Wetlands and the Ecological Services that They Provide on Multiple Spatial Scales, from Landscape Down to Soil

Abstract

In this chapter, we present a hierarchical framework to consider wetlands and their ecosystem services in landscape planning. Wetlands are important in a landscape setting as they are intricately linked to the water cycle, and they provide many ecosystem services. Collectively, wetlands can be regarded as wet ecological infrastructure. Wetlands can be categorized as different hydrogeo-morphic types, which all play a different role in the overall hydrology and lead to different ecosystem services. Ecosystem services can act on various spatial levels, and all of these levels need to be considered when conserving wetlands and securing their ecosystem benefits. The levels that can be recognized for this are the catchment level, the individual wetland (hydrogeomorphic unit) level, the wetland habitat level, and at the smallest scale even the soil level, as some of the most important ecosystem services are related to the biogeochemistry associated with wetland soils.

1. Introduction

Wetlands are vulnerable ecosystems because they find themselves in the center of a wide range of human economic activities. In most cases, historically, the first settlements of humans have always appeared around places that have easy access to water for irrigation. People want access to the resources that wetlands can provide, but they also drain wetlands to transform the land for different uses. For this reason, people have had an ambiguous relationship with wetlands.

Today, in an age of high population densities, intense industrialization and large-scale changes in land use, many wetlands have been lost, and the remainder have been often degraded, with their original hydrology and water quality compromised. This is a problem in its own right, but this loss of wetlands also plays an important role in the degradation of the landscape as a whole. Wetlands provide many ecosystem services and as such they are often regarded as the most valuable ecosystems [1]. Ecosystem services can be defined as the ways in which the presence of ecosystems affects human well-being, and their existence usually becomes evident only after ecosystems degrade or disappear [2].

Another way of describing the utility of ecosystems for human well-being is by stating that wetlands form an important part of the “ecological infrastructure” [3]. Ecological infrastructure is a term that encompasses all the natural elements of a landscape that are vital to its overall functioning and thereby to the provisioning of ecosystem services. Civil infrastructure can be built to enhance the utility of the landscape for human purposes but generally society benefits from leaving critical ecological infrastructure intact as far as is possible and from mitigating eventual losses. Historical or current losses of ecological functioning are often used as a justification for spending resources on ecological restoration [4, 5]. This is because sustainable livelihoods are dependent on the productive capacity of ecosystems and thereby on their natural functioning. This applies particularly to aquatic ecological infrastructure, as many of the most important ecological processes are tied in with the water cycle. The way in which water moves through a landscape and how water gets distributed throughout it determines the ecological functions of the landscape.

In order to optimally utilize the landscape for human well-being, we need to have an overview of the ecosystem services that all wetland ecosystems within it provide. This depends on the type of wetland as well as on the position that that wetland occupies within the broader water catchment. All wetlands together perform certain tasks that account for outcomes at the level of the catchment, such as flood retention and the maintenance of base flow, and that need to be optimized for the sake of water resource planning. A hierarchical view of ecological infrastructure helps in planning land use and civil infrastructure around the existing ecological infrastructure.

The valuation of wetland ecological infrastructure does not only depend on the wetlands themselves but also on the “demand” for that service, which depends on where people are living within the catchment, where are the needs for certain goods and services coming from the wetland, and on where is the industrial activity and other sources of pollution.

In this chapter, we would like to argue how the presence of wetlands in a landscape is an important aspect of the ecological functioning of that landscape as a whole and we would like to present a hierarchical approach into assessing the health of a landscape in terms of its wetland “ecological infrastructure.”

2. A hierarchical view of wet ecological infrastructure

The benefits and ecosystem services that wetlands provide can be felt at different spatial scales. The highest spatial scale is that of a river catchment. People depend on rainfall in order to be able to grow sufficient food, but this rain will partially percolate into the soil or drain into channels that carry away the excess water. Sustainable livelihoods depend on the capacity of the wetland infrastructure to retain water during dry periods and to remove excess water quickly during wet periods, as flooding will damage civil infrastructure. In integrated catchment management, this often means that the wetland infrastructure needs to be supplemented with gray infrastructure outside of the floodplain areas to properly spread out the flood events.

