Coastlines + water + oysters = climate resilience.
Oyster reefs have remained a quiet yet steadfast line of defence against the forces of climate change, particularly the rising sea levels and eroding coastlines. In these challenging times, oysters emerge as more than just a prized delicacy; they transform into a natural defence against the advancing tides. The very same shells that adorn dinner plates become the calcified shields that protect our shores from the relentless grip of the sea. They are often overshadowed by the beauty of coral reefs, because they are less charismatic as a species but are just as valuable for ecosystems.
(Also Read: Corals: a story of death and rebirth)
What are oyster reefs?
Oyster reefs are unique structures formed by clusters of oysters that create hard surfaces in areas where soft sediments dominate. These reefs serve as vital habitats for a wide variety of marine life, including different types of mollusks, worms, crustaceans, and juvenile fish. The presence of these reefs provides shelter and foraging grounds, allowing many species to thrive.
One of the key roles of oyster reefs is their ability to filter water. Oysters consume nutrients, sediments, and tiny plants called phytoplankton from the water, which helps keep the water clean and clear. This filtering process not only improves water quality but also supports the health of nearby habitats, such as seagrass beds, by allowing more sunlight to penetrate the water. This is particularly important in preventing problems like eutrophication, where excess nutrients lead to harmful algal blooms.
Oyster reefs protect coastlines by absorbing wave energy, which reduces erosion. They act as natural barriers, breaking the waves coming towards the shore using their unique uneven shapes, rough texture and strong base. They help shield valuable habitats like salt marshes from the impacts of waves and storms when the tides are at their most powerful. As oysters grow, they sequester carbon from the water by forming calcium carbonate shells. They help increase the capacity of oceans to absorb carbon dioxide from the atmosphere, which can help lower greenhouse gas levels in the atmosphere.
Moreover, these reefs promote a process called denitrification. They reduce nitrogen levels in the water by concentrating waste materials like feces,which can enhance the growth of benthic plants and improve overall ecosystem health. The positioning of oyster reefs can also influence larger landscape processes, acting as corridors that connect different habitats for various marine species.
Oyster reefs are not just important for the oysters themselves; they play a crucial role in maintaining the health of coastal ecosystems by providing habitat, improving water quality, protecting shorelines, and contributing to carbon sequestration and nutrient cycling.
How can oyster reefs protect coastlines?
Oysters are a nature based solution for coastline management. They are not picky eaters at all. In fact, they love to feast on the tiny plants and nutrients that are suspended in the water, like algae and phytoplankton. The more of these "goodies" in the water, the better the oysters like it. To feed, oysters have a clever system called filter feeding. They take in a lot of water through their gills, which are lined with tiny hair-like structures called cilia. These cilia act like a sieve, trapping all the solid and semi-solid particles in the water, including the plankton and nutrients that oysters love to eat.
Once the oysters have filtered out the good stuff, they mix it with a special sticky substance called mucus to form a tasty meal. They then swallow this food and digest it, using the nutrients to grow, repair themselves, and even reproduce.
But what happens to all the stuff that oysters can't digest? Well, they have a couple of ways to get rid of it. First, they poop it out as long, stringy faeces that sink to the ocean floor. Second, they expel it in mucus-covered clouds or sprays, called pseudofaeces, that also settle on the seabed. These faeces and pseudofaeces, called biodeposits, are like a feast for the bacteria and microorganisms that live on the ocean floor. These tiny creatures then work their magic, transforming the trapped pollutants in the biodeposits into harmless nitrogen gas. This gas is then released into the atmosphere or used by other organisms in the ocean.
This process is called the Nitrogen Cycle, and it's essential for keeping the ocean in balance. Without it, the water would become polluted and unhealthy for all the amazing creatures that call the ocean home.
Another recent study indicates that oyster reefs may have historically played a significant role in protecting coastlines from flooding. Following Hurricane Sandy, researchers from UMass Amherst examined coastal ponds near New York to assess sediment deposits from the storm and compared them to sediment layers from previous storms. They were surprised to discover that similar sediment deposits only dated back 200 to 400 years.
Historically, oyster beds covered the estuaries of the Hudson and Raritan rivers, and the researchers believe these reefs provided vital coastal protection before they were largely destroyed by early European settlers. A computer simulation conducted to test this idea revealed that the wave energy during extreme storms is now between 30% and 200% higher than it was when the oyster reefs were intact.
