A succession which had its origin in watery habitats (i.e. marshy grounds, ponds, lakes, or any other aquatic environment) is called Hydrosere.
It may be studied in a pond or pool. The water is deep in the middle and gradually becomes shallow towards the edge or shore. A Hydrosere presents seven stages in succession and these are :
(a) Plankton stage
The pioneer stage of hydrosere is the plankton stage, It consists of migrated spores of phytoplankton, e.g., diatoms, one celled colonial or filamentous green and blue-green algae, green flagellates, etc.
They are the pioneer species of hydrosere. They remain floating or swimming at or near the surface of the water.
(b) Submerged stage
In this stage, water is more than 3-6 m in depth. The pioneers of submerged stage are angiospermic plants like Hydrilla, Vallisneria (Hydrocharitaceae), Potamogeton (Potamogetonaceae), Najas (Najadaceae), etc are found to grow.
They are rooted in the soft mud at the bottom and forms dense growth. The spaces in between those plants are occupied by Chara, and often Vallisneria.
When the water becomes shallow owing to the deposition of humus and eroded soil particles from the shore and unfit for the submerged species, then this area is occupied by new invaders.
(c) Floating stage
Here the water is 1.6–2.5 m in depth. Plants of floating stage are grouped into two types, e.g., (i) floating leaved anchored hydrophytes and (ii) floating hydrophytes.
Floating leaved hydrophytes are attached in the mud at the bottom. These plants possess either long petioles or erect shoot so that their leaves are brought to the surface of water e.g., Nymphaea, Nelumbo, Nuphar etc of the family Nymphaeaceae.
Floating leaved hydrophytes have their leaves floating on the surface of the water. here the entire plant floats freely in water e.g., Pistia, Lemna, Eichhornia etc. These plants grow very fast and almost cover the water surface.
These plants further constitute the pond bottom due to their death and decay—as a result water becomes too shallow and unfit for the growth of floating plants which would disappear.
(d) Reed swamp stage
The water now becomes 0.3-0.1 m in depth. In this stage, marshy plants like Typha (Typhaceae), Sagittaria (Alismaceae), Scirpus (Cyperaceae), Phragmites (Gramineae) etc. grow.
These plants are semi-submerged with their roots attached at the bottom and shoots well above the water. These plants further make the water shallow by the deposition of plant humus and other sedimentary materials, as a result water becomes unfit for the growth of plants belonging to this stage but suitable for the growth of the plants of the next stage.
(e) Marsh meadow stage
In this stage, the substratum changes from aquatic to marshy soil with surface water to the depth of a few centimetres during rainy season but surface water disappears during other season.
Plants growing in reed swamp stage can not tolerate this dry habitat and therefore this habitat is invaded by Cyperus and Carex (Cyperaceae), Juncus (Juncaceae), Polygonum (Polygonaceae), and many species of Gramineae.
Ultimately humus collect, meadow becomes lowland and dry – as a result plants of marsh meadow stage are replaced by shrubs and trees.
(f) Woodland stage
This stage is composed of woody shrubs and trees. Plants of this stage can tolerate both bright sunlight as well as water-logged conditions around their roots. This stage includes Terminalia, Populus, Alnus, Salix etc.
(g) Climax forest
In this stage shade-loving trees develop. The soil becomes rich in humus and micro-organisms – the air is humid and shady. This condition results in unfavourable growth for sun-loving trees – consequently shade-loving trees form the pioneer vegetation.