Normally, the towering green crane in the Everport Terminal at the Port of Los Angeles would be busy unloading hulking container ships. Longshoremen below would flit around in “bomb carts” used to ferry containers from the ship. Big rigs would carry off imported furniture, car parts and clothing to other parts of the country.

But on a recent Thursday morning, the 300-foot crane sat idle, a casualty of the tariffs that President Trump has imposed to curb foreign trade. Almost a fifth of the 99 boats that Gene Seroka, the port’s chief executive, had expected to arrive in May were canceled.

“It’s a very quiet day,” Mr. Seroka said. “This is the impact that the tariffs have had.”

The Port of Los Angeles, along with a nearby facility in Long Beach, makes up a shipping complex that stretches across nearly 75 miles of Southern California shoreline. The ports are a bellwether for trade and the U.S. economy. Together, they move an astonishing 40 percent of the goods that come into the United States via containers. They also account for 30 percent of what the country exports.

As Mr. Trump’s chaotic and aggressive tariff strategy has seesawed this year, activity here has, too. That has threatened the livelihood of the roughly 100,000 workers at the port complex and complicated life for the hundreds of thousands of companies that bring goods through the port each year. The trends at the port hint at the pain that will ripple through the broader economy in the coming months, as fewer and higher-priced goods travel from ports and warehouses to American stores and consumers.

The ports experienced a surge of activity this year when shippers rushed to bring in goods ahead of tariffs that reached their highest levels in a century. That rush has faded, and trade has become more sluggish. With higher tariffs set to snap back within weeks, both importers and port workers remain cautious, unsure of what their futures will hold.