Review
TRENDS in Ecology and Evolution
Vol.21 No.2 February 2006
Regional and global concerns over
wetlands and water quality
Jos T.A. Verhoeven1, Berit Arheimer2, Chengqing Yin3 and Mariet M. Hefting1
1
Landscape Ecology, Institute of Environmental Biology, Utrecht University, PO Box 800.84, 3508 TB Utrecht, the Netherlands
Swedish Meteorological and Hydrological Institute (SMHI), SE-601 76, Norrköping, Sweden
3
Research Center for Eco-Environmental Science, Chinese Academy of Sciences, Beijing, P.R. China
2
Water quality in many stream catchments and river
basins is severely impacted by nutrient enrichment as a
result of agriculture. Water-resource managers worldwide are considering the potential role of riparian zones
and floodplain wetlands in improving stream-water
quality, as there is evidence at the site scale that such
wetlands are efficient at removing nutrients from
through-flowing water. However, recent studies have
highlighted disadvantages of such use of wetlands,
including emissions of greenhouse gases and losses of
biodiversity that result from prolonged nutrient loading.
Here, we discuss the water purification function of
wetlands at the site and catchment scale and suggest
ways in which these disadvantages could be overcome.
Introduction
Nutrient loading of terrestrial, freshwater and coastal
ecosystems occurs as a result of human waste disposal and
agriculture at a global scale. Although technological
purification plants are the best option to reduce the
nutrient fluxes to the environment, nutrient loading
owing to intensive agricultural practices typically occurs
through diffuse or ‘non-point’ sources (Box 1), which are
hard to tackle technologically. Measures to reduce diffuse
sources of nutrient loading are: (i) the reduction of
fertilizer application; (ii) the use of nitrogen-fixing crops;
and (iii) the restoration or creation (hydrological connections to) of wetlands in the landscape [1].
Here, we critically evaluate the practice of using
wetlands to manage water quality in catchments dominated by agriculture. We first consider this practice from a
functional perspective and evaluate under what conditions of loading intensity and relative wetland surface
area in the catchment wetlands are effective. We pay
special attention to undesired effects such as enhanced
greenhouse gas production and breakthrough of nutrients
stored earlier in the system as a result of overloading.
Then, we evaluate the ecological consequences of nutrient
loading on the species composition and structure of
wetlands. This issue has been ignored in most catchment
water-quality enhancement initiatives, where wetlands
are considered simply as systems with a high potential for
nutrient retention. We review evidence that prolonged
Corresponding author: Verhoeven, J.T.A. ([email protected]).
Available online 15 December 2005
nutrient loading can have profound effects on the nutrient
dynamics of the wetland, leading to shifts from one stable
state to the next, often involving structural changes in the
vegetation and losses of plant species diversity. We discuss
the functionality and biodiversity of wetlands from the
perspective of sustainable land and water resource-use
management in agricultural catchments to demonstrate
that certain conditions must be fulfilled for wetlands to
have a substantial effect on water quality without losing
their ecological integrity or providing additional risks for
global warming.
The water purification function of wetlands
Wetlands are in use worldwide to reduce concentrations of
nutrients in through-flowing water. Many studies at the
site scale have demonstrated that wetlands have a high
and long-term capacity to improve water quality and this
evidence has resulted in many initiatives to restore or
even create wetlands for this particular purpose. The most
‘human-controlled’ examples are the so-called ‘treatment
wetlands’, which are constructed, planted and hydraulically controlled for the purpose of removing pollutants from
wastewater [2,3]. Apart from these constructed wetlands,
(semi)natural wetlands in landscapes, such as riparian
(stream-side) wetlands, also reduce the nutrient load of
through-flowing water [4–8] by removing nitrate and
phosphorus from surface and subsurface runoff. With
water quality in streams severely deteriorating in densely
populated areas and intensively farmed regions worldwide,
the interest from natural resource managers in the
purifying functionality of wetlands in river catchments is
increasing. In Europe, future water-quality standards will
become stricter as a result of the implementation of the EU
Water Framework Directive (http://europa.eu.int/comm/
environment/water/water-framework/index_en.html). In
Canada and the USA, there is also pressure to tighten
water-quality standards.
