It is pollen. This pollen had blown around in the air and then landed in the water. It then collected on the rocks, making the yellow rings around all of these rocks. Pollen is a very important part of plant reproduction. Bees also depend on pollen for food.
Clue #1 This was blowing around in the air.
Clue #2 It is made by plants.
Clue #3 Bees make honey out of this.