Alpena (ălpēˈnə) [key], city (1990 pop. 11,354), seat of Alpena co., N Mich., on Thunder Bay, an arm of Lake Huron; inc. 1871. Limestone quarried nearby is used to make cement, Alpena's chief manufacture. Cement and limestone are transported on the Great Lakes by way of Alpena's harbor. The city lies in a year-round resort area and has an annual winter carnival.