Landowners, farmers and environmentalists from Maryland's three southern counties turned out in force tonight to voice concerns about the state's plan to create and manage land and water areas critical to the preservation of the Chesapeake Bay.

Some of the 100 residents of St. Mary's, Charles and Calvert counties told members of the governor's Critical Areas Commission that they feared the state would become an all-powerful zoning board and supercede local control in its efforts to clean up the bay and its tributaries.

But most speakers at the public hearing supported the general goal of cleaning up the bay and preserving its rich fishing industry. A spokesman from the Maryland Forest, Park and Wildlife Department said the state has lost 30 percent of its forest land since 1961 and urged the commission to make retention of trees along all tributaries the "highest priority."

Industry, sewage treatment plants, erosion and run-off from road construction "pose a much, much more serious threat than new development," said Frank A. Jakitsch, Calvert County director of planning and zoning.

Many farmers, like Al Swann, a fifth-generation landowner in Lower Marlboro, said their livelihood would be severely curtailed if across-the-board zoning and farming regulations are put into place as a result of the commission's findings. "Some farmers will simply not exist if many more restrictions are placed on them," said Robert Jarboe, president of St. Mary's County Farm Bureau.

"It's easy to say farmers are the ones polluting the bay . . . but some of us are doing a good job of seeing that nitrates and phosphates don't wash into the Chesapeake," said a farmer who has lived next to the Patuxent River for 60 years.

The 25-member commission was formed last June, mandated by a Maryland law that requires all local planning and zoning boards in the state's nine tidewater counties to adopt specific plans by 1986 aimed at making sure that new development along the bay and in the wetlands does not further pollute water quality or adversely affect fish, wildlife and plants.

The Bay's resources along the Eastern Shore and the three southern Maryland counties are particularly stressed by a population boom and an explosion of commercial and industrial growth in the Washington-Baltimore corridor, legislators said.

The law calls on local jurisdictions to create "buffer zones" along shorelines where agriculture will be permitted only if the best management practices are used and to limit the amount of concrete parking lots, roads, buildings or other structures impervious to water. Cluster development should be encouraged wherever possible, the law says.