I browsed thru these documents and I believe it was the North lakes. It mentions a ferry going between St. Ignace and Mackinac City. There is no such ferry as far as I know with exception of Labor Day to accommodate walkers and runners for the annual event. Otherwise;
"Bicyclists are not allowed to cross the Mackinac Bridge on their own. The Mackinac BridgeAuthority will transport your group across the bridge in Mackinac Bridge Authority vehicles. ... The administration building is located on the north end of the Mackinac Bridge on the east side of the toll plaza."