Wetland ecosystems for
denitrification processes in rivers
A comparison for implementation in the lower
Seine and its estuary (France)
ADVIESRAPPORT
AUTEUR: NICO DECONINCK
ONDERZOEK UITGEVOERD IN OPDRACHT VAN CENTRE NATIONAL DE LA RECHERCHE SCIENTIFIQUE, PARIJS,
FR.
HEERLEN, okt-15
WETLAND ECOSYSTEMS FOR DENITRIFICATION PROCESSES IN RIVERS
Colofon
Naam document
Opdrachtgever
Uitgave
Projectteam (auteurs)
Projectteam (nummer)
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Datum (laatst
bijgewerkt)
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PWA-NicoDeconinck-IM2010nj.doc
Dit onderzoek is uitgevoerd in opdracht van Centre National de la Recherche
Scientifique (CNRS), Mevr. Laverman Anniet.
InCompany Milieuadvies,
faculteit Natuurwetenschappen,
Open Universiteit Nederland,
Postbus 2960,
6401 DL Heerlen, NL.
www.ou.nl/nw
Nico Deconinck
Team-105, Denitrificatie, IM2010nj
Jikke vanwijnen, Open Universiteit Nederland – InCompany Milieuadvies
26-5-2011 12:10
in bewerking
voor review (intern: team/projectcoach; extern: opdrachtgever)
ter beoordeling
definitief
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VMAB2010nj-M105-DENITRIFICATIE-PWADeconinck-20110526.doc
WETLAND ECOSYSTEMS FOR DENITRIFICATION PROCESSES IN RIVERS
Projectgegevens
Titel (nl)
Title (in English)
Opdrachtgevende instantie
Omschrijving opdracht (nl)
Description of the order (in English)
Trefwoorden
Key words
Betrokkene(n) bij opdrachtgeversorganisatie
Projectleider team IM
Projectcoach
Opdrachtgever
Examinator
Referentie naar dit rapport
Wetland ecosystemen voor denitrificatie processen in rivieren.
Een vergelijking voor de implementatie in het estuarium van de
Seine (Frankrijk)
Wetland ecosystems for denitrification processes in rivers.
A comparison for implementation in the lower Seine River and its
estuary (France)
Centre National de la Recherche Scientifique (CNRS)’, Université Pierre
et Marie Curie, Parijs, FR.
De opdracht bestaat erin een overzicht te geven van mogelijke
wetlands, zoals moerassen en overstromingsgebieden die voor
denitrificatie in rivieren in aanmerking komen. Hierbij worden een
aantal praktische voorbeelden vergeleken en worden de mogelijke
milieuomstandigheden besproken. De resultaten van deze studie
moeten een overzicht geven van de wetlands, die in functie van de
omgevingsfactoren, de denitrificatie of verwijdering van nitraten in het
estuarium van de Seine mogelijk maken.
This study concerns an overview of potential wetlands, like marshes
and floodplains that could be implemented for denitrification in rivers.
A few practical cases, related to the environmental conditions will be
discussed. The results of this study should give an overview of
potential wetlands, related to environmental conditions, which could
be considered for implementing in the estuary of the river Seine for
denitrification or the removal of nitrates.
Nitraat – nitrificatie – denitrificatie – ecosysteem – rivier – wetlands –
estuarium – overstromingsgebied
Nitrate – nitrification – denitrification – ecosystem – river – wetlands –
estuarine – floodplain
Mevr. Anniet Laverman, contactpersoon CNRS
Nico Deconinck, cursus Virtueel milieuadviesbureau, N5021,
837544374, wo-bachelor Milieu-natuurwetenschappen (B.Sc.), is
verantwoordelijk voor contact met Mevr. Laverman + tussentijdse
rapportage.
Mevr. Jikke vanwijnen, faculteit Natuurwetenschappen, Open
Universiteit Nederland, telefonisch (00-31-20-5788425) of per email
[email protected]
Mevr. Anniet Laverman, UMR 7619 Sisyphe, Université Pierre et Marie
Curie, Parijs, FR
Dr. Wilfried Ivens, faculteit Natuurwetenschappen, Open Universiteit
Nederland, Heerlen, NL
Deconinck, N. (2011).Wetland ecosystems for denitrification processes
in rivers. A comparison for implementation in the lower Seine River
and its estuary (France). Onderzoek uitgevoerd in opdracht van het
Centre National de la Recherche Scientifique (CNRS). Parijs, FR.
