Denver International Airport soared to a passenger record last year despite a brutal run-up in fuel prices, a stomach-churning economic crisis and the onset of a steep decline in demand.

The nation's fifth-busiest airport announced Wednesday that it handled 51.2 million travelers in 2008, up 2.8 percent from 2007. It ended the year on a strong note, posting a 4.8 percent spike in December, though much of that had to do with spillover travel from a later-than-usual Thanksgiving holiday.

DIA chalked up its annual growth primarily to heavy competition fueled by the expansion of Southwest Airlines, which doubled its presence here with nearly 60 new flights.

"Clearly we believe we have some inherent advantages . . . but I would not be truthful if I didn't say the Southwest factor was a major contribution," said Kim Day, DIA's aviation manager.

DIA also reached a symbolic milestone by surpassing the 50 million passenger mark. The airport was initially designed to handle that many passengers annually before it would need to embark on a major expansion.

And, as envisioned, the airport is operating near capacity, while gate space is at a premium.

DIA has been updating its master plan in an attempt to prepare for the next phase of growth. Officials estimate the airport will need another runway, nearly two dozen more gates, additional parking and other upgrades over the next eight years.

Its short-term plans are less clear.

DIA must walk a fine line, balancing the need to prepare for growth with the challenges of today's economic environment.

Travel demand is down sharply nationwide, and DIA's largest airlines have been axing service. Both United Airlines and bankrupt Frontier Airlines trimmed dozens of flights here in 2008. United has announced plans for more cutbacks across its system this year, while Southwest eliminated some service last month and will cancel four more daily departures in June.

"United is basically on hold in Denver, Frontier is doing well but probably won't (expand right now) and Southwest is pretty much at a standstill," said Mike Boyd, an aviation consultant based in Evergreen. "I don't see a whole lot of new growth."

The airport expects a 7.5 percent dip in seat capacity this June compared with a year earlier, based on published airline schedules.

Boyd said DIA's passenger traffic could fall by about 2 million this year and another 2 million in 2010. It won't be until 2014 until traffic returns to 2008 levels, he predicts.

"We're seeing some ugly stuff with airlines right now. We could even see something similar to what happened to the auto industry, where there's a total plunge in sales over a very short period of time," Boyd said. "But airports have to plan for growth, and they have to plan for it now."

DIA officials said flexibility is the key to weathering the storm.

The airport laid down plans in 2007 to spend more than $1 billion through 2013 on its concourses, concession areas, security checkpoints, parking lots and baggage system. It has since scaled that back, putting some projects - such as an expansion of the C Concourse - on hold for now.

"There are a couple of projects that we'll sit on until we're really sure they merit investment," Day said. "We're really only doing the must-do projects at this time."

DIA also has created several contingency plans involving various scenarios, such as the loss of a significant amount of service.

At the same time, however, Day said the airport is moving forward with projects such as a FasTracks station. It's also forging ahead with construction of a hotel attached to the main terminal, a project that Day hopes will break ground later this year.

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