Despite netting its third best year in port history in 2014, the Port of Los Angeles saw ongoing congestion affect its cargo numbers during the second half of last year.

The nation’s busiest seaport moved 8.3 million container units in 2014, which was 6 percent more than 2013, according to statistics released Friday by the port. That made it the third busiest year at the Los Angeles port, behind 8.4 million units in 2007 and 8.5 million in 2006.

But that news was tempered by issues related to congestion, which has been brought by the arrival of larger ships carrying more cargo for various shipping lines, the lack of available chassis (the trailers that allow trucks to tow cargo containers) and unresolved contract talks between the longshore workers union and their employers that have grown contentious.

“(The issues related to congestion) are creating dynamics we’ve never seen before,” said port spokesman Arley Baker.

During the first half of the year, the Port of Los Angeles was No. 1 in productivity, according to the Journal of Commerce. That dropped off around the latter six months, around the time longshore contract negotiations began and bottlenecks along the supply chain began to increase.

Los Angeles’ port moved 658,567 units in December, up only 1 percent from the same time last year.

“The 2014 numbers are an encouraging indication that the national economy continues to improve,” Port Executive Director Gene Seroka said in a statement. “Beyond that, the second half of the year ushered in a mix of unprecedented challenges due to transformational changes in the shipping line business.”

He also encouraged the International Longshore and Warehouse Union and Pacific Maritime Association to work quickly and in good faith toward a contract.

“We are working hard to help our customers and supply chain partners overcome those challenges and urge them to work together with us to find solutions,” Seroka said.

The Port of Long Beach expects to release its cargo numbers next week.