The Metro board approved cutbacks and other changes in bus service yesterday that will affect more than 50 routes in Northern Virginia in a move to reduce costs by more than $800,000 a year.

The changes, scheduled to take effect June 30, were aimed at curtailing service on "marginal" routes with relatively low ridership. Officials initially had proposed sharper cuts on some routes, but their plans were revised because of protests by bus riders at recent hearings.

Under the plan, some routes will be eliminated, while service on others will be reduced or modified. At the same time, bus service is to be increased in some developing areas, mainly in Fairfax County.

In a related move, the board approved an experimental expansion of service on several Maryland routes.

The cutbacks, which had been recommended previously by Northern Virginia officials, will affect bus routes in Arlington and Fairfax counties and in Alexandria, Falls Church and Fairfax City.

The major Virginia bus lines on which changes are planned are 1, 2, 3, 5, 6, 7, 8, 10, 13, 17, 18, 19, 22, 23, 24, 26 and 29.

A Metro report said the shifts will "improve Metrobus productivity" and reduce the bus system's deficit. "Some of the proposed changes would reduce underutilized or duplicative service where ridership is not sufficient to support the current level of service," it said.

According to Metro officials, service will be expanded on routes 17 and 18, which serve Kings Park, Burke, Springfield and Orange Hunt. Other experimental increases in Virginia service are to include extensions of routes 1V and 29H along with a new route, 19L, serving the Lorton area along U.S. Rte. 1.

In southern Prince George's County, a new route, W13, is to operate as a six-month experiment from Allentown Road and Old Fort Road West in Friendly to downtown Washington. The P17 will be extended to serve Fort Washington.

In Montgomery County, a new T4 route is to serve the Bethesda, Medical Center and Twinbrook subway stations, running mainly on Old Georgetown Road. It will replace the T4 route that was eliminated in January. The T6 route is to be revised to improve service in Rockville.