Marshes beat droughts with mussel power

Co-operation between species can make ecosystems more resilient, so that marshes beat droughts far faster than they could otherwise, US scientists find.

LONDON, 23 August, 2016 – Florida scientists have worked out what it takes to help a coastal saltmarsh survive drought. It takes an ecosystem of just two species: the prevailing grasses, and a colony of mussels.

With these two partners clustered together in the drying mud, a coastal wetland can recover from prolonged drought in less than a decade. Without the mussels, recovery could take more than a century.

“It’s a story of mutual benefit between marsh grass and mussels,” says the lead researcher, Christine Angelini, an environmental engineer at the University of Florida.

Since 40% of the planet lives by the coasts, and since drought in the US southeast seems to be on the increase, with three severe droughts since 1999, the research is of more than academic interest.

Natural defence

Coastal wetlands are important both environmentally and economically: they represent a natural defence against coastal flooding and tropical windstorm damage as well as a wildlife resource and a nursery for coastal fisheries.

Researchers have warned that, if global warming is not contained, the world’s coastal cities could be losing $1 trillion a year by 2050. And things could get worse as coastal developments are hit ever harder.

After the devastation of New Orleans and New York by hurricanes in 2005 and 2012, scientists have repeatedly confirmed that natural barriers – healthy marshlands, mangrove forests and coral reefs – offer the best and most cost-effective buffer against rising sea levels and roaring winds.

The Florida scientists report in the journal Nature Communications that at the end of a severe, two-year drought in June 2012, they studied nine stretches of marsh along 150 miles of Atlantic coastline from Georgia to South Carolina. These had been hit by serious losses of the cordgrass Spartina alterniflora that holds the wetland sediments together.

Root reservoirs

They found that, wherever there were clusters of the bivalve Geukensia demissa, a mussel, in the mud around the stems of the plants, the grass had a 64% chance of surviving. Without additional mussel-power, the probability of survival fell to 1%.

It has become orthodoxy in ecosystem science that the lower the diversity, the less resilient the habitat. This study shows that even with just two species, life can go on.

Why this should be so has still to be established. But the reasoning is that the hard shells of the mussels pave the surface and serve as attractions to burrowing crabs that create little water-filled reservoirs under the plants’ roots.

“Without co-operation, US southeastern saltmarshes would likely be in a dramatic, spiralling decline because of increasing drought stress”

In the absence of shellfish partners, the once-water-logged marshes dry and oxidise. Salts concentrate, and acidity and heavy metal concentrations increase. Snail damage to the cordgrass intensifies. The terrain becomes increasingly at risk of erosion, and a natural habitat perishes. But if mussels cling on through the drought, the odds alter in favour of recovery.

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

And her research colleague Brian Silliman, Rachel Carson associate professor of marine biology conservation at Duke University, explained how two species together mean marshes beat droughts. He said: “This work highlights how co-operation is key for ecosystems’ stability to withstand and bounce back from climate change.

“Without co-operation, US southeastern saltmarshes would likely be in a dramatic, spiralling decline because of increasing drought stress.” – Climate News Network