Humble shellfish, called a 'blue superfood,' nourishes humans, cleans water, and stores carbon

Most foods leave a mark on the environment, but farmed oysters can tip the balance the other way. They grow without feed, they tidy up coastal waters, and their shells hold carbon in a mineral form.

A new study on Irish Pacific oysters measured what farms emit and what oysters remove, then added up the difference.

The team identified large nutrient removals from water, meaningful carbon stored in shells, and a small, well defined footprint from farm operations.

Oysters are a healthy food source

“Sustainable food production that meets consumer demands while reducing environmental impact is a critical societal challenge,” wrote Paula Costa Domech, the study lead from the University of Galway.

The researchers sampled oysters from two farms and gathered operations data from three farms along Ireland’s northwest coast.

They dried and analyzed tissues and shells for carbon, nitrogen, and phosphorus, then paired those results with the energy, fuel, and materials the farms used.

This pairing matters because it connects what oysters do in the water to what farmers do on land and boats. It gives a single balance for nutrients and for carbon, rather than treating each in isolation.

How oysters clean water and carbon

Oysters filter out tiny particles from the water, and incorporate nutrients into their tissues and shell. The water column thus loses excess nitrogen and phosphorus.

In this study, one metric ton of oysters removed about 3.05 kilograms (6.7 pounds) of nitrogen and 0.35 kilograms (0.8 pounds) of phosphorus, and the net result was a nutrient removal that exceeded modeled nutrient pollution by 228 percent.

Scientists also look at eutrophication to describe nutrient pollution that fuels algal blooms and low oxygen levels.

Here, nutrient removals pushed the eutrophication balance into negative territory, which signals a clean-up effect rather than a burden.

Another pathway is denitrification, which is the microbial conversion of reactive nitrogen to nitrogen gas that leaves the system.

Evidence shows oysters and their biodeposits can stimulate this process in sediments, adding a second route for nitrogen removal.

A broader meta-analysis of oyster habitats found consistent denitrification potential, though the strength depended on site conditions and methods.

That variability matches what farmers see, since water flow, temperature, and sediment type all shape outcomes.

Math behind oysters and carbon

Production at the farm gate released about 374 kilograms (825 pounds) of carbon dioxide equivalent per metric ton of oysters.

Electricity for grading and packing, along with diesel for harvesting, made up most of that number.

Shells changed the picture by locking up carbon through biomineralization.

About 274 kilograms (604 pounds) of carbon dioxide equivalent per ton ended up in the shell material, which offset roughly 73 percent of those production emissions.

On a protein basis, the footprint was 5.7 kilograms (12.6 pounds) of carbon dioxide equivalent per kilogram (2.2 pounds) of oyster protein.

That figure includes the shell’s carbon storage, so it captures both sides of the ledger.

What oysters do for coasts and diets

Scaled to Ireland’s annual production, the sector could remove about 33.9 tons of nitrogen, 3.9 tons of phosphorus, and store an estimated 834.3 tons of carbon in shells each year.

Those are direct, in-water benefits that are tied to the farms’ output.

The nutrient clean-up service is valued at about 2.13 million dollars per year, when using standard wastewater treatment benchmarks from prior work.

That is money a utility would otherwise need to spend to achieve the same reductions.

Looking beyond one country, a large cross-species analysis found that unfed shellfish are among the lowest emission aquatic foods.

A complementary research effort that combined nutrition and climate metrics reported farmed bivalves as strong options for high nutrient density with modest emissions.

What the methods measured

The team used life cycle assessment (LCA) to count the energy, fuel, gear, and consumables needed to grow, harvest, depurate, grade, and pack oysters.

That approach assigns emissions to electricity, diesel, steel trestles, and plastic bags, then rolls them up into a single footprint.

At the same time, they measured the oysters’ ecosystem services, which are the benefits nature provides, such as nutrient removal and carbon storage.

The analysis converted nitrogen and phosphorus in shells to standard eutrophication units, and shell carbon to carbon dioxide equivalent to line up with the life cycle results.

This combination produced a net number for nutrient pollution and a net number for warming impact. It also showed which steps on the farm drive impacts, making it easier to target improvements.

Limits, tradeoffs, and what comes next

Every estimate depends on local conditions and choices that farms make. Electricity sources, engine efficiency, and the service life of gear can shift the footprint in either direction.

Shell fate matters too because carbon stays put only if shells are reused in ways that prevent fast dissolution. Reef restoration, soil liming, and building materials all have different time horizons for storage.

“These findings show how oysters can be a sustainable food source with local environmental benefits,” wrote Domech.

Models that convert nutrients to eutrophication units carry uncertainty, especially across sites with different water chemistry. 

Knowledge cannot erase the weight of cultural food traditions. Only time will show whether this sustainable food source becomes part of the mainstream diet or remains on the margins.

The study is published in npj Sustainable Agriculture.

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