Mud Bay’s Mysterious Bubbles

My sister first noticed the Mud Bay bubbles (see photo) during a visit a couple of years ago to see my new house. Since then she keeps asking “Have you found out what causes them?” When conditions are right – high tide and calm water – an area maybe 40 yards out from shore seethes like the cauldron tended by Macbeth’s witches (OK, an exaggeration but you get the idea). And apparently this has been going on for years, at least according to the former owner of my house. “It’s just air bubbling to the surface,” he said. Sure. But why there and what causes it to be released?

Figuring the bubbles have to come from somewhere, I borrowed my neighbor’s rowboat, rowed out, and marked the exact spot with a brick (see photo below). Then I went back at low tide and looked for any sort of clue: holes in the mud, pipes, an underground spring. Nothing. The ground under the bubble area doesn’t look any different from the ground 15 yards away. Just a slightly raised area covered by pickleweed. There’s no apparent source for the turmoil that appears at high tide.

Lately I have turned to experts for help. A marine habitat specialist at the University of Washington speculated that the source could be decomposition or simply air trapped in the sediments at low tide. He also suggested that I make more observations regarding time and duration and get a sample of the gas from the bubbles. I plan to do the latter when the weather is a little warmer. I also plan to make additional queries using the convenience of email and the Internet.

To review what is known about the Mud Bay bubbles:

• They are most noticeable when the water is slack at high tide.
• They don’t appear at every high tide (or at least they aren’t noticeable).
• They don’t seem to be seasonal (the photo was taken in January).
• They are most concentrated in one spot (call it Macbeth point), but on a particularly bubbly day other smaller release areas can be seen. 

I will post their source if I learn it. Feel free to comment.