Nitrogen Removal by Shellfish Measured in First Comprehensive Study

Josh Reitsma collects shellfish samples in Barnstable Harbor during fieldwork in 2012.
Photo by Diane Murphy, Woods Hole Sea Grant.

WOODS HOLE – A study conducted by the Woods Hole Oceanographic Institution’s Woods Hole Sea Grant will provide towns with some useful information regarding the capability for shellfish to remove excess nitrogen in local waters.

It is widely understood that shellfish have the capability to naturally process nitrogen as part of their growth process, but regionally specific numbers regarding how much have until now been unavailable.

The research team gathered samples of wild and farmed oysters and quahogs from various Cape Cod bodies of water and analyzed them for nitrogen stored in shells and tissues.

Individual oysters averaged .28 grams of nitrogen, more than the average quahog which contained .22 grams.

Wild oysters and those grown on pond bottoms held .32 grams, more than those grown in floating cages.

Season of harvest also had an impact on nitrogen intake. Lead author of the study Josh Reitsma said that shellfish harvested in the fall take in more nitrogen than those taken in the spring.

“The seasonal part of this is the biggest variation and I think that’s something any sort of town approaching this method would need to keep in mind,” he said.

The seasonal difference, he said, is because oysters and quahogs tend to “fatten up” during the fall in preparation for winter. The larger their shells, the more nitrogen they can theoretically contain.

Just because scientists and environmental organizations are concerned about nitrogen in the water does not mean consuming the creatures which cycle it is a danger to public health.

Reitsma said to think of eating shellfish in the same way one thinks of eating Swiss chard or tomatoes, which have also incorporated nitrogen from the soil in their growing process.

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