Manuals to restore riparian wetlands have been
produced (e.g. http://www.chesapeakebay.net/info/forestbuff.cfm) and there have been studies to determine the
most optimal location or spatial arrangement of wetlands
in agricultural catchments [9]. The use of riparian zones
and other types of wetlands for water-quality enhancement has been advocated as a mitigation procedure for
ever-more intensive land-use practices, involving denser
livestock rearing and increased fertilizer application
www.sciencedirect.com 0169-5347/$ - see front matter Q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.tree.2005.11.015
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TRENDS in Ecology and Evolution
Vol.21 No.2 February 2006
97
Box 1. Diffuse nutrient sources in agricultural areas
Figure I indicates the areas of the world that are intensively farmed. In
North America, most of the Mississippi basin is used for intensive
agriculture, and most parts of Europe, except for latitudes above 558 N,
are also used intensively for agriculture, resulting in nutrient loading in
the basins of the major river catchments all across the continent (e.g. the
Rhine and Danube). In Asia, the major areas with intensive agriculture
are southeast China, India and Indonesia, involving the catchments of
major rivers such as the Yellow River, the Yangtze and the Ganges. In
South America, large parts of the eastern Amazon basin are cultivated,
as well as a large area draining towards the Pantanal in Brazil and
Argentina. In Africa, a system of subsistence agriculture is common,
which is less intense and often does not apply artificial fertilizer.
This uneven distribution of agriculture is reflected in the location of
areas with diffuse nutrient loading and eutrophication. A study of
nutrient loads in some of the major rivers of the world has shown that
the European rivers and the Mississippi have increased their nutrient
loads ten- to 20-fold from 1960 to 1990 [1]. Severe eutrophication
problems involving blooms of cyanobacteria and fish kills have
occurred in shallow and deep freshwater lakes [67], in the southern
part of the North Sea [68] and in the coastal areas of the Baltic [64] and
the Mediterranean [69]. Repeated, prolonged hypoxia has occurred in
parts of the Gulf of Mexico, with major fish kills and adverse effects on
fisheries [70]. Severe eutrophication problems involving algal blooms,
loss of seagrass beds and major damage to blue crab production have
also occurred along the east coast of North America, particularly in the
Chesapeake Bay estuary [71]. More recently, increasing fertilizer use
and diffuse pollution problems have been reported for southeast China
[72]. Expectations are that, with the current rapid economic development, agricultural practices will become much less sustainable in this
area and will involve severe fertilizer overuse, with detrimental effects
on freshwater systems and coastal waters.
Here, we focus on the three areas of the world where diffuse nutrient
pollution is most severe and where the need for finding solutions
including wetland nutrient sinks in the landscape is most pressing (i.e.
Europe, eastern North America and southeast China). That does not
imply that eutrophication is insignificant in other areas. For instance,
the Amazon is the river carrying the highest load of nutrients globally,
but its load has, as a result of agriculture, merely doubled [1]. In Africa,
there are eutrophication problems in Lake Victoria [73] and Lake
Malawi [74] as a result of diffuse nutrient inputs.
Figure I. Cultivated areas of the world. Brown regions indicate areas in which at least 30% of the landscape is cultivated. Reproduced with permission from the Millennium
Ecosystem Assessment 2005 (http://www.MAweb.org), UNEP.
and manure dumping. In many cases, the conservation or
restoration of habitat for plant and animal species is seen
as an additional benefit of such initiatives [10].
Using wetlands to improve water quality: how does it
work?
River catchments in which the dominant land-use type is
agricultural often have lower-order stream subcatchments that are strongly influenced by runoff from fields
or grasslands. In intensively farmed areas, nutrient
loading is often so high that large quantities of nitrate
leach into the groundwater, which discharges into streams
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as seepage or subsurface runoff (Box 2). In intensively
farmed catchments, phosphorus (P) and nitrogen (N) are
also transported to streams in surface runoff [11]. Between
agricultural fields and streams, one often finds riparian
areas that can influence surface and subsurface runoff
before it reaches streams.