Unpublished Bachelor’s Thesis, Open Universiteit Nederland, Heerlen,
NL.
VMAB2010nj-M105-DENITRIFICATIE-PWADeconinck-20110526.doc
WETLAND ECOSYSTEMS FOR DENITRIFICATION PROCESSES IN RIVERS
Content
Colofon
Projectgegevens
Inhoud
Voorwoord
Samenvatting
Abstract
1. Introduction
Background
1.1.
1.2.
Problem definition
1.3.
Target of the research
1.4.
Criteria for the results
1.5.
Conditions
2. Methods
2.1.
Literature search
2.2.
Analysis
3. Results
3.1.
Inventory of types of wetlands
3.2.
Denitrification results and wetlands in rivers
4. Discussion and conclusions
4.1.
Conclusions
4.2.
Discussion
5. Suggestions
Literature
Annex A Description 8 rivers and their wetlands
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WETLAND ECOSYSTEMS FOR DENITRIFICATION PROCESSES IN RIVERS
Voorwoord
Dit adviesrapport bevat de resultaten van een literatuuronderzoek met betrekking tot de denitrificatie
in de Seine (Frankrijk) met behulp van wetlands of natte gebieden. Het onderzoek werd in opdracht
van het Centre National de la Recherche Scientifique (CNRS) te Parijs uitgevoerd.
Met de hulp van Mevrouw Anniet Laverman, tewerkgesteld bij de Université Pierre et Marie Curie te
Parijs en met de ondersteuning van de projectcoach Mevrouw Jikke vanwijnen kwam deze studie tot
stand.
De vrucht van dit werk moet ertoe leiden dat enerzijds het inzicht in de werking van wetlands
verbeterd wordt en dat anderzijds de relaties tussen de factoren, die bij de nitraatverwijdering
bepalend zijn, verder onderzocht worden. De nadruk komt in dit adviesrapport te liggen bij de Seine
als studieobject en meer in het bijzonder het estuarium met een aantal wetlands.
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WETLAND ECOSYSTEMS FOR DENITRIFICATION PROCESSES IN RIVERS
Samenvatting
Wetlands (natte zones) zijn algemeen gekend voor hun denitrificatie en nitraat
verwijderingeigenschappen, maar hun efficiëntie kan nogal verschillen volgens het type. Er is
bovendien geen algemeen akkoord over de definities van de verschillende wetlands. In deze studie
werden 5 indelingen van wetlands gevonden en onderverdeeld in verschillende types en subtypes.
Verder werden 5 rivieren met wetlands in de Verenigde Staten en 3 in Europa geselecteerd. Hierbij
werden verbonden en geïsoleerde, overstromingszones en estuarium wetlands gedetecteerd.
Daarnaast werden in deze studie de denitrificatie efficiëntie resultaten in de wetlands vergeleken op
basis van de schaal van Janssen et al. (2005). Verbonden wetlands, overstromingszones en estuarium
wetlands gaven de beste resultaten betreffende nitraat verwijdering. De nitraatstromen werden voor 2
wetlands berekend op basis van het NANI model (totale som van antropologische stikstof instroom) en
gaven lagere waarden dan deze, bepaald door de auteurs. Als gevolg van een trage waterstroming en
een langere contacttijd in verbonden wetlands, werd een hogere nitraat verwijdering en retentie
vastgesteld. Deze parameters kunnen door een gecontroleerd pompen van rivierwater in de verbonden
wetlands beheerst worden. Verder werd een betere verwijdering in verbonden dan in geïsoleerde
wetlands vastgesteld. Zuurstofarme sedimenten toonden de beste denitrificatie waarden in
overstromingszones. Relatieve grote oppervlakten en een korte afstand tot de rivier leverden de beste
nitraat verwijderingefficiëntie op. In het estuarium gaven een hogere redox potentiaal en een lagere
ammonium instroom de beste nitraat verwijderingefficiëntie. Laag moeras (Nuphar advena) gaf betere
denitrificatie resultaten dan hoog moeras (Phragmites australis). De kenmerken van de
geïdentificeerde wetlands werder verder vergeleken met de wetlands in de zone tussen Caudebec en
Honfleur in het estuarium van de Seine. Spijtig genoeg zijn er geen vergelijkbare nitraat
verwijderingresultaten te vinden in de literatuur. Marais du Hode, Grande Vasière Nord en Marais
Vernier zijn hoofdzakelijk voorzien van verbonden wetlands door een netwerk van kanalen en de
waterstroom wordt gecontroleerd door een sluis of openingen in een dam. Marais de la Risle heeft een
aantal geïsoleerde wetlands, een modderbank en een kunstmatige meander. In overeenstemming met
de resultaten bij de andere rivieren, worden goede prestaties bij Marais du Hode, Grande Vasière Nord
en Marais Vernier verwacht. Een aantal kleinere stromen, die verbonden zijn met de Seine, zoals de
Eure, de Oise en de Marne, zouden bestudeerd moeten worden om de effecten stroomafwaarts in het
estuarium te kunnen bepalen. Als gevolg van hun lagere waterstroming zouden deze rivieren
potentieel kunnen bezitten voor het implementeren van optimale retentie en vertragingsmiddelen. Om
de complexe samenhang tussen de nitrificatie en denitrificatie parameters te begrijpen, dient meer
onderzoek te gebeuren en in het bijzonder in het estuarium door de invloed van de dynamiek van de
getijdenstroming.
