Wetland: Types and General Management Considerations

A wetland is defined as being an area of land where the water table never, drops below 15cm from the surface or exceeds 6 metres depth above the surface of the land. The British climate is suitable for sustaining wetland habitats though climate change could lead to a reduction in habitat size. There are several wetland types and habitats vary even within the habitat. There are six main wetland habitats.


Wetland Types

Marsh
Marshes are often dominated by grasses rushes and sedges, they occur near lakes and rivers where the water table is predominantly high and close to, but rarely above, the surface. Marshes do not contain a layer of peat and are composed on mineral soils. Marshes can be fresh or salt water.

Reed Swamp
Reedbeds/swamps are wetlands dominated by stands of the Common Reed (Phragmites australis), it occurs on mineral soils where the water table is at, or above ground level for most of the year Reed swamps are amongst the most important habitats for birds in the UK, despite this they are largely fragmented.

Wet Meadow
Wet meadows are similar to marshes but receive seasonal flooding rather than continual, this often results is more dominant grasses. They often occur in poorly drained areas such as shallow lake basins, low-lying farmland, and the land between shallow marshes and upland areas.

Fen
Fens develop on alkaline, mineral rich soils, vegetation builds (sedges, grasses and rushes) up and when the plants die, they become caught in amongst the other vegetation, forming layer after layer of dead organic matter, the built up DOA and water saturated soils prevent/ delay the decaying process and eventually peat forms.

Carr
A Carr is a transition to woodland, it is part of the process of ecological succession occurring when the area has developed to being dominated by alder or willows, this succession will continue until the area becomes dry and a new succession will occur.

Bog
A bog is characterised as being an acid mire, vegetation accumulated in waterlogged soils due to a lack of oxygen required for aerobic decomposition. As a result peat accumulates, bogs can accumulate metres and metres of peat in one site, though this forms over thousands of years.
Bogs can be split into different types:

Ombrogenous Mires
These are created by extensive rainfall and rely on a reasonably wet climate to survive as all water replenishment comes from the atmosphere.

There are two forms of ombrogenous mire: blanket bog and raised bog. Blanket bog is so called because its development is mostly independent of basins or topographical features where water collects; it simply covers the landscape like a blanket. Raised bogs develop from a basin or dip in the land where rainwater collects into and builds up layers until the peat sits above the surrounding land.

Topogenous Mires
These are dependant on the formation of the land for existence they are influenced by topography and receive relief water from drainage and seepage. Water rarely comes above the surface or falls far below. Valley bogs receive a flow of water wick keeps the bog continually wet. Basin bogs have no flow of water and water seeps out slowly or evaporates in the basin.

Soligenous Mires
Here the wetness of the ground is maintained by slow lateral gravitational seepage of water through the substrate or the peat. Topography is still the main determining factor but the high water table results not from the concentration of water as in a topogenous mire, but by a sustained, slow flow through the site; usually along some definable drainage or seepage line. As the moving water flowing through the substrate is more oxygenated and hence decomposition is more effective in soligenous mire sites, the depth of peat accumulation is usually less than in topogenous mires, although there are considerable similarities in peat types


Factors Important to the Management of Wetlands
Wetlands are a varied set of habitats with waterlogged soils for some part of the year, the extent of the water-logging varies from habitat to habits and often they overlap and appear in close proximity of each other. Wetlands provide habitat, food, cover for a huge range of invertebrates, wildfowl and waders and so many wetlands are subsequently identified and protected under designation of Ramsar or SPA to prevent decline and work towards improving the quality of the habitat. These important species co-exist with (and often rely upon) specific vegetation present within the community typical to waterlogged soils.

The species found on the wetland should influence the management of the site for example; if wildfowl are present manage for the benefit of waders, who are dependant on winter flooding for feeding opportunities. If waders are present manage for waders etc. In the interest of diversifying a wetland, a mosaic of flooded and non-flooded provides benefits to a wider variety of species.

We have already established that wetlands rely on regular water logging to sustain the habitat; centuries of land drainage have contributed to the loss of wetland sites and their current conservational importance. Any alterations to the habitat will effective the competitive balance between plant species and sudden change could lead to a change of habitat rather rapidly.
The water table should not be altered by draining, or diversion of water into the site from the surrounding area.

Most wetlands rely upon the low nutrient availability that accompanies waterlogged soils; enrichment from agriculture could lead to domination of fewer more competitive species rather than a diverse, rich community that can thrive on low nutrients. This means that no fertilisers, slurry or manure should be used where it can drain into wetlands and pesticide spray must occur away from wetland and surface surrounding it.

Scrub needs to be managed to prevent encroachment and succession towards a climax community, grazing in certain seasons provide vegetation management as well as the removal of nutrients.

Trees and hedges planted near wetlands consume much larger amounts of water and also increase transpiration rates, reducing water and leading to dryer, more successive areas. The seeding of trees and hedges encroaches in to the wet areas and lays down deep layers of litter annually.


Reference:
http://www.envf.port.ac.uk/geog/teaching/ecol/b6notes.htm
http://www.sac.ac.uk/mainrep/pdfs/tn519wetlandswildlife.pdf.