Question

How does the vegetation at the mouth of the river compare to the vegetation in the flood zone

Answer 1

Vegetation when it is in the ecological zone, it has macrophytes or aquatic plants.

The reason here is that both zones are being regularly or constantly being submerged in the water. The species of plants will differ because some grow in brackish water.
For example, as at the mouth of rivers but the freshwater aquatic plants will be able to grow in floodplain places.

Answer 2

Vegetation in both ecological zones has aquatic plants/macrophytes. This is due to the fact that both zones are constantly or regularly submerged in water. However, the species of plants would differ since some grow in brackish waters (such as at the mouth of rivers) while freshwater aquatic plants will grow in the floodplains.