Inside a low-ceilinged basement of the Smithsonian's National Museum of Natural History, electric pumps hum and thunk, fans whir and water sloshes from troughs.

Female blue crabs work their way from freshwater to saltwater tanks to hatch their young in the spring, just as they do in the wild. Oysters grow in brackish water, snails cling to slender stalks of marsh grass and fish swim in the shallows.

It's a good show, and one that deserves an audience, says Walter H. Adey, the creator of this model of the Chesapeake Bay's life-support system. But Adey's bay-in-the-basement is an experimental system that is not open to the public.

Adey, who built the system five years ago, is on a quest to build a bigger bay for public display as an instructive ecological showcase.

"How can you really understand an ecosystem?" he said. "I think people can, but they have to be shown how. They have to see it and walk through it."

Adey, the creator of a complex in Arizona in which eight scientists will spend two years, has been the Smithsonian Institution's marine systems director for 14 years. He is preparing a detailed proposal for a permanent Chesapeake Bay exhibit that, if financed, would likely open at the museum in 1992. The Ethyl Corp., of Richmond, provided Adey with a $300,000 grant to research the project, and will decide later this year whether to finance the $1 million-plus exhibit.

Adey is an enthusiastic promoter and leading practitioner of an emerging science that he calls synthetic ecology. During his studies of ocean reefs in the 1970s, he tried to duplicate the complex aquatic system in a closed model. He calls such models mesocosms -- bigger, more elaborate versions of microcosms used in laboratory research.

"It turned out to be simpler than I thought," he said. Models he has built since then include a coral reef exhibit for the Australian park authority and a model of the Florida Everglades, closed to the public, that occupies a greenhouse at the U.S. Soldiers' and Airmen's Home in Northwest Washington.

His most ambitious work is the construction of a marsh, 2-million-gallon ocean and underwater reef for Biosphere 2, a scientific venture staged in a four-acre glass enclosure north of Tucson. Biosphere 2 contains thousands of species of plants and animals arranged in seven interconnected environments. Late this year it also will become home to eight scientists, who will work and live in the five-story enclosure for two years.

Adey's Biosphere 2 work depended on the success of his miniature Chesapeake Bay, his first attempt to model an estuary. Estuaries are a soup of ingredients that form shifting combinations from upstream to the bay's mouth. Adey concocted recipes of water salinity and temperature, nutrients and light to duplicate the bay's shift in character throughout its 200-mile length. He developed algae scrubbers, sheets of mesh coated with seaweed and scum that remove excess nutrients from the water and produce oxygen. His bay even empties into an "ocean," an assortment of boxy equipment that traps sediments and pumps cooled saltwater back into the bay.

Adey populated the 2,500-gallon tanks with several hundred species of plants and animals that breed, die and coexist largely on their own. Adey and his staff play the role of top predator, and each spring they pluck out the biggest blue crabs to keep them from overwhelming the system and, incidentally, to have a nice meal.

The elaborate artifice has created a healthy ecosystem that offers a glimpse of how the real Chesapeake Bay functioned before centuries of pollution.

Models such as his, he said, could stand in for the bay in experiments tracing how pollutants weaken the ecosystem. Adey intends to pursue that research in addition to building a public bay exhibit.

Adey thinks the world's chaotic diversity is worth imitating, not avoiding. After all, this is a man who cuts his lawn in patterns to encourage its growth as a meadow, and whose artistry includes collecting mud and bugs and water and shaping them into a living community.

"It's just wonderful to make things happen," he said. "You can work with your environment. You don't have to manage everything."