The Fairfax Board of Supervisors voted yesterday to ask the State Water Control Board to hire an independent consultant to determine the cause of the sudden appearance of blue-green algae in the Potomac River.

"Fairfax County's good name is at stake," said Supervisor Audrey Moore (D-Annandale), who said accusations that the county's Lower Potomac sewer plant was responsible for the algae were false. "None of us wants to see algae return to the river."

In other actions, the board also voted to go to court to try to block the Federal Aviation Administration from testing its controversial scatter plan to spread flight paths--and aircraft noise--from National Airport over a wider area. But the board declined to go to court to attempt to force the District of Columbia to reduce the number of prisoners at Lorton prison or increase the number of guards.

The board's decision to seek an independent study of the algae in the Potomac was intended to quell a controversy between county officials, some environmentalists and some residents along the Potomac, most notably Noman Cole, who was head of the water control board during the early 1970s.

Blue-green algae, which can be toxic to fish, flourish in water containing large concentrations of certain nutrients such as phosphorus, particularly in warm weather. Cole says discharges from the county's sewage plant are responsible for the nutrients in the Potomac. Fairfax officials say runoff from heavy spring rains is the culprit.

The board did vote yesterday to delay for three months the adoption of less-stringent winter standards for phosphorus that were scheduled to take effect next month. But they stressed the delay was not an admission of guilt, merely a reaction to the unseasonably warm autumn and possibility of increased algae growth.

The board voted 8 to 1 to go to court over the FAA's scatter plan, which is scheduled to begin sometime after Thursday. Board member Sandra L. Duckworth (D-Mount Vernon), whose district would benefit from the proposed flight path changes, was the only supervisor to oppose the suit.

Supervisor Joseph Alexander (D-Lee), who proposed the action, said the board left it up to the county attorney to decide whether to join with Arlington and Alexandria, which also have voted to sue the FAA, or to file a separate suit.

The board rejected, 5 to 4, a proposal by Duckworth that the county go to court to attempt to force the District of Columbia to reduce the number of prisoners at Lorton or increase the number of guards. Duckworth argued that Lorton needs 93 more guards to adequately control the prisoners and protect the neighboring communities, which Duckworth represents.

Republican Chairman John F. Herrity, one of those who opposed Duckworth's proposal, argued that the board could not make a credible legal case.

The board did, however, vote to ask the District for access to records of security at Lorton, which houses District prisoners but is located in southern Fairfax. There has been renewed concern over Lorton security since two escapes last week.

The supervisors voted last night to give a developer permission to build a road connecting two planned residential developments in Loudoun County with Route 7 in Fairfax.

The road, called Holly Knoll Drive, would cut through a residential subdivision, which angered many of the subdivision's residents, but it was supported by other communities, which would have otherwise been inundated with traffic from the planned developments in Loudoun.

The vote means that Washington developer Warren K. Montouri can proceed with plans to build the $2.5 million road on land he has purchased. Supervisor Nancy Falck, a Republican who represents the area, said in proposing the controversial road: "I am most regretful that I have been totally unable to come up with a viable alternative . . . It is with absolutely no pleasure and a great deal of regret that I make this proposal."

The board's action caps a decade-long controversy over traffic congestion anticipated from subdivisions along the Loudoun-Fairfax border.