Staples tumbled $3.18, or 16 percent, to $16.47 at 11:42 a.m. in Nasdaq Stock Market trading, the biggest intraday decline since March 2000. Before today, the shares had fallen 14 percent this year.

Profit for fiscal 2011 will rise to as much as $1.45 a share, the Framingham, Massachusetts-based company said today in a statement. Analysts had projected $1.54, the average of 16 estimates compiled by Bloomberg.

Staples, led by Chairman and Chief Executive Officer Ron Sargent, said it’s adopting a “more conservative” outlook for earnings and sales after first-quarter results were weaker than expected. The company predicts “very little improvement” in the economy this year, according to the statement.

“Clearly I’m not happy with our first-quarter performance,” Sargent, 55, said today on a conference call. “I’m disappointed by the slowness of the economic recovery. It looks to us that the economy is still stuck in neutral.”

Sales this year will increase in the low single digits on a percentage basis. In March, Staples projected full-year earnings to increase to as much as $1.60 a share on sales in the low to mid-single digits.

Staples is scaling back expansion plans, reflecting “soft” demand for office products, Sargent said. This year, it will open about 20 stores in the U.S. and 10 in Canada. The company will close about 10 outlets in North America, where it had previously projected to open 40 stores, he said.

The retailer also plans to reduce square footage as it renews the leases of about 500 stores over the next three years.

First quarter net income rose 5 percent to $198.2 million, or 28 cents a share, from $188.8 million, or 26 cents, a year earlier. Analysts projected 32 cents, the average of 15 estimates compiled by Bloomberg.