Bees on water lettuce? why?

we had a ton of bees on our water hyacinth and flying around the pond lately. so we took them out. we removed all of the hyacinth from the pond, and left only the water lettuce, because the bees were leaving them alone. after the hyacinth gone, the bees now are starting to converge on teh lettuce and also on moist moss ridden rocks. what in the world is going on here?