Vanguard Group Inc., the world’s largest mutual-fund company, sees the deepening plunge in equity markets today as a return to volatility.

“You’ve had this steady march upward from 2009 until this year when we were going sideways, treading water,” said Tim Buckley, chief investment officer at Vanguard, which manages $3.3 trillion in assets. “Now you’re seeing a return of volatility to the markets.”

Most of the trading activity today was in stocks, Buckley said. The Dow Jones Industrial Average tumbled more than 1,000 points as the market opened and then rebounded to recoup much of the loss. It was down 126 points at 12:42 p.m.

Trading volume in the equity markets was 160 percent of what Vanguard has seen in the past 100 days, Buckley said. Trading on the fixed-income side was calmer, about 80 percent to 85 percent of a normal day, he said.

While there have been signs of China’s economic growth slowing, the market slide shows “a tipping point in the fear” around the country’s outlook, said Buckley. Long-term, the world’s second-largest economy will be a source of growth and the U.S. economy is on “very solid ground,” he said.

The markets may not settle any time soon without a more aggressive policy response from China or clear signal that the Federal Reserve is likely to delay a rate hike, Russ Koesterich, a global chief investment strategist at BlackRock Inc., the world’s largest money manager, said today in a report.

Don’t Panic

“That said, most leading U.S. and European indicators are still suggesting a continued economic expansion. In addition, the selling has restored value to certain parts of the market,” Koesterich wrote.

Individual investors shouldn’t panic or try to time the market, said Buckley. Reacting to such news destroys value in a portfolio on average, he said.

Trading volumes were “up significantly” among clients of TD Ameritrade Holding Corp. going into the open of markets today, Kim Hillyer, a spokeswoman for the company, said in an e-mail. “Clients’ trades are still being executed,” she said.

Some wealth managers saw the decline in stocks as an opportunity to buy for clients. Sam Katzman, chief investment officer at Constellation Wealth Advisors, bought “a bit” at the open of the U.S. markets today for some clients who are on an averaging program or were looking to increase equity exposure, he said.

“We do not see this as a sign that the U.S. is entering a recession,” said Katzman, whose firm manages about $6 billion. “Getting to purchase the names you like, especially those that are down much more than the index, is an opportunity. Almost everyone says they are waiting for a pullback to buy, but when it comes, it never feels good, so it is hard to do.”