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WETLAND ECOSYSTEMS FOR DENITRIFICATION PROCESSES IN RIVERS
Abstract
Wetlands are generally recognized for their denitrification and nitrate removal capacities, but their
performance can vary seriously. Moreover, a general agreement about the definition of the several
kinds of wetlands is still lacking. In this study 5 ways for categorizing wetlands and subdividing them
in several types and subtypes. Next, a selection of 5 rivers with wetlands in the United States and 3 in
Europe was made. As result, these rivers were associated with connected and isolated, floodplains and
estuary wetlands. Furthermore, during this study, A comparison of the denitrification performance
results in these wetlands, by using the denitrification performance ranges by Janssen et al. (2005) was
made. Connected wetlands, floodplains and estuary wetlands showed the best nitrate removal results.
The nitrate fluxes, calculated for 2 wetlands by using the NANI (net sum of anthropological nitrogen
inputs) model, were lower than the results observed by the authors.
Due to a low water flow rate and a longer contact time in connected wetlands, they show a better
nitrate reduction and retention. These parameters could be controlled by pumping river water in the
connected wetlands. A better removal performance for connected than isolated wetlands has been
found. Oxygen poor flooded sediment showed the best denitrification rate in floodplains. Furthermore,
a relative great surface area and a small distance from the riverside enhanced the nitrate removal
performance. In the estuary wetlands, a higher redox potential and lower ammonium fluxes gave
satisfactory nitrate removal rates. Low marsh (Nuphar advena) showed better denitrification results
than higher marsh (Phragmites australis). The characteristics of the identified wetlands were further
compared with the wetlands in the region between Caudebec and Honfleur in the estuary of the river
Seine. Unfortunately, no comparable nitrate removal results were identified in the literature. Marais du
Hode, Grande Vasière Nord and Marais Vernier are mainly provided with connected wetlands by a
network of ditches and controlled by a sluice or openings in a dam. Marais de la Risle has a number of
isolated wetlands and comprises mudflats and an artificial meander. In accordance with the results of
the other rivers, I expect the best nitrate removal performance for Marais du Hode, Grande Vasière
Nord and Marais Vernier. A number of smaller streams, which are connected with the Seine, like the
Eure, Oise and Marne, should be studied in order to examine the effects downstream in the estuary.
Due to their lower water flow rates, these rivers could show potential for implementing optimal
retention and velocity decreasing means.
Due to the complex interaction between the different parameters in nitrification and denitrification
processes, more research should be done for better comprehension, especially in the estuary because
of the influence of tidal dynamics.
