Can shellfish farming clean our coastal waters?

Feature Image: Tending to bottom-cultured oyster cages in Rhode Island. Photo Credit: L. Green-Gavrielidis

Source Article: Reitsma, J., Murphy, D.C., Archer, A.F., York, R.H. 2017. Nitrogen extraction potential of wild and cultured bivalved harvested from nearshore waters of Cape Code, USA. Marine Pollution Bulletin 116: 175-181. https://doi.org/10.1016/j.marpolbul.2016.12.072

Our coastal nutrient problem

Roughly 66% of coastal waters in the United States have nutrient (primarily nitrogen) pollution issues. Nitrogen is transported to the coastal environment through run-off from a variety of sources including lawn fertilizers, agriculture, and wastewater treatment plants. An excess of nitrogen in the coastal environment can lead to traumatic changes in the abundance and presence of organisms in coastal habitats and low oxygen levels that threaten the survival of many animals. This makes nutrient pollution “one of America’s most widespread, costly and challenging environmental problems”, according to the US Environmental Protection Agency (EPA).

Due to the widespread nature and severity of nutrient pollution domestically and globally, significant money and effort have been invested to remove excess nitrogen from our coastal environments. Recently, there has been a growing interest in promoting shellfish farming as a method for removing excess nitrogen from the environment. Shellfish are filter-feeders, meaning that they remove particles such as microscopic algae from the environment as they feed, and incorporate the nitrogen trapped in the algae in to their shells and tissue. In fact, a single adult oyster can filter over 50 gallons of water in a day! When the shellfish are harvested, the nitrogen in the shells and tissue is removed from the coastal ecosystem. To understand whether shellfish farming is an effective method of removing excess nitrogen from coastal environments, scientists needed to determine how much nitrogen each shellfish could remove.

Can shellfish farming help clean our coastal waters?

Josh Reitsma and his colleagues at Cape Cod Cooperative Extension and the Mashpee Department of Natural Resources in Massachusetts set out to determine whether shellfish farming is an effective method of removing excess nitrogen from coastal environments. Specifically, the researchers aimed to determine how much nitrogen two commonly farmed shellfish species, the eastern oyster Crassostrea virginica and quahogs Mercenaria mercenaria, removed from coastal waters. The team also wanted to know how nitrogen removal by shellfish is impacted by cultivation type, location, and season.

Reitsma and his colleagues visited twelve sites on Cape Cod to collect oysters and quahogs. At the sites, they collected wild populations of shellfish and farmed shellfish. To determine if the nitrogen content changed between seasons, oysters were collected from all sites in spring and fall. The shellfish were then analyzed to measure how much nitrogen was in the shell and tissue.

For both, oysters and quahogs, Reitsma and his colleagues found that the majority of the nitrogen (66-75%) was in the tissue (e.g. meat) even though the shell was the heaviest part of the animal. Reitsma and colleagues found that oysters removed 0.28 grams of nitrogen per animal, while quahogs removed 0.22 grams of nitrogen per animal. The nitrogen content of oysters varied depending on cultivation method (on versus off-bottom, with the former having higher nitrogen) and the content of wild oysters was similar to that of on-bottom cultured oysters. Both species had higher total nitrogen in the fall, since soft tissue composition changes over the course of the year from spawning (spring) to preparation for winter (fall).

Coastal nutrient management efforts should be diverse

While Reitsma and his colleagues showed that oysters and quahogs do remove nitrogen from coastal environments, it is unlikely that shellfish aquaculture alone will be a solution to reduce coastal nitrogen pollution. This is because the amount of nitrogen that is being added to the coastal environment is massive. For example, roughly 300 million pounds of nitrogen pollution is added to the Chesapeake Bay every year! Based on the total nitrogen removed by each oyster (0.28 g) and quahog (0.22 g) reported by Reitsma and his colleagues, you would need to culture and remove over 485 billion oysters or 618 billion quahogs every year to remove this excess nitrogen. That simply is not realistic, but it doesn’t mean that shellfish farming can’t be a part of the solution. Depending on the needs of the community, shellfish farming can be combined with other efforts including improvements to stormwater and wastewater treatments. There are ongoing efforts across the country, including the Billion Oyster Project, which aims to plant 1 billion oysters in New York Harbor by 2035 to improve water quality.