Study determines how much nitrogen can molluscs eliminate at Cape Cod
2017-02-10 19:17:07   copyfrom:    hits:

Samples of wild and farmed oysters and quahogs from various water bodies in the Cape Cod region (Photo: Diane

Samples of wild and farmed oysters and quahogs from various water bodies in the Cape Cod region. (Photo: Diane Murphy, Woods Hole Sea Grant)


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Thursday, February 09, 2017, 02:40 (GMT + 9)


A new study by Woods Hole Sea Grant, Cape Cod Cooperative Extension and the Mashpee Department of Natural Resources provides the first complete measurement of nitrogen removed by molluscs harvested from Cape Cod waters.

Cities along Cape Cod and islands no longer only see seafood as culinary delights, but also as cleaners of waters degraded by excess nitrogen in the region.

While nitrogen is essential for all plants and animals, excess nitrogen in ponds and waterways, often caused by runoff from fertilizers and septic tanks, can fuel algae growth and cause low oxygen levels.

Seafood such as oysters and quahogs help remove excess nitrogen by incorporating it into shells and tissues as they grow.

However, the exact amount of nitrogen they are able to remove in local waterways and how such amounts may vary according to location or season has been unknown due to the lack of region-specific data.

"We did this project specifically as a service to local municipalities to get them accurate data to utilize, if they're going to go with the approach of seeding and growing shellfish as part of their water quality management plans," says Woods Hole Sea Grant Extension Agent Joshua Reitsma, lead author of the paper published in the journal Marine Pollution Bulletin.

During fieldwork in 2012, the research team gathered samples of both wild and farmed oysters (Crassostrea virginica) and quahogs (Mercenaria mercenaria) from various water bodies in the Cape Cod region.

Following, the samples were then analyzed for nitrogen stored in the shells and tissues, which represent how much nitrogen would be removed from the water when harvested.

On average, the researchers found individual oysters contained .28 grams of nitrogen, slightly more than the average quahog with .22 grams. Wild oysters and those grown on pond bottoms contained an average of .32 grams of nitroge, more than those grown in floating cages.

"Wild oysters or bottom-grown oysters, which deal more with predators, have to grow thicker shells for protection," Reitsma explains. "More shell means greater weight, and with greater weight, the animal can take in more nitrogen."

The nitrogen values also varied by season, with shellfish harvested in the fall taking in more nitrogen, which Reitsma says wasn't a surprise since oysters and quahogs tend to "fatten up" in the fall in preparation for the winter season.

"Theoretically, you could maximize the amount of nitrogen removed by harvesting in the fall rather than in the spring, when the shellfish tend to be skinnier," he adds.

Some might wonder if the oysters and quahogs used for cleaning are still safe to eat, given the absorption of excess nitrogen. Overall, the answer is yes, says coauthor Diane Murphy, a specialist in fisheries and aquaculture at Woods Hole Sea Grant and Cape Cod Cooperative Extension.

In addition to Mashpee, other local towns testing or utilizing shellfish projects for water quality improvement include Falmouth, Barnstable, Yarmouth, Dennis, Orleans, and Wellfleet on Cape Cod, and Edgartown on Martha's Vineyard.

Towns on the north side of Cape Cod, which have fewer salt ponds, bays or estuary-type environments, have less problems with excess nitrogen than those on the south side, Reitsma says.

"The north side of the Cape also has much bigger tides that tend to flush areas out much more than the inlets on the south side," Richard York from the Mashpee Department of Natural Resources, explained. "The combination of smaller estuaries and bigger tides helps remove excess nitrogen." 

The Woods Hole Sea Grant program is funded by the National Oceanic and Atmospheric Administration (NOAA) and is conducted by the Woods Hole Oceanographic Institution.


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