Hydrosere is the primary succession sequence which develops in aquatic environments such as lakes and ponds. It results in conversion of water body and its community into a land community. The early changes are
allogenic as inorganic particles such as sand and clay are washed from catchment areas and begin filling the basin of the water body. Later, remains of dead plants also fill up these bodies and contribute to further changes in the environment. If a water body is large and very deep, a strong wave action is at work, therefore in these bodies a noticeable change cannot easily be observed. However, in smaller water bodies such as a pond the succession is easily recognizable. Different plant communities occupy different zones in a water body and exhibit concentric zonation. The edges of the water body are occupied by rooted species, submerged species are found in the littoral zone and plankton and floating species occupy the open water zone.
Phytoplankton stage Phytoplanktons (cyanobacteria), green algae (Spirogyra, Oedogonium), diatoms, etc. are the pioneer colonizers in the initial stage, starting from a water body, such as a pond. Their
spores are carried by air to the pond. The phytoplankton are followed by
zooplankton. They settle down to the bottom of the pond after death, and decay into
humus that mixes with silt and clay particles brought into the basin by run off water and wave action and form soil. As soil builds up, the pond becomes shallower and further environmental changes follow.
Reed swamp stage The pond is now invaded by emergent plants such as
Phragmites (reed-grasses),
Typha (cattail), and
Zizania (wild rice) to form a reed-swamp (in North American usage, this habitat is called a marsh). These plants have creeping rhizomes which knit the mud together to produce large quantities of
leaf litter. This litter is resistant to decay and reed peat builds up, accelerating the autogenic change. The surface of the pond is converted into water-saturated
marshy land.
Sedge-meadow stage Successive decreases in water level and changes in substratum help members of
Cyperaceae and
Graminae such as
Carex spp. and
Juncus to establish themselves. They form a mat of vegetation extending towards the centre of the pond. Their rhizomes knit the soil further. The above water leaves
transpire water to lower the water level further and add additional leaf litter to the soil. Eventually the sedge peat accumulates above the water level and soil is no longer totally waterlogged. The habitat becomes suitable for invasion of
herbs (secondary species) such as
Mentha,
Caltha,
Iris, and
Galium which grow luxuriantly and bring further changes to the environment.
Mesic conditions develop and marshy vegetation begins to disappear.
Woodland stage The soil now remains drier for most of the year and becomes suitable for development of wet woodland. It is invaded by
shrubs and
trees such as
Salix (willow),
Alnus (alder), and
Populus (poplar). These plants react upon the habitat by producing shade, lower the water table still further by transpiration, build up the soil, and lead to the accumulation of
humus with associated microorganisms. This type of wet woodland is also known as
carr.
Climax stage Finally a self-perpetuating climax community develops. It may be a
forest if the climate is humid,
grassland in case of sub-humid environment, or a
desert in arid and semi-arid conditions. A forest is characterized by presence of all types of vegetation including herbs, shrubs, mosses, shade-loving plants and trees.
Decomposers are frequent in climax vegetation. The overall changes taking place during development of successional communities are building up of substratum, shallowing of water, addition of humus and minerals, soil building and aeration of soil. As the water body fills in with sediment, the area of open water decreases and the vegetation types moves inwards as the water becomes shallower. Many of the above-mentioned communities can be seen growing together in a water body. The center is occupied by floating and submerged plants with reeds nearer the shores, followed by sedges and rushes growing at the edges. Still further are shrubs and trees occupying the dry land. == Examples ==