The salt marsh may look like a uniform prairie, but it is a busy place. Grassy Bayou, a tidal creek, flows in and out with the tide through a salt marsh dominated by smooth cordgrass (Spartina alterniflora). Other grasses, wiregrass (Spartina patens), salt grass (Distichlis specata), black rush (Juncus roemarianus), and salt wort (Batis maritima) may also be present.
The salt marsh environment is harsh but productive, even though cordgrass, the primary food source, is low in nutrition. Leaf-chewers: insects, spiders, weevils, and squareback crabs feed on its leaves. Sap-sucking insects have a taste for its stems. Butterflies–skippers, the great southern white, and the western pygmy-blue–drop down on the leaves for a nibble. The long-billed marsh wren, the salt marsh sparrow, and the seaside sparrow weave nests from its stems and feed on the insects and the crabs. When the grass dies, microscopic bacteria and fungi move in and start the process of decay. Meiofauna–protozoa, nematodes, copepods, and larval stages of large invertebrates–living between grains of sand, take over and feed on the bacteria and fungi. Foragers follow, filter feeders and deposit feeders, oysters and blue crabs. The crabs browse meiofauna, the decaying grass, and algae on the surface of the sediment. Oysters and the ribbed mussel filter food particles out of the water.
Fish move in and out of the marsh, feeding along the edges of tidal creeks. Shrimp, menhaden, redfish, speckled trout, and crabs spawn either upstream or offshore, migrate to the marsh as juveniles, where food and shelter are abundant. As they reach adulthood, they return offshore. Wading birds–egrets, herons, woodstorks, and roseate spoonbills–pick through the marshes in search of fish. Waterfowl–mallards, widgeons, gadwalls, redheads, and teals winter there. Pelicans, gulls, and terns–diving birds–navigate the tidal creeks from the air, on the lookout for fish.
Finally, the mammals–the critters with an insatiable appetite for salt grass–the nutria and the muskrat are capable of reducing vast areas of salt marsh to open water. They may prefer freshwater marshes, but as those disappear they are moving on to salt marshes, where they eat the growing grass in the summer and rip up and feed on the tubers in the winter.
Freshwater supplies the sediment for building a delta lobe. Once abandoned, its supply of sediment comes from marine processes, which rework the lobe. The tide washes some sediment in from the gulf, which the grass catches and holds in place, anchored by its roots and rhizomes. Hurricanes and other major storms rework the floor of the gulf and wash more sediment into the marsh. And, the growth and decay of the grass builds up the marsh. When the land under the marsh sinks beneath it, in time the cycle of growth and decay of the marsh can no longer keep up. The marsh submerges.
The meandering bayous of of the Louisiana wetlands, which which once facilitated the exchange of freshwater for saltwater and spread sediment throughout the marshes, are the channels that could spread oil from the BP spill throughout the marshes.
[i] Morton, Robert A., Personnal communication, March 20, 2007; Mitsch, William J., and Gosselink, James G., Wetlands, New York: John Wiley and Songs, Inc., 1993, 226-248; Louisiana Department of Wildlife and Fisheries, “Conservation Habitats and Species Assessments: Salt Marsh,” December 2005, http://www.wlf.louisiana.gov/pdfs/experience/Salt%20Marsh.pdf.