There is a large body of literature based on studies at
individual sites that indicates that riparian habitats
remove nutrients from the water flowing through them
on its way from the agricultural land to the stream. The
most frequently documented function is the removal of
nitrate from subsurface run-off in wetland zones with
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TRENDS in Ecology and Evolution
Vol.21 No.2 February 2006
Box 2. Nutrient loading in an agricultural landscape: pathways, processes and effects
Heavily fertilized agricultural fields and grasslands show substantial
leaching of the very mobile nitrogen species NOK
3 . These nitrates leach
downward to the groundwater and are transported laterally to streams,
mainly through subsurface runoff and deeper groundwater fluxes
(Figure I). Simultaneously, phosphates are transported from the
agricultural field in particulate form through overland flow (surface
runoff). In the absence of riparian buffer zones in the landscape, these
high nutrient loadings flow into the stream water and cause severe
eutrophication of the streams, rivers, lakes and coastal
areas downstream.
In landscapes with riparian zones, the superficial groundwater layer,
which is rich in nitrates, comes into contact with the wetland soil,
which is highly organic and covered by wetland vegetation. Nitrogen
and phosphorus are taken up by the wetland vegetation and are cycled
in the ecosystem, which increases their residence time. Thus, that
peaks in their concentrations are levelled out before the water enters
the stream. If the vegetation is herbaceous, nutrients can be exported
although harvesting [17].
Nitrates entering anaerobic soil zones are easily subject to
denitrification. The nitrate is first reduced to N2O, and then further to
dinitrogen gas. The denitrification process removes nitrogen permanently from the through-flowing water and contributes highly to the
sink function of the wetland. If the bacterial process is in some way
hampered, the major end product might be N2O, which is a strong
greenhouse gas. This might occur when buffer zones are overloaded
with nitrates.
Greenhouse gas emission
Riparian buffer zone
Agricultural field
off
Water table
Denitrification
Leaching
NO3–
NO3–
(Su
b)s PO4
urf
ac
er
un
Plant uptake
N2, N2O
Eutrophication
Lakes,
ocean
Stream
NO3–
PO4
Water saturated zone
Confined layer
Loss of biodiversity
TRENDS in Ecology & Evolution
Figure I. Cross-section of a riparian wetland showing hydrological fluxes, nutrient processes and environmental impacts of nutrient loading. Thicker arrows with warmer
colours indicate a higher nutrient loading rate.
anaerobic soil conditions [12–17]. Denitrification is generally the most important process for nitrate removal,
whereby dead organic matter is decomposed by bacteria in
the absence of oxygen, using nitrate as an electron
acceptor. Nitrate is converted to nitrous oxide (N2O) and,
subsequently, to atmospheric nitrogen (N2), which is
emitted by the wetland [18–23]. Nutrient uptake in
vegetation as water passes through the riparian zone is
also important and results in long-term nitrogen storage
[16,24–26]. However, its removal from the system only
occurs if the vegetation is harvested as part of the
management of that system [18]. Phosphorous removal
in riparian habitats has also been reported, with
sedimentation, soil adsorption and plant uptake being
the most important mechanisms [6,27,28].
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Research on the nutrient removal capacity of wetlands
in the temperate zone has revealed that the maximum
potential rate of nitrogen and phosphorous removal
typically ranges from 1000 to 3000 kg N haK1 yK1 and
from 60 to 100 kg P haK1 yK1 [2,20,29]. These are high
values, if one considers that they are an order of
magnitude higher than fertilizer applications in intensively farmed areas. The capacity of riparian wetlands
to remove nitrogen and phosphorous is, however, only
important if it can be demonstrated that there has been
a significant reduction of the load of nitrogen and
phosphorous that reaches stream ecosystems. What is a
‘significant reduction’? In some cases, it has been
argued that riparian wetlands contribute in a ‘significant’ way if they remove at least 30% of the total
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TRENDS in Ecology and Evolution
Sw
ed
en
Se
a
Söderköpingsån
(882 km2)
Ba
l ti c
Rönneå
(1900 km2)
(b)
N
0
5
99
the USA (http://www.epa.gov/seahome/wqs.html), and
have requirements in terms of the presence of indicator
species rather than strict limits for nitrogen and
phosphorous concentrations in Europe (‘ecological quality’, EU Water Framework Directive). For practical
reasons, therefore, we use the first approach by
adopting a standard of 30% removal of nitrogen and
phosphorous loading as the boundary between ‘significant’ and ‘insignificant’ reductions.
