Both Los Angeles area ports continued showing major year-over-year cargo gains last month, though volumes were slightly lower than August.

The September growth was most pronounced at the Port of Long Beach which handled 574,790 container units, a 30.5 percent increase over the same month in 2009. The Port of Los Angeles handled 711,613 container units, about 22 percent more than a year earlier.

Long Beach port spokesman Art Wong attributed the growth to continued shipping for the upcoming holiday season, reflecting “a strong rebound in retail over the last couple of months.

“It’s been surprisingly strong this year,” Wong said. “Throughout the year people have been saying that the numbers would taper off because the economy has been weak, but we’re through nine months now and it’s still exceptionally strong.”

Both ports announced last month that they were reinstating late-night and weekend operating hours at marine terminals to handle the increased loads. The move signaled the return of hundreds of part-time dock workers let go when the recession began.

Los Angeles port spokesman Phillip Sanfield said the question is whether the growth can be sustained.

“Overall it’s been a very strong year, better than we forecast, but not completely unexpected given the declines of 2009,” he said. “Consumer demand in the months head remains relatively uncertain.”