PORTLAND – Officials at the Portland International Jetport held a ribbon-cutting Thursday to mark the start of service from the second of two low-cost carriers. But the arrival of AirTran Airways already had a big impact on air fares.

Continental Airlines, Delta Air Lines, Northwest and U.S. Airways already lowered prices to compete with AirTran. On Wednesday, Delta was offering a round-trip flight from Portland to Seattle for $144, taxes and fees included.

“Portland should be very happy,” said Tom Parsons, founder of Bestfares.com, a discount travel Web site.

AirTran’s service from Portland includes three daily flights to Baltimore-Washington International Airport with service to points beyond. It’s also operating a weekly flight to Orlando, Fla., where the company is based.

On Thursday, Gov. John Baldacci was on hand with gifts as the first AirTran flight arrived from Baltimore. Baldacci exchanged lobster with Jim Walsh, director of the Baltimore airport, who brought with him Maryland blue crabs.

“Maine is poised for exciting growth, and thanks to AirTran we will become a bigger travel destination,” Baldacci said.

Lower fares will probably mean more traffic at Portland’s airport, as bargain-hunting travelers choose to travel from Portland instead of going to Boston’s Logan Airport or the Manchester-Boston Regional Airport.

Air travel experts say last year’s arrival of discount airline JetBlue in Portland already has helped to fuel the boom at the jetport.

Air travel jumped nearly 30 percent in the first four months of this year compared with the same period last year, with 220,237 passengers boarding airplanes in Portland from January through April. The trend will likely continue, Parsons said.

The reason for bargains at the Portland International Jetport is simple: competition. Parsons said the legacy airlines slash prices to protect some routes from competition from the low-cost airlines. The low-cost airlines can offer lower prices on their hub routes.

“As soon as we announced the new city, the fares dropped about 50 percent that day,” said Judy Graham-Weaver, an AirTran spokeswoman.

The deals typically are only on those routes served by AirTran and JetBlue. It’s still cheaper to fly out of Boston to many destinations, said Steve Hewins, vice president of travel for AAA Northern New England.

But Manchester is no longer the bargain it was when nine years ago Southwest Airlines arrived. “Southwest fares have crept up quite a bit, so the advantage Manchester had has disappeared,” Hewins said.