PORTLAND – Some York County clam diggers could be back on the flats by the weekend as New England’s largest red tide outbreak in 30 years weakens in the western Gulf of Maine, officials said.

But the bloom of toxic algae appears to be strengthening in waters off eastern Maine and the Bay of Fundy, where Canadian officials last Friday closed shellfish beds along most of the New Brunswick coast.

The red tide now stretches from the Nova Scotia border to Cape Cod in Massachusetts, although some areas around Penobscot Bay, the Jonesport area and the St. Croix River have thus far escaped contamination.

But those “clean pockets” may disappear as the summer sun warms the cold waters Down East and the algae there gain toxicity, said Darcie Couture, director of biotoxin monitoring for the Maine Department of Marine Resources.

“We kind of got a preview in western Maine for what’s coming for eastern Maine,” Couture said. “The red tide we have gotten was so intense and bad in western Maine, there is no reason it won’t become that bad in eastern Maine when the water is warmed up.”

Toxicity levels in Cape Cod Bay are declining, although shellfish harvested there are still considered unsafe to eat, said Dave Whittaker, a senior marine fisheries biologist with the Massachusetts Division of Marine Fisheries.

There also was a drop last week in a small stretch of waters near Cape Porpoise in York County. If tests this Friday show similar results, the state will reopen that area to shellfish harvesting, Couture said.

Red tide taints shellfish such as clams and mussels, making them unsafe for people and animals to eat, but poses no risk to people who consume fish, lobsters, scallops and shrimp.

Toxicity levels remain high in Casco Bay, where algae feed on higher nutrient levels fueled by pollution. Couture also noted that Casco Bay doesn’t flush out the nutrients as well as waters off York County.