On the alluvial plains of North America's eastern seaboard, sheltered by barrier beaches and dunes, exists a unique community: the tidal salt marsh. In the minds of many vacationers, this ecosystem registers as no more than the several acres of waving grass they've driven through en route to the beach. This perception, however, minimizes the ecological importance of the tidal salt marsh, and the nearly unmatched productivity of the plants that grow there.

Tidal salt marshes begin in most cases as mud or sand flats. These flats are first colonized by algae and, if the water is deep enough, by eelgrass. As organic debris and sediments accumulate, the eelgrass is replaced by the quintessential salt marsh plant: Spartina alterniflora, or Spartina grass. Standing three to four feet tall and partially submerged in salt water at every high tide, Spartina grass forms the strip between open sand or mud flat to the front and taller grasses behind -- a strip that feeds surrounding coastal waters and estuaries.

The salt marsh is one of the most productive of all ecosystems, rivaling even tropical rain forests in net primary production (the amount of plant biomass produced in a given area per year). The marsh owes its high level of productivity to several factors. First, tides that infiltrate the marsh twice daily bring nutrients in and carry wastes away. Second, the meeting of fresh water (relatively high in nitrogen) and salt water (relatively high in phosphorus) provides a concentrated blend of important nutrients, increasing natural fertility. Third, cyanobacteria (formerly called blue-green algae) in the marsh mud fix and transform nitrogen into a form that plants can easily use. These bacteria, combined with algae, grow rapidly on the nutrients released from decaying Spartina grass, and are washed into coastal waters and estuaries, where they are consumed by filter feeders that, in turn, are eaten by fish.

Not surprisingly, the most productive tidal salt marshes along the eastern coast of North America are those farthest south. Salt marshes along the coast of Georgia are, on average, more than three times as productive as New England marshes. This owes, in part, to the South's longer growing season. Whereas plant growth in New England marshes ceases in late fall, production continues throughout the winter in the South -- albeit at lower levels than in summer.