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What is a wetland?
According to the Ramsar Convention on Wetlands, an international treaty on conservation and wise use of wetlands and their resources, “wetlands are areas of marsh, fen, peatland or water, whether natural or artificial, permanent or temporary, with water that is static or flowing, fresh, brackish or salt, including areas of marine water the depth of which at low tide does not exceed six metres" and their boundaries may incorporate “riparian and coastal zones adjacent to the wetlands, and islands or bodies of marine water deeper than six metres at low tide lying within the wetlands" (Ramsar Convention, 1971). This broad definition covers indeed very different ecosystems, but all of them have in common the element of water as a principal determiner of their ecology, from how the soil develops to the types of plant and animal communities living in it. Wetlands are found in every country and climatic zone, from the tundra to the tropics, covering an area that is 33% larger than the USA (www.wetlands.org, 2012). Hence, the regional and local differences in soils, climate, topography, hydrology, vegetation, human presence and many other factors, originate a wide variation of wetlands, even among those of a similar type. --

Types of wetlands
There are many types of wetlands and different classifications depending on the aspects considered, like soil type, salinity of water, and plants and animals living in it. The traditional classification distinguishes between mineral soil wetlands (marshes and swamps) and organic soil wetlands (fens and bogs), also referred to as peatlands. They can be described as follows (EPA, 2001; www.personal.ceu.hu, 2012):

Marshes:
Marshes are a type of wetland ecosystem characterized by poorly drained mineral soils and vegetation dominated by herbaceous plants such as reeds, cattails and rushes. They can be further characterized as tidal (coastal) marshes, which can be both salt and freshwater, and non-tidal (inland) marshes, which frequently occur in poorly drained depressions, floodplains, and shallow water areas along the edges of lakes and rivers.

Swamps:
Swamps are wetland ecosystems characterized by mineral soils with poor drainage and trees as dominant vegetation type. They are found throughout the world, most often in low-lying regions next to rivers. Some swamps develop from marshes that slowly fill in, allowing trees and woody shrubs to grow. They can also be distinguished between freshwater or saltwater. Mangrove forests would be an example of the former.

Bogs:
They are characterized by waterlogged, spongy, poorly drained peaty soil, dominated by the growth of mosses, especially the gender Sphagnum, and heaths. Bogs are usually acid areas, formed in lake basins or depressions, frequently surrounding a body of open water. They receive water almost exclusively from rainfall.

Fens:
This type of wetland is characterized by peaty soil, dominated by grasslike plants. They are neutral or alkaline areas which receive water mostly from surface and groundwater sources, rather than rainfall. Both fens and bogs tend to occur in northern latitudes, often appearing together with no clear borderline between them.

This traditional classification has the disadvantage of taking into account comparatively few wetlands. That is why more detailed scientific classifications have been developed in an attempt to take into account the many wetland types all over the world. One of them is the Ramsar Classification System for Wetland Type (Ramsar Convention, 1971), which distinguishes 12 types of marine or coastal wetlands, 20 types of inland wetlands and 10 types of human made wetlands. The complete classification can be seen here. --

Importance of wetlands: functions and value
Wetlands are complex ecosystems that support many ecological functions, most of which provide very valuable ecosystem services as well as natural resources, key to local economies. The most important of these are:

Water Quality
As runoff and surface water pass through, wetlands remove or transform pollutants through physical, chemical, and biological processes:

Nutrient Removal
Human activities, especially inputs used in agriculture, often release excessive amounts of nutrients into the environment and water. Wetlands play an important role in removing this excess by encouraging sedimentation, adsorbing nutrients to sediments, taking up nutrients in plant biomass and enhancing denitrification (Fisher and Acreman, 2004). The nitrogen removal function of wetlands, carried out primarily through the denitrification process, has been shown to be robust by a wide variety of studies at the local, landscape and river basin scales (Hefting et al., 2012). Wetlands can also store significant quantities of phosphorus through its use by plants and microbes, adsorption by aluminum and iron oxides and hydroxides, precipitation as phosphates, and burial of phosphorus adsorbed to sediments or organic matter (Richardson 1985; Johnston 1991; Walbridge and Struthers 1993). Estimated mean retention of phosphorus by wetlands is 45% (Johnston 1991). Wetland soils can, however, reach a state of phosphorus saturation, after which it may be released from the system (Richardson 1985). The efficiency of wetlands in reducing phosphorus and nitrogen depends primarily on the degree of water logging and the rate and duration of nutrient loading (Fisher and Acreman, 2004).

