If the person immediately above you on the masthead has given his or her notice, don't take for granted that you'll automatically be moved up, even if it is the logical step. At the very least, speak to the department head to make sure he or she is aware of your interest. And you may even want to go as far as making an official application, with a cover letter and clippings – as if you weren't already working for the magazine. It honestly just may not occur to your boss to consider you for the job; you want to make sure he or she does. Plus, even if he or she was thinking of you, it shows them how much you want it and reinforces the fact that you're a good candidate.