Correct Response: D. Large bodies of water can be separated into layers according to temperature. A mixed layer of relatively warm water is near the surface, a cold deep layer is at the bottom, and a transition layer called a thermocline separates the mixed and deep layers. Ecotones (A) are transition areas between ecological communities and often contain characteristics of both. Although an inverse relationship between temperature and elevation exists in the troposphere (B), this is not a thermocline. Zonation (C) is the distribution of organisms into specific geographical regions that occurs naturally based on the physical characteristics of the region. Zonation occurs in many habitats, including intertidal habitats. However, this phenomenon is distinct from thermoclines.
