How oysters are restoring the health coastlines around the world
The Chesapeake Bay once had oyster reefs so large they were seen as navigational hazards, but it has now lost as much as 99% of its native oyster population. Around the world, four-fifths of all oyster reefs have disappeared in the last hundred years.
hat is a problem for multiple reasons. Oysters purify water, filtering out pollutants. Oyster reefs provide a habitat for all kinds of sea creatures, and food for humans. And oyster reefs help with adapting to climate change: much like purpose-built structures, they can help to prevent erosion of shorelines by dissipating the power of waves.
Oyster reefs are formed when baby oysters, or spat, attach to and grow on other oysters’ shells. The resulting structures, comprising living and dead oysters, can grow to hundreds of metres. New oyster reefs can be artificially encouraged to form by depositing a mass of oyster shells – or another hard substance, such as concrete – seeded with spat.
This is an example of an approach known as ecological engineering. Researchers from Wageningen University studied its effectiveness by growing an oyster reef in Kutubdia Island on the southeast coast of Bangladesh. Based on a successful experience in Oosterschelde in the Netherlands, it was the first time the idea had been tested a sub-tropical, monsoon climate.
As sea levels rise and storms become more extreme, protecting coastlines from erosion and flooding becomes ever more important – especially in Bangladesh, where an estimated sixth of the country’s land could be submerged by 2050, displacing 20 million people.
As explained in the below video by the Wageningen team, the reefs not only break up the waves, they trap sediment between themselves and the shoreline. This can enable the planting of mangroves, which further help to protect shorelines from wave damage.
In Oosterschelde, the artificial reef was constructed using oyster shells held together initially by mesh – by the time the mesh decayed, the reef was well enough established to hold together. In Bangladesh, the construction needed to be more robust from the start, so the researchers used a row of precast concrete rings 80cm high and wide.
The results, published last year in Nature, showed that the artificial reefs reduced erosion by over half compared to control sites, and were effective in dissipating energy from waves.
Another study suggests that oyster reefs may have historically played a large role in protecting against coastal flooding. After Hurricane Sandy, researchers at UMass Amherst looked at coastal ponds near New York to see how much sediment the storm had deposited, and compare it to layers of sediment deposited by storms in previous years. To their surprise, they found that similar deposits dated back only to between two and four hundred years ago.
Oyster beds used to cover the estuaries of the Hudson and Raritan rivers. The researchers hypothesise that they protected the coastline, before their destruction by early European colonists. A computer simulation to test this hypothesis suggested that wave energy in extreme storms is between 30% and 200% greater today than when the oyster reefs existed.
The researchers warn that the technique they used in Bangladesh will not be suitable everywhere: the country’s central and southwestern coasts have too much discharge of sediment-laden freshwater from the Ganges, Brahmaputra and Meghna rivers to be suitable for oyster reefs. However, it could be applied along almost 400km of the south-eastern coast.
As one of the Netherlands researchers, Marijn Tangelder, explains: “The great thing is that such solutions are flexible and can grow with sea level. This is different with rigid structures such as dikes.” Indeed, researchers from the University of North Carolina and Northeastern University found that oyster reefs have the potential to grow vertically by up to 11 centimetres per year.
Oyster reef projects are underway all around the world – from the Solent in the UK to New South Wales in Australia. Meanwhile, the Chesapeake Bay Foundation is asking local restaurants to save empty shells for recycling, placing them in water tanks that contain oyster larvae, and giving them to community volunteer “oyster gardeners” to plant and tend. Each recycled shell can be home to dozens of spat, slowly restoring the Bay’s oyster population – with all its many benefits.
The ideas presented in this article aim to inspire adaptation action – they are the views of the author and do not necessarily reflect those of the Global Center on Adaptation.
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