A hydrosere is a plant succession which occurs in a fresh water lake. In time, an area of open fresh water such as a lake will naturally dry out, ultimately becoming woodland. During this change a range of different land types such as swamp and marsh will succeed each other.
Stages of Aquatic Succession or Hydrosere
Hydrosere is primary succession sequences which develop 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 edges and move down to the basin to fill up. Later dead remains fill up these bodies and change the environment.
If water bodies large and very deep a strong wave of action at work therefore in this bodies a noticeable change cannot be observed. However in similar water body 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 the planktons and floating species occupy open water zone.
Stage 1: Phytoplankton Stage
Unicellular floating algal plants such as diatoms are pioneer species of a bare water body, such as a pond. Their spores are carried by air to the pond. The phytoplanktons are followed by zooplanktons. They settle down to the bottom of the pond after their death, decay into humus that mixes with silt and clay particles brought to the basin by run off water and wave action and help in soil building. The result in swallowing of pond and change in environment.
Stage 2: Rooted Submerged Stage
The new habitat helps light penetration at the edges. The submerged aquatic vegetation develops in the regions of ponds and lakes where the water depth is about 10 feet or more. This is suitable for growth of rooted submerged species such as Chara, Utricularia, Vallisneria, and Hydrilla. These plants root themselves in mud. Once submerged species colonize the successional changes more rapid and are mainly autogenic as organic matter accumulates. Inorganic sediment is still entering the lake and is trapped more quickly by the net of plant roots and rhizomes growing on the pond floor. Consequently the pond becomes sufficiently shallow like 2-5 ft for floating species and less suitable for rooted submerged plants.
Stage 3: Rooted Floating Plants
The water depth is by now 2 to 5 feet. The plants colonize the habitat with their rhizomes. The floating plants are rooted in the mud, but some or all their leaves float on the surface of the water. These include species like Nymphaea, Trapa. Some free-floating species also become associated with root plants like Wolffia, Pistia and Salvinia. The large and broad leaves of floating plants shade the water surface and conditions become unsuitable for growth of submerged species which start migrating. The plants decay to form organic mud which makes the pond shallower like 1 to 3 feet.
Stage 4: Reed Swamp Stage
When the water depth is 1 to 3 feet the pond is invaded by plants that are rooted with most of their shoots exposed to the air. Some examples are reed-grasses, cattail, and wild rice to form a reed-swamp. 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.
Stage 5: Sedge Meadow Stage
Successive decrease in water level and changes in substratum help members of Carex and Juncus to establish them. They form a mat like vegetation towards the centre of the pond. Their rhizomes knit the soil further. The above ground leaves transpirate water to level and add leaf litter to the soil. The sedge peat accumulates above the water level and soil is no longer totally waterlogged now. The habitat becomes suitable for invasion of herbs such as Mentha, Iris which grows luxuriantly and brings further change in the environment. Mesic conditions approach the area and marshy vegetations begin to disappear.
Stage 6: Woodland Stage
The soil remains drier for now most of the time of the year and becomes suitable for development of wet woodland. It is invaded by shrubs and trees such as willow, alders, Poplus. These plants react upon the habitat by producing shade, lowering the water table by transpiration, building up the soil and accumulation of humus with associated microorganisms.
Stage 7: Forest Stage
Finally a self perpetuating climax community develops. It may be 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 vegetations 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 sub stream, shallowing of water, addition of humus and minerals, soil building and aeration of soil. As the water body fills in with sediment, the open area decreases and the vegetation types moves inwards as the water becomes shallower. Many of the above mentioned communities can be seen together in a water body often. The center is occupied by reeds, followed by sledges and rushes growing of edges. Still further are bushes or small trees and trees occupying the dry land.
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