An oyster reef is a dense aggregation of oysters forming a large colony. Because oyster larvae need to settle on hard substrates, new oyster reefs form on stone or hard marine debris. Later, the oyster reef will grow by spat settling on the shells of older or nonliving oysters.
The Role of Oysters as Ecosystem Engineers
Oysters are known as ecosystem engineers because of their formation of three-dimensional reef structures. The oyster reef structures support over 300 species of organisms such as fishes, crabs, and birds that depend directly or indirectly on the reefs for habitat, nursery, and as a food source. The complex three-dimensional interstitial spaces within oyster reefs provide refugia for prey or juvenile species, which increases prey biomass and thereby enhances trophic transfer.
These structures not only support the ecosystem but also people. The crevices within reefs that form when oysters attach to each other provide shelter for other sea life, such as fish, crabs, worms, shrimp, snails, and clams. Oyster reef habitat also provides food for many predators, such as shorebirds. Additionally, reefs act as nurseries for juvenile crab and fish species, which can hide there from larger predators.
Ecological Functions of Oyster Reefs
Water Quality Improvement
Oysters feed by filtering and eating particles in the water column. One single oyster is capable of filtering up to 50 gallons of water in 24 hours! Oysters filter nutrients, phytoplankton, sediments, heavy metals, and toxins out of the water column. This filtering improves water quality and clarity as well as increases the penetration of light into the water column, which enables the growth of submerged aquatic vegetation (e.g., seagrass).
The filter feeding behavior of oysters can buffer against environmental degradation caused by human-induced eutrophication of estuary systems. Disruption of the filter feeding by oysters can lead to a decrease in the elimination of organic matter from the water column and increase phytoplankton abundance.
Carbon Sequestration and Nutrient Uptake
Oyster reefs can also impact the carbon sequestration and excess nutrient uptake. As filter feeders, oysters help remove excess nutrients from the water, including nitrogen and phosphorus, the two primary pollutants degrading water quality in many estuarine systems.
Shoreline Protection
Reefs also slow shoreline erosion by buffering the coast from boat wakes, storms, and sea level rise. Working together with other natural infrastructure like marshes and living shorelines, oysters can further reduce vulnerability to waves and protect adjacent marshes from erosion.
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Formation and Development of Oyster Reefs
Eastern oysters (Crassostrea virginica) are found throughout coastal waters of the Atlantic Ocean and Gulf of Mexico, with a habitat range extending from Canada to Mexico. Oyster spawning in Florida generally takes place from late spring through the fall with environmental cues such as increases in water temperatures and salinities initiating oysters to release their eggs and sperm into the water column. The fertilized eggs then develop into larvae over the course of two to three weeks.
During that time, larvae will grow a foot-like appendage which they use to find suitable substrate to attach to. Larvae generally attach to other oyster shell, thus creating oyster reefs; however, oysters can attach to any hard substrate such as mangrove prop roots, docks, or seawalls. Upon finding a suitable substrate, larvae secrete a glue which allows for permanent attachment to the substrate. The attached larvae are then referred to as “spat”.
Free swimming oyster larvae, called "spat," often attach to live oyster shell. This constant addition of new oysters on top of existing oysters can result in dense mounds of live animals and dead shell. Because oysters grow quickly, they can stay above the surrounding soft sediments that would bury other stationary organisms. So, not only do these reefs offer ideal habitat for oyster spat, the resulting structure becomes a vital micro-environment within estuaries.
Historical Context and Decline of Oyster Reefs
OYSTER REEFS were once a common feature of many coastal environments. In the United States, Crassostrea virginica, the eastern oyster, was a major reef builder in the Chesapeake Bay until the late 19th century. Because of overfishing, environmental degradation, and disease, populations of C. virginica underwent a drastic reduction in population size.
There is an established pattern connecting human fishing practices, such as dredging, to oyster population collapse across the globe. For example, the conversion of large areas of forest to farmland in the Chesapeake Bay contributed significant amounts of sediment flowing into the Bay that silted over oyster reefs and river bottoms. By the end of the 19th century, Maryland was the world’s largest oyster producing region, but even during the 1800s, scientists were already recording overfishing and depletion of oyster populations.
The overexploitation of oyster fisheries in New England pushed additional fishing pressure into the Chesapeake Bay. Two parasitic diseases, Dermo and MSX, contributed to significant die-offs of oysters in the Chesapeake Bay that intensified during the 1980s. Dermo reduces an oyster’s ability to grow and reproduce, eventually killing it if the infection is severe. MSX can quickly infect oysters and lead to their death.
Over the past two centuries, overharvesting, pollution, habitat destruction, and disease have caused 85% of oyster reef habitat all over the world to be lost. Salinity fluctuations, sediment accumulation, increasing temperatures, and rising sea level due to climate change are adding additional stressors to reefs.
Threats and Stressors to Oyster Reef Ecosystems
Disease and Environmental Factors
The disease die-offs in the 1980s were initially linked to droughts that drove higher salinity in the Bay, but more recent scientific findings suggest the Dermo disease itself became more virulent. Climate change is an additional threat to the Bay’s oyster reefs. Ocean acidification makes it increasingly difficult for shellfish, like oysters, to create the shells they need to survive and grow. More frequent, intense storms caused by climate change also wash more sediment and pollution into waterways, fueling algal blooms and low-oxygen “dead zones” that oysters cannot move from to escape.
Recruitment Failure and Shifting Baselines
An indicator of a stressed reef is the absence of baby oyster spat, which means conditions are not allowing for successful reproduction. Disease, pollution, burial by sediments, predation, and overfishing are primary causes of recruitment failure. Without newly settled oysters to the reef, the three-dimensional reef structure will not grow. With few living oysters, the reef may also break down over time and be less effective in providing ecosystem services like shoreline protection.
Comparing older and more recent oyster reef death assemblages reveals a significant decrease in species diversity. This illustrates the phenomenon of a shifting baseline, where successive generations have different impressions of what a healthy natural environment looks like due to a gradual loss of knowledge about past conditions.
Oyster Reef Restoration Efforts
The Chesapeake Bay is home to some of the largest oyster reef restoration projects in the world. Many organizations have contributed directly to these projects by adding oysters to the Bay through restoration programs. There has been extensive advocacy for the resources and policies needed to achieve and sustain large-scale reef restoration through initiatives like the Chesapeake Oyster Alliance.
Management of the Bay’s oyster fishery is critical to restoring the oyster population, but it has long been fraught with deep divisions. Advocacy for science-based decision-making and policies that build collaboration and trust between industry, government, and scientists is crucial. Through restoration and oyster gardening programs, community members are engaged in the effort to bring back the Bay’s oysters. Volunteers help build reef balls, grow oysters, and recycle shells to boost restoration on sanctuary reefs.
Oyster restoration in the Chesapeake Bay depends on many partners working together at all levels of government. Support for large-scale oyster restoration can be shown by contacting elected officials. Restoration programs provide a hands-on way to help oysters return to the Chesapeake Bay. Volunteers assist with tasks such as building reef balls and preparing oyster shells for use in restoration projects. Oyster shells are the foundation on which baby oysters attach and grow, making them a critical part of sustaining oyster populations and their reef habitat.
Recycling the shells of oysters eaten by consumers can significantly aid restoration efforts. In many regions, consumers can dispose of shells at local shell recycling locations, eat at restaurants that recycle their shells, sponsor a shell recycling bin, or arrange for collection during oyster roasts or seafood festivals.