That's pretty cool! Saw them in the Adirondacks last year, along with the freshwater jellyfish. I cannot imagine they use much of the water's nutrient load as they are mostly jelly, but are probably helped by the good aeration and circulating water.
As far as I can tell, nothing touches them as far as being eaten, and their growth rate is odd, as they seem to just "appear". I wonder if the "blobs" are in some deflated state while maturing, and then when conditions are right, they absorb water and get into the ball shape? Did you notice them before as smaller objects?
