The bubbles can be other fish, or it can be just gas being released if its a muddy bottom. But I would say its definately fish at this time of year. Earlier in the season I would say gas from the presure changes in the atmosphere. Next time, before you chum use a pair of polarized sun glasses and try to look and see whats the source of the bubbles if the water is clear or cloudy. I would be trying to looking for dark shadows like carp under the bubbles still searching for your chum, left overs. Look for the tip of carp tails moving under the bubbles. If you see them even before you chum. You can literally drop a rig down if there is a carp there (silently no splashing) and they will take the corn without chumming..By now definately carp will be holding in your spot. Even when your not fishing there. I had fish stay in one spot for about a week after I stopped chumming.
Just hope no on else sees your spot. You can now use this spot for next year to chum cause the fish will come back looking for a food source.
Have Fun
DSN
I'm hoping I'll be able to get out with MJL next week. But I have to prepare the place with chum before hand as you have done.