Sea level-induced oyster reef accretion causes minimal net carbon burial change

Intertidal oyster reefs live for millennia and prevent shoreline erosion but not climate change.

Overfishing has severely damaged oyster reefs. Today, their total area is only a small fraction of what it was 150 years ago. Scientists and coastal managers have found strong evidence that estuaries would have cleaner water, and coastal communities would experience less severe hurricane damage if oyster reefs were still abundant.

Oyster reefs are an estuarine foundation species because they form hard structures on the seafloor, they help stabilize shorelines, reduce wave energy, filter the water, and provide food, shelter, and habitat for many estuarine animals. These benefits have encouraged a growing number of oyster reef restoration projects. However, poor water quality, the spread of oyster diseases, and increasing ocean and coastal-water acidity can make it difficult for restored reefs to survive. As a result, many reef-construction projects do not deliver the benefits people expect.

Reefs can form in both subtidal and intertidal realms. Oysters harvested to be served on the half shell are typically grown on subtidal reefs, which remain underwater all year and form a thick tear-drop shaped shell. These reefs generally thrive in water that is less salty than the open ocean and not suitable for oyster predators and competitors. Because they sit lower in the water, subtidal reefs are less effective at reducing wave energy than intertidal reefs that are exposed during low tide.

Intertidal reefs are often found in saltier water and typically occur in two settings: along the edges of salt marshes, where they are called fringing reefs, or as isolated patches on sandy, shallow areas called patch reefs. Intertidal oysters benefit from being exposed to the air during low tide. Exposure can give them an advantage over competing organisms, but too much exposure can cause the oysters to dry out. On the other hand, if intertidal oysters remain underwater for too long, competitors may become more abundant and restrict oyster growth. These conditions create a “Goldilocks zone” along the sides of an intertidal reef, a level that is neither too dry nor too wet. This zone supports the greatest number of oysters and is the area of most rapid growth. The Goldilocks zone also helps explain the relationship between oyster reefs and sea-level rise. As sea level rises, the zone shifts upward and expands, creating more space for oysters to grow. The reef can then accumulate sediment and build upward over time.

Our work shows that oyster reefs grew faster as the rate of sea-level rise increased after about 1800 CE. This raises the important possibility that restored reefs could serve as natural breakwaters that reduce wave energy, provide habitat for fish and other wildlife, improve water quality, and adjust their elevation as sea level rises. In theory, these reefs could help protect shorelines for centuries. The challenge is that many estuaries are becoming increasingly inhospitable to oysters. As a result, projects promoted as nature-based solutions to climate change may fail unless the underlying water-quality and environmental problems are addressed. Restoring oyster reefs can be valuable, but restoration alone cannot compensate for conditions that oysters cannot survive.

Ongoing thread. More from Antonio B. Rodriguez to follow.