One of the reasons the art market recovered so quickly from the recession was China's continuing boom. Just as Wall Streeters and hedge-funders pulled back their auction paddles and gallery visits, the growing numbers of Chinese rich poured in.

The market in 2009 and 2010 was largely about China. Last year, China surpassed the United States to become the world’s largest art market, according to some estimates.

Now, that’s changing.

According to data from ArtTactic, the British art-market research firm, auction sales this spring fell to $1.5 billion, a drop of 43 percent compared to the fall of 2011.

Contemporary art – which had been going gangbusters – led the decline, with a 58 percent drop year-over-year.

The decline was driven by weakness at the very top of the market – a dangerous trend, since the high end has been the main source of growth in the art market. The number of lots selling for $1 million or more fell 63 percent this Spring. (Read More: Car Auction Beats Pebble Beach Record)

The question is whether this weakness is a one-off or the sign of a more secular decline in the spending of the Chinese rich.

Guan Yu, Beijing-based director at the research firm AMMA (Art Market Monitor of ARTRON), told the China Economic Review that the art boom of 2009-2010 was the result of liquidity from the government stimulus “washing up in art.” He said that the economic slowdown in China could cause the tide to recede. (Read more: The 'Fairy Tales' of Art, Wine and Jewels)

Either way, China’s slowdown could put pressure on the big auctions coming up in New York – and around the world – this fall.