For the sake of environmental impact assessments of any human activity, one core activity often revolves around the identifying and mapping of wetlands, as many countries recognize that wetlands require special attention in conservation. These mapping activities take place by looking at soil and vegetation characteristics which differentiate the wetland from the surrounding uplands [6]. Of course, the first characteristic to look at is the presence of standing water, but since water tables fluctuate in the course of the seasons, soils and vegetation provide a good proxy for the presence of water that can be seen throughout the year [7], but does require some expert judgment and good local knowledge.

Once wetlands are mapped, we need to consider various spatial scales of wetland configuration. Firstly, we need to consider where wetlands are within the context of an entire catchment. The catchment can be defined at many spatial scales, such as the primary catchment, secondary catchment, tertiary catchment, and quaternary catchment, depending on the size of the main drainage channel and the number of tributaries that are contained within the catchment. Where wetlands are located within the overall configuration of drainage channels has an effect on what role these wetlands may play for the flow regime of the river as a whole, but may also have an impact on water quality as water flowing through a wetland may be filtered and nutrients may be removed.

After wetlands are mapped, they should also be typed in order to understand what is going on in the wetland as not all wetland types have the same functions within the overall hydrology of the catchment [8]. When classifying a wetland as belonging to a certain type, individual wetlands should rather be recognized as hydrogeomorphic (HGM) units [9, 10] as the landform and hydrology of a wetland determine the ecological processes that may take place in the wetland. In defining an HGM unit, a researcher has to address the question why it is that water accumulates in that place and what are the sources and losses of water, and how does it flow through the wetland. Each HGM unit can be regarded as a spatial unit that has a uniform hydrological functioning, but it is also possible that wetlands of different HGM units are organized together into a wetland complex whereby water that is released from one HGM unit can flow becoming the source of water for another HGM unit. Larger wetland areas may often actually be wetland complexes even though many characteristics of the wetland such as water flow, vegetation, and soils are probably distinctive between the units that make up the complex [8]. Many HGM units are typically linked to the drainage network, such as floodplains, channeled and unchanneled valley bottom wetlands, and a large part of the seepages. Others are more typically separate from the drainage network or at least the main drainage channels, although they may be connected to it by means of interflow and aquifers: these are the depressions, wetland flats, and some of the seepages [10].

Even while hydrogeomorphic units determine the hydrology and ecological functioning of wetlands as a whole, many ecological processes are also determined at a finer scale. The hydrology of wetlands creates environmental gradients and along these environmental gradients we find many different habitats that create different conditions for plant growth. Within a wetland, we can therefore recognize different vegetation types or also called different habitat types [8]. These habitat types are mostly determined by hydroperiod, which is the fraction of time that a site is inundated with water [11]. Other environmental conditions, such as pH, nutrient status, salinity, soil texture, and organic matter contents, also determine the habitat type. Because wetlands exhibit such wide environmental conditions on a small scale, they play an important role in evolutionary adaptations and speciation in many groups of plants and animals [12]. The categorization of wetland habitat types is often done on the basis of plant communities, which are the most visible and consistent aspect of the overall biotic community in a wetland.

3. The link between wetland typology and ecosystem services

When talking about wetlands, it is useful to categorize wetlands in order to cover the wide variety in ecosystems, the species that occupy them, and the processes that take place within them. Over time, many different wetland classifications have been developed, but they are only considered useful if they make accurate descriptions of ecosystem processes and ecosystem functions within those wetlands [8]. For this reason, classifications based on hydrogeomorphic types (HGM types) are regarded as the most suitable way of classifying wetlands, and the type to which a wetland is allocated already tells us about which ecosystem services the wetland will provide [9, 10].

Hydrogeomorphic types are determined by looking at the source of water in a wetland as well as the shape of the basin in which this water is retained. A wetland is a place where there is a net surplus of water for at least part of the year, which means that the water entering the wetland exceeds the water exiting the wetland. Water can enter or exit the wetland in three different ways, namely the atmosphere (entry by means of precipitation and exit by means of evaporation), surface water (entry by means of flooding and exit by means of drainage), or subsurface to deep groundwater (entry by means of recharge and exit by means of discharge) [13]. The source of water, how this water is retained within the wetland and the way it is released from the wetland determine the role of this wetland within the overall water cycle and thereby the contribution it makes toward ecosystem services (Table 1) [8].

