He has helped stage audacious outdoor art exhibitions and supported cultural groups through expansions and renovations — while quietly contributing hundreds of millions of dollars from his own personal fortune.

Mayor Michael R. Bloomberg has been good to the arts in New York City, leaving a legacy that includes transformative projects like the BAM Cultural District and tourism-generating events like “The Gates,” which adorned Central Park in 2005 with its billowing saffron-colored panels.

There is, however, a problem: He’s leaving.

With less than six months remaining in Mr. Bloomberg’s tenure, a cloud of unease has descended over arts executives, fund-raisers and artists. Over the last 12 years, they have grown accustomed to a City Hall that was receptive to their needs, as well as to a billionaire mayor who could privately bolster their budgets.

Now these organizations are facing two unknowns: Will a new mayor continue to support them? And, just as important, will Citizen Bloomberg?