The subtidal zone it the zone close to shore, but constantly submerged. Coastal waters, i.e. the intertidal and subtidal zones, experience the most extreme changes due to land processes. For example, the salinity can fluctuate between high salinity in times of hot, dry winds, to low salinity during times of heavy rain and runoff. Temperature extremes can also exist, as the temperature of the air changes seasonally. Of course, subtidal communities are less affected than intertidal communities, but they still experience these changes.
Previously, this textbook has discussed important subtidal ecoystems- kelp forest and algae beds, seagrasses and coral reefs. These ecosystems are found in shallower areas of the subtidal zone, where sunlight penetrates to the seafloor. Beyond the shallower subtidal, communities resemble that of oceanic communities, in which the primary producers are phytoplankton.