(a)
Genevadsån
(224 km2)
Vol.21 No.2 February 2006
10 km
TRENDS in Ecology & Evolution
Figure 1. Agricultural catchments in Sweden (a) Location of the catchments
Söderköpingsån, Genevadsån and Rönneå. These catchments have heavy
agricultural use and cause substantial nitrogen enrichment of the Baltic Sea. (b)
The 148 riparian wetlands under construction (black dots) in the Rönneå catchment.
nitrogen and phosphorous load [30,31]. Another
approach is to investigate whether nitrogen and
phosphorous removal by riparian wetlands contributes
to meeting the water-quality standards in the receiving
surface water. These standards are, however, often
pragmatic compromises and subject to change. For
example, water-quality standards differ regionally in
How much land area is required?
Studies evaluating the removal of nitrogen and phosphorous by wetlands at the catchment level have been carried
out at different scales. For the whole of the Mississippi
basin, Mitsch et al. [32] calculated that 20–50% of the total
nitrogen load carried by the river into the Gulf of Mexico
could be removed by restoring a major proportion of all
riparian zones and wetlands associated with the small,
lower-order streams, together covering 1–2% of the total
catchment area. An additional 20–50% of this total
nitrogen load to the Gulf of Mexico could be removed if
bottomland hardwood forests associated with the river
floodplains were restored to the point that they would
cover 3–7% of the total Mississippi basin.
Similar estimates were made in catchments in
southern Sweden (Figure 1; Table 1), which are a major
source of nitrate enrichment of the Baltic Sea. In the
Rönneå catchment, restoration of 148 wetlands covering
0.6% of the catchment area failed to improve river nitrate
concentrations [33]. A modeling study revealed that 40%
nitrogen removal would require a wetland area covering
5% of the total catchment [34]. The traditional Chinese
‘multipond systems’ (Box 3) indicate that effective
nitrogen and phosphorous retention occurs when the
water in a catchment is directed through a system of
ditches and created wetlands covering around 5% of the
catchment area, preventing high nutrient loading of
the streams and rivers and associated eutrophication
problems [35–37]. These examples from the USA, Sweden
and China all suggest a global rule that wetlands can
contribute significantly to water-quality improvement at
the catchment level if they account for at least 2–7% of the
catchment area.
Effects of nutrient loading on wetland ecosystems
Given that many wetlands in agricultural catchments are
continually enriched with nutrients, it is relevant to
consider the consequences for the species composition and
functioning of the wetland ecosystem. Enriched wetland
systems can lose species and can show drastic changes in
nutrient cycling rates.
Table 1. Modelled nitrogen removal by wetland constructions on arable land in three catchments in Swedena,b
Catchment
Söderköpingsån
Genevadsån
Rönneå
a
Catchment
size
(km2)
882
224
1900
Present
situation
(mt N yK1)
234
267
1990
With potential
wetlands
(mt N yK1)
132
251
1753
Removal by wetlands
(mt N yK1)
102
16
137
(%)
43%
6%
7%
Wetland
area
(% of total)
5.0
0.4
0.6
See also Figure 1, main text.
These studies imply that 5% of the total catchment must be considered for wetland restoration to have a significant impact on nitrogen reduction.
b
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Refs
[65]
[33]
[34,66]
100
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TRENDS in Ecology and Evolution
Vol.21 No.2 February 2006
Box 3. Multipond systems in China: ancient water management in agricultural catchments
Multipond systems, which are large artificial irrigation networks
dating back 20 centuries, are common in southeast China. They
consist of many tiny ponds, scattered in agricultural catchments and
connected by ditches and streams [36,37]. Typically, the pond area is
6–10% of the total catchment and each pond has an area of 0.05–
1 ha, serving an area of 0.5–10 ha.
