How much nitrogen is currently being extracted from harvested shellfish in Wellfleet Harbor?

(Updated 4.28.20) How much nitrogen is currently being extracted from Wellfleet Harbor when our shellfish are harvested? Shellfish contain nitrogen within in their meats and their shells. In shellfish, meat nitrogen is stored in proteins, and in their gut primarily as chlorophyll that they are feeding on. Their shells also contain nitrogen. When shellfish are harvested and removed from the water, the nitrogen that they contain is removed, simple really. The total nitrogen content varies to some extent, based on their growing conditions and location. It is important to note that no two locations are exactly the same. Also, this calculation is solely counting the total weight of the nitrogen when the shellfish are landed. It does not include, denitrification, burial, or export

These are the approximate nitrogen content of oysters dry weight.

Joshua Reitsma, Et el, Nitrogen extraction potential of wild and cultured bivalves harvested from nearshore waters of Cape Cod, USA, 2017

Welllfeet’s shellfish landings are measured in wet weight. It is necessary to covert wet weight to dry weight. There is a difference between wild vs off bottom. I’m using the cultured off the bottom as the representative sample for cultured oysters. On bottom cultured weigh more and triploids weigh less and each has a different ratio of dw vrs ww. Cultured oysters have a great deal of variability due to how they are grown by the growers, the local conditions, diploids vrs triploids. There data for the wieght of Wellfleet’s oyster landing that are triploids.


OystersWet Weight
(Whole) g
Dry Weight
(Tissue & Shell) g
Dry Weight as
% of Wet Weight
Wild71g48.24g67.94%
Off Bottom60g38.03 g63.38%

These are the approximate nitrogen content of quahogs by dry weight.

There is little difference in the wet weight to dry weight between cultured vrs wild quahogs.

QuahogsWet Weight
(Whole) g
Dry Weight
(Tissue & Shell) g
Dry Weight as
% of Wet Weight
Wild54.4g35.03g64.39%
Cultured48.6g31.49g65.00%
mean51.7g33.42g64.64%

For the sake of simplicity for estimated quahog nitrogen content I am using the mean value for wet weight to dry weight conversion.

Approximate nitrogen content of blue mussels by wet weight.

Hedberg, Nils et al. “Nutrient removal capacity and potential ecological consequences of blue mussel farms for nutrient abatement in the Baltic Sea.” (2018).

For the purposes of estimating the total amount of nitrogen contained within Wellfleet’s shellfish catch, I applied the following values to the total weight of landings:

Nitrogen as a %
of Dry Weight
Conversion factorNitrogen as a %
of Wet Weight
Quahog (Wild).67%64.64%.4412%
Quahog (Cultured).66%64.64%.4321%
Oyster (Wild).67%67.94%.4366%
Oyster (Cultured).70%63.38%.5333%
Blue Mussel.71%

The below graph shows the estimated total nitrogen contained within Wellfleet’s shellfish landings.

What are the conclusions? Overall, Wellfeet’s shellfish catch contains a significant amount of nitrogen. In terms of overall removal rates, Oysters and Quahogs have swapped positions. Quahogs were a huge driver in terms of the total amount of nitrogen removed. The gains in terms of total nitrogen physically removed from the increase in oyster landings are largely offset but the severe decline in quahog landings. It would be beneficial to have information on the nitrogen content of the shellfish species with smaller catch totals (blood Arc Clams, Softshell clams, razor clams, and bay scallops).

Data tables are below

References:

Joshua Reitsma, Diane C. Murphy, Abigail F. Archer, Richard H. York,
Nitrogen extraction potential of wild and cultured bivalves harvested from nearshore waters of Cape Cod, USA,
Marine Pollution Bulletin,
Volume 116, Issues 1–2,
2017,
Pages 175-181,
ISSN 0025-326X,
https://doi.org/10.1016/j.marpolbul.2016.12.072.


Hedberg, Nils & Kautsky, Nils & Kumblad, Linda & Wikström, Sofia. (2018). LIMITATIONS OF USING BLUE MUSSEL FARMS AS A NUTRIENT REDUCTION MEASURE IN THE BALTIC SEA. 10.13140/RG.2.2.15804.49285.

Wellfleet Shellfish 2005-2018, Nancy Civetta, Wellfleet Shellfish Department, 2019

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