Magna Global’s Bob Coen has downgraded his forecast for U.S. online ad spend to 8 percent growth from his earlier call this summer for web ad dollars to rise 12 percent. At this point, it looks like 2009 growth in online will slow significantly, rising only 5 percent, he said. Coen made his forecast at the UBS Global Media and Communications (PDF) conference alongside execs from ZenithOptimedia (which cut its global online spending forecast to 21.2 percent this year) and GroupM (which projected 22 percent gains for ‘08). Last year at the same conference, Coen predicted a 16.5 percent rise in internet ad revenues over 2007.

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While online certainly looks considerably stronger than the rest of the ad market—he forecasts a 3.2 percent decline for total ad spending for this year, much more bearish than the 0.2 percent drop expected by Zenith and GroupM—Coen doesn’t have much more confidence in online. In his presentation he said, “Aside from search, which is still fairly robust, the online ad space doesn’t look much different from the total market. It will be a tough year for everyone.” Specifically, for 2009, Magna anticipates that total U.S. ad spend will be down 4.5 percent. Release (PDF)