Miles before arriving at a seacoast, I begin sniffing the air, hoping for a scent I consider fragrant, but others call a stench. It is the rich aroma of saltmarsh.

For many years I studied a great Eastern estuary, a major river descending into an Atlantic bay. Like all coastal plain estuaries, it is bordered by soft alluvial shores: sand in the lower reaches, everywhere else, mud. Estuarine mud is seldom firmly packed, for despite its weight and stickiness, it is 50 percent water and only 10 percent organic material. The rest is silt and clay.

For mud to remain, it must somehow be held in place. In nature this means anchorage by plants. Tropic estuarine shorelines are buttressed by mangroves, but our temperate New England latitudes support very special grasses and a few other coastal plants to do the job.

The odor I detect miles away is organic decomposition -- rotting vegetation. The enormous quantities of saltmarsh cordgrass, Spartina, cut and chewed by crabs, shredded by shore birds, dying naturally and broken down by bacteria, eventually turn into a fine flocculent detritus that is swished back and forth by the tides. Decayed further, it eventually settles down between living grass stems as dark, soft, sticky mud.

Walking across a saltmarsh is not for everyone. Who wants to slip and slide, pull feet out of thick cloying mud (losing a shoe), bat away vicious green-eyed horseflies, swat more hungry saltmarsh mosquitoes than you can imagine? It is hot and humid with likelihood of a fiercer sunburn than on an inviting beach not far away. The ground quakes under your feet. Upon emerging from a saltmarsh, your boots and clothes are covered with globs of black stickiness. Who wants to enter such a place? I do.

There is no more productive natural crop in the world than cordgrass in a saltmarsh. Sugar cane cultivated by man and machine is the only crop to surpass its level of primary production per acre.

A saltmarsh not only is self-sustaining, it grows outward into shallow water that shoals ever more as detritus accumulates. More grass, more mud, more marsh. The place fertilizes itself. Because it develops from overwash by tidal ebb and flow, it is utterly flat.

More nutrients leave a saltmarsh than return with the tide, so there is a net flow of organic matter into bay and ocean. It is not an overstatement to say saltmarshes nourish the coastal seas, the world\'s greatest fishing grounds.

Cordgrass is an unusually tolerant grass that grows in water possessing as high a salt content as the ocean itself. Furthermore, spots exposed at low tide in a saltmarsh become increasingly salty as evaporation removes water and leaves behind concentrated minerals. Glance at cordgrass on a summer day and you\'ll find blades sparkling with crystals of secreted salt. In this way it preserves a normal internal environment typical of inland grasses that absorb only fresh water.

What underlies these vast meadows of cordgrass? At the surface is a foot or more of densely packed, rotted plant material and firmly packed clay. Next are several feet of soft mud containing grass blades, still intact in this oxygenless environment. Below that lies peat, followed by more clay and sand, the latter indicating the ancestral shore.

The dynamics of water movement through a saltmarsh are complex, the single most visible corridor being a tidal creek. Water entering on a flood tide from a bay is relatively clear and cool, but on an ebb tide it is warm and opaque as it carries vast quantities of plant detritus from the marsh. Saltmarsh creeks build natural levees along their banks, although these do not prevent water from flowing across the flats during the highest tides and in storms. Most of the time, however, the mud levees restrict incoming and out-flowing tides to the channels. This means the flats themselves only periodically exchange salt water and whatever organic richness it carries. It\'s a nicely balanced system that remains exceptionally stable over time.

At first glance a saltmarsh seems to consist only of cordgrass and no visible animal life. Approach closer and suddenly a willet calls or a great blue heron takes off. In spring and fall, marsh and creek are alive with migrating waterfowl and shorebirds. Across the wide expanse sturdy mounds of packed dry grass blades tell muskrats are at work and live in these secure houses. Perhaps a shy diamondback terrapin slides into the creek as you approach.

Among thick cordgrass you\'ll come across sea lavender and saltwort, and farther from the tidal creek there may be marsh mallow, swamp rose, black needlerush, and a great many other small plants, most of them less tolerant of salty conditions than Spartina.

At low tide the banks of a tidal creek come alive with rushing hordes of fiddler crabs that leave their burrows to forage. Ribbed mussels -- 3 million to an acre -- attach to one another, providing their own secure attachment. Also present may be \"hands\" of oysters clustered together.

Beyond the levee and onto the flats purple marsh crabs stand under their burrows\' overhanging mud porches, munching short bits of cordgrass they have harvested. Marsh periwinkles and battered-looking black marsh snails are underfoot, and burrows of marine worms pockmark the surface. Resident birds know very well a saltmarsh offers endless quantities of food, but in spring and fall it is the migrating birds that gorge excessively on smaller creatures to sustain their long ocean flights.

It is difficult to comprehend the intensity of life in each square meter of saltmarsh. At the microscopic level, a thin layer of billions of tiny plants and animals can tinge the mud with yellow and green. But even this pales in comparison to what is riding the tidal flow and swimming in the brown, opaque creek-water.

First there are fish. Many are juveniles, for saltmarsh waters are nurseries for a host of oceanic species, ranging from the mighty tarpon down to herring and even smaller gobies. The croaker, striped bass, summer flounder, spot, American eel, shad, and Atlantic sturgeon are some species that spawn here, grow here, or are passing through. Of all the fishes you see, it is the little killifish, or mummichog, that prefers saltmarsh creeks to all other habitats. Its various species are ubiquitous inhabitants of almost every marsh along the Eastern seaboard.

What brings so many fish into these murky waters? An astonishing population of invertebrates is nearly invisible among the flotsam and detritus in a tidal creek. Some are larvae of almost any coastal marine creature you can name, while others are adult blue crabs, brown shrimp, and grass shrimp. Collecting plankton is difficult in such a place, for nets clog up instantly with tiny plant fragments. Pour a net\'s contents into a glass bowl and you will see nothing but a frenzied jittering among the detritus.

Close study reveals thousands of little inhabitants. Any juvenile fish needing constant nourishment and a fair degree of protection from predators is lucky if it finds its way into a saltmarsh creek. Many adult open ocean fish populations would lessen were youngsters unable to enter these extraordinary feeding grounds.

Often saltmarshes are drained, ditched, and sprayed by those who understand little of their role in the world\'s bioeconomy. Would-be developers speak disparagingly of \"muskrat economy.\" Knowing a bit of what goes on in a saltmarsh, I sniff with glad anticipation as we drive toward the coast. For me, it is the good odor of nature at its best.

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