Rising Sea Levels

Oysters as protection agains rising sea levels

How sand, oysters and sea otters can help against rising sea levels and climate change


When we talk about the dangers of climate change, one of our biggest concerns is often the rising of sea levels. Although my previous article showed that in the near future it is more important to worry about the shortage of fresh water, there is an important difference. It is easier to obtain fresh water than to stop the seas from rising. But people can be creative and there are different ways we might be able to protect ourselves.

The ecological Hondsbossche Zeewering

Hondsbossche zeewering PettenI grew up in a small town called Petten. It is located below sea level, but protected from the sea by a massive dike. The dike itself was around 5 km long between Petten and Camperduin and was called the ‘Hondsbossche Zeewering’.

In 2013 ‘Hollands Noorderkwartier’ part of Rijkswaterstaat’ the governmental institution in charge of water management in the Netherlands, calculated that the massive dike still wasn’t strong enough to resist a so called ‘Super Storm’ that occurs once every few hundred years. This time, instead of rising the height of the dike and increasing its strength, they opted for a more natural approach.  More than thirty-five million cubic meters of sand has been pumped up from the seafloor to create a wide beach and new dune in front of the exiting dike. It provides protection from the North Sea for at least another 50 years, after which it will be relative easy and cheap to increase the amount of sand again. The advantages of this new approach include that it is more environmentally friendly. Instead of creating massive amounts of unnatural ugly concrete, the pumped-up sand created a new natural environment for different species of flora and fauna, including sea lions. The new beach also created additional attractions for natural and recreational tourism, which is good for the economy of Petten.

Artificial oyster reefs as an answer to rising sea levels

In Bangladesh the environment is different than off the coast of Petten and their budget is very different as well. 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. Kutubdia 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 could not, stayed, inching back, as the island they had called home for generations changed around them. In 2012 Mohammed Shah Nawaz Chowdhury, research associate at the Institute of Marine Sciences at the University of Chittagong worked together with researchers at Wageningen University in the Netherlands. Subject of their research were artificial oyster reefs as a defense mechanism against coastal erosion. The idea is that these reefs calm the waves before they reach the shore. It has worked before in the Netherlands, and was also successful in Louisiana in the US. But Bangladesh, the Netherlands, and 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 skeptical, but we had to plan. A lot.” Chowdhury spent the next six years, including more than 600 days with 27 students living on Kutubdia Island, to find out if ecological engineering through oysters could save the dynamic coastline of his homeland?

Ecological engineering

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 the growth of sea grass, which in return increases the absorption of CO2 from the air. But 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 Haskoning DHV. “It’s a dynamic process, not just 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. Oysters as protection agains rising sea levelsHowever, to start this new reef Chowdhury and his team still had to use concrete as a base. “We wanted to use what was locally available and sustainably priced,” he says. The structure of choice became circular rings that the local residents stack and top with a commode to create latrines. These would not only be readily available, but could withstand the rough weather during the monsoon.

The best people to help find the right spot for the artificial reef were the indigenous coastal community. They know their coastal line best and are already attuned with the rhythms of the rise and fall of the tide across seasons. The local islanders’ knowledge of the kostura (the local word among islanders for oysters) also proved invaluable for Chowdhury and his research associates. In 2014 the first 300kg concrete rings went into the water at a test site. Once in the right place with the right conditions, the reef would be able to grow upwards and keep pace with sea-level rise. Minor damage to the reef, especially by natural causes, would be self-repaired, the new population grows again to fill the space and account for any oysters lost.

The test-reef completely dissipated waves lower than 50cm, and those over 1m also had their force considerably reduced. It worked even in intense weather – observed when tropical cyclone Roanu, with wind speeds of 70-110kph, hit the study site in May 2016. On top, the reef helped sediment to accumulate for as far as 30m (98ft) behind it! Even during the monsoon season silt was stabilized. The vegetation nearby flourished too compared with the control locations. This vegetation could be crucial in helping restore the coast, as it does in other parts of the country. We could have the oyster reef first, and behind that, the mangrove “Near the Sundarbans area, in the south-west of Bangladesh, you don’t see erosion, because mangrove forests act as a bio shield,” says Chowdhury. This is consistent with research by Susmita Dasgupta, an environmental economist at the World Bank. Mangroves are an effective way to protect against storm surges, she writes. The mangroves obstruct the flow of water with roots, trunks and leaves, reducing water flow velocity by 29-92%.

Secondary benefits of oyster reefs

The researchers also hoped that the oysters could become a nutritious source of food and rich harvesting opportunities for the local community. Unfortunately this went somewhat against the grain of local tastes. “Not all of our people are interested in oysters, some don’t even know you can eat them” says Chowdhury. Therefore they aren’t widely eaten in Bangladesh. However, the oyster reef also provided an indirect increase in the food supply. The Kutubdia fishermen became well aware of the fertile habitat and additional fishing ground the oyster reef provides. “We started finding mud crabs in the rings, drawn by the reef,” says Chowdhury. “Mud crabs have immense export value. 1kg can be sold for $10. Two to three families can easily make a livelihood off a small oyster reef by adding traps,” Chowdhury says. Other species the research team found included barnacles, sea anemones, gastropods and polychaetes (marine worms), all of which can attract fish.

Fishing in BangladeshOsman Ali, a 55-year-old fisherman who has spent his life on the island, says after the reefs began to grow, he no longer needed to work such strenuous hours to make a living. “We find a higher abundance of fish, shrimp and crabs near the kostura,” he says. And while the reef spanned only 100m, Ali is convinced: “If a larger reef had been created, it would increase our likelihood of getting more fish.” Chowdhury proposed a partnership with the local fishermen. “If you take care of the reef, it will take care of you”. So far, the oyster reef at Kutubdia Island seems to have held up the “natural” part of this partnership, but the necessary upkeep by humans has been somewhat neglected. The reef has grown, but a few portions have been broken down by boats at high tide. “The site has lost the sign pole we put up to avoid this,” says Chowdhury. “We don’t have the funds to put it back and monitor further.” He hopes to establish a larger testing site of at least a kilometer in length to really show the reef’s potential. Let’s hope Chowdhury soon receives the necessary financial support and enough local support to maintain and extend his project.

Sea otters as climate heroes

Meanwhile the (re)introduction of sea otters might help with maintaining the newly created mangrove in a sustainable way. Unlike many other marine mammals, sea otters lack blubber, so they keep themselves warm by eating a quarter of their body weight in food a day. Sea otters also play a singular role in supporting kelp forests ecosystems in the North Pacific. They help ecosystems capture carbon from the atmosphere and store it as biomass and deep-sea detritus, preventing it from being converted back to carbon dioxide and contributing to climate change.

Eating sea otter to reduce rising sea levelsIn order to maintain their high metabolic rates, seaotters must eat constantly. Among their favorite foods are sea urchins, which are easy to catch and dense in calories. When present, sea otters eat so many urchins that the invertebrate’s population stays low. “They have a disproportionately large impact on the ecosystem relative to their abundance,” says Heidi Pearson, a marine biologist at the University of Alaska Southeast. This is what makes them a “keystone” species. Without their presence, the stability of the entire ecosystem can be lost. When sea otters disappear from an ecosystem, sea urchin numbers spike. The herbivorous urchins then basically clear cut the kelp, chewing through the holdfasts at their base and sending the rest of the giant algae to wash away. Along with it goes the habitat for numerous species, including fish, invertebrates and other mammals.

Sea urchins even stick around after slashing through kelp beds. They enter a dormant state, biding their time under new kelp sprouts, and then revving back into action to eat the young seaweed. These invertebrates have become known as “zombie urchins” for this ability.

Even before the sea otter effect was studied, kelp has been thought of as a possible climate solution. That’s because it can grow really fast – up to 60cm a day. That means it’s pulling carbon from the atmosphere faster than a slower growing plant (though kelp is technically an alga). When Sea urchins cut-off the kelp, it washes ashore and dies on the surface. The carbon from this kelp then returns to the atmosphere during decomposition. But when kelp dies naturally and sinks to the sea floor, it might not surface. “A kelp frond that drifts to the seafloor and decomposes under water, that carbon might be trapped in the sediments for millennia, even millions of years,” says Pearson. If sea otters return, their feasting can put the sea urchins in check and allow the kelp to flourish once again. But that isn’t all. These hungry mammals do not only protect the kelp ecosystems. Sea otters can also benefit sea grass. In these zones, otters mostly feed on crabs. When the mustelids bring down the numbers of crabs, grazing organisms that the crabs eat rebound. These slugs and snails often don’t eat the sea grass; instead they scrape away the algae that grows on the grass, which allows the sea grass to absorb more sunlight and grow more efficiently. “They miraculously don’t eat the sea grass,” says Hughes. “They have this little radula that gently scrapes the sea grass and takes off all the epiphytes growing on it. And so it essentially protects the sea grass.”

In 2012, a team of ecologists including Estes published a study on the potential for sea otter carbon sequestration in the North Pacific between the Aleutian archipelago and Vancouver Island. Using data on the rate of kelp growth and its density at sites with and without otters, they found that the presence of sea otters across rocky reef habitat in the study area, covering 51,551 square kilometers (19,900 square miles – an area about the same size as Costa Rica), is capable of storing 4.4 to 8.7 million tons of carbon compared to if an otter-free condition. That’s more carbon than that emitted from a million passenger cars for a year.

The economical value of sea otters

Sea otters were once widespread across coastal waters in the Northern Pacific Ocean, from Baja California to Alaska, all the way to rocky reefs in Russia, Japan and Thailand. However, in the 1700s and 1800s, fur traders hunted their population down to about 2,000 animals. Since then, conservation efforts have allowed otters populations to slowly start recovering. But reintroducing otters might not be a win for everyone. Their massive appetites can diminish fishing opportunities for commercial operations and subsistence-based Indigenous communities.

Relaxing sea otterQuantifying the effect of otters might help ease some of the impacts to fisheries. Based on the December 2012 price on the European Carbon Exchange ($47 per ton of carbon), Estes’ paper that year estimated that sea otter presence in their North Pacific study area was worth up to $408m. Carbon prices have climbed since then, recently exceeding $71 a ton, which would bring that estimate even higher in today’s market. A 2020 study found that the monetary benefit of sea otters could outweigh the losses to shellfish fisheries. This due to their restoration of kelp habitat and associated increase in fish stocks, carbon sequestration, and ecotourism value. A win-win strategy could be to use money from otter-generated carbon offsets to compensate fishers for losses in their catch 
As you can read above, everything in nature and society is connected. If the value of otter carbon banking is recognized by our human banks, maybe more sea otters will once again swim across more reefs and estuaries, fighting climate change and rising sea levels while they’re at it.

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