Hey catfish, is that nut part of the assembly? And is the shaft itself threaded on the outside? Sometimes the impellers are threaded like a dredge pump. If the nut is a separate part...the shaft is tapered pressed on only by the nut. Loosen the nut a little like 63bkpkr said. I used to repair small engine equipment like this, you may need to use a prybar (carefully) put pressure towards the nut from behind. Once you have firm but careful pressure, whack the nut square with a dead blow or a ball peen (flat side). It should pop right off. Pry a lil harder if it doesn't come off the first time.
Good luck and get out there to find some yella