NEW YORK, Sept 17 (Reuters) - Microsoft founder Bill Gates is the richest person in the United States for the 15th year in a row, but economic woes have claimed some members of Forbes magazine ranking of the 400 wealthiest Americans.

Dropouts this year include former American International Group chief executive Maurice Greenberg, and former eBay chief Meg Whitman, while among those on the list some 126 fortunes declined -- six times more than last year.

Casino magnate Sheldon Adelson saw his fortune dwindle by $13 billion in the past year, the biggest loss of anyone on the list, while Kirk Kerkorian's fortune shrank by $6.8 billion as shares in casino operator MGM Mirage tumbled.

"The fact that the rich are not getting richer means that the economy is locked," said Matthew Miller, editor of the Forbes list. "Credit is not being extended, there is very little liquidity in the markets and deals can't get done, so the economy contracts."

Forbes used publicly traded stock prices on Aug. 29 when compiling the list, signaling that this week's stock plunges -- part of the worst financial turmoil since the Great Depression -- have further eroded many of these fortunes.

Although Gates topped the list with a personal fortune estimated by Forbes to be about $57 billion, this was down from the $59 billion the magazine said he was worth last year.

Investor Warren Buffett came in at No. 2 with $50 billion, down from an estimated $52 billion last year, followed in third place by Oracle Corp founder Lawrence Ellison, who is said to be worth $27 billion.

HIGH BARRIER

After being booted out last year by Google Inc co-founders Sergey Brin and Larry Page, four members of the Walton family, descendants of Wal-Mart Stores Inc founder Sam Walton, return at positions Nos. 4 through 7, all with fortunes of about $23 billion.

Rounding out the top 10 are New York City Mayor Michael Bloomberg at No. 8, with a fortune estimated at $20 billion from his news and financial data empire, and brothers Charles and David Koch, who run manufacturing and energy company Koch Industries, and are valued at $19 billion each.

The list of the Top 400 Richest People in America can be seen at (http://www.forbes.com/forbes400).

But for all the the economic and financial turmoil, membership in this exclusive club remains as tough as ever.

For a second straight year, the minimum net worth needed to get on the Forbes list is $1.3 billion, while the average net worth of the 400 richest Americans is $3.9 billion.

Collectively, that means that the 400 richest Americans have a net worth of $1.57 trillion, exceeding Canada's gross domestic product.

The youngest member on the Forbes' list is 24-year-old Facebook founder Mark Zuckerberg, whose $1.5 billion could swell if ever his social networking company goes public.

Harvard University in Cambridge, Massachusetts has the largest contingent on the list among elite schools, with 49 alumni, followed by Stanford with 30, Yale with 17 and Princeton with seven.

Of the 400 richest Americans, Forbes said 271 made their fortunes themselves, while 75 people inherited their money.