Hydrogeomorphic type	Main water source	Water losses	Main ecosystem services provided
Floodplain	Surface flow	Runoff	Flood attenuation, sediment deposition, and water quality enhancement
Channeled valley bottom	Surface and subsurface flow	Runoff	Erosion control
Unchanneled valley bottom	Surface and subsurface flow	Runoff	Erosion control and water quality enhancement
Seepage	Subsurface flow	Runoff	Sustenance of base flow
Depression	Precipitation	Evaporation	Provisional services
Wetland flat	Subsurface flow	Evaporation	Provisional services

Table 1.

Hydrogeomorphic types, their inputs and outputs of water, and the main services that they typically provide.

Ecosystem services are those ecological processes that take place that directly benefit human well-being and that tend to be taken for granted. In recent times, efforts have been made to attach values to ecosystem services by means of various valuation techniques, and it is consistently found that wetlands are among the ecosystems with the highest economic value. This is because they are intricately linked to the water cycle and humans depend on the water cycle for their food production and hygiene but can be harmed by excessive flooding and the associated erosion. At the same time, the water cycle is strongly tied to other biogeochemical cycles, and wetlands play an important role in the natural cycling of carbon, nitrogen, phosphorus, sulfur, and other minerals. Overall, ecosystem services can be subdivided into four main categories, namely regulating, supporting, provisioning, and cultural services (Table 2) [14].

Ecosystem services		Spatial level supply of service	Demand for service	Properties that facilitate this service
Regulating and supporting services	Flood attenuation	HGM unit, position in catchment	When there are downstream properties that need protection	Surface roughness is influenced by vegetation structure, longitudinal slope of the wetland, and storage capacity of depression basins.
	Streamflow regulation	HGM unit and position in catchment	When rainfall is episodic downstream	The properties of the vegetation and its influence on evaporative losses, subsurface flow in the wetland
	Sediment trapping	HGM unit and position in catchment	When there is excessive erosion upstream	The same features that assist in flood attenuation: surface roughness and storage capacity, as slowing down of floodwaters facilitates deposition of sediments
	Phosphate assimilation	HGM unit	When there are sources of phosphate upstream	Most phosphate assimilation takes place where sediments as deposited as phosphates are attached to sediments.
	Nitrate assimilation	HGM unit	When there are sources of nitrogen upstream	The presence of vegetation
	Toxicant assimilation	HGM unit	When toxicants are released upstream	Factors that facilitate sediment deposition. Presence of plants capable of assimilating heavy metals (Typha, Pontederia, etc.).
	Erosion control	HGM unit	When the landscape has steep slopes and high runoff	Vegetation and its stem density of vegetation as it holds soil together as well as canopy cover as it protects soils from the impact of drip.
	Carbon storage	Habitat unit	Cumulative effect of all wetlands	Productivity of vegetation, permanence of inundation as anaerobic conditions slow down organic breakdown
Provisioning services	Grazing resources	Habitat unit	When the surrounding landscape is grazed	High productivity of wetland vegetation, especially when it endures during the dry season
	Provisioning of medicinal plants	Habitat unit	When there are few medical services around	Depending on species present in the wetland
	Harvestable thatch and other materials	Habitat unit	When people rely on traditional materials	Depending on species present in the wetland
	Provision of water	HGM unit and position in catchment	When there are no other sources of water in the area	Depending on easy access, presence of open water
Cultural services	Cultural heritage	HGM unit	When there are communities with cultural practices	Presence of places with special cultural significance within the wetland
	Tourism and recreation	HGM unit and habitat unit	When the wetland is on touristic routes	Presence of high conservation value and biodiversity, scenic beauty of the wetland, and the surrounding landscape
	Education and research	HGM unit	When there is good access to the wetland	Presence of existing research infrastructure

Table 2.

Various ecosystem services that wetlands provide, the scales at which they are relevant, and the properties that determine their effectiveness.

