. Oysters engineer their environment by clustering on hard, submerged surfaces and fusing together to create reef structures. Their role in filtering and retaining nutrients in water, providing spawning and shelter for fish, and so boosting biodiversity is well-documented. Oyster reefs provide a habitat for other animals, improve water quality and enhance seagrass growth.
But Mohammed Shah Nawaz Chowdhury and his colleagues in the Netherlands were particularly interested in their role as natural breakwaters. Oyster reefs can provide relief to a coast constantly buffeted by strong waves. That is not their only benefit, though. "What you want is the sedimentation behind the reef structure that the oysters naturally form. The reefs give you a more extensive foreshore and [consequently] calmer waters," says Petra Dankers, senior consultant for morphology and eco-engineering at Royal HaskoningDHV, which partnered with Wageningen University, in the Netherlands, on the Kutubdia Island project.
Such fluidity is the hallmark of the building with nature approach. And rather than being seen as a setback, it becomes part of the plan.
"It's a dynamic process – not hard concrete. And that's the new understanding of using natural forces to achieve our goals," says Aad Smaal, emeritus professor of sustainable shellfish culture at Wageningen University.
The artificial reefs provided a home for many other species besides oysters (Credit: M. Shah Nawaz Chowdhury)
The natural forces shaping Kutubdia were especially important to understand because, unlike many oyster reef projects across the world (for example, in the Gulf of Mexico), this was not a restoration of a declining reef. It was an introduction of new reefs as engineering structures.
Fortunately, the region presented many of the ideal conditions necessary for the reef, Chowdhury found in his initial survey. There was the appropriate water temperature, water flow velocity, pH levels, salinity and dissolved oxygen. The marks of life already existing in the water, such as phytoplankton, also indicated that this could be a place where oysters would thrive.
Next, they needed to check if oyster larvae were already present in the Bay of Bengal waters. "It's not just a case of putting something in the water and expecting it to work," says Smaal. One of the best substrates (surfaces on which organisms like molluscs grow), he says, is existing oyster shells. "It's also about successful recruitment, meaning the passively drifting larvae will settle and not move anymore," he adds. "Even in an open system with strong currents."
In fact, Kutubdia Island showed large clusters of oysters growing naturally on concrete pillars, especially near a jetty. And as studies suggest, concrete is one of the most successful alternative substrates after natural oyster shells – with reefs on concrete equalling or in some cases surpassing the size, biomass and density of oysters.
Regularly pelted by storms, cyclones and sea level rise, Bangladesh's coast is susceptible to many climate stresses. Kutubdia Island, with its fast receding shoreline, is a microcosm of these challenges at their fiercest. "Potentially, it has been estimated that by 2050, one in every seven Bangladeshis will be displaced by climate change," says Chowdhury. "We're getting more ferocious waves due to global warming and heating waters."
Traditional structural interventions, such as concrete embankments or dykes, have been a common response to protect the shoreline. According to Rezaul Karim Chowdhury, executive director of the Bangladeshi NGO Coast, 60% of the country's coast is protected by embankments. But some argue that building a living defence, rather than one made of concrete, could be more effective.
Oyster breakwaters
Ecological engineering, or eco-engineering, involves designing sustainable ecosystems with both natural and human value. Oysters engineer their environment by clustering on hard, submerged surfaces and fusing together to create reef structures. Their role in filtering and retaining nutrients in water, providing spawning and shelter for fish, and so boosting biodiversity is well-documented. Oyster reefs provide a habitat for other animals, improve water quality and enhance seagrass growth.
But Mohammed Shah Nawaz Chowdhury and his colleagues in the Netherlands were particularly interested in their role as natural breakwaters. Oyster reefs can provide relief to a coast constantly buffeted by strong waves. That is not their only benefit, though. "What you want is the sedimentation behind the reef structure that the oysters naturally form. The reefs give you a more extensive foreshore and [consequently] calmer waters," says Petra Dankers, senior consultant for morphology and eco-engineering at Royal HaskoningDHV, which partnered with Wageningen University, in the Netherlands, on the Kutubdia Island project.
Such fluidity is the hallmark of the building with nature approach. And rather than being seen as a setback, it becomes part of the plan.
"It's a dynamic process – not hard concrete. And that's the new understanding of using natural forces to achieve our goals," says Aad Smaal, emeritus professor of sustainable shellfish culture at Wageningen University.
The natural forces shaping Kutubdia were especially important to understand because, unlike many oyster reef projects across the world (for example, in the Gulf of Mexico), this was not a restoration of a declining reef. It was an introduction of new reefs as engineering structures.
Fortunately, the region presented many of the ideal conditions necessary for the reef, Chowdhury found in his initial survey. There was the appropriate water temperature, water flow velocity, pH levels, salinity and dissolved oxygen. The marks of life already existing in the water, such as phytoplankton, also indicated that this could be a place where oysters would thrive.
Next, they needed to check if oyster larvae were already present in the Bay of Bengal waters. "It's not just a case of putting something in the water and expecting it to work," says Smaal. One of the best substrates (surfaces on which organisms like molluscs grow), he says, is existing oyster shells. "It's also about successful recruitment, meaning the passively drifting larvae will settle and not move anymore," he adds. "Even in an open system with strong currents."
In fact, Kutubdia Island showed large clusters of oysters growing naturally on concrete pillars, especially near a jetty. And as studies suggest, concrete is one of the most successful alternative substrates after natural oyster shells – with reefs on concrete equalling or in some cases surpassing the size, biomass and density of oysters.
Post continues at: https://www.bbc.com/future/article/20210827-the-unlikely-protector-against-rising-seas-in-bangladesh
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