Removal of metals
Wetlands can remove metals from water as a result of the presence of clays, humic materials, aluminum, iron, and/or calcium (Gambrell 1994). Metals either bind to the negatively ionized surface of clay particles, precipitate as inorganic compounds, complex with humic materials, or adsorb to precipitated hydrous oxides.

Removal of Pathogens
Fecal coliform bacteria and protozoans, which are threats to human health, enter wetlands through sewage waters, leaking septic tanks, and agricultural runoff. Bacteria attach to suspended solids that are then trapped by wetland vegetation (Hemond and Benoit 1988). In this way wetlands have an important role in removing pathogens from surface water.

Deposition of sediments and suspended solids
Sediment deposition in wetlands prevents a source of turbidity from entering downstream ecosystems. Typically wetland vegetation traps 80-90% of sediment from runoff (Johnston 1991). Deposition of suspended solids, to which polluting substances like nutrients, organics or metals are adsorbed, removes these pollutants from the water.

Water supply and regulation
Wetlands act as natural reservoirs for the watershed. Their biogeophysical features make them able to hold back water and then slowly release it to downstream areas, groundwater, and the atmosphere. This makes them act as an important source of groundwater recharge, and so, if they are drained, the hydrology of the watershed may potentially change because it will receive less water from groundwater discharges. It has been also shown that wetland basins that are not already filled to capacity with water can reduce the magnitude of flood peaks, not just by storing surface water but also through flood mitigation by wetlands soils, which are highly porous and can store water three to nine times of their own weight (Ming et al., 2006). This is particularly valuable in wetlands within and upstream of urban areas. Finally, wetlands play a role in the regulation of local and global climate, being able to moderate temperature extremes in adjacent lands (Ming et al., 2006).

Erosion control
Riparian wetlands and marshes located at the margin of lakes protect shorelines and streambanks against erosion. Wetland plants hold the soil in place with their roots, absorb wave energy, and reduce the velocity of stream or river currents. Coastal wetlands buffer shorelines against the wave action produced by hurricanes and tropical storms (Mitsch and Gosselink 1993).

Carbon storage
The waterlogged anaerobic wetland soils, with high productivity and low decomposition rates, provide unique conditions for carbon storage within peat and soil, thus playing an important role in mitigating global warming (Whiting and Chanton, 2001). Short-term stores are in existing biomass (plants, animals, bacteria and fungi) and dissolved components in the surface and groundwater (Wylynko, 1999). Paradoxically, wetlands can contribute to greenhouse effect since they are a source of methane emissions, yet over extended time horizons, the CO2 sink function of wetlands outweighs the release of methane into the atmosphere (Whiting and Chanton, 2001). When wetlands are drained, the aerobic conditions increase organic matter decomposition, thus increasing the release of carbon dioxide, which is another good reason for preserving or restoring wetlands.

Wildlife habitat
Wetlands are among the most productive ecosystems in the world (Mitsch and Gosselink 1993) and they have been shown to enhance biodiversity at the landscape as well as regional scale (Hefting et al., 2012). Immense varieties of species of microbes, plants, invertebrates, amphibians, reptiles, birds, fish, and other wildlife depend on wetlands for food, habitat, or temporary shelter.

