What a difference a season makes. Back in May, a $91.1 million Jeff Koons sculpture and $110.7 million Monet created the impression that business at top-end art auctions was booming.

But this week, against a backdrop of presidential impeachment hearings, Brexit and other geopolitical distractions, numbers at New York’s marquee fall sales of Impressionist, modern and contemporary art were significantly down. The change in mood was clear on Monday and Tuesday evening as sales of Impressionist and modern art dipped 52 percent at Christie’s and 40 percent at Sotheby’s over their equivalent sales last May.

By Wednesday, Christie’s fall auction of contemporary art raised $325.3 million, versus the $539 million it achieved in the spring, when the Koons and other pieces from the S.I. Newhouse collection were included. In the absence of a prestigious estate, and with owners reluctant to release blue-chip masterworks, there was little to whet collectors’ appetites.

“This was a season with few trophies,” said Thomas Danziger, an attorney specializing in art law at the New York firm Danziger, Danziger & Muro, LLP. “The auction houses did the best with what they had.” From the seller’s point of view, he added, “The mood is not good, but the art market as we know it is not coming to an end.”