This time of year the Smithsonian lays on a few tours to show off its Chesapeake Bay Center for Environmental Studies down in Edgewater, Maryland.

A National Estuarine Sanctuary, the property consists of the 2,600-acre watershed of the brackish Rhode River, including 14 miles of shoreline. Visitors catch up on phytoplankton and mosquito research and the center's study of land use: how cows and pesticides and boats affect life on marshland and mudflat.

I tagged along on a tour through woods and along Muddy Creek, out to Fox Point with its view of the center's pier and its sampling stations, and across a marsh to Hog Island, which is virgin forest. Our guide was the kind of knowledgeable volunteer who introduced us to four species of local crabs and three of killifish (Indian name, mummchugs). The group gave as good as it got, offering him recipes for snapping-turtle soup.

In the woods, where we marveled at a 50-year-old poison ivy vine, sturdy as an oak, the guide alerted us to evidence of center studies -- litter boxes (for monitoring leaf and fruit-fall), weirs and water-collecting stations.

We'd have missed it, hidden in cattails across the creek, if our guide hadn't pointed to the marsh railroad, built to give access to the marsh without casting plantlife in shade as a boardwalk would do. The tin-roofed station, reached by snubnosed punt, is big enough to shelter a scientist's notebook from the rain, but not the scientist. The tracks run 200 feet with, midway, a spur of the same length. The cars are 3-by 4-foot platforms on caster wheels.

We didn't see the railroad in action -- it's sometimes called the Muskrat Junction Line -- but I could picture the pleasure of kicking my way along the track with fine views of sky and bay. Here, water samples are taken from groundwater wells spaced along the line, to see how nutrients, microfauna and bacteria vary. Surface mud is scooped up to measure populations of amphipods, little crescent-shaped crustacea that graze their way across plants feeding on micro-organisms.

Raccoons, we were told, run along the rails as a shortcut. They haven't yet discovered the fun-ride of downtown Edgewater.

Our group crossed the marsh single-file via boardwalk. At the edge of Hog Island our guide identified threesquare grass, cordgrass and the plumed phragmites and invited all of us to jump up and down to feel the turf quake.

Dr. Roland Limpert met us on the island. Just minutes earlier, he'd spotted black ducks, ruddy ducks, puddle ducks and mallards. We didn't -- they'd vanished. But he showed us the neckbands he uses on waterfowl: collars three and four inches high, black with white numbers so large he can read them with a spotting scope from half a mile. They're loose enough to permit swallowing.

Limpert stains some canvasback ducks with picric acid so he can keep track of them. Their diet also interests him: The birds have shifted from vegetarianism (their Latin name, Aythya valisneria, refers to a fondness for wild celery) to clam- and mollusk-eating. It's a good thing, too; the Bay's celery has long been blighted.

Poplar Island, which hosts a bald-eagle nest and a heron rookery, is off-limits, but near the entrance are the ruins of the 1720 brick home of John Contee, who bought the land with his share of the booty awarded to the crew of the USS Constitution for sinking HMS Java in the War of 1812.

The island was planted in tobacco, later in corn. A 20th- century dairy retained the name Contee gave his farm -- Java Farm Dairy. Its calf-barn now houses laboratories, while the cow-barn with twin silos serves as Center headquarters. MEANDERING THE MARSH Tours of the Smithsonian's Chesapeake Bay Center for Environmental Studies are set for November 13 and December 12. Call 261-4190 for reservations.