Decaying Organic Material
If your filter or skimmer is clogged or has stopped working, you may be noticing bubbles because of an accumulation of plant matter decaying on the floor of the pond. Use a pond vacuum to remove as much debris as you can, and see if the bubbles cease.
If they do, you will need to add an additional filter or skimmer to prevent future buildup and a decrease in water quality.
If you have fish in your pond, especially larger fish such as koi, you may be noticing bubbles from them. Often times while feeding, koi will expel air from their mouths and create bubbles.
As long as your fish are not at the surface gulping air, there is nothing to be concerned about.
If your liner or prefabricated form has a hole, you may be noticing a steady stream of bubbles rising from the location of the puncture or crack. These holes will be uniformly spaced and continually emerge in the same part of your pond.
Monitor your pond for water loss to confirm a leak. Drain and seal the leak and refill the pond.
If the bubbles appear to be floating on the surface of your pond and not rising from the bottom of the pond, you may be seeing insect larvae. Some insects form air pockets around their bodies while they are on the surface of the water to breathe and aid in floating.
Look closely at the bubbles and see if there is a small worm-like mosquito larva or dragonfly in the center. If you find the insects problematic, add fish to your pond to control populations.