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WETLAND ECOSYSTEMS FOR DENITRIFICATION PROCESSES IN RIVERS
1. Introduction
1.1. Background
Many rivers are overloaded with nutrients, including nitrogen compounds, like nitrates, ammonium or
urea from natural and anthropological sources, such as wastewater and agriculture fertilizer by runoff
water from agriculture fields. These nutrients can cause harmful algal blooms, especially in the coastal
zone. Due to degradation of the algal biomass by heterotrophic bacteria, oxygen depletion in this zone
can appear. This process is called eutrophication and has already been reported in 1955 by Ohle (Ohle,
1955). Actually, eutrophication is defined as ‘a change of the natural position of organisms in the food
chain in a water body by anthropological influences with negative consequences’ (Van Ballaer et al.,
2006). The oxygen depletion, called anoxia or hypoxia, will affect oxygen dependent organisms and so
disturb the biodiversity of the ecosystem (Gardner, 2008). Furthermore this riverine nutrient flux will
proceed into the sea. The nitrogen flux from the Seine, the Somme and the Scheldt to the sea for the
year 2000 has respectively been estimated to 1975, 1333 and 1464 kg N km-2 y-1. In comparison with
a more environmental friendly agriculture and without less fertilizer usage (= business-as-usual), an
estimation for respectively the Seine, the Somme and the Scheldt to the sea will be 2245, 1623 and
1570 kg N km-2 y-1 (Thieu et al., 2010).
Two processes, namely nitrification and denitrification play an important role in nitrogen fluxes in
aqueous and terrestrial ecosystems. Nitrification, that converts ammonium into nitrite (NO2-) and
nitrate (NO3-), needs oxygen rich conditions. In the contrary, denitrification transforms nitrate into N2O
and N2 in oxygen poor concentrations < 0.2 mg O2 L-1, which is indicated with the term ‘suboxic’ or
anoxic (Seitzinger et al., 2006). More specific, denitrification is defined as the transformation of nitrite
or nitrate into a nitrogen gas, as opposed to the reduction of nitrite or nitrate to ammonia. These
gases are nitrous oxide (N2O), considered as a greenhouse gas and NO that contributes to the ozone
production, both intermediate products of the reduction from NO3- to N2 in the following sequence NO3→ NO2- → NO → N2O → N2. Contrarily to the previous gases, nitrogen gas (N2) is harmless (Verhoeven,
2002).
Overall, microbial denitrification and plant uptake have been identified as the most important natural
processes to remove nitrates (Kelly, 2006).
Wetland ecosystems, natural or artificially constructed, are able to remove nutrients, such as nitrogen
from aquatic systems. Basically, wetlands are distinguished by a) the presence of water at the surface
or the root zone b) soil conditions and c) vegetation (Hogan et al., 2000).
The indication ‘wetland’ is a very general and broad term that is used for several kinds of ecosystems.
In 1971 a convention, the Ramsar Convention was established. The goal of this intergovernmental
treaty was to define a better use of wetlands. Articles 1.1 and 2.1 of the convention define wetlands
respectively as ‘For the purpose of this Convention 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
meters’ and ‘may incorporate riparian and coastal zones adjacent to the wetlands, and islands or
bodies of marine water deeper than six meters at low tide lying within the wetlands’ (Commonwealth,
State and Territory Governments of Australia, 2001).
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WETLAND ECOSYSTEMS FOR DENITRIFICATION PROCESSES IN RIVERS
1.2. Problem definition
Denitrification is a critical process that can remove bioavailable nitrogen (N) but it also affects global
climate directly by the greenhouse gas nitrous oxide (N2O). Three conditions are required for
denitrification: a) available nitrate b) reduced oxygen conditions and c) presence of electron donors,
like organic carbon (Seitzinger et al., 2006).
Fig.1: Nitrogen pathways (Gardner, 2008).
Figure 1 illustrates the major nitrogen pathways. Nitrification and denitrification processes are often
coupled.
The occurrence of aerobic close to anaerobic zones in shallow surface water and wetlands could
promote coupled nitrification/denitrification (Gardner, 2008). This coupling mechanism is regulated by
diffusion, which enables to set-up a concentration gradient. NH4+ must diffuse upward in the aerobic
zone to be nitrified and NO3- downward in the anaerobic area (Gardner, 2008). Consequently, due to
the limited oxygen diffusion in constructed wetlands, nitrification is often the limiting step in the N
removal process (Maltais-Landry et al., 2009). Nevertheless, growing plants and tidal influence
enhance the oxidizing environment of the rhizosphere and sediment pores, which increases oxygen
availability to support nitrification and decrease the diffusion distance between oxygen rich and anoxic
sites for respectively nitrifying and denitrifying bacteria (Hamersley & Howes, 2005).
