Oyster Denitrification Research Overview

The Chesapeake Bay’s native oyster—Crassostrea virginica—is a great multitasker. Oysters are a valuable fishery, but also have important roles in the ecosystem: as they grow, they form reefs that provide habitat for other Bay species; and by filter feeding, they help clean the Bay’s water.

That oysters remove nitrogen from the water is well known, too. Because nitrogen is one of a number of nutrients that fuel algae blooms that lead to reduced water clarity and low dissolved oxygen, many efforts to restore health to estuaries focus on reducing the amount of these nutrients—especially nitrogen—that enter the water.

Here on the Chesapeake, nutrient reductions to increase estuary health recently became part of the Environmental Protection Agency’s regulations in the Chesapeake Bay, through the Total Maximum Daily Load (TMDL) process. Efforts like the TMDL to reduce nutrient inputs have worked in some places. For example, in the Potomac River from the 1970s through the 2000s, in response to decreasing nitrogen input thanks to better wastewater management systems, as algae blooms declined, dissolved oxygen levels went up, and underwater grasses came back.

Are there other ways to lower the amount of nitrogen in the Chesapeake Bay? Some scientists are researching whether oysters can remove substantial amounts of nitrogen from the water. If they can, perhaps restoring oysters can promote the removal of nitrogen that is already in the Bay.

Oyster denitrification research

Oysters are filter feeders; they remove particles from the water that contain nitrogen, primarily in algae (phytoplankton), and use it to build their tissues and shells or move it to bottom sediments in their biodeposits. Once the nitrogen is in the sediments, microbes may break it down and release nitrogen gas into the atmosphere, a process called denitrification, removing nitrogen from the estuary. Also, all the nitrogen that is in any oysters that are harvested is removed from the system as well.

Oysters’ ability to improve water quality by filtration has been well known for more than 20 years, but many related questions remain. Does oyster filtration always lead to nitrogen removal in nearby sediments? How much nitrogen is removed, and under what conditions? Do wild oysters have the same amount of nitrogen as those raised in aquaculture, and does this vary by location?

And, can oysters’ work to remove nitrogen count toward nutrient reduction goals set forth in the TMDLs? Both oyster restoration and oyster aquaculture have increased in the Chesapeake Bay, and some people wonder whether restoration and/or aquaculture—because they mean more oysters in the water—could count toward nutrient reduction goals.

Written for the Virginia Secretary of Natural Resources, a 2012 white paper summarized information from published sources about the capacity of aquaculture oysters to remove nitrogen. The white paper concluded that while the nutrient content of harvested oysters can be quantified, there is not enough information to quantify any enhancements of denitrification associated with oysters that remain in the Bay.

To answer questions related to oysters and denitrification, the NOAA Chesapeake Bay Office provided a grant to fund research on denitrification rates, and NCBO held a workshop in January 2013 to see what the latest research had shown on nitrogen removal by oysters. The workshop, held at the VIMS Eastern Shore Lab in Wachapreague, Virginia, was attended by an impressive list of experts in this field, including several who work in other estuaries.

Overall, workshop attendees concluded:

  • Consistent estimates of nitrogen and phosphorus content of oyster soft tissue and shell exist. These estimates can be used to estimate nutrient removal by harvest, if there is actual harvest data (oyster dry weight).
  • There are no estimates of the amounts of nutrients buried by oyster biodeposits, or of denitrification rates associated with on-bottom cage aquaculture.
  • Measured sediment denitrification rates associated with oyster aquaculture in floats have not revealed any consistent enhancement above background levels.
  • Restored oyster reefs often enhance denitrification in the reef and in sediments near the reef, compared to control sites, but the rates are too variable to make any general conclusions about those rates without more studies.

Four communications products about the workshop, which emphasize and detail these conclusions, are available:

Researchers, including some working on NCBO-funded projects, continue to measure denitrification rates associated with restored oyster reefs, which will help scientists understand why those rates are so variable. Additional studies are needed to measure the amounts of nutrients buried by oyster biodeposits and the denitrification rates associated with on-bottom cage aquaculture. These and other studies may allow scientists to reliably estimate denitrification from restored oyster reefs and aquaculture operations in the future.