Regulating ecosystem services refer to the benefits that are derived from the natural functioning of ecosystems, whereby the ecosystem in question ensures the continuation or regulation of a process. Wetlands may play a role in the ensurance of baseflow in rivers by storing water for extended periods of time although the evaporative losses in wetlands may be quite large as well [15], but also in the retention of floodwaters, which lead to a reduction in the risk of flooding for downstream properties. Other regulating services include the removal of nutrients from water that flows through a wetland and erosion control by slowing down the flow of water. From the perspective of the climate, wetlands can help in climate regulation by means of the storage of carbon.

Supporting ecosystem services are the underlying natural processes that take place that support the regulating ecosystem services, such as photosynthesis at the basis of primary productivity and the carbon and water cycles at the basis of climate control and streamflow regulation. These services are often not specifically considered when monetary valuations are calculated, because they benefit humans mostly indirectly by supporting specific regulatory and provisional services. One specific supporting service that may be considered here as well is the support of species diversity, by providing a habitat where many species can thrive and reproduce.

Provisioning ecosystem services refer to the goods and materials that are provided by ecosystems that benefit people directly, either by the collection of construction materials, subsistence farming, or by commercial operations that benefit a wider community of people. In the case of wetlands, this of course includes the provision of water, either for oneself or for domestic animals that graze around the wetlands. Water can also be used for irrigation of nearby farmlands whereby water is pumped out of the wetland. Other goods that people may harvest from wetlands are fish, waterfowl, edible plants, construction materials, or medicine. The agricultural crops that are planted in wetlands or in areas that are irrigated by water from wetlands can also be regarded as goods that the wetland provides.

Lastly, cultural ecosystem services refer to the ways in which ecosystems have influenced our culture and our use of the landscape, either traditionally or in modern times. Across the world, wetlands have often been at the center of our civilization, as people have always settled around areas where easy access to water was guaranteed and successful harvests could be made to support a growing population. For this reason, many cultural and spiritual practices have emerged that have centered around wetlands, such as traditional cleansing and baptizing ceremonies. Today, wetlands are also central to many recreational practices and many wetlands provide opportunities for tourism. Lastly, wetlands can also be important for research and education, and in some places, research centers and education facilities have been centered around wetlands.

The reality of the ecosystem services that wetlands provide means that people will always have to live with wetlands in their environment. In the following section, we will consider the ecological processes that are part of wetland environments at various spatial scales.

4. Ecosystem services at a catchment scale

Wetlands are scattered throughout the landscape and are connected in various ways to the water cycle. Several of the ecosystem services that wetlands are known to provide are directly linked to this water cycle and help to ensure the accessibility of water to many communities. The position of the wetland within the catchment and in relation to residents living within that catchment has a big influence on ecosystem service supply. For example, the location of wetlands within a catchment has an influence on the collective nitrate removal for the catchment as a whole [16]. Some hydrogeomorphic types of wetlands are also typically found in specific areas within the catchment; for example, seepages are often found close to the upper reaches of the catchment, close to the water divide. In some cases, when such ecosystems contain peat, they may slow down the flow of water into the drainage system as peat bodies hold water that trickles gradually into the streams so that water continues to be released into the fluvial system long after it has precipitated down in the form of rain or snow. Snowfall in the high mountains is another mechanism by which the release of water into rivers is delayed as it will only start percolating into the soil after it starts to melt toward springtime.

Flooding risk is ameliorated by various types of wetlands that are found along the stream channel, especially floodplains, that retain water for extended periods of time, thereby protecting properties downstream. Especially floodplains that contain oxbow lakes and abandoned side channels form basins that can store excess water. One of the main reasons to restore floodplain wetlands and to provide rivers with the space to overtop their banks is to provide storage for floodwaters in order to protect towns and farmlands downstream.

The presence of seepage wetlands, floodplain wetlands, and valley bottom wetlands in a catchment is therefore crucial to ensure the resilience of the livelihoods within that catchment. Many of these wetlands combined can be regarded as a form of “ecological infrastructure” that combined with the river channels that connect them ensure that water remains available throughout the catchment. When wetlands are lost by drainage for agriculture, the need arises to mitigate these losses by creating artificial wetlands within the same catchment.

