Consider the following. Which of following is not an ecotone?
Grassland
An ecotone is a transition area between two biomes. It is where two communities meet and integrate. It may be narrow or wide, and it may be local (the zone between a field and forest) or regional (the transition between forest and grassland ecosystems).
Mangrove refers to both a habitat type in the intertidal zone and to the specialized, salt-tolerant trees and shrubs that grow there.
Estuaries and the lands surrounding them are places of transition from land to sea, and from fresh to salt water.
Lagoons are shallow, often elongated bodies of water separated from a larger body of water by a shallow or exposed shoal, coral reef, or similar feature.
Grassland is an ecosystem by itself.