Purple crabs clobber blue carbon
Millions of purple marsh crabs are churning through salt marshes along the East Coast, significantly disrupting the storage of carbon within these ecosystems. The small crabs’ constant burrowing and consumption of the cordgrass lead to erosion and a 40-70% loss in carbon, according to the results of a study by a team of marine ecologists and coastal geologists at William & Mary’s Batten School & VIMS.
Serina Wittyngham led the study as a postdoctoral researcher working in the lab of coauthor Matthew Kirwan, coastal geologist and professor at the Batten School & VIMS. She notes that blue carbon, or carbon captured by oceans and coastal systems, is considered an important factor in meeting future climate goals. However, the impact of animals on carbon cycling in coastal ecosystems is not well documented.
“Most researchers who study blue carbon cycling stop at the plants, but we asked, ‘what about the creatures removing the plants?’” said Wittyngham, who leaned on the marine ecology expertise she gained while earning her Ph.D. at the Batten School of Coastal & Marine Sciences under the guidance of academic advisor and study coauthor David Samuel Johnson. “Part of what made this study unique was how we combined the expertise of the coauthors to look at carbon cycling in a way that hadn’t been fully considered before.”
The impact of the small invertebrates on salt marshes is visually dramatic and can be seen in satellite imagery. Known as consumer fronts, the grazing crabs leave expansive, fan-shaped mudflats as they burrow into the marsh and consume large swaths of cordgrass.
The researchers identified consumer fronts in Virginia, South Carolina and Georgia, evaluating four distinct sites in each state. They measured carbon loss, gain and recovery by calculating the aboveground biomass of the plants and taking 30cm-deep core samples of sediment in disturbed and undisturbed areas.
The scientists also used a remote sensing technique developed by coauthor and former Batten School & VIMS Postdoctoral Researcher Yaping Chen to track the long-term movements of the crabs. Chen identified 50 additional consumer fronts using high-resolution aerial images taken between 1993-2019.
The team found that all three states experienced a net loss in carbon stocks when comparing ungrazed and recovering areas. Estimated times for carbon recovery varied significantly between states. Salt marshes in South Carolina were estimated to take approximately five years to recover their carbon losses, while Georgia marshes recovered in 17 years and Virginia marshes may never regain their lost carbon. The remote sensing data showed that the rates of consumer front movement are accelerating, with an approximately 30% increase in their formation and movement since the early 1990s.
The different recovery periods are thought to be caused by the elevation of the salt marsh and the amount of water that moves in and out during a tidal cycle. Salt marshes in locations featuring higher elevations and greater water inundation better facilitate the growth of cordgrass.
“What’s happening is that the crabs are lowering the elevation of the marsh through sediment loss as they burrow and consume the grass. This causes more water to flood the marsh, which is good for the grass but causes the crabs to move to higher ground,” said Johnson.
“Our findings are a great example of why, when possible, science must account for regional variability when considering the impacts of various factors on ecosystems,” said Wittyngham. “Additionally, salt marshes are incredibly resilient and an exciting takeaway from our study is that the marshes themselves always recovered. Whether or not they regained their lost carbon, the plants always grew back.”
The full manuscript of the study is available online at: https://esajournals.onlinelibrary.wiley.com/doi/10.1002/ecy.4385