The great flats of salt marshes, green in summer, golden in fall, are an impressive feature of the Georgia coast. Two-thirds to three-fourths of the estuary lying behind the sea islands is salt marsh, and over 90 percent of the marsh is covered by just one species of plant: smooth cordgrass, Spartina alterniflora. Its genus name, Spartina, derives from a similar salt grass growing along the shores of the Mediterranean Sea. The ancient Greeks found that fibers from the dried grass could be woven into a strong rope. "Spartine" is Greek for cord, so both the scientific and common names of the plant refer to this practical use.
Photo of Chuck Hopkinson wading up a soft muddy Spartina creekside marsh along the Duplin River by Sapelo Island in the late 1980's. Note that the Spartina plants are green and tall. In the upper right there is a raft of dead Spartina stems resting on the top of the creekbank marsh, deposited by a previous high tide. Chuck was a research scientist at the University of Georgia Marine Institute in the 1980's, and is currently directing the Georgia Sea Grant program at the University of Georgia in Athens. By E. Sherr.
Grasses are particularly hardy; various grass species thrive over a wide range of conditions. The grasses that grow in salt marshes have had to adapt to harsh conditions of salty water and soil, high summer temperatures, and water loss through the leaves, which can lead to desiccation. Marsh plants have evolved various ways of handling these problems. Spartina copes with the stress of salt in marsh muds by excreting it from glands in the leaves. On a hot summer day, salt crystals sparkle on the sides of the shoots. Spartina also has an efficient C-4 biochemical pathway of photosynthesis, which allows the plant to take in carbon dioxide from the air with minimum evaporative loss of water. (The C-4 pathway gives Spartina a distinctive stable carbon isotope ratio of 13-C and 12-C, which allows scientists to trace marsh grass carbon through estuarine food webs).
A sampling station in a short cordgrass marsh plain. Note that the Spartina plants here are shorter and yellower than creekside plants due to greater stress. By E. Sherr.
Cordgrass has yet another problem: it grows in anoxic marsh sediments, but the roots need oxygen to live. This difficulty is solved by the hollow stems of Spartina, which funnel oxygen from the air down to the rhizomes and roots in the anoxic mud. Like lawn turf, cordgrass has an extensive system of underground roots and rhizomes, from which new shoots emerge as the rhizomes extend through the sediments. It is hard to dig into the salt marsh because the tough rhizomes are so dense. The thick tangled underground growth supports the muddy soil, so it is easy to walk over the marsh plain without sinking into the surface.
A thick, tangled mass of Spartina roots and rhizomes dug up, with difficulty, from a marsh plain. This dense root mass makes the marsh plain firm enough for easy walking. By E. Sherr.
An additional problem caused by marsh mud is not so easily overcome. Marsh soils are full of toxic sulfide, produced by sulfate-reducing bacteria living on the abundant sulfate in seawater brought in by the tides. Sulfide can kill delicate plant roots. Experiments have shown that the growth of Spartina is inhibited by high concentrations of sulfide around its roots. In fact, the black marsh sediment reeks of a rotten-egg smell resulting from sulfide. Along with salt stress, large amounts of sulfide in higher-elevation soils contribute to the stunting of marsh plain plants.
These varying conditions across the marsh shows in the difference between the lush creekside stands six to eight feet high and the swards of stubby Spartina plants less than a foot tall in the upper reaches of the marsh.
Aerial view of Spartina salt marsh around Sapelo Island. Note the lush, dark green cordgrass lining the tidal creek, and the sparse, yellower cordgrass in the marsh plain on either side of the creek. By E. Sherr taken from a small plane.
One thing cordgrass doesn't have to worry about (usually) is being eaten alive. Endemic marsh grasshoppers do chew the leaves, and plant hoppers suck sap out of the stems. But, despite the abundance of these insects, research has shown that they consume less than 10 percent of the annual production of Spartina. (Although occasional mass outbreaks of periwinkle snails in the marsh can result in overgrazing of cordgrass - that is another story.)
Most of the summer growth of Spartina leaves and stems is destined for decay. Cordgrass stems, more fibrous than the leaves, are the last parts of the plant to decompose. In fall, dead stems of creekside Spartina are washed out of the tidal creeks by extreme high tides, forming great grey-brown stem rafts that drift around the estuary. These rafts are often carried up back onto the marsh plain on high tides, killing the cordgrass plants beneath them. Another high tide that later lifts the stem rafts away will reveal a patch of mud in the marsh, which will stay bare until the rhizomes below sprout new Spartina shoots. Although the stem rafts are impressive, they are only a small part of the dead mass of Spartina, most of which is fragmented into small, microbe-laden particles.
A second species of Spartina lives in less salty habitats along coastal rivers. In the lower Altamaha River, where estuarine water is fresher than in the barrier island marshes, grow stands of giant cordgrass, Spartina cynosuroides. This grass resembles the most robust smooth cordgrass plants. Small marshy islands in the river mouths are often covered with giant cordgrass. In early spring, when most of the cordgrass leaves are still brown and dry, it used to be common practice to burn these islands in order to promote the growth of new cordgrass as well as to kill the larvae of biting flies and mosquitos living in the marsh mud.
Salt hay in a New England marsh. By V. Howard, UCGS - http://nas.er.usgs.gov/queries/factsheet.aspx?SpeciesID=1128, Public Domain, $3
Dune habitats on the sea lslands offer one more species of cordgrass: salt hay, Spartina patens, a bushy plant with long narrow leaves, unlike the other species of Spartina. In New England salt marshes, salt hay grows in extensive stands and was once used as fodder for cattle. In Georgia estuaries, salt hay is found only in isolated clumps along the edge of the cordgrass marshes or at the edges of sloughs in the beach dunes.