The positions of high, mid and low intertidal pools are shown below: In fact, insects and insect larvae, typical freshwater forms are common in upper intertidal pools.
Since pools seldom reached by seawater frequently contain water of low salinity as a result of freshwater runoff and rainwater accumulation, one would expect to find freshwater organisms that are adapted to low salinity or marine organisms that can tolerate exposure to freshwater. On the other hand, tide pools in the lower intertidal provide a relatively constant chemical and physical environment since they are not exposed to the elements as much as upper pools. During winter, water of low salinity may freeze while in summer pool water temperature may rise above that of the air. In either case, eventual contact with the ocean restores the original pool salinity. Rainwater dilutes the more saline pool water, whereas evaporation increases the salinity. Pools located highest in the intertidal, because of the extended time they are exposed to the elements, undergo considerable fluctuations in temperature and salinity. On this basis, tide pools can be distinguished as dominated, respectively by green algae, calcareous red algae, mixed brown and red algae, and mixed green, brown and red algae. Tide pools may be classified either according to their position within the intertidal zone-Low middle or high-or according to the dominant plant form within the pool. These pools are colonized to varying degrees by intertidal animals and plants, many of which have been previously described. Tide Pools in the Rocky Intertidal When the tide recedes, water often remains trapped in rock depressions.
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