The coastal ocean acts as a highly efficient trap for materials from land. The traditional notion of the coastal ocean as being defined from the high-water mark on shore to the shelf break is essentially correct but ignores the fact that a dynamic continuum exists between land and the open sea. Boundary layers are formed when turbid inshore and estuarine waters are mixed and trapped along the coast, particularly in calm weather; there is much slower mixing between coastal and offshore waters. The coastal zone, therefore, varies in breadth depending on the strength and salient characteristics of local ocean circulation, river discharge, shelf width, climate, and latitudinal position. Coastal circulation is driven by energy derived from solar heating or gravity, barometric pressure, and the density of oceanic waters. Physical processes have until recently been ignored when considering trophic energetics within the microbial loop in coastal waters.