Oyster aquaculture can remove nutrients from Bay using native oysters (C. Virginica) in three ways (see Figure below).  Nutrients entering the Bay are energy sources for phytoplankton and algal growth.  These organisms, in turn, are the primary food source for oysters.  A portion of the nutrients contained in the phytoplankton becomes part of the oyster and when the oysters are harvested, nutrients are also removed from the Bay as oyster meat and shell (1 in the Figure).

As the oysters feed, a portion of what is consumed is deposited to the bottom as biodeposits.  Oyster biodeposits are digested or undigested material expelled by the oysters. Bacteria and other microorganisms then begin a process of breaking down these nutrient rich biodeposits.  Bacteria in aerobic conditions break organic nitrogen into nitrates and nitrites (NO2 and NO3). In the anaerobic sediments, a portion of this nitrogen undergoes a denitrification process sediments where this nitrogen is converted into inert N2 gas.  N2 makes up about 78 percent of the earth’s atmosphere. Nitrogen in this form is unavailable for plant growth.  The conversion of organic biodeposits into N2 gas is the second way nutrients are removed from Bay waters. Finally a portion of the nitrogen and phosphorus becomes sequestered in sediments (3 in the Figure).

Every new oyster aquaculture facility potentially removes nutrients from the waters of the Chesapeake Bay.   Unlike other forms of aquaculture, oyster aquaculture does not require the use of feed inputs. The oysters feed on naturally occurring phytoplankton.  Since no feed energy is required or added to the system, no new nutrients are ever added to Bay waters because of an oyster aquaculture facility.

Because oyster aquaculture is an actively managed production system, oyster aquaculture also has the potential to accelerate and improve on the nutrient removal capacity of the native oyster.  For example, selecting disease resistant and fast growing oysters can increase nutrient processing and removal processes.  Locating oyster production facilities in areas with favorable conditions for nitrification and denitification processes can also accelerate nitrogen removal.

Further Reading:

Newell, R. 2004. “Ecosystem Influences of Natural and Cultivated Populations of Suspension-Feeding Bivalve Molluscs: A Review.” Journal of Shellfish Research. 23(1):51-61.

Dame, R. 1996. Ecology of Marine Bivalves, an Ecosystem Approach, Boca Raton, FL: CRC Press.