While it may seem they are forming at the surface, this is usually the plants on the bottom streaming off very small bubbles of gas, usually O2 during photosynthesis. I see this in my aquarium regularly, especially after a water change. It does make the water surface sparkle! I doubt the gas is all O2 when it reaches the top, but if you collected the gas right at the plant with a test tube, and then stick something burning into it, you will clearly see it has a high concentration of O2.
