The cleaning power of oysters

Tasty bivalves provide clear water and denitrification, and play an important part in local marine ecosystems.

Oysters naturally clean the water column, reduce nitrogen, and are a vital part of the marine ecosystem on Martha’s Vineyard. — Lucas Thors

Oysters are a delicious shellfish that folks on Martha’s Vineyard are lucky enough to have grown fresh, practically in their backyard. But some are unaware of the benefits these bivalves can have on marine ecosystems. 

The Times spoke with Emma Green-Beach, executive director of the Martha’s Vineyard Shellfish Group (MVSG), who wants the entire world to know about the filter-feeding power of oysters, and how they can clean the water, reduce nitrogen, and provide a vital food source for larger creatures (including us).

On-Island, oysters are grown in Katama Bay, Menemsha Pond, and outside Oak Bluffs Harbor, and Green-Beach said the shellfish group has participated in shellfish restoration programs in both the Tisbury and Edgartown Great Ponds over the past 20 or so years. “The purpose is both ecological restoration and fisheries restoration,” Green-Beach said of the program, which sees millions of seed oysters being distributed in the estuaries each year.

Edgartown receives anywhere from 1 million to 2 million oysters each year from the shellfish group, and Tisbury receives 5 million to 10 million. 

Although Edgartown Great Pond doesn’t open to commercial shellfishing every year, Green-Beach said individuals are still fishing the embayment recreationally. And even if those oysters aren’t being harvested, she said, they play an important role in achieving an ecological equilibrium. 

Oysters and all other bivalve shellfish, Green-Beach said, are filter feeders. This means they filter water through their gills, trapping food particles and micronutrients for consumption, and spitting out any suspended sediment.

In spitting out the sediment, Green-Beach said, they are taking those particles out of the water and clearing the water column.

“They are taking suspended particles that make the water cloudy, or green, or brown, and they ingest it, then deposit it on the bottom of the pond,” she explained. In doing so, oysters are increasing the potential for biodiversity — allowing the sun to shine down through the water and nourish the aquatic vegetation (like eelgrass).

Another way that oysters improve the health of our local water bodies, Green-Beach said, is denitrification. When oysters deposit their waste into the muck at the bottom of estuaries, beneficial microorganisms can then utilize that waste for their own metabolism. That organic matter is turned into nitrogen, which rises up through the water and is released into the atmosphere. “That is called denitrification, and that is really what we are going for,” Green-Beach said.

One program that has been offered by MVSG since 2011 is the shell recycling program. Green-Beach noted how much she wants folks on-Island to buy into this program, because of how beneficial it is for our marine environment. “This is one where I really need everyone to know that it exists in order for it to grow, just like composting,” she added.

Primarily, the shellfish recycling program involves members of the shellfish group going around to various Island restaurants and taking the empty shells from their raw bars.

The shells are aged for one year, (in order to kill any diseases like Vibrio and Dermo), then used as substrate to grow new oysters on.

“Those recycled shells come into the hatchery; we get little baby oysters to cement onto them in these large clusters,” Green-Beach explained. 

The shellfish group will also take bare oyster shells (that have been aged) and put them in both the Great Ponds. 

The shell itself hardens the bottom of the pond, and increases its “buffering capacity,” which refers to the water’s ability to keep the pH stable as acids or bases are added.

Acidification of local water bodies has been an issue on-Island for decades. Largely due to nitrogen loading from septic systems, acidification occurs when excess nutrients throw off the pH balance in the water.

Green-Beach said this form of “coastal acidification” happens when excess nutrients create too much seaweed and microalgae, which eventually die and sink to the bottom.

As this muck is degrading, it releases acidic compounds into the water. And when baby oysters are trying to find a solid place to call home, they sometimes fall into this mud and are dissolved. 

Since shells are made of calcium carbonate (just like Tums), Green-Beach said, placing them in the water balances the pH and promotes the delicate equilibrium our estuaries rely on so heavily.

And of course, oysters are an important link in the food chain. They take microbes out of the water, use them to make energy, and ultimately serve as dinner for larger aquatic creatures (and hungry tourists).

So next time you’re about to slurp down some tasty oysters, give thanks for the health and sustainability of our local estuaries.