Referring to Thieu and colleagues (2010), nitrates create an important problem in the Seine River in
France. As well as in the lower river Seine as in the freshwater estuarine, oxygen deficit has been
determined due to respectively degradation of organic matter and nitrification of ammonium (Cébron
et al., 2003).
Furthermore, Billen and colleagues (2007) used a model (the Riverstrahler) to describe nutrient
transfer and transformation in the Seine. The model predicts nitrogen retention of 40-60.106 kg N y-1,
representing 20 à 30% of the total inputs, with denitrification as the major process.
Denitrificaton processes or the microbial transformation of NO3- into N2O and N2 in the Seine have
already been examined by a number of scientists. Denitrifying bacteria in aqueous and sedimentary
conditions have also been studied universally. Not only the presence of denitrifying bacteria, such as
Pseudomonas, Alcaligenes and Flavobacterium is important, but also the environmental conditions will
determine the denitrification processes (Gardner, 2008). Three alternate microbial pathways have
been identified. Anammox, that concerns the production of N2 (= nitrate reduction) from anaerobic
oxidation of ammonium with nitrite, assimilatory nitrate reduction to ammonium (ANRA) and
dissimilatory nitrate reduction to ammonium (DNRA) (Gardner, 2008). Denitrification mainly occurs in
the benthic phase. Curie and colleagues (2009) found that the riparian zone of the Beauregard stream
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WETLAND ECOSYSTEMS FOR DENITRIFICATION PROCESSES IN RIVERS
(large alluvial floodplain of the Seine) supported denitrification all year long due to river water with
levels of dissolved oxygen, high organic carbon content and high temperature.
Approximately 65% of the increasing levels of N2O into the atmosphere are due to N transformations
in the soil of the Seine basin (Garnier et al., 2006). More particularly, Garnier and colleagues (2006)
concluded that N2O emission from 1997 until 2003 was higher in the Seine estuary (between Poses
and Caudebec), where salt or brackish water is dynamically mixed with freshwater, than upstream
from Paris WWTP (wastewater treatment plant) effluents, but lower than in the effluent’s downstream
zone (see fig. 2)
The use of wetlands or wetland ecosystems for purifying domestic, agriculture, recreational and
industrial wastewater is known since 1950. Besides denitrification by bacteria, still three other
processes are important for the nitrate removal in these ecosystems. Plants in the wetland take up
nutrients and store them in the upper parts. Next, at the bottom of the wetlands, sedimentary
absorption can take place, but ammonium and nitrate absorb rather difficult to soil particles. At last,
difficult solvable compounds, due to biochemical reactions can bind to nutrients and store them in
sediments (de Vries, 2002). Consequently, different plant species can influence N removal by variation
in oxidation and respond differently to seasonal changes (Maltais-Landry et al., 2009) and the oxygen
concentration of the substrate of the wetlands, can be determined by the vegetation and more
particularly the rhizosphere of the plants (Fisher et Acreman, 2004). Finally, nutrients can be liberated
when erosion of the wetland occurs or when plants die (de Vries, 2002).
1.3. Target of the research
This study is requested by CNRS (Centre National de la Recherche Scientifique) in Paris in order to
identify wetlands and their potential for denitrification and their role in eliminating nitrogen from the
Seine rivery (France). The functioning of the Seine river is studied intensively since 1989 by the
PIREN-Seine program (Programme Interdisciplinaire de Recherche sur l’Environnement de la Seine).
Closely related to this initiative, is the Seine-Aval program that contributes since 1995 to increase the
knowledge and the understanding of the physical, chemical and biological processes in the Seine.
The goal of this study is to examine the functioning of nitrate removal processes in existing wetlands
(natural or artificial) for nitrate removal in the river Seine (France) by comparing examples of wetlands
nearby great rivers and determining a number of influencing factors.
The removal of nitrates by plant uptake and regularly harvesting these plants as well as waterlogged
soil or sediment with a microbially mediated reduction of nitrate (=denitrification) are important
processes in wetlands, but this study is not focused on ‘denitrification processes’ as such.
