St. Louis officials learned last month that the city's population had declined 8.3 percent since 2000 -- a loss of about 29,000 residents.

The move could have serious implications for the city. State and federal funding for cities is pegged to population, and fewer residents means less money at a time when St. Louis -- like most municipalities -- is facing financial pressure.

Other major metro areas dealing with declining populations include Chicago, whose population is down nearly 7 percent; Cincinnati loss 10 percent of its population; and Cleveland's fell 17 percent.

For St. Louis, the news is particularly troubling since it thought it had finally beat a long-term trend of population decline. The Census had previously estimated the city's 2009 population as 356,587 -- a 2.5 percent increase from 2000. Instead, the 2010 results indicate residents are leaving St. Louis for smaller cities in the suburbs, some of which are enjoying enormous population growth.

At its peak in 1950, St. Louis had a population 856,796 and was one of the largest cities in the country. Today, St. Louis isn't even the largest city in Missouri. The new Census pegs population at 319,294 in 2010 -- its lowest total in 140 years.

Francis Slay, the city's mayor, called the news "a wake-up call."

"The numbers themselves have really hit home hard," says Slay. "We're taking them very seriously."

Still, Slay hasn't tried to sugarcoat the results. On the day they were released, he posted an entry to his blog calling the numbers "absolutely bad news" that would require an "urgent and thorough rethinking of how we do almost everything." A day later Slay's site featured detailed tables charting population change along various geographic and demographic parameters.

Slay says his city will develop a formal plan to address the population decline that will involve input from academics and other experts who can provide insight on exactly why people are leaving, and what the city needs to do to retain them. "These numbers are going to confirm some of the stuff we suspected but also help us craft an approach, a plan, a message to help be more aggressive and revisit some of the things we've been doing."

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(Census 2010 figures not yet available for New York and Boston. Graphic uses data from 2009.)

In St. Louis, the population decline is largely among children, indicating that families are fleeing the city. St. Louis's under-18 population declined by 22,100 over the decade, while the number of adults declined by less than 7,000. "We know the demographics of those coming and leaving," Slay says. "When you have 22,000 fewer kids, that tells you something pretty clearly."

While young adults may enjoy the urban lifestyle in St. Louis, families appear to prefer other locales. Slay believes improving the city's schools will be key to retaining residents. "Maybe we're not being aggressive in demanding more from St. Louis Public Schools," Slay says.

About 20 new charter schools have opened in recent years, Slay says, and he wants to add more.

He's also calling for more regional cooperation as a way to address the declining population. The city of St. Louis is entirely independent of St. Louis County -- which also saw a population decline -- which means the city is duplicating work done at the county level. Meanwhile, the city and county are in competition for jobs and tax dollars, as are the area's surrounding cities, which causes a fragmented approach to growth. "We need to reinvent ourselves as a region," says Slay, who has previously advocated for a city-county merger.

The decline comes even as the city has worked on revitalization downtown and improving transit service, considered key aspects of any modern city that hopes to attract newcomers. Slay says those steps have indeed helped attract young people back to St. Louis -- but just not at the same rate families are leaving. Anecdotally, Slay says he has heard from constituents who enjoy the city life but once they have children feel like they must leave.

"They like city living," Slay says. "They're looking for a good, quality public education, which they aren't comfortable with in the city."