Because each type of plant is limited to a range of depth, the plants form zones. Although unseen, phytoplankton is a significant marsh producer limited only by the availability of sunlight. Zonation, or the pattern of growth according to depth, is visible in the enhancement marshes. Succession, the series of replacements of one community by another, occurs in a pond or marsh as organisms die and fall to the bottom. Gradually it fills in and becomes more shallow, sometimes over thousands of years.
Species previously excluded by depth are able to colonize.
The presence of a substantial amount of emergent vegetation marks the transition from pond to marsh. The stages of succession are reflected by the pattern of zonation from open water to dry land. Imagine starting at the center of the marsh and traveling outward. The progression can be seen as a travel through time, from pond community to more advanced successional communities. Nevertheless, it is useful to point out to students that visible stages of succession as a reminder that ecosystems are continually changing.