The Seine river (France) is 780 km long and has a temperate-oceanic regime, with high flow in winter
and low flow in summer. In order to compare the nitrate removal results for the river Seine, a heavy N
load zone between Poses (198 km from Paris) and Honfleur (355 km from Paris), with a distance of
157 km, as illustrated in figure 2 has been selected. The Seine river can be divided in a tidal (estuary)
and non-tidal (from Poses to Paris) zone, with 4 main basins (upstream Seine, Marne, Oise and Eure
rivers) and a dam about 100 km downstream, at Poses forms an obstacle to tidal water movement
upstream (Cébron et al., 2003). The Seine river receives treated effluents of 10.106 inhabitants of
Paris and its suburbs, as well as nitrogen from intensive agriculture activity (Servais et al., 2007). One
of the main sources is the Archères WWTP downstream Paris.
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WETLAND ECOSYSTEMS FOR DENITRIFICATION PROCESSES IN RIVERS
Fig.2: Lower Seine river + estuaries (Cébron et al., 2003).
The zone between Poses and Caudebec has already been studied, according to table 1, that shows
total N input (Poses, Eure river and lateral point sources) and output (Caudebec) as well as
denitrification rate as result of the difference between N input and output (in 103 kg N d-1) in the river
Seine for the years 2000-2003 and differentiated between summer and annual (average) sampling
(Garnier et al., 2006).
Table 1: Total N input (from Poses, the Eure river and lateral point sources) and output (Caudebec)
and denitrification rates obtained as a difference between summer and annual (average) sampling in
the Seine river (estuary) in 103 kg N d-1.
N input annual
N output annual
Denitrif. annual
N input summer
N output summer
Denitrif. summer
2000
535
411
-124
299
248
-51
2001
537
462
-75
314
265
-49
2002
375
340
-35
171
142
-29
2003
272
243
-29
109
82
-27
More particularly, this study is focused on the zone between Caudebec and Honfleur, in the Seine
estuary, provided with a number of wetlands or ‘wet zones’, like illustrated in the following figure 3.
These zones are Grande Vasière Nord (1), characterized by intertidal mudflats, Marais du Hode (2)
with reedbeds, Marais de la Risle (4) with river meadows, Marais Vernier (5) with marshy grassland,
and Heurteauville, Grand-Mare(6) by peat (Beaumais & Laroutis, 2006).
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WETLAND ECOSYSTEMS FOR DENITRIFICATION PROCESSES IN RIVERS
Fig.3 : Wet zones Seine (Beaumais & Laroutis, 2006).
1.4. Criteria for the results
In order to evaluate the results of the nitrate removal processes in wetlands in this literature study, a
number of criteria have been determined for comparing the characteristics of the study subjects.
Firstly, this study needs a clear description of the characteristics of the types of wetlands for
comparing the results within different rivers.
Next, the nitrate removal results from different studies need to be discussed by critically comparing
the measuring methods. Herewith, a division between field and laboratory studies as well as between
the nitrate removal paths shall be made. Very often, modeling is used to determine and predict nitrate
removal.
Fourthly, for determining the impact of vegetation, a short overview will be given.
Finally, the areas of the river Seine, that form part of this study will be described.
1.4.1. Terminology ‘wetlands’
Scientists disagree about the characteristics of a wetland due to dynamic processes and their
boundaries. More than 50 definitions are used (Turner et al., 2000). A universally acceptable wetland
definition has not yet been agreed.
Generally, wetlands are defined as ‘intermediate areas between dry land and aquatic ecosystems with
several characteristics common with both’ (Hogan et al., 2000). A buffer zone is a more general term.
It is described as ‘a piece of land, often having rough or semi-natural vegetation, situated between
agricultural land and a surface water body such as a river or lake, and acting to protect that water
body from harmful impacts such as high nutrient, pesticide or sediment loadings that might otherwise
result from land use practices. It offers protection to a water body through a combination of physical,
chemical and biological processes, and some of the most beneficial processes for water quality
improvement occur optimally in wetland’ (Hogan et al., 2000).
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WETLAND ECOSYSTEMS FOR DENITRIFICATION PROCESSES IN RIVERS
Not all buffer zones are wetlands and not all wetlands act as buffer zones. Wetlands are frequently
placed to interact with pollutants. Hydrological connectivity is important for the efficient operation of
buffer zones (Maltby et al., 2005).
Broadly, Hogan and colleagues (2000) divide river marginal wetlands in those located on floodplains
and those on slopes. Floodplains are susceptible to flooding from a river or a stream, while slope units
are at more distant locations from rivers or streams. Furthermore, ditches often occur around field
margins, minor depressions, scrub or woodland. Backlands are often very wet soils, for holding overbank flooding from the river for prolonged periods and additionally provided with groundwater
discharging from foot slope seepage zones. Figure 4 gives an overview of the different locations of
wetlands along a river, according to the WFD Guidance Document no.12 (2003) (Maltby et al., 2005).
