It is okay for me to post this here, because squid are vertical migrators, too. They're just another step up the food chain from the phytoplankton and zooplankton Hannah is singing about.

A quick linguistic postscript (which I can assure you will not be nearly as entertaining as that song was): I knew someone once who had a pet peeve about "diurnal" versus "diel" migration. The two words do in fact have different meanings: diurnal refers to something that happens during the day, rather than during the night. It's the opposite of nocturnal. Diel refers to something that happens over a 24-hour period, including both day and night. It's a synonym for daily.

Therefore (goes the argument) the daily vertical migration of zooplankton (and other things) should be called diel migration. The problem is that there's already a large body of literature referring to it as diurnal migration. And though I am an English geek, it actually doesn't bother me! Because (goes another argument) the migration which is daily is also happening during the day, when these animals migrate from the surface down to depth.

So diel migration is also diurnal, and everyone is right.