Another way in which wetlands provide services on a landscape scale or catchment scale is in the maintenance of biodiversity. Wetlands are scattered throughout the landscape, often as small pockets of habitat for aquatic biota. In some cases, wetlands are connected by means of fluvial channels, but they can also be cut off from each other by terrestrial habitat. Species that require wetland habitats have to migrate across the landscape to colonize new wetland habitats, and the metapopulation of a species can best be maintained if wetland habitats are large and wetlands are proximate to each other.

5. Ecosystem services at a wetland scale

At the scale of an individual wetland, wetlands serve as sinks for sediments from the surrounding landscape. This leads to conditions at the level at the individual wetland that may create local ecosystem services. It means that many nutrients, but also pollutants can accumulate in the wetland. Wetlands have the capacity to filter the water that flows through them, due to plant uptake and bacterial conversions. Artificial wetlands called helophyte filters are often constructed to process mine effluents, sewage, and other industrially produced pollution. The filtering capacity of the wetland depends on the size of the wetland, the rate of flow through it, and the type of plants that grow in the wetland. In many large wetlands, because of this filtering effect, the water quality is guaranteed for local subsistence farmers who also get their water needs from the wetland.

Wetlands also form a sink for sediment. This can be either clastic sediments, in areas where there is sufficiently strong flow, but this can also be organic sediments in places that are permanently inundated where the water is stagnant. The storage of a large mass of sediment makes the wetlands soak up water after rains, but it also makes them vulnerable for erosion [15]. Wetlands that develop on steep slopes are unstable as excessive surface runoff may wash away all stored sediment in the wetland.

Wetlands keep soil in place and prevent erosion by spreading water over a larger surface area which reduces the energy of concentrated channeled flow. The trade-off between slope and surface area of wetland follows a function where larger wetlands run a larger risk of eroding and turning into a channel, losing all their sediment.

In the case where wetlands store large amounts of peat, wetlands also perform a role in climate regulation. This is a two-edged sword as it also means that degrading wetlands may emit large volumes of carbon dioxide and methane. In order to emphasize their relatively high global importance as C stores, it is important to note that although wetlands occupy only 4–5% of the land area of the globe, and they hold approximately 30% of the carbon in the terrestrial biosphere [17, 18].

The scenic beauty of a wetland determines its use as a tourist destination or even as a place of worship or spiritual inspiration. Larger waterbodies and waterbodies that attract large amounts of waterfowl can be regarded as attractive tourist destinations, and the economic activities that may arise around such wetlands can be regarded as cultural ecosystem services.

6. Ecosystem services at a habitat scale

Many of the provisioning services that a wetland provides depend on there being suitable habitat for the species that is to be harvested. For a wetland to provide sufficient amounts of fish, there needs to be suitable habitat for fish, which is generally the parts of the wetland that are permanently inundated. Some fish may seek shelter in the inundated plains of a floodplain, especially if there are large predators around. Such shallow water habitats are suitable hunting grounds for fishermen.

Reeds and other tall graminoids are regularly harvested for construction materials or for weaving, arts, and crafts. The diversity of plant morphologies of wetland plants contributes to the diversity of different weave types, e.g., the fine culms of Juncus kraussii have particular value for twining and straight sewing, while the robust Cyperus latifolius leaves are well suited for plaiting [19]. Sometimes, the plants from wetlands can provide materials to thatch roofs. Wood can be harvested from swamp forests, but generally this can also be harvested from upland areas. Swamp forests may become dominant in floodplain areas in tropical climates and may become dominant along rivers in temperate climates, but are rare in many other parts of the world.

Some wetlands are utilized for small-scale agriculture. The crops that are harvested in this manner can also be regarded as provisioning ecosystem services as it is the wetland soils and soil moisture that provide the fertility to grow crops. There are only few crops that can cope with permanent inundation, but it may be quite feasible for farmers to use the temporary or seasonally wet areas of wetlands, whereby water from the wetland can be used to provide water to the crop. Traditional wetland cultivation systems have characteristically been closely coupled with the natural hydrological regimes of wetlands. This is illustrated by the rice farmers of central Sierra Leone, who through their rich knowledge of the soil moisture requirements of different rice varieties are able to match the different rice varieties with the soil moisture variation along the wetland soil catena [20].

