Seaweeds exist as individuals, but they also live together in communities with other seaweeds and animals – communities that affect and are affected by the environment. Ecologists and physiologists alike are drawn to coastal marine ecosystems because of the easy access to strong environmental gradients over short spatial scales. Marine organisms grow in often distinctive vertical or horizontal “zones” or “bands” along these gradients, thereby providing “natural laboratories” in which to study environmental (abiotic) and biological processes shaping the communities. Zones of vegetation are also found in terrestrial habitats, but here the spatial scales are typically much greater. On a mountain, for example, vegetation is zoned with altitude, but the vertical distance over which changes occur can be in the order of 1000 m rather than several meters in the intertidal zone (Raffaelli and Hawkins 1996). Vertical gradients in the intertidal are easily observed at low tide, but also extend underwater where the surface irradiance can be reduced to 1% at 15 m depth in many coastal waters (Lüning and Dring 1979; sec. 5.2.2). Horizontal gradients include the salinity gradients of estuaries and salt marshes, and wave exposure (Raffaelli and Hawkins 1996).

In Chapters 1 and 2, we reviewed the morphologies, life histories, and developmental processes of seaweeds as species. In this chapter we consider the patterns and processes in marine benthic communities as a starting point for later factor-by-factor dissection of the environment. We open with an overview of zonation patterns seen in the intertidal and subtidal environments.