Ahydrosere is aplant succession which occurs in an area of fresh water such as inoxbow lakes andkettle lakes. In time, an area of open freshwater will naturally dry out, ultimately becomingwoodland. During this change, a range of different landtypes such asswamp andmarsh will succeed each other.[1]
The succession from open water to climax woodland takes centuries or millennia. Some intermediate stages will last a shorter time than others. For example, swamp may change to marsh within a decade or less. How long it takes will depend largely on the amount ofsiltation occurring in the area of open water.
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 areallogenic 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.[1]
There is nevertheless still debate about whether dry woodland is always the final climax community, or whether a watery, bog community can also be the final, stable, climax community.[2]
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. Theirspores are carried by air to the pond. The phytoplankton are followed byzooplankton. They settle down to the bottom of the pond after death, and decay intohumus 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.[1]
As the water body becomes shallower, more submerged rooted species are able to become established due to increasing light penetration in the shallower water. This is suitable for growth of rooted submerged species such asMyriophyllum,Vallisneria,Elodea,Hydrilla, andCeratophyllum. These plants root themselves in mud. Once submerged species colonize the successional changes are more rapid and are mainlyautogenic 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. The pond becomes sufficiently shallow (2–5 ft) for floating species and less suitable for rooted submerged plants.[1]
The floating plants are rooted in the mud, but some or all their leaves float on the surface of the water. These include species likeNymphaea,Nelumbo andPotamogeton. Some free-floating species also become associated with root plants. The large and broad leaves of floating plants shade the water surface and conditions become unsuitable for growth of submerged species which start disappearing. The plants decay to form organic mud which makes the pond more shallow yet (1–3 ft).[1][3]
The pond is now invaded by emergent plants such asPhragmites (reed-grasses),Typha (cattail), andZizania (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 ofleaf 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-saturatedmarshy land.[1]
Successive decreases in water level and changes in substratum help members ofCyperaceae andGraminae such asCarex spp. andJuncus 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 leavestranspire 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 ofherbs (secondary species) such asMentha,Caltha,Iris, andGalium which grow luxuriantly and bring further changes to the environment.Mesic conditions develop and marshy vegetation begins to disappear.[1]
The soil now remains drier for most of the year and becomes suitable for development of wet woodland. It is invaded byshrubs andtrees such asSalix (willow),Alnus (alder), andPopulus (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 ofhumus with associated microorganisms. This type of wet woodland is also known ascarr.[1]
Finally a self-perpetuating climax community develops. It may be aforest if the climate is humid,grassland in case of sub-humid environment, or adesert 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.[1]
An example is a smallkettle lake called Sweetmere, inShropshire,UK.[3] Sweetmere is one of many small kettle lakes which formed at the end of thelast glacial period when thetemperatures began to increase. The ice began to melt and retreat approximately 10,000 years ago.
As theclimate slowly began to warm this allowedalgae,water lilies and floating aquatic plants to begin to colonise the lake. These, in essence, were thepioneer species. Once these began to die it provided organic matter to the lake bed sediment and therefore increased fertility and reduced depth. As a result, this allowed deeper rooted species to develop such asreed,bulrush andreedmace. At this point there is a growing floating raft of thick organic matter within the lake. Because the bulrushes and reeds have relatively deep roots, this encouraged bioconstruction which traps moresediment, allowing sedges, willow and alder to become established. This process further decreased the water depth and raised the lakebed thus making it drier.
Drier conditions now meant that a wider range of species could inhabit the area.Birch andalder came into dominance. All species which have grown have occurred because ofseed transfer either by animals, birds, wind, or water transfer.Water level is further reduced as a result of further bioconstruction and also due to increasing temperatures there is increased evaporation from the lake.
Underneath the birch canopy developed terrestrial shrubs and grasses. This then increased theacidity which increased the rates of nutrient exchange. The area has been artificially drained and this allowed theoak andash community to develop. This is theseral stage.
The lake is now being managed by cutting down certain species in order to stop the whole lake becoming dried up and dominated by the oak and ash woodland.
Another example of a hydrosere is Loch a' Mhuilin, located on theIsle of Arran,Scotland. This small lake lies behind a ridge of material deposited towards the end of thelast ice age. The lake exhibits characteristic features of a hydrosere, the succession from a fresh water surface with smallpioneer plant species to a sub-climax vegetation of alder andwillow. The climax vegetation of oak andbeech woodland has not been achieved due to the impact ofhuman activities of clearing grazing land, as well as grazing byred deer andrabbits.
