A new study shows that the world's oyster reefs are a precious resource that's vastly underestimated and underutilized. The authors set out to calculate the reefs' potential gross ecosystem product (GEP), the ecological counterpart of the gross economic product, and arrived at trillions of dollars of ecosystem goods and services delivered each year to different aspects of planetary health, including human wellbeing. But for their full potential to be realized, oyster reefs around the world will need to be restored and protected.
"Here we estimate that fully restored oyster reefs in the top ten hotspot countries worldwide would yield a GEP of $12 trillion," said Dr Matteo Convertino, an associate professor at Tsinghua University and the corresponding author of the publication in Frontiers in Marine Science .
The UN's Sustainable Development Goal 'Life below water' calls for the conservation and sustainable use of coastal and marine resources to safeguard human wellbeing. But today, oyster reefs — like all coastal ecosystems — are under growing pressure from overexploitation, pollution, and climate change. Worldwide, an estimated 85% of such reefs have been lost over the past century, especially in estuaries and river deltas.
More than just food
Convertino and colleagues began by assessing where suitable habitats for oyster reefs exist around the world today and where reefs could spontaneously form, given the right protection and interventions for restoration. They then estimated the potential value of the entire 'portfolio' of ecosystem services these reefs could deliver. Besides oysters for human consumption, these include water filtration, shoreline protection, the recycling of nutrients through the food web, and carbon sequestration. Oyster reefs also provide food, shelter, and nursery grounds for a plethora of fish and invertebrates.
The scientists imported thousands of geotagged records of oyster reefs from two public databases. They then used a tried-and-tested machine learning algorithm to predict geographic distributions and habitat suitability around the world, and identify potential new locations for reefs. Their estimate of suitability weighed a slew of environmental measures such as local water temperature, salinity, light, nutrient-richness, and oxygenation. Any location with a habitat suitability score of 65% or greater was classified as suitable. By combining a given area's score with a measure of the ecosystem services that a reef there could provide, the authors rated the 'ecopotential' of each location. The higher the ecopotential, the greater the potential benefit.
The results showed that the predicted ecopotential is greatest in ten 'hotspot nations': the US, Australia, Chile, China, Japan, South Korea, Argentina, France, Germany, and Italy. Across these countries, approximately 295,000 square kilometers, or 6% of their coastal zones, were deemed to be suitable. Oyster reefs are currently known to occupy only 10% of this area, mainly in lagoons and bays, estuaries, and river deltas: this indicates that there is a lot of room for expansion. Importantly, only 23% of the suitable area is currently under any form of marine protection.
Given adequate protection, the world's your oyster
If the entire area with positive ecopotential were occupied by reefs, the authors' calculations show that these could filter prodigious volumes of water: 77.58 quadrillion liters in the US, 69.92 quadrillion liters in Australia, and 46.58 quadrillion liters in Chile. These reefs could also sequester as much as 12.3 million metric tons of carbon a year. In the US alone, the combined potential value of nitrogen removal, carbon sequestration, and water purification by oyster reefs was estimated at $25.1 billion a year.
However, the authors stressed that these are estimates, which infer ecosystem services from global averages of key parameters, and which could be affected by changes in oyster populations over time or by the climate crisis. Significant work on habitat restoration and protection would also be needed to unlock reefs' potential.
"We estimated that 90% of sites suitable for oyster reefs require restoration, while 80% need protection through conservation efforts. In other words, the current oyster situation is very suboptimal but there is a huge opportunity, considering the benefits," said Convertino.
"Because global warming generally decreases the habitat quality of oyster reefs, we expect that their global GEP will go down from the current potential value of $12 trillion. But some countries may see an expansion of reefs due to more favorable conditions for the growth and dispersal of oysters: in particular this could happen in Australia, Indonesia, South Korea, Japan, and France."
