By Grace Bukowski-Thall

Nutrients allow organisms to survive, grow, and reproduce. However, an excess of nutrients can have dire ecological consequences. Eutrophication is when an aquatic ecosystem becomes overly enriched with nutrients such as nitrogen and phosphorous. Nutrient excess is typically caused by sewage system leakage, air pollution, and runoff from agricultural and urban developments. An excess of nutrients in waterways increases plant and algae growth, which has a negative domino effect on the entire ecosystem; first, algal blooms reduce dissolved oxygen levels in the water, which kills off many types of fish and plants. Then as the algae, plants, and fish die and decompose, large amounts of carbon dioxide are released, lowering the pH of the water. Water acidification reduces fish and shellfish growth as well as inhibits bivalve mollusk shell formation. This is problematic for both ecological food chains and commercial fisheries.

Eutrophication is a prominent issue in many of America’s estuaries. Every year about 331 million pounds of nitrogen flow into the Chesapeake Bay, which is the biggest estuary in the United states. This is about six times the amount of nitrogen found in the Bay in the early 1600’s when it was estimated to be at its healthiest. In 2010 the EPA established the Chesapeake Bay Total Maximum Daily Load (TMDL). The TMDL sets annual Chesapeake Bay watershed limits to 185.0 million pounds of nitrogen and 12.5 million pounds of nitrogen. However, due to overharvesting, pollution, habitat destruction, and parasitic disease, Chesapeake Bay has also lost its natural water purifiers: oysters.

Oysters are filter feeders, which means that they eat by drawing water over their gills where they trap bits of plankton, algae, and other particles in a mucus. These food particles are then transported to the mouth to be eaten and digested. Since oysters can filter 50 gallons of water a day, they are very effective and efficient at removing algae and excess nutrients from the water.

*Chesapeake Bay on Map, image by Kmusser*

Since many bodies of water are too hypoxic to recover naturally, researchers have started re-introducing oysters to waterways to combat eutrophication. In the past decade, the Nature Conservancy (TNC) has spearheaded a major oyster reef restoration project in the Chesapeake Bay. This year in 2020, the TNC has constructed 33 acres of oyster reef in the Piankatank River of the Chesapeake Bay in Virginia. These reefs are made out of slabs of granite or concrete and are deposited in the river using cranes and barges. The Piankatank River was selected for this project because it has circular water patterns that help oyster larvae attach to the reefs. In Collaboration with the Army Corps of Engineers, Virginia Marine Resources Commission (VMRC), and the National Oceanic and Atmospheric Administration (NOAA), the TNC hopes to build 428 acres of oyster reef in the Piankatank by 2025. With time, oyster populations are expected to grow and become self-sustaining.

Studies show that for every 100,000 oysters, six pounds of nitrogen and phosphorous nutrients are removed from Chesapeake Bay annually. However, these numbers show that millions of oysters need to be grown and harvested every year to bring nitrogen levels in Chesapeake Bay to the proper TMDL.

Oyster reef restoration has proven to be both eco-friendly and economical. Oysters are a keystone species that are an important food source for animals like rockfish, and crabs. As a result, re-introducing oysters to waterways can restore food chains. Data indicate that oyster aquaculture also provides an animal protein source with low greenhouse gas emissions. Additionally, oyster reef restoration has economic benefits with a $7 dollar return for every $1 spent.

Re-introducing oysters to eutrophic water bodies is also expected to mediate some of the effects of climate change. This is because oyster reefs reduce the size of storm waves as well as help slow erosion. Because oyster reefs grow vertically, they are also expected to keep up with sea level rise and continue to lessen the effects of coastal storms. In order to protect the coastline, oyster reefs were built at sites in the Chincoteague National Wildlife Refuge that had undergone significant Hurricane Sandy damage.

Oyster restoration in places like the Chesapeake Bay and the Chincoteague National Wildlife Refuge are examples of how we can use naturally occurring processes to economically and efficiently rehabilitate ecosystems, and help protect coastlines from the effects of climate change. Hopefully oyster reef restoration projects will continue in other estuaries and water bodies across the country.

Using Oyster Reef Restoration To Mitigate Eutrophication