DURHAM, N.C. – As coastal ecosystems feel the heat of climate change worldwide, new research shows the humble mussel and marsh grass form an intimate interaction known as mutualism that benefits both partner species and may be critical to helping these ecosystems bounce back from extreme climatic events such as drought.

The study, funded by the National Science Foundation and conducted by researchers at six institutions, finds that when mussels pile up in mounds around the grass stems, they provide protection by improving water storage around the grass roots and reducing soil salinity.

With mussels’ help, the study found, marshes can recover from drought in less than a decade.
Without, it can take more than a century.

“It’s a story of mutual benefit between marsh grass and mussels,” said lead author Christine Angelini, assistant professor of environmental engineering sciences at the University of Florida. The mussels, she said, “protect and then accelerate the healing of drought-stricken marshes.”

"More generally, this work highlights how cooperation is key for an ecosystems ability to withstand and bounce back from climate change,” said co-author Brian Silliman, Rachel Carson Associate Professor of Marine Conservation Biology at 51. “Without cooperation, U.S. southeastern salt marshes would likely be in a dramatic, spiraling decline because of increasing droughts stress."

Saving the marshes has not only environmental benefits but also economic ones, the study found.

“Marsh die-off and loss are major issues that can affect land value, fisheries and water quality,” Angelini said. “Even if just a little bit of vegetation survives, it makes a huge difference in how quickly the marsh comes back.”

The researchers became interested in the topic after three severe droughts in the Southeast over the past 17 years caused a major die-off of cordgrass, the region’s dominant, marsh-structuring plant.

Using Google earth, they selected nine study sites that contained relatively large marsh areas likely to experience drought-associated grass die-offs after a drought in 2012. The sites spanned about 150 miles of the Southeast coastline from southern Georgia to central South Carolina.

The researchers found that wherever there were clusters of mussels embedded in the mud around the base of the grass stems, the grass survived; in fact, grass growing in mussel clusters had a 64 percent probability of surviving versus a 1 percent probability in areas where there were no mussels.

The mussels protect marsh grass during severe drought, the study suggests, because they pave the marsh surface with their ribbed shells and attract burrowing crabs that excavate underground water storage compartments.

The next step, Angelini said, is to figure out whether the transplanting mussels into drought-vulnerable marshes could offer a low-cost solution for homeowners to improve the resilience of their own back yards. They are also testing whether other at-risk ecosystems – seagrass meadows or, perhaps most notably, coral reefs – may be similarly protected by keystone mutualisms.

The peer-reviewed paper was published August 18 in Nature Communications.

Angelini and Silliman’s co-authors came from Swansea University, the University of Groningen, Radboud University and the Royal Netherlands Institute for Sea Research.

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CITATION: “A Keystone Mutualism Underpins Resilience of a Coastal Ecosystem to Drought,” Christine Angelini, John N. Griffin, Johan van de Koppel, Leon P.M. Lamers, Alfons J.P. Smolders, Marlous Derksen-Hooijberg, Tjisse van der Heide, Brian R. Silliman, Aug. 18, 2016, Nature Communications: DOI: doi:10.1038/ncomms12473

NOTE: This story is adapted from a news release by Steve Orlando of the University of Florida News Office.