The nature of the rock between the tide marks may exert a certain selective influence on the organisms that attempt to colonize it, in the sense that some rocks are more susceptible to the activities of boring organisms and others are not and certain minor variations in zonation may also be attributed to it. You see, porous sandstone will hold water longer during low tide than granite and other non-porous rocks and therefore may exhibit a different type of zonation than other rocks nearby. The proximity of sand also affects populations of adjacent rocky areas. In some cases, sands shift a good deal, so that a low rocky reef may be buried under sand at one, and emerged at another time. This does not always kill everything on the reef unless it lasts for a long time.
Predation is also important but less so than in the Pacific.
This zone extends higher than the highest tide where there is great exposure with spray from the various waves causing the organisms to extend higher into the littoral zone. Mussels and seaweeds can crowd out the barnacles but the barnacles can tolerate surf and desiccation better, giving them the upper position in the Mid-littoral. The rockweeds in the middle of the mid-littoral zone provide a nice moist protective canopy when the tide is out, so many inverts. The green crab hides under it during the day and comes out at night. Mussels will be preyed on by certain snails, seastars, crabs, shorebirds, lobsters etc.