A salt marsh is an incalculable resource.

By the numbers

334,500- 365,900Estimated acreage of South Carolina’s salt marshes, said to be the most of any Atlantic Coast state.6%Percentage of creeks in fair condition in a long-term study of the overall health of the state’s tidal creeks.34,962Acres of brackish water marshes.61,531Acres of freshwater marshes.1,280Number of permit requests for coastal alterations in the past three years, most of which are for building private docks.938Combined number of fish (163) and invertebrate (775) species living in the tidal creeks, estuaries and nearshore areas.4%Percentage of creeks in poor condition.$14.81 millionTotal value to fishermen in 2011 of commercially important marsh-dependent species. This includes shrimp, oysters, hard clams and blue crab.$533millionTotal economic impact of recreational fisheries in South Carolina in 2006. The majority came from fishing salt marsh-dependent species such as red drum, flounder, spotted sea trout, whiting and croaker.Source: David Whitaker, S.C. Department of Natural Resources

It’s a vital fisheries habitat, providing food, structure and refuge from predators for smaller sea life. It regulates the amount of freshwater, nutrient and sediment “input” into the estuary, according to the S.C. Department of Natural Resources.

The marsh also plays an important role in the health and water quality of the estuary. Along the margins of estuaries, the marshes and their dense stands of plants help stabilize the shoreline and contain floodwaters during coastal storms.

It is estimated that nearly 400,000 acres of coastal marshes sweep across the South Carolina coast. But the marsh, like so much else in the natural world, is at risk.

Development and urban rain runoff pollution are depleting it. As seas keep rising, there is less to deplete.

The evidence of deterioration is compelling, said David Whitaker, a DNR marine biologist.

“Intuitively, one would guess that developing areas next to marshes would have negative effects on the salt marsh, tidal creeks and the animals that live there,” Whitaker said. “However, it has only been within the last decade or so that quantitative evidence has accumulated, clearly showing the negative impacts of development.”

The marsh is “a biological factory without equal,” said environmental writer Charles Seabrook, a Johns Island native, in his new book “The World of the Salt Marsh” (University of Georgia Press).

“What makes the salt marsh so productive is that the tides ferry untold amounts of nutrients into the system that cause it to flourish,” Seabrook said. “Rivers also flow into the estuaries bringing nutrients from upstream. Twice a day this good stuff comes in that system needs to thrive.”

But there are those who still regard marshes as vast wastelands to be drained, diked or “improved,” he said.

“Basically, it is willy-nilly growth that is the biggest threat facing the Southeast coast, though due to the economy’s sharp downturn, a lot of these developments have been put on hold and in some cases these companies have gone bankrupt.

“But that is temporary. I have faith that the economy will come back and when it does many of these development projects are going to rise again. They will come back with a vengeance.”

While no enemy of development in principle, Seabrook insists growth can’t be unlimited or unplanned.

“How do you plan growth so you don’t ruin the environment and the very things that attract people: the beauty, the climate and the serenity of a place? How do you hold on to that and still accommodate growth?”

Maintaining the quality and diversity of lifestyles that make the Southeast coast a desirable and unique place to live will require action on the part of all citizens, from politicians to conservation agencies to businesses and homeowners, he said. But he views the prospects with a skeptical eye.

“I was born and reared on Johns Island. Most of the roads in my youth were of dirt. You could lie down on River Road for an hour before a car came by. Over where the Limehouse Bridge is today, one could see only trees, Tommy Grimball’s farm and John Limehouse’s old family store.

“But now when you look to your right crossing the bridge there are scores of houses occupying what used to be a lush old farm. Of course, people need a place to live, and more and more people are moving to the area because it is so desirable. But how long will it remain so?”

Reach Bill Thompson at 937-5707.