Fig.4: Positions of wetlands in the landscape (Maltby et al., 2005).
The Water Framework Directive 2000/60/EC (2003) define the following zones:
Riparian zone: is defined as land immediately adjacent to a river and includes parts, like islands and
floodplains. It further includes a variety of wetland habitats that rely on over-bank flows.
Shore zone: land immediately adjacent to a lake.
Intertidal zone: zone between high water spring and low water spring tides.
River diversion wetland: a wetland on the adjacent floodplain or behind artificial levees that receives
water by pumping or gravity flow from the main channel of a river and includes such floodplain
features as oxbow lakes, backwater swamps and other riparian wetlands (Mitsch et al., 2005)
Due to the Ramsar Convention (see introduction), a general division in wetland ecosystems is based on
natural and constructed wetlands systems (CWS). Maltby and colleagues (2005) classify wetlands,
established in artificial water bodies (AWB), as ‘constructed wetlands’ and defined as ‘a wetland which
has been and which has not been created by the direct physical alteration of an existing wetland or
water body’. Further, Heavily Modified Water Bodies and wetlands (HMWB) are ‘ones which as a result
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WETLAND ECOSYSTEMS FOR DENITRIFICATION PROCESSES IN RIVERS
of physical alterations by human activity are substantially changed in character and cannot, therefore,
meet the ‘good ecological status’ (Water Framework Directive (WFD) 2000/60/EC, 2003).
Fisher and Acreman (2004) define wetlands that show nutrient retention, those where the nutrient
loading of waters that leave the wetlands is lower than the nutrient loading entering the wetlands.
50% of the international studies concerning wetlands and nutrient loading, originate from the US,
while approximately 30% has been done in Europe (Fisher & Acreman, 2004). Our examples will be
focused on these geographical areas with similar conditions as the river Seine.
1.4.2. Determining nitrate reduction rates in wetlands
In order to measure and predict the removal of nitrates from rivers by wetlands, a number of
distinctions have to be made. First of all, different measuring methods (1) or models (2) are used.
Secondly, several ecosystems are differentiated by their vegetation (3) and the circumstances for the
research. At last, in order to compare different environmental conditions retention, accumulation and
conversion processes (4), according to 1.2, need to be mentioned.
1.4.2.1 Measuring methods
Groffman and colleagues (2006) concluded that several methods for measuring denitrification vary
greatly in different environments and at different scales. They evaluated 9 different methods. In the
following table 2, we only mention the most frequently used nitrate measuring methods based on a
number of literature citations in this study.
Table 2: Overview of 4 nitrate measuring methods for wetlands with their references to a number of
studies.
Nitrate measuring method
Short description
Reference(s)
Acetylene blockage
Inhibit reduction N2O/N2
Olentangy River (US),
Hernandez & Mitsch, 2007.
Des Plaines River (US),
Sirivedhin & Gray, 2006.
Rhine River (DE), Venterink
et al., 2003.
Great Ouse River (UK),
Machefert & Dise (2004).
15
N Tracer
Labeling 15N to NO3Mississippi river (US), Kreiling
et al., 2010.
Mincio River (IT), Pinardi et
al., 2009.
Neva estuary (FI), Gran &
Pitkänen (1999).
Po/Oglio/Mincio (IT),
Racchetti et al., (2010).
Mississippi River (US), Yu et
al., 2006.
Seine River (FR), Curie et al.,
2009.
Mass balance N fluxes
Estimating inputs/outputs
Mississippi River (US), James
et al., 2007; Kreiling et al.,
2010; Mitsch et al., 2005
Stoichiometric
Mass balance, comparing N
7 rivers Chesapeake Bay
with C and P
(US), Noe & Hupp, 2009.
Delaware river (US), Patrick
Center for Environmental
Reasearch et al., 1998
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WETLAND ECOSYSTEMS FOR DENITRIFICATION PROCESSES IN RIVERS
1.4.2.2. Models
A number of models have been developed to predict nitrogen fluxes in rivers. One of these models is
the Seneque/Riverstrahler, which makes a difference between landscape and in-stream retention of N
and further takes account of