Whilst trying to locate all the invasive Brittle Naiads (Najas minor) growing in my cove, I came across something exciting - a jellyish, donut-shaped mass attached to a plant stalk. I thought it was a primitive animal life form, as it initially seemed to be moving and pulsing, but it turns out it was just the water currents making it sway about.
It took a while for me to be able to take a reasonable (non-blurred) photograph, even though I had it captive in a bucket - I was excited by my find, and couldn't hold my breath long enough to keep my phone still while I pressed the button. After returning the jelly mass to the lake, I was so dissatisfied with how my photo had turned out, that I went back to the lake to look for it again, picked it up in my hand this time, and got a better shot.
I've found out that the ring is the egg sac of the Caddisfly.