The ponds collect surface runoff and agricultural drainage water,
which is used to irrigate rice and other crops. Each pond is
connected via ditches to three–five other ponds and the water
cascades from one pond to the next. The multipond systems have a
large retention capacity for nutrients. In the Liuchahe Watershed
(Figure I), there is a high variation in nutrient concentrations
exported from different land-use types. Runoff nutrient concentrations are highest in water discharging from villages, croplands
and rice paddies. After capture of the runoff in the ditches and
ponds, the nutrient concentrations were reduced significantly and
nutrient concentrations at the watershed outlet were consistently
lower than in the runoff water [35].
The retention and removal mechanisms of nutrients in the
multipond systems include sedimentation, adsorption, recycling
through irrigation and uptake by aquatic plants. Nutrient budgets
compiled for phosphorus during five years in Liuchahe catchment
showed reductions of nutrients in runoff by the multipond systems
of O90% [35].
In some regions of China in recent years, there has been a
tendency to ignore the importance of the multipond systems owing
to the development of modern reservoirs and irrigation channels.
The number of ponds throughout the country was 8.3 million during
the 1950s, and the irrigation area was 133 million ha, which
accounted for 39% of whole irrigation area. The number of ponds
is now only 6.3 million, and the corresponding irrigation area is 80
million ha [75]. Some ponds were destroyed to increase arable land
and housing, whereas others became filled with sediments that were
no longer excavated to fertilize arable land. The distribution area has
been reduced significantly and this is already having negative
impacts upon water quality in the rural areas of the country.
(a)
(b)
N
0
1000 km
Liuchahe
river
Figure I. ‘Multipond’ landscape in southern China. (a) Typical landscape showing
ponds, rice fields, woodlands and villages near Chaohu Lake, P.R China. (b) Water
system with connected ponds (in blue) and ditches (blue lines) in the Liuchahe
catchment area at the same location.
Critical loads
Nutrient inputs to ecosystems have increased over the
past century in many parts of the world. The resulting
nutrient enrichment often has significant effects, including increased productivity, higher rates of nutrient
leaching and shifts in the dominance, and composition,
of species [38–40]. Most ecosystems can incorporate
higher loading rates, which have only minor effects as
long as they do not surpass a certain limit. However, when
nutrient loading rates surpass this critical level, species
composition and ecosystem functioning change dramatically over short periods of time and the systems often move
to a different stable state. Among the best-known
examples are shallow lakes that shift from water with
low turbidity and abundant submerged macrophyte
vegetation to a turbid state with prolonged phytoplankton
dominance [41]. Wetlands that are characterized by low
productivity and high plant diversity dominated by slowgrowing, nutrient-conserving species shift to systems
dominated by large, fast-growing helophytes following a
strong increase in nutrient-loading rates.
The degree to which the species composition changes
depends on the natural nutrient richness of the system.
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Naturally nutrient-poor (i.e. oligotrophic and mesotrophic) systems react more drastically than do naturally
nutrient-rich (eutrophic) systems. Nutrient-poor systems
show a complete shift in plant species composition as well
as a drastic change in nutrient dynamics, whereas
nutrient-rich systems might show only further increased
productivity. All systems, however, show a characteristic
breakdown of the nutrient retention function after
prolonged high nutrient loading [42]. An example of
a drastic shift in plant species composition and
nutrient dynamics is the effect of agricultural runoff on a
large mesotrophic wetland system in the Everglades.
Species-rich communities dominated by sawgrass Cladium jamaicense have been replaced by tall species-poor
cattail Typha domingensis stands in areas enriched with
nutrients. In the same areas, the rate of phosphorus
cycling has increased as a result of higher decomposition
rates [38,43]. This reinforces the nutrient richness of the
system and creates a situation that is difficult to reverse to
mesotrophic conditions.
Such shifts in structure (i.e. species composition) and
functioning (nutrient cycling and retention) have spurred
scientists to propose critical loads of nitrogen and
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TRENDS in Ecology and Evolution
phosphorous for ecosystems. A critical load of a nutrient is
defined as the loading rate below which the system
remains all but unchanged, but beyond which it exhibits
sudden, drastic changes, including a shift in species
dominance and species composition and a major change
in ecosystem functioning, in terms of carbon (C) and
nutrient outputs, trophic interactions and/or nutrient
cycling rates. Such situations of nutrient overloads cannot
only be detected as a major shift in species composition or
structure, but also as a distinct increase in the nutrient
concentrations of water that is exported from the
ecosystem (e.g. in wetland outflow). A drastic increase in
output nutrient concentrations is another indicator that
can be used to establish critical loads for ecosystems. For
wetlands, a critical loading rate of 10 kg P haK1 yK1 has
been proposed, based on the analysis of a large database of
wetlands enriched with nutrients [44,45]. For nitrogen,
much research has been carried out along gradients of
atmospheric nitrogen deposition in Europe [46–48].