Cultural, recreational and economic values
Uncountable human settlements have developed for thousands of years along bodies of water, incorporating into their economy the natural resources wetlands provide, from drinking water to fishery, hunt, timber or wicker, to name a few. They also have important historical and cultural values, being historically linked to local traditions and artistic expressions (www.water.ncsu.edu/watershedss/info, 2012). Nowadays the development of ecotourism and an increasing concern and appeal for the environment has widened the economic and recreational value of wetlands by attracting more visitors interested in nature observation, photography, birdwatching, sport fishery, aquatic sports and many other activities.

Wetland conservation and protection in the EU.
Over the last 30 years wetlands have been one of the most important conservation subjects in the European Union. The Convention on Wetlands of International Importance, known as the Ramsar Convention, was the first major international agreement on wetlands. This intergovernmental treaty, signed in the Iranian city of Ramsar in 1971, provided the framework for national action and international cooperation for the conservation and wise use of wetlands and their resources (www.ramsar.org, 2012). The Birds Directive in 1979 (79/409/EEC) also explicitly recognizes the need for the protection of wetlands as a vital habitat for water birds. Member States are required under the directive to designate wetlands of international importance for birds as Special Protection Areas (SPAs), included in the Natura 2000 network. The 1992 Habitat Directive, on the other hand, highlights lagoons, Mediterranean temporary ponds, active bogs, wet meadows and calcareous fens as habitat conservation priorities. A large number of proposed Sites of Community Interest (SCIs) include wetland habitats and species such as amphibians, water plants and invertebrates (Silva et al., 2007). With the aim of encouraging international collaboration among 25 Mediterranean countries, specialized wetland centers and NGOs in protecting regional wetlands, The Mediterranean Initiative on the Ramsar Convention on Wetlands, or ‘MedWet’, founded in 1991, tries to indentify key issues and take positive action to protect wetlands. In 2002, MedWet became a formal inter-regional structure for the implementation of the Ramsar Convention (Silva et al., 2007). The more recent Water Framework Directive (WFD - (2000/60/EC) provides a new framework for integrated river basin management, thus offering a platform to address wetland-related issues. The directive clearly identifies the need for the protection and restoration of wetlands. There are other European directives relevant to wetland conservation, in particular when it comes to the prevention of pollution. Examples include the Nitrates Directive (91/676/ EEC), the Groundwater Directive (2006/118/EC), and the Urban waste water (91/271/EEC) Directive.

What is ecological restoration?
Ecosystems comprise a complex network of relationships between their biotic and abiotic elements that sustain a series of ecological processes and functions that define them. When an ecosystem is altered and some of its elements and processes are lost, natural recovery may take place with time, but if degradation is too deep and the natural conditions are unfavorable, time may not be enough and actions may need to be taken in order to recover the health and functioning of the ecosystem. Thus, the Society for Ecological Restoration (SER) defines ecological restoration as “the process of assisting the recovery of an ecosystem that has been degraded, damaged, or destroyed.” Restoration activities must understand the complexity of the specific ecosystem and address the causes of its degradation, otherwise degradation will not stop and the actions taken will have no chance of success. Therefore, ecological restoration requires extensive scientific knowledge of the environments that sustain biological communities and ecosystem processes, as well as an understanding of the social context, taking into account the needs and interests of local communities and other actors involved and the interrelationships between nature and culture. Restoration is an essential tool for achieving biodiversity conservation, enhancing ecosystem services and fostering a sustainable socioeconomic development (www.ser.org, 2012). There are different activities that can also be considered as ecosystem restoration (wetland restoration, in this case) or that at least are closely related to it (www.epa.gov, 2012): - Creation: it consists of placing a wetland where there was not one in the recent past. - Enhancement: modification of specific structural features to increase one or more functions or to produce conditions that did not previously exist, in order to accentuate values of the site or to achieve management objectives (Gwin et al, 1999; Lewis 1989). The problem of this approach is that sometimes increasing one function may have the effect of decreasing another. - Relocation or replacement: activities that convert an existing wetland into a different type. - Mitigation: it refers to the restoration, creation or enhancement of wetlands to compensate for permitted wetland losses somewhere else (Lewis, 1989).