NEW ORLEANS (AP) — Until Monday, New Orleans had escaped effects from the oil disaster spreading across the Gulf Coast.

That delicate balance changed when balls of tar were found in the Rigolets, one of two passes that connect Lake Pontchartrain with the Gulf of Mexico.

‘‘So far it’s scattered stuff showing up, mostly tar balls,’’ said Office of Fisheries Assistant Secretary Randy Pausina. ‘‘It will pull out with the tide, and the show back up.’’

Pausina said he expected the oil to clear the passes and move directly into the lake, taking a backdoor route to New Orleans.

Prevailing east winds since Hurricane Alex have steadily pushed the oil toward the city’s eastern coastline along an arm of the Gulf. On Monday, the state Department of Wildlife and Fisheries closed the area to fishing.

Oil spilling from the BP well would be the most significant environmental challenge since a massive recovery effort lifted Lake Pontchartrain from near death by pollution.

A playground for boating and fishing for many decades, urban runoff and the dredging of the lake bottom had chased away many species by the 1970s. Swimmers were warned of high counts of bacteria in the lake.

But after years of efforts by lake boosters, pollution was stemmed by new regulations and dredging of the lake bottom was halted.

Dolphins, fish in abundance and even an occasional visiting manatee came back to the lake, which is connected to the Gulf by two narrow passes and is a mixture of fresh and salt water.

For New Orleans, oil in the lake threatens the second major disaster to sweep in from the Gulf in five years. On Aug. 29, 2005, a massive storm surge driven by Hurricane Katrina swept into Lake Pontchartrain, contributing to the destruction of levees. An estimated 80 percent of the city flooded.

Until Monday, the oil’s impact on the city proper was largely confined to fears tourism would suffer as the closure of seafood harvesting grounds threatened the cuisine at the city’s internationally famous restaurants.

In communities surrounding the city, the impact has been greater. A massive cleanup effort has been mounted in St. Bernard, Plaquemines and Jefferson parishes, where oil has fouled marshes, stressed the economies of fishing villages and eroded tourism.

The BP-operated rig Deepwater Horizon exploded April 20, killing 11 people, and sank two days later. The first oil reached the U.S. mainland on April 29 at the mouth of the Mississippi River.

On Monday, the Coast Guard confirmed oil found in Texas was from the BP well.

It has now been found in every state along the U.S. Gulf Coast.