Concerned about the growth of an aquatic weed and sediment and nutrients in Centennial Lake, the Howard County Department of Recreation and Parks plans to conduct a water quality study of the 54-acre Ellicott City lake.

The 18-month test will "give us an idea of what condition the lake is in," said Robert Linz, manager of Centennial Park. The lake is not polluted, parks officials said, but must be tested to determine how best to care for it.

Parks officials are hiring a certified laboratory to conduct water tests and expect the study to begin within the next several months. The department will finance the study with a $44,600 grant it received in 1992 from the Clean Lakes Program of the U.S. Environmental Protection Agency.

The study will consist of 14 tests measuring several substances, including nutrient and

sediment levels, phosphorus, water clarity and acidity. It will take 12 months to conduct the tests and six months to perform follow-up tests.

Once the tests are completed by the laboratory and parks employees, Coastal Environmental Services Inc. of Linthicum, an environmental consulting firm, will analyze the data and create a long-range management plan for the man-made lake, created in 1987.

"Ecologically speaking, the lake is healthy and offers excellent fish habitat and recreational opportunities," said John Byrd, chief of the Bureau of Parks. "In a nutshell, this is like getting an annual physical. The results of this study will set the stage for a Phase 2 project, which will be a management plan to correct problems that may exist and reduce impact."

Tests will examine the lake's nutrients and sediment, which nourish a hardy water weed called hydrilla.

Parks officials believe the weed was introduced into the lake in 1987 from a boat that picked it up in the Potomac River, where it has been reproducing since the mid-1980s. Officials fear that the plant, which can grow 30 to 40 feet from the bottom to the water's surface, will clog the lake.

Since the weed was first discovered in Centennial Lake, the county has spent between $7,000 and $10,000 a year fighting its growth, Mr. Linz said.

To eliminate the plant, Coastal Environmental Services will examine methods to control water runoff from nearby farms, housing developments and construction sites.

The plan could consist of creating wetlands, buffer zones, or building storm water detention basins that hold water and prevent it from seeping into Centennial Lake.