Intertidal zonation.

The highest zone is called the supratidal fringe, or splash zone. The supratidal fringe is seldom covered by high tides but is often wetted by waves. Below this fringe is the intertidal zone proper. The upper intertidal zone is covered only during the highest tides, and the lower intertidal zone is uncovered only during the lowest tides. Between the upper and lower intertidal zones is the middle intertidal zone, which is covered and uncovered during average tides. Below the intertidal zone is the subtidal zone, which remains covered by water even during the lowest tides. As we shall see in the next two sections (Physical Conditions and Chemical Conditions), tidal fluctuation produces steep gradients of physical and chemical conditions within the intertidal zone.