The large, fresh-water lake at Washington's three-year-old Constitution Gardens is being converted into a wildlife habitat with fish, underwater grasses, lilies and lots of zooplankton.

The lake, which was drained for cleaning recently, had become so clogged with yellow-green algae that a Park Service official called it a "disgrace that we simply had to do something about." The park service created the 45-acre park near the Lincoln Memorial during the Bicentennial.

Most of Washington's ponds, fountains and reflecting pools are cleaned at least once a year to remove trash, coins and algae. But Constitution Gardens, the Park Service says, has had more problems than most: Nothing seems to grow on the soil and everything seems to grow in the water.

Most of the 700 large trees and hundreds of azelea bushes planted in the park in 1976 died - drowned was the official diagnosis - in the first year, because water collected in the holes dug for them "like a tea cup," Park Service officials said. The park was created out of the heavy clay and rubble left when several World War I office buildings on the site were demolished in 1970 and 18 inches of topsoil was spread over the almost impenetrable clay.

Some of the problems at the lake apparently are caused by the heavy doses of fertilizer that have been used on the surrounding grassy knolls. Some of the fertilizer has drained into the lake, speeding the growth of the algae, according to Ed Drotos, the park service resident chief of resource management. The park service now plans to spread composed sludge in the park instead of chemical fertilizers.

Creating a wildlife habitat out of the lake - or at least part of the lake - is an experiment the park service hopes will help stem the growth of algae, as well as save thousands of dollas a year in maintenance and repair costs, said Drotos. The miniscule zooplankton are expected to eat the algae.

The habitat also will add interest to the somewhat baren looking park, says John Hoke, the park service's urban parks specialist, who has made a pet project of stocking Washington's ponds with fish, turtles and wild grasses to attract wildfoul.

"Look at the Bolivar Pond (at 18th and C streets NW). That hasn't had to be cleaned since we made a habitat there three, four years ago," Hoke said. "The water's crystal clear and only this morning, in the bushes on that little island in the center, a mallard hen hatched nine ducklings."

The underwater grasses, fish and other aquatic life that will be placed in the Constitution Gardens lake will be chosen and provided by the Aquatic Gardens, the Fish and Wildlife Service, the National Aquarium and other wildlife experts, said Hoke. Some items, like the grass, will be grown in underwater planting boxes until enough slit has built up to give the grass a firm rooting medium, which should take a couple of years, said Drotos.

"We're trying to condense time and to create a natural system in unnatural conditions . . . with a concrete bottom," he said.

Much to Hoke's disappointment, few of the additions will be visible. To maintain the lake as a reflecting pool and to allow large areas of unobstructed water for toy sailboats, a popular pastime at the lake, most of the additions will be wnderwater.

However, Hoke and Drotos say if the experiment "with the zooplankton, etc." is successful, the park service may expand the lake habitat.

"I'd like to see this looking like London's Hyde Park, which they turned into a wildlife habitat a number of years ago and now have 43 species of waterfowl there, in the heart of London," Hoke said. "The architect, everyone, would like to see ducks and geese and swans in Constitution Gardens.But ducks and geese won't come unless there's something for them to eat."

Hoke already had taken a few things into his own hands during the past year, such as bringing corn to entice sucks to the lake and putting sime "gambusia fish in there to eat mosquito larvae. That's the fish that saved the Panama Canal. It's the guppy family - they adore mosquito eggs."

But there is another "mad environment alist " somewhere in Washington who has been putting things in the lake, says Hoke. $"Not only are there tons of fish and goldfish, in there with the algae," Hoke said, "but there even are things like pickerel. Somebody put them in there."

But neither he nor Drotos seem displeased with the unexpected fish - which were drained out into the Potomac along with the water for the cleaning operation - or worried about their reappearance.

"People want something to look at . . . not this algae, and the bottles and trash, said Hoke. "When it looks like this, people trash it, but when it's beautiful they respect it." CAPTION: Picture, Wildlife habitat is planned for Constitution Gardens lake. By Criag Herndon - The Washington Post