Many wetlands are key grazing resources, particularly those located in arid to semi-arid climate, where they are especially valuable in dry years and at the end of the dry season in most years, owing to their residual moisture. Thus, wetlands are often critical forage sources over the dry season for both livestock and wild grazers [21].

Wetlands are also rich in other biodiversity that plays a role in people’s livelihoods, for example, in the form of medicinal plants such as sweet flag (Acorus calamus) which is widely used for oral hygiene and the treatment of digestive complaints. The strong environmental gradients create many niches for plant species, and a substantial section of all useful plants can be found in wetland habitats [22].

Different wetland habitats do not only lead to different provisioning ecosystem services, but they can alter the hydrological aspects of the wetland and thereby have an effect on regulating services. For example, if the vegetation in certain sections of the wetland consists of shrubby vegetation, it will lead to more resistance to flow, which will impact ecosystem services that have to deal with erosion control or floodwater retention. Such ecosystem services are mainly impacted by larger-scale processes, but are modified on a local scale by different habitat types [23].

The services most strongly associated with different habitats in wetland potentially complement each other, thereby enhancing the wetland’s overall supply for a variety of ecosystem services. For example, a temporarily flooded alluvial fan occupying the inflow of a valley bottom wetland may be particularly important for the trapping and retaining of sediment and the phosphates adsorbed to these sediments, given that phosphates are generally strongly absorbed by sediments, and therefore, phosphate removal tends to be strongly associated with the trapping of new sediment. Further downstream in the same wetland, where permanently flooded habitat and associated low redox conditions and high primary productivity of emergent vegetation and abundant organic sediments predominate, the storage of carbon, and the assimilation of nitrates may be particularly important, given the favorable biogeochemical conditions of this habitat for these particular services.

7. Ecosystem processes at a soil or sediment scale

At the most basic level, most of the processes that are unique to wetlands play out within an inundated soil. The reason for this is that wetlands provide a wide variety of metabolic pathways as there is no oxygen available and bacteria will have to find an alternate electron acceptor for the energy-gaining reactions that drive their life-cycle. This reflects the fact that wetlands today are among the habitats where the original metabolic pathways at the basis of life prevail, from the period before oxygen started to accumulate in the Earth’s atmosphere as a waste product of photosynthesis. The primary way in which the soil biochemistry is affected by these inundated conditions is by the inhibition of the breakdown of dead organic matter that accumulates in the soil. This organic matter forms the substrate where many of the redox reactions in anaerobic soils take place.

Within these soils with anaerobic conditions, there is also variation in redox potential as water levels fluctuate and additionally wetland plants also have their influence. The root systems of these plants transport oxygen into the inundated soils as it may leak around the roots, creating narrow aerated zones around the smaller roots. The existence of steep gradients in redox potential are the conditions in which important biogeochemical processes such as denitrification take place [24]. This has an important reaction within the nitrogen cycle, but it is only one example of the wide range of redox reactions that can take place in wetland soils.

The wide variety of metabolic pathways arise from the combination of reactants that can exchange electrons in redox reactions that release free energy to be exploited by bacteria. The type of metabolism that is favored depends on the free energy released by the two half-reactions in the redox exchange, on the amount of organic matter that is available to be broken down and on the concentration of products and reactants that is available in the substrate [25]. This situation creates a multitude of possible reactions, and therefore, there is a wide range of niches at various depths of the substrate available to be exploited by different types of bacteria that each utilize a different energy source.

