Ahh - I read your first post too fast!
Nevertheless, my comment is still applicable. Unless you invest in a thick-walled poly carboy specifically designed for fermentation and aging (check out "Better Bottles"
if you want to go that way), you will have enough air infiltration through the thin walls to eventually oxidize the mead. Especially if you have a lot of airspace above the liquid in one of those large plastic carboys, oxidation will eventually affect your mead.