Increased levels of atmospheric nitrogen deposition
occur as a result of air pollution owing to fossil fuel
combustion (NOx) and agriculture (NH3). In Western
Europe during the 1980s, levels were higher than
45 kg N haK1 yK1, which is ten times the background
value [48]. Metadata sets on the effects of increased
deposition have proposed a critical nitrogen load of
w25 kg N haK1 yK1 for wetlands, but there is little
published information available about differences among
different wetland types in this respect [46–48].
It is striking that the proposed critical loading rates of
nitrogen and phosphorous for wetlands are several orders
of magnitude lower than the typical loading rates in
natural or constructed wetlands used for improving water
quality (Table 2). Only in a few cases, for example in
wetlands used for water treatment in the Mississippi [49],
loading rates of nitrogen and phosphorous have been close
to these critical values. This implies that critical nutrient
loads are easily surpassed in many natural wetlands and
that, depending on their original trophic status, shifts in
species composition and/or increases in nutrient concentrations in the outflow will occur, in spite of rates of
nutrient retention remaining high. As an example,
forested riparian zones in the Netherlands typically
receive nitrate-rich runoff from heavily fertilized fields
[50,51]. Many of them lost their original plant diversity
decades ago [52], but continue to retain high quantities of
nitrate, even though the surface and subsurface water
discharged from them still have high concentrations of
Vol.21 No.2 February 2006
101
nitrate [13]. Hence, wetlands in agricultural catchments
can contribute significantly to water-quality improvement, but their loading rates often surpass critical values.
Enhanced nitrous oxide emissions
The major process responsible for nitrate removal in
wetlands is denitrification. However, in situations where
the reduction of the nitrate to N2 is incomplete, the
denitrification process can also be a major source of the
greenhouse gas N2O, which has a global warming
potential 310 times that of CO2 [53]. N2O accounts for
w6% of the total greenhouse effect and also has an
important
role
in
the
destruction
of
the
stratospheric ozone.
N2O emission in wetlands is generally promoted under
conditions that are suboptimal for denitrification, such as
low pH or soil moisture. However, N2O production is also
promoted by high nitrate availability, because it is
energetically favorable for denitrifyers to reduce nitrate
instead of N2O. There is much recent information about
the increase in N2O emission after nitrogen addition to
agricultural soils [54–56]. Much less data exist on the
indirect effect of nitrogen addition by agriculture on N2O
emissions from riparian wetlands [57–61]. Recently,
Barnard et al. [62] reviewed studies of nitrogen addition
to natural ecosystems and concluded that nitrogen
addition significantly increased N2O emissions in both
laboratory and field experiments.
Recent publications [57–60,62], strongly suggest that
nitrogen transformations in buffer zones receiving high
levels of nitrate result in a significant increase in N2O
emissions. For example, riparian buffer zones in Twente,
the Netherlands, showed high N2 O emissions
(20 kg N haK1 yK1) at sites with high nitrate loads [63].
This implies that the nutrient retention benefit of riparian
zones comes at an environmental cost, in particular when
nitrate-loading rates are also high. This potential negative
consequence of loading wetlands with nitrate is often
ignored or downplayed. Thus, there is a great need for
additional information about the risk of N2O emissions
from nitrogen-loaded wetlands and about management
options to balance that risk against the environmental
benefit of water-quality improvement.
