Bubbles in the sea

On August 15th I visited Dornie on the west coast of Scotland.

These bubbles were coming from the bottom of a tidal sea loch in many places where the water was quite shallow. They moved from place to place as the tide level changed.

Some of them were just small strings of bubbles, but others were leaping up to around 20cm above the surface of the water.

When I dug into the pebbles around the origin of the bubbles, I was unable to find any source.

Fred has seen similar bubbles at a different location.

I would be interested in any responses as to their possible cause.