Ocean water pulled by the gravitational attraction of the moon and the sun rises and then falls as the attraction decreases. When the water reaches its lowest point (low tide) an expanse of rocky shoreline is exposed to the elements. Certain animals and plants call this home and are only able to remain because they are adapted to withstand environmental extremes such as fluctuating temperatures or the force of crashing waves. When the water is at its highest point (high tide) intertidal organisms are covered by a protective layer of water and are no longer exposed to such extreme conditions. As the tide goes out, surfaces in the upper part are exposed longer than those in the low tide areas. Heavy wave action often prevents organisms from attaching to the substratum. Such areas are often sparsely populated. Those that live here are usually modified to withstand the effects of crashing waves.
The limpet, a gastropod mollusc, for example, has a conical shell that can be pulled downward tightly against the rock surface. The shell shape minimizes water drag that would otherwise might pull it loose.
The blue mussel, a bivalve mollusc, uses its byssal threads to attach strongly to the rock surface. Blue mussels in exposed areas tend to be smaller at the same age as their counterparts in sheltered spots. What factors might be responsible for the smaller size and what advantage does small size confer to mussels in exposed areas?
Distinct horizontal bands of organisms form visible zones within the rocky interidal. The photographs below record these bands in a number of locations along the coast of Maine. They are named by the dominant organism that occupies the band. The uppermost zone is characterized by Lichens, followed by the Bluegreen Zone, the Barnacle Zone, The Brown Algal Zone and finally by the Red Algal Zone. Each zone is discussed in more depth in later pages. The Subtidal Zone that lies below the Red Algal Zone is also discussed even though it is not part of the Rocky Intertidal.
The two photographs above were taken at about mean low tide. There is a vertical drop of about 9 feet between the high and low tide lines, exposing horizontal bands of marine organisms. The uppermost band is composed primarily of bluegreen bacteria. It is followed by the barnacle, brown algal and red algal zones.
Zonation of marine organisms is also found on vertical rock faces in Acadia National Park as shown above.
Zonation can occur on individual boulders in an intertidal boulder field as shown above. Bluegreen bacteria are most evident on the tops of boulders on the right side. White barnacles can be seen on lower boulder surfaces on the right hand side and brown algae are attached to the sides and tops of boulders.
Zonation (two views) on a shoreline in Crows Neck, Lubec Maine. Both are situated above mudflats.
Zonation along a deep water channel on Crows Neck, Lubec Maine in November is shown above.