The perception that coastal salt marshes are 'wastelands' good only for draining, has been contradicted by observations over many years that such areas are rich in biodiversity. Salt marshes provide habitats and food for insects, fish, molluscs, and mammals, and are known to be important nurseries for numerous species — and these organisms can help maintain or restore the health of salt marshes. A new study published in Nature Communications explores how mussels can help restore a salt marsh after the depredations of a long drought.
Dr. Christine Angelini of the University of Florida, Gainesville and colleagues investigated salt marshes along about 155 miles of the southeastern coast of the United States — an area that had suffered three severe droughts over the previous 17 years. They were particularly interested in learning about the factors that helped salt marshes withstand and recover from such prolonged droughts, and one of those factors was the presence of mussels that attached themselves to clumps of marsh grass.
In biological terms, the investigators wanted to "investigate the potential for a mutualism between cordgrass, Spartina alterniflora, and the ribbed mussel, Geukensia demissa, to increase salt marsh resistance to drought and fuel subsequent recovery." By a mutualism, the authors meant a relationship that is beneficial to both partners. They explained that under conditions of drought, the soil around grasses can dry, and the salinity and metal concentrations of the water around the roots can increase, impairing the ability of the plants to survive and regenerate.
Previous research had established that cordgrass serves as a substrate for clumps or mounds of mussels. The investigators surveyed areas that had become denuded of cordgrass during droughts. They found that the probability of cordgrass surviving in such areas was 65 percent when the grass was associated with mussels, but only 1 percent when it was not.
They also assessed the effect of mussels on cordgrass patches to see how they affected soil characteristics, and found that water stored per unit of soil volume was significantly greater in clumps of cordgrass associated with mussels than in those that were not. And the salinity of the water was lower in the clumps associated with mussels. When they added mussels expeimentally to clumps of cordgrass that had not had them, the investigators again found a decrease in salinity in the mussel-associated soils. They hypothesized that this effect of the mussels occurred because crabs excavated burrows near the mussels, thus providing underground water water-retention compartments.
The authors pointed out that stress-resistant patches can have a disproportionately large affect on ecosystem recovery. And they concluded "[t]his finding reveals that: (1) a surprising level of ecosystem resilience can result even when only a limited area of dispersed patches of habitat-forming species remain after episodes of severe stress (that is, a system with low overall resistance), and (2) the time it takes for such ecosystems to recover and, hence, their resilience8 may not be predicted simply by the size of the disturbance."
Although, as they note, the presence of an ameliorating factor such as the mussels in this case is important to understand from the points of view of both the possibility of mutualism and the practical aspects of encouraging recovery from stress, other factors could still overwhelm such effects.