The atmosphere is the part of the carbon and water cycles but absent from the phosphorous cycle. Carbon cycle depends on photosynthesis and respiration. Water cycle connects aquatic and terrestrial biomes.
The barnacles are only able to attach them selfs to surfaces in the water. Since the surface is limited, barnacles are attaching themselves to the other barnacles, crowding the rock. The muscles were not able to grow or attach themselves off the surface of the rock. Same thing with the mussels that are growing on the rock. So not only barnacles are crowding the rock, but Mussels as well are crowding the rock.