* About 80 percent of stocks on NYSE, Nasdaq in red
* U.S. employers hired at the slowest pace in nine months in March
* F5 Networks plunges after weak outlook, weighs on peers
* Indexes down: Dow 0.5 pct, S&P 0.7 pct, Nasdaq 0.9 pct
By Caroline Valetkevitch
NEW YORK, April 5 (Reuters) - The S&P 500 was on track for its worst weekly performance of the year on Friday after weaker-than-expected U.S. jobs data added to worries the U.S. economy could be losing momentum.
The S&P 500 was down about 1.5 percent for the week. Losses were broad, with about 80 percent of stocks on the New York Stock Exchange and Nasdaq falling.
The government said 88,000 jobs were added in March, less than half economists' average forecast of 200,000. The unemployment rate dipped to 7.6 percent from 7.7 percent, largely due to people dropping out of the work force.
The unemployment report follows similarly disappointing readings on the manufacturing and services sector as well as other poor labor market data.
Next week reports on first-quarter earnings unofficially get started, another concern for investors. Analysts' estimates for earnings growth in the first quarter have fallen since late last year, according to Thomson Reuters data.
"They're really waiting for the earnings season on balance to disappoint. Given the macro background, I think that really is the worry," said Bruce Zaro, chief technical strategist, Delta Global Asset Management in Boston.
The S&P 500's biggest percentage decliner was network gear maker F5 Networks Inc, which dropped 16.8 percent to $73.58 a day after forecasting quarterly earnings and revenue well below Wall Street's expectations.
The Dow Jones industrial average was down 78.95 points, or 0.54 percent, at 14,527.16. The Standard & Poor's 500 Index was down 10.34 points, or 0.66 percent, at 1,549.64. The Nasdaq Composite Index was down 29.15 points, or 0.90 percent, at 3,195.83.
Several of F5's competitors were also sharply lower, with Juniper Networks off 2.9 percent at $17.60 and Citrix Systems down 1.7 percent at $68.53.
Airline stocks were hit after J.P Morgan Securities cut its revenue expectations for U.S. airlines by 2 percent to 3 percent for 2013 and 2014 and said it expects monthly revenue per available seat mile to turn negative for some airlines, partly due to the federal government's automatic spending cuts.
Delta Airlines Inc fell 4.8 percent to $14.04 and United Continental Holdings was off 2.9 percent at $28.46.
If the S&P closes down on the week, it will be only the third weekly loss this year. The benchmark index has gained about 8 percent so far this year without a significant pullback, leading many analysts to expect one.
Energy shares were pressured as the group is closely tied to economic growth expectations and crude oil futures fell for a third straight day. Chevron Corp fell 0.8 percent to $117.08.
Earnings forecasts have been scaled back heading into first-quarter reports, due to start on Monday with Alcoa. S&P 500 earnings are expected to have risen just 1.6 percent from a year ago, according to Thomson Reuters data, down from a 4.3 percent forecast in January.