Ocean acidification is destroying oyster reefs – here's how that could affect the environment and the economy

In the intricate tapestry of coastal ecosystems, the threat of ocean acidification has emerged as a menacing force. It transcends boundaries, leaving a distressing yet still delible mark on many habitats and species, including oysters. This challenge threatens not only their existence, but the delicate ecosystems these molluscs support. Recent environmental conservation efforts show the benefits of oyster reef construction and how these often overlooked bivalves – molluscs with hinged, two-part shells – are pivotal to marine environments.

As ocean acidification intensifies, the future of the oyster hangs in the balance – and with it, environmental and economic ecosystems. Addressing this problem will require policymakers, industries and people around the world to comprehend the interconnectedness of our oceans and act accordingly.

As a foundation species and ecosystem engineer, oysters are crucial to marine life. Their reefs are more than mere collections of shell; they serve as a bedrock of biodiversity and deliver benefits to the living and non-living things around them.

From shoreline stabilization to water filtration and the creation of habitats for a myriad of marine species, oysters’ ecological significance is immense. According to one US study conducted in Alabama, the installation of even loose shell oyster reefs can mitigate shoreline retreat by over 40% and increase the population of species like blue crab by 297% and flounder by 79%. In another experiment conducted in the US state of Louisiana, oyster reefs contributed to an average shoreline erosion reduction of 1.07 meters per year at coastal sites already at high or intermediate exposure to erosion.

If used instead of expensive engineered shoreline stabilization devices, an ideally placed hectare of oyster reef provides up to $85,998 in value, according to a 2012 academic study.

As well as protecting valuable coastal land, oysters also remove nitrogen from the water, preventing harmful algae blooms and making our shores safer and more enjoyable. Oysters eat the waste of other organisms, including human nitrogen waste from fertilizers, for example. Research shows it could cost as much as $6,716 to remove the same amount of nitrogen that a hectare of oyster reef would digest.

And while it's difficult to capture the full monetary value that something like oysters can provide to an ecosystem, it starts with the small animals that are attracted to, and live in reefs. When they get eaten by bigger animals, their energy and nutrients wind all the way up the food chain into our fishing stock.

Oyster reefs protect these fish – research shows 1,000 square meters of oyster reef can be home to around 2,600 kilograms of larger crustaceans or fish per year. That can generate as much as $4,123 per hectare of (ecologically healthy) reef per year for the fishing industry (less any additional costs associated with fishing this extra population).

The same study from 2012 valued the average oyster harvest of a pristine reef in the US states of North Carolina and Virginia at $51,217 per hectare (all combined, in 2011 dollars), or $17,072 after harvesting costs. Meanwhile, the ecosystem services provided by an unharvested reef were worth $99,421 per hectare, per year, according to this research.

Costs and values can change over time, of course, but these figures give some indication of the economic loss to the fishing industry of not protecting reefs from ocean acidification.

Ocean acidification occurs when excess carbon dioxide is absorbed into seawater. This increases the concentration of free positive hydrogen atoms in the ocean and reduces water pH. This undermines the very structure of oyster reefs.

Oyster shells are primarily composed of calcium carbonate, a compound which essentially dissolves in low pH environments. Oyster shells are prone to damage in pH levels below 7.2. Current ocean pH levels are at 8.1 and are projected to drop to 7.8 at the surface by the end of this century, if present trends do not change.

Juvenile oysters have smaller, thinner shells and so are particularly vulnerable to the effects of ocean acidification. For oysters growing protective shells in an environment where they simultaneously break down, shell formation could be compromised by ocean acidification, hindering their survival and potentially reducing overall oyster populations.

A decline in oyster populations could have cascading effects on other species that depend on oyster reefs for shelter and food. The delicate balance of these ecosystems is in danger. But climate change and its environmental impacts are multifaceted, and warming temperatures caused by greenhouse gases such as carbon dioxide in the atmosphere change our ecosystems in many ways.

A study on how mangrove trees affect oyster reefs shows another way that climate change is affecting these fragile ecosystems. Mangroves are being pushed poleward by warming temperatures and their northern range now intersects with the habitat of oysters. Mangroves, a threatened species, now grow on top of oyster reefs and acidify the water below the 7.2 pH level oysters need to thrive.

Such intertwined vulnerabilities are underpinned by economic stakes tied to fisheries, aquaculture and tourism, showing the increasingly complex web of effects that climate change can cause.

Ocean acidification could translate into economic losses for the fishing industry due to a decrease in catch levels, production and increased mortality rates for sea life. A ripple effect could extend to jobs, businesses and local economies that depend on the industry. For oysters specifically, weakened and thinner shells due to acidification render them more susceptible to damage from predators, parasites and environmental stress, as well as damage during handling, harvesting and processing, resulting in further losses.

The shellfish industry may face challenges in adapting to changing environmental conditions. Efforts to mitigate the impacts of ocean acidification, such as adjusting aquaculture practices or developing more resilient oyster varieties, would require significant investment and research. Oysters are also a globally traded commodity, so changes in oyster populations and quality due to ocean acidification could influence market dynamics, affecting local and international trade.

Our actions have a significant impact on the wellbeing of our oceans, as well as ourselves. By addressing the root causes of climate change and investing in adaptive strategies – such as building oyster reefs now, while they can still survive and act as carbon sinks – we will not only shield our coasts, but also ensure the resilience and sustainability of marine ecosystems and economies for generations to come.

Just under the water, a delicate symphony of life hinges on the choices made above. The time to act to protect these ecosystems is now.