Not all fireworks on the Mall figure into celebrations. Some pyrotechnics in the paal Park Service's way of saying goodbye to the methane gas that collected under huge domed formations of algae floating on Constitution Gardens Lake near the Lincoln Memorial.

"I lit one went off with quite a crack," recalls Park Service official John Hoke. "These were giant floating mats of algsting."

Hoke and his colleagues have been searching ever since for ways to drive the obnoxious plant creature away from the monuments, and now they think the problem is just about licked.

The answer to all that unsightly green, it see the water almost black. Last fall the Park Service sprayed 20 pounds of blue and black dye into the lake alond the water in the Reflecting Pool, as well as Bolivar Pond outside Interior Department headquarters and another pond on the grounds of Park Service headquarters on Hains Point.

Officials say the disappearance of algae is only one benefit of the dye, which inhibits algae growth by cutting back sunlight. The ominous looking water also discourages tourists from wading off-limits, hides the trash that settles to the bottom, saves tens of thousands of dollars in annual cleaning costs and establishes a stable urban wetlands environment where hundreds of ducks and fish can freely roam.

"You ought to go out and look at them now," boasts Hoke of his chemically assisted aquatic creations. "They're absolutely exquisite."

For years, algae plagued officials and disgusted tourists. According to Hoke, the clear water that used to be pumped into the Reflecting Pool and Constitution Gardens Lake was perfect for the unruly weed.

The pool and the lake had to be drained and cleaned regularly--the lake several times a year--at a cost of between $10,000 and $20,000 for each scrubbing.

"We were treating them like swimming pools," said Hoke. "We used to have bulldozers in there pushing all that green stuff around."

Officials first tried dye on a grand scale in Constitution Gardens, which had become a "Sargasso Sea of green goo," in Hoke's words.

Park Service workers placed planters on the concrete bottom filled with aquatic plants that are tolerant to less sunlight and promote animal life that feed fish and waterfowl.

The first tinting agent was insoluble and eventually settled on the bottom, so scientists switched to two soluble dyes--acid blue No. 9 and acid black DCJ--that stay mixed with the water without harming animals, Hoke said.

The ecosystem created on the lake is stocked with bass and has attracted between 300 and 400 ducks, as well as some visiting geese and other birds, Hoke said.

Tourists, apparently more sensitive to pools supporting fish and bird families, now throw less trash into the water, and Hoke says the darker water makes for better photography.

But the algae problem didn't quite end with the dye. Park Service scientists have begun attacking on a biological front, adding a natural agent that also seems to keep algae from growing. Hoke unceremoniously dubbed the agent "Swamp Muck."

Now officials are trying to get a grip on the elm leaves that westerly winds deposit in one end of the lake and Reflecting Pool.