Wetlands in the landscape for water-quality improvement: finding the right balance
Wetlands in nutrient-loaded agricultural catchments have
a significant role in improving water quality. Our analysis
Table 2. Characteristic nitrogen and phosphorous loading rates of wetlands in agricultural catchments in relation to relevant
loading thresholds
Catchment
Liuchahe
Regge, Twente
Everglades
Mississippi
Various
Treatment wetlands
Maximum loadb
Critical loadc
a
Location
Wetland type
P.R. China
Multipond
Netherlands
Riparian
USA
Marsh
USA
Forested
USA
Riparian
in USA and Europe
Origin
Constructed
Natural
Natural
Natural
Natural
Constructed
N load (kg haK1 yK1)
O500
200–1140
N/Aa
19–39
20–155
500–9000
1000
25
P load (kg haK1 yK1)
O50
2–40
2–9
N/A
100–2000
60
10
No data available.
Beyond this limit, the wetland will show substantial leaching and associated high concentrations in the outflow.
c
Beyond this limit, wetlands with a species-rich vegetation will show a dramatic increase of productivity and associated change in species composition.
b
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Refs
[35–37]
[13,63]
[38,43]
[49]
[32,49]
[2,32]
[2,20,32]
[44,45,47,48]
102
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TRENDS in Ecology and Evolution
shows that this potential is only realized in catchments
with a minimum area of wetlands relative to total
catchment size. Measurements from different regions
around the world indicate that at least 2–7% of the total
catchment needs to be in wetland habitat to see a
significant increase of water quality at the catchment
scale, a remarkably narrow range. This minimum value
has already raised much dispute among policy makers as
to the practicality of restoring or creating such a large
area of wetlands as a management tool, particularly in
many European catchments, where the proportion of
wetlands has often become close to zero.
In Sweden, resource managers advocate other
measures such as less drainage of fields and grasslands,
or lower fertilizer applications as less costly and easier to
realize than wetland restoration [34,64]. In areas with
intensive land use and very heavy use of fertilizer, such as
the Netherlands, nitrate concentrations in the groundwater are so high that high N2O emissions in the riparian
wetlands are to be expected. Again, restoration of riparian
zones should be accompanied by lower fertilizer applications to avoid adverse effects on the environment.
The implications for land-use management in areas
under intensive agricultural use (e.g. Europe, South Asia
or the Mississippi basin) are that only a combination of
measures will result in acceptable environmental quality,
that is, (i) fertilizer levels have to be reduced significantly;
(ii) riparian zones and floodplain wetlands should be
rehabilitated; and (iii) their location in the landscape
should be carefully selected on the basis of hydrological
studies. This will only be possible at great economic cost,
which will delay implementation.
Another clear outcome of our discussion is that many
wetlands in agricultural catchments are loaded beyond
the ‘critical’ limits, and some even beyond the ‘maximum’
limits (Table 2). This means that many riparian zones will
lose plant species or have already done so. Riparian
wetlands in such catchments will all converge towards a
narrow set of ecosystem types characterized by high levels
of nutrient richness, a high primary productivity and low
species density. The example of wetlands used for water
treatment in the Mississippi Delta [52] shows that the
nutrient retention function can be combined successfully
with conservation of the original status of the ecosystem
and its biodiversity under the condition that loading rates
are low and not surpassing critical limits.
Conclusions
Land-use planners and environmental resource managers
are facing many challenges to ensure good surface-water
quality in agricultural catchments. In their development
of sustainable water-quality management options, they
must give consideration to the existence of loading limits
as discussed here. Because of their high potential for
nutrient retention, it is still a good idea to use wetlands in
catchment water resources management for water-quality
improvement. Restoring wetlands and their hydrological
connections to the upland and the stream can be a
rewarding activity in this respect.
Most importantly, however, the combination of waterquality improvement with wetland biodiversity
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Vol.21 No.2 February 2006
conservation requires loading rates of wetlands to remain
below critical thresholds. In many agricultural catchments, these limits have been surpassed and biodiversity
restoration would require a decrease in the loading
through decreased fertilizer applications or the use of
larger areas of wetlands for nutrient retention. In
catchments with intensive land use, loading rates could
surpass another, much higher critical limit, beyond which
the wetland ecosystem no longer performs its retention
function properly but releases nutrients or emits the
greenhouse gas N2O. In such catchments, the only feasible
measure is to decrease fertilizer levels.
Acknowledgements
We thank Dennis Whigham for his constructive remarks about the article.
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