The largest container gateway into the U.S. remained closed April 7 as a shortage of dockworker labor that halted operations on the evening of April 6 went into a second day.

“ILWU Local 13 withheld labor again for this morning’s shift. The action by the union has effectively shut down the Ports of Los Angeles and Long Beach,” the Pacific Maritime Association said in a statement.

The terminal closures come as cargo volumes have dropped sharply at the Southern California ports from peak levels a year ago. The threat of delays during contract talks has spurred retailers, manufacturers and other importers to avoid potential shipping snarls by diverting goods to the East and Gulf coasts.

Still, an extended shutdown could disrupt supply chains. The National Retail Federation said it has renewed a call to the White House to engage in the labor negotiations and prevent disruptions to cargo flows in a statement April 7.

“The West Coast ports, especially those in Los Angeles and Long Beach, are a pivotal entry point to the U.S. that allow American consumers access to global products and essential goods,” David French, senior vice president of government relations.

The White House declined to comment.

The PMA, which represents ocean carriers and terminal operators in contract negotiations that have been ongoing since last May, said that while some workers showed up on the evening of April 6, a majority of the jobs went unfulfilled.

“The workers who did show up were released because there was not a full complement of ILWU members to operate the terminals,” the group said.

The closures coincide with a changeover in leadership as Gary Herrera, who previously served as vice president of the International Longshore and Warehouse Union’s Local 13 chapter, took over for outgoing President Ramon Ponce de Leon.

The ILWU declined to comment. The ILWU Local 13 president’s office didn’t return a call seeking comment.