The elements that may become involved in these redox reactions occur in a specific sequence from the surface to the deeper soil layers or through time [25, 26] (see Table 3). Firstly organic matter is degraded in aerobic respiration as oxygen is energetically the most favored electron acceptor (reaction 1). When the oxygen is depleted, nitrate (NO₃⁻) is depleted as an electron acceptor in which it is either reduced to ammonium (NH₄⁺) or released as nitrogen gas in denitrification (reactions 2 and 3). When the nitrate is gone, the next electron acceptor is manganese (Mn⁴⁺) that is reduced to Mn²⁺ (reaction 4), followed by Iron (Fe³⁺) that is reduced to Fe²⁺, creating a grayish color in the soil (reaction 5). When none of these ions are available, sulfate (SO₄²⁻) is reduced to form H₂S gas, which can be recognized by the foul odor of rotten eggs (reaction 6). Lastly, when there is still organic matter available, it can be broken down in the methanogenesis reaction (reaction 7), which is a very inefficient reaction and therefore happens at a very slow rate [27].

Reaction	Electron acceptor	Reaction	Redox potential
1	Organic matter	C6H12O6  CO2 + H2O	250 mV
2, 3	Nitrate	C6H12O6 + 4NO3^−  6CO2 + 2N2 + 6H2O; C6H12O6 + 4NO3^− + 4H^+  6CO2 + 4NH4^+ + 6H2O	250 mV
4	Manganese	CH2O + 2MnO2 + 4H^+  CO2 + 2Mn^2+ + 2H2O	225 mV
5	Iron	CH2O + 2FeOOH + H^+  CO2 + 2Fe^2+ + H2O	120 mV
6	Sulfate	2CH3CHOHCOO^− + 2SO4^2− + 3H^+  4CO2 + 4H2O + 2HS^−	−75 mV
7	Organic matter	CH3COO^− + 4H^+  2CH4 + 2H2O	−250 mV

Table 3.

Sequence of redox reactions taking place in inundated soils.

The complexity of the many reactions taking place in the anoxic soils of wetlands showcases the importance of wetlands in global biogeochemical cycles, especially for carbon, nitrogen, manganese, iron, and sulfur as they are directly involved in the redox reactions. Many other nutrients, such as phosphate, potassium, sodium, and chloride may become concentrated in wetlands in specific circumstances, which may provide either opportunities (phosphate and potassium) or limitations for life forms living in the wetland environment (sodium and chloride). The importance of wetlands for the global cycling of many of the elements that are deeply involved in various life cycles means that these ecosystems provide important supporting ecosystem services that extend beyond the border of the wetland itself.

This means that, even though we recognize these processes as taking place at the smallest scale, their impact is actually on a much larger scale, as it is the cumulative effect of all biogeochemical processes in all wetlands in an area that determine the fluxes of exchange between wetland and atmosphere. For these reasons, people living the direct environment may not be able to “value” the services that the wetland provides, especially when it comes to a service such as “climate regulation.” Another problem is that these services may easily be altered when wetlands get degraded where they actually make the problem worse, such as with methane emissions [28].

8. Considering ecosystem services in the prioritization of wetlands for restoration or conservation

Historically, humans have always had an ambiguous relationship with wetlands: they acted as the centers of many civilizations where the first cities emerged on riverbanks and around other places with a steady access to water, but they have also always been regarded as a place where diseases spread and as places that need drastic changes by means of drainage before they can be rendered useful as agricultural land. Some of the world’s most productive crops, such as rice and taro, can be grown in wetlands, but for many other crops, we need access to wetland water for irrigation, but the actual wetland itself is unsuitable for cultivation as the soil also requires aeration.

The importance of wetlands in ecosystem services and in the overall biogeochemical pathways across our planet means that as humanity we have to aim to design our landscapes in such a way that it provides space for wetlands and where we even may construct artificial wetlands if natural wetlands have disappeared. For this reason, urban planners and landscape architects should always keep considering the wet ecological infrastructure as all of our lives are strongly intertwined with the flow of water through our landscapes. This implies that the supply of ecosystem services must match the demands that society requires at each of the places where wetlands are designated or restored [29].

9. Conclusion

As wetlands are increasingly under threat worldwide, much has been written about the ecological services that they provide and there is a growing realization that there needs to be a presence of wetlands across any living landscape. The mapping of wetland ecosystem services requires us to understand that the different ecosystem services play out on different spatial scales. Some ecosystem services can take place within a single wetland, whereas for most of the hydrological services, the configuration of wetlands across the entire catchment is crucial. All these aspects of wetland ecology need to be considered in any landscape policy plan or natural resource management plan.