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Quantifying Nitrogen in Shellfish Helps Planning Efforts on Cape Cod

Woods Hole Sea Grant works on water quality in Cape Cod bays

By Abigail Archer and Josh Reitsma, Woods Hole Sea Grant

Cape Cod’s economy depends on its estuaries and coastal bays. Commercial fishing, recreational shellfishing, and tourism activities like boating and swimming all require good water quality to entice people to the shore.   Unfortunately, many of the coastal bays on Cape Cod are experiencing issues with water quality as a result of activities on land.  One of the main issues is large algae blooms that look unpleasant and are bad for other plants and animals in the water.  A major cause is residential septic systems that discharge nitrogen into the groundwater which then flows into coastal bays.   Nitrogen promotes plant growth – and too much of it causes the excessive algae blooms. 

Fortunately, robust efforts are being made to discuss, evaluate, and find solutions to the problem of excess nitrogen in coastal bays.  Several methods are being explored such as centralized wastewater treatment, innovative septic system designs, permeable reactive barriers, and increased tidal flushing to coastal marsh systems (Draft Cape Cod Area Wide Water Quality Management Plan Update).  Another method receiving attention is the growing and harvest of shellfish.

How can shellfish remove nitrogen from the water?  Part of the answer lies in how they eat. The water that shellfish live in is teeming with microscopic plant life known as phytoplankton which can take in dissolved nitrogen directly from the water.   Shellfish are filter feeders and as they pump water into their shell cavity they graze on the phytoplankton, and then incorporate the nitrogen into their tissues.  The final step is for the shellfish and the nitrogen within them to be taken out of the water and eaten by people. 

In order to evaluate this method Cape Cod citizens and natural resource managers wanted to know how many oysters (Crassostrea virginica) or hard clams (Mercenaria mercenaria) they would have to grow and harvest to make a positive impact on water quality.  The first step is to determine how much nitrogen an average oyster or hard clam takes in from its environment, but the existing research on the topic was not clear.  Most of the studies were done on oysters from the Chesapeake Bay region, and less information was available for hard clams which is an important species in southeastern Massachusetts aquaculture. Also, the information available in the scientific literature varied based on size of the shellfish, growing method, and season of harvest.

To answer this question for Cape Cod shellfish, Woods Hole Sea Grant partnered with Boston University to sample and analyze both oysters and hard clams of harvestable size including both wild and cultured stock, and in both spring and fall.  The samples were collected with the help of municipal shellfish departments, shellfish growers, and shellfish harvesters from a variety of sites. 

Results showed local oysters have an average of 0.69% nitrogen by total dry weight (about 0.28gN/animal) and hard clams an average of 0.67% nitrogen by total dry weight (about 0.22gN/animal). When compared in terms of % of the total dry weight of both tissue and shell combined, the oysters from Cape Cod had a higher percentage of nitrogen than those from the Chesapeake.  The hard clam nitrogen values were all higher than values in the published literature. Also, the way in which an oyster was grown showed differences in shell mass corresponding to increased mass of nitrogen per animal.

By season, a large difference was seen in the potential for nitrogen removal from spring to fall.

Shellfish meats tend to be ‘lean’ in the spring and ‘fatten” into the fall with extra storage in tissues for winter.  Oysters in October had 98% more meat tissue compared to those of similar length in June.  Over the same period hard clams averaged a 63% increase in tissue, leading to increased quantities of nitrogen stored in biomass for fall harvests of both species.

The results are published in a Marine Bulletin called, “Shellfish, Nitrogen, and the Health of our Coastal Waters” and the information has already been used for one local town’s planning efforts.   Town of Mashpee Shellfish Warden Rick York incorporated this information in the Town of Mashpee Draft Comprehensive Watershed Nitrogen Management Plan to calculate the projected nitrogen removals for the shellfish part of the plan.


To read the full report and learn more about shellfish propagation efforts on Cape Cod follow the links below:

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