Baton Rouge, La. — Here in Louisiana, terram is not always so firmam. The state has some 40 percent of the nation's coastal wetlands, and they are disappearing at a rate of some 40 square miles a year, says Dr. Walter Sikora, an ecologist at Louisiana State University here. (He finds the often-cited figure of nearly 50 square miles too high.)
This is a ''very dynamic area,'' geologically speaking, he says. The Mississippi River is continually changing its course; the deltaic sediments that make up much of southern Louisiana are dewatering and hence subsiding. And the sea level is gradually rising.
But there is concern that the natural forces redesigning the shoreline are getting rather too much help from human activities.
There are dire predictions made for the wetlands, which are the result of river delta formation in the several thousand years since the melting of Ice Age glaciers brought the shoreline more or less where it is today.
Parts of certain parishes downriver from New Orleans are widely expected to be under water within 50 to 150 years.
One of the first industries to be affected is expected to be the coastal fisheries, an important renewable resource for the state.
Fishing - including crabs, shrimp, and oysters, plus menhaden, which are caught for oil and meal - is easily a $200 million annual industry for Louisiana , according to Dr. Sikora's colleague, Dr. Eugene Turner.
The wetlands are crucial to these species. For all their evident mushiness, the marshes are ecological bedrock, the bottom of the food chain. Two-thirds of the fish caught along the Gulf and Atlantic coasts require the wetlands for habitat, food, or breeding.
The problem is still being studied, says Dr. Sikora; it's hard to make a case that the shellfish harvest is in long-term decline. Most data on the subject are ''soft,'' or anecdotal. ''People remember that back in the '30s they could go crabbing in Lake Pontchartrain and feed the family easily and still have something left to sell, for example.''
Louisiana's problem with its wetlands may be the result of ''superabundance, '' he adds. ''They've been so vast, there's been no concern about them. It's like the American bison. People thought, 'We'll never make a dent in that thundering herd.' ''
So just what's been eating into the marsh? For one thing, the river naturally changes its course every millennium or so. It carries silt in its course, building up a delta, which slows the river down. Eventually there comes a time when gravity tempts the Mississippi down a straighter, steeper channel, and the river makes the switch. When the new channel silts up, the process is repeated. It's happened half a dozen times or so in the last 5,000 years.
Abandoned deltas naturally decay over time. But navigation channels cut into the marsh, as well as canals dug to enable energy firms to get to gas and oil wells in the wetlands, are thought to speed the entry of saltwater and the deterioration of the marsh. Spoil banks made of silt dug out when canals are built interfere with the cyclical inundation and drainage of the marsh grass.
The oil and gas firms, arguing that they already bear a heavy tax burden, have so far fought off Gov. David C. Treen's efforts to tax them to mitigate the effects of their operations on the wetlands.
Another difficulty is the levee system, much of which dates back to the aftermath of serious floods in 1927. Silt that would ordinarily build deltas is forced all the way to the end of the river, where it is dumped in a channel 600 feet deep.
Dr. Sikora tells of the visiting Kansan who found out about this and commented, ''If we knew that's what you're doing with all this farmland, we wouldn't have sent it to you.''
But don't get the idea the Pelican State is one big vanishing act. There has been an interesting new real estate development at the mouth of the Atchafalaya River: A new delta is emerging. It's indicated on maps with a broken line, like a highway under construction. It has become ''subaerial'' (visible above the water line) just since the major floods of 1973.
The Atchafalaya is where most of the Mississippi water would be flowing now if the US Army Corps of Engineers would let it. But letting nature take its course would mean saltwater intrusion up the present channel as far as New Orleans, which relies on the Mississippi for its drinking water.