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.

Oyster reefs form in estuaries as mounds and are sometimes labeled hard bottoms or rock bottoms on nautical charts. These areas are avoided by boats to prevent running aground and causing damage. That hardness makes reefs resistant to erosion, which is why they are being restored along estuarine shorelines to protect communities from storm waves. Reefs are also promoted as carbon-burial hotspots that can help mitigate greenhouse gas emissions. Oysters capture microscopic algae suspended in the water as they filter-feed, excrete that food as organic carbon-rich feces, and those larger particles rapidly settle to the bottom, where they are ultimately stored within the reef. Shoreline protection and carbon burial are valuable ecosystem services that reefs provide, but both require the reef to gain elevation over time.

Earlier work shows that reefs have the potential to grow rapidly, up to 10 cm per year, and that information is fueling large investments in intertidal oyster reef restoration projects. The function of oyster reefs as naturally expanding breakwaters and carbon sinks is mainly supported by measurements taken from young, less than decade-old, restored reefs. Coastal managers expect and plan for restoration projects to last much longer than a decade, though. Furthermore, reef expansion truly depends on the accumulation of oyster shells and organic-rich sediment, which is a long-term process operating over decadal–millennial timescales. Understanding reef growth and carbon burial requires a geological perspective on legacy reefs that formed naturally.

To expand the timescale over which carbon burial and reef growth are constrained, we collected cores from 25 intertidal reefs in North Carolina to track reef development from when the first oyster colonized and formed the reef foundation, to the present day. We used a concrete vibrator to drive thin-walled aluminum pipes into the reefs and pulled the sediment-filled tubes back out with a hand winch. Back in the lab, we sampled oysters in the cores for radiocarbon dating to develop a timeline and measured down-core variations in the amounts of shell and organic carbon.

The results showed that over the long-term reefs don’t grow as rapidly as we thought and their rate of growth is closely tied to sea level. After 1800 CE when humans started the process of warming the planet, sea-level rise accelerated. Intertidal reefs grew vertically and accumulated sediment three times faster after 1800 CE than during the previous 18 centuries. Reef growth resulted from the accumulation of shell and organic-rich sediments; however, none of the 25 reefs sampled ever functioned as complete carbon sinks.

Organic carbon was being buried and preserved in the reefs about as efficiently as in other coastal “Blue Carbon” habitats, such as salt marshes, mangroves, and seagrass beds. Unlike those vegetated environments, oysters are shelled animals, and the reaction that forms calcium carbonate naturally liberates carbon dioxide. The emission of carbon dioxide during shell formation offsets the burial of organic carbon during filter-feeding. As sea-level rise continues to accelerate, the vertical growth of healthy intertidal reefs should respond rapidly. Reefs will become larger sources of carbon dioxide but will maintain their effectiveness at damping waves.

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