Stocks traded mostly lower by Monday's close, with the S&P 500 and Nasdaq giving back earlier gains in choppy trading as big tech names struggled. The S&P 500 was down over 3% and the Nasdaq fell over 5%, while the Dow Jones Industrial Average closed at a positive flatline.

Here's how the market settled to start the week:

S&P 500 Index (SPY  ): -0.37% or -16.61 points to 4,483.92

Dow Jones Industrial Average (DIA  ): 0.00% or +1.12 points to 35,090.86

Nasdaq Composite Index (QQQ  ): -0.58% or -82.34 points to 14,015.67

Bitcoin prices climb for a fifth day:

Bitcoin prices rose for a fifth straight day on Monday, as gains in the broader market overflow into the cryptocurrency market, setting the coin up for its longest winning streak since last September.

Prices for Bitcoin, the world's largest cryptocurrency by market capitalization, rose over 3.4% on Monday to $43,866, or its highest level since mid-January.

Frontier Airlines, Spirit Airlines to merge in $6.6 billion deal:

Two of the largest low-cost airlines in the United States, Frontier (ULCC  ) and Spirit (SAVE  ), have agreed to merge in a $6.6 billion deal, the companies announced Monday. The resulting company will become the fifth-largest airline in the U.S.

Under the deal, Frontier will have a 51.5% controlling stake in the combined company. Spirit shareholders will receive 1.9126 shares of Frontier plus $2.13 in cash per share, resulting in an implied value of $25.83 per share, the companies said.

"This transaction is centered around creating an aggressive ultra-low fare competitor to serve our guests even better, expand career opportunities for our team members and increase competitive pressure, resulting in more consumer-friendly fares for the flying public," said Ted Christe, president and CEO of Spirit, in a press statement.

Peloton rises on reported takeover interest:

Peloton Interactive (PTON  ) shares rose over 25% higher Monday morning following reports that the fitness technology company had garnered potential takeover interest from several major companies, including Nike (NKE  ) and Amazon (AMZN  ).

The Financial Times and Wall Street Journal reported Friday that several companies were considering making Peloton a purchase offer. Peloton shares have fallen nearly 83% in the past year as the company warned of waning demand for its connected at-home stationary bike and treadmill products.

Here's how market benchmarks started trading soon after open:

S&P 500 Index: +0.15% or +6.67 points to 4,504.12

Dow Jones Industrial Average: +0.16% or +57.38 points to 35,147.12

Nasdaq Composite Index: +0.11% or +15.79 points to 14,113.80