Mussels can supercharge recovery from drought in marshes

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. This benefits both partner species and may be critical to helping these ecosystems bounce back from extreme climatic events such as drought.

The research, published in Nature Communications, was conducted by an international team, including Dr John Griffin of the College of Science at Swansea University, along with colleagues from the University of Florida and from the Netherlands. 

The team discovered 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.

They found that:

  • With mussels’ help, marshes can recover from drought in less than a decade. Without that help, it can take more than a century
  • Wherever there were clusters of mussels embedded in the mud around the base of the grass stems, the grass survived
  • Grass growing in mussel clusters had a 64 per cent probability of surviving versus a 1 per cent probability in areas where there were no mussels

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

600 x 449Picture:  standing dead stalks of cordgrass remaining in a Sapelo Island, Georgia salt marsh. This vegetation died during the 2011-2012 drought that affected the southeastern US.  Credit:  Christine Angelini, University of Florida

Dr John Griffin, senior lecturer in biosciences at Swansea University, said:

“The most exciting finding is that mutualisms can super-charge the recovery from an extreme climate event like a drought.

Under normal, non-drought, conditions, there is nothing very remarkable about the interaction between mussels and cordgrass.   Its importance only truly comes to light once the drought hits.  We found that mussels determined the very survival of the grass and the recovery of the entire ecosystem.

With climate change and the extreme shocks it brings, other mutualisms that have perhaps been overlooked or assumed to be relatively unimportant, may turn out to be crucial to ecosystem survival.”

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

Using Google Earth, the team considered 13 sites that contained relatively large marsh areas likely to experience drought-associated grass die-offs. They selected nine that spanned a little more than 150 miles of southeastern U.S. coastline from southern Georgia to central South Carolina at the conclusion of a severe, two-year drought in June 2012.

600 x 450Picture:  a mound of ribbed mussels embedded in the mud around cordgrass stems. Grass growing in mussel clusters had a 64 per cent probability of surviving versus a 1 per cent probability in areas where there were no mussels.  Credit: Christine Angelini, University of Florida

Christine Angelini, assistant professor of environmental engineering sciences at the University of Florida, and lead author on the paper, said that the mussels “protect and then accelerate the healing of drought-stricken marshes”:

“It’s a story of mutual benefit between marsh grass and mussels.  Saving the marshes has not only environmental benefits but also economic ones.

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

The next step, Angelini said, is to figure out whether the transplanting mussels into drought-vulnerable marshes could offer a low-cost solution to improve the resilience of this valuable ecosystem in the face of climate change.

The team are also testing whether other at-risk ecosystems – seagrass meadows or, perhaps most notably, coral reefs – may be similarly protected by keystone mutualisms.

Read the paper in Nature Communications.

Institutions involved were University of Florida (US), Swansea University (UK), The University of Groningen and Radboud University in the Netherlands; the Royal Netherlands Institute for Sea Research; and Duke University (US).