That wasn't -my- alternator but all these air-cooled units will have this problem to some degree. If you visit your local home improvement center and get a small roll of that flexible plastic netting (BLACK, about 6 in. wide) for rain gutters, you can cut a piece to size and strap it with small zipties into the front of the alt duct mouth but behind the grille piece on the bumper. It will eliminate most of this and no one will notice it's there. You can also pull the plastic tube connection off the plastic part on the rear of the alt. I've had my alt apart a couple times since doing the netting on the grille end and it was debris-free. First time I remove my alt, it had lots of maple "copters", pine needles, bee and hornet butts, butterfly wings, and even some Wrigley's chewing gum foil (probably from a jaded e39 owner....).
The alt has cooling blades on the front and rear edges of the spinning stator.