Bay communities exist where seawater is separated from the ocean by a physical barrier, such as a point of land or a sandbar.

Coastal lagoons are small bodies of water separated from the ocean by a barrier beach and joined to the ocean by a narrow channel. Estuaries are areas where freshwater on its way to the sea meets ocean water.

Marshes and nearly barren mudflats are areas exposed to the air as the tide goes out. Man-made substrates, such as floats, pier pilings, and shore rip-rap are also locations for bay communities.

Each of these communities is affected by sedimentation, tidal fluctuation, freshwater runoff, and the kinds of organisms living there.*

*Extracted Natural History of Southern California: Ancinec, Radford, Schwenkmeyer