Cost-effectiveness of oyster reef restoration projects
Shoreline armoring refers to the use of artificial structures to protect coastlines from erosion. When natural protections like oyster reefs are missing, many people turn to seawalls and bulkheads made from materials like wood, concrete, or metal, which are often called "hard" armoring. These structures are usually built vertically, like walls.
Vertical armoring can cause problems because the water can erode the base of these structures over time, which can weaken them and harm local habitats. Additionally, these hard structures reduce the area of the intertidal zone, which is crucial for many plants and animals that live in estuaries. Installing and maintaining these armoring solutions can be expensive for property owners.
Artificial structures such as bulkheads, jetties, and breakwaters come with a hefty price tag, often exceeding $1 million per hectare to construct. In contrast, the average cost for restoring oyster reefs in the United States is around $2,260 per hectare, although this can vary widely. For homeowners with a budget of $10,000, this amount could protect approximately 68 metres of shoreline using an oyster reef. However, the same budget would only provide about 14 linear metres of protection if spent on a bulkhead. This highlights the cost-effectiveness of oyster reef restoration compared to traditional shoreline armoring methods.
Despite the costs, hard armoring often does not provide effective long-term protection against flooding and erosion caused by rising sea levels. Unlike natural solutions, these man made structures cannot adapt to changing conditions, such as rising water levels. They also don’t have the ability to repair themselves like living shorelines, such as oyster reefs, which can regrow after being damaged. This means that shoreline armoring projects, like seawalls and bulkheads, can lead to ongoing maintenance costs, which can diminish their overall value.
Oyster reefs offer a more economically friendly option for combating shoreline erosion. They help reduce wave height and absorb wave energy, which protects the shoreline from erosion. In natural settings, oyster reefs act as shields for salt marshes, absorbing the energy from incoming waves and helping to stabilise sediments, which prevents marsh loss.
One of the biggest advantages of oyster reefs is their ability to grow and adapt to changing conditions, such as rising sea levels. Healthy, unharvested oyster reefs can grow vertically at a rate that keeps pace with or even exceeds the rise in sea levels. This vertical growth is crucial because it allows oyster reefs to act as living barriers against erosion, increasing in size as water levels rise. In contrast, artificial shoreline armoring typically needs to be replaced with taller structures over time. Because of these qualities, oyster reefs can often be just as effective, require less maintenance, and be more cost-efficient compared to artificial structures in many cases.
(Also Read: Feeding the Future)
Case studies
Oyster breakwater reefs in Kutubdia Island, Bangladesh: Researchers in Bangladesh conducted an experiment to test this by building three oyster-covered breakwater reefs on an eroding mudflat. They studied how the reefs affected wave energy, shoreline changes, sediment patterns, and the growth of nearby salt marshes over four seasons, including the rainy monsoon period.
The reefs were able to block up to 100% of small waves when the water level was less than half a metre deep. Even for higher water levels between 0.5-1 metre, the reefs reduced wave heights by 95-100%. This wave reduction helps shield the shoreline from erosion, especially during storms.
On the sheltered side of the reefs, there was a significant buildup of clay sediment, with an average of 29 cm accumulating. This sediment trapping was key in lowering erosion rates in reef areas compared to control sites without reefs. The reefs also enabled nearby salt marshes to expand seaward by about 1.4 metres per year, while marshes in areas without reefs actually retreated.
References
Jonathan H. Grabowski and Charles H. Peterson. (2007). Ecosystem Engineers - Plants to Protists. RESTORING OYSTER REEFS TO RECOVER ECOSYSTEM SERVICES, 281–298. https://sci-hub.st/10.1016/s1875-306x(07)80017-7
Walles, B., Troost, K., van den Ende, D., Nieuwhof, S., Smaal, A. C., & Ysebaert, T. (2016). From artificial structures to self-sustaining oyster reefs. Journal of Sea Research 1–8. https://sci-hub.st/10.1016/j.seares.2015.11.007
Oyster Reef Habitat. (n.d.). Https://Www.Fisheries.Noaa.Gov/National/Habitat-Conservation/Oyster-Reef-Habitat. https://www.fisheries.noaa.gov/national/habitat-conservation/oyster-reef-habitat
Goelz, T., Vogt, B., & Hartley, T. (2020). Alternative Substrates Used for Oyster Reef Restoration: A Review. Journal of Shellfish Research, 39(1), 1. https://doi.org/10.2983/035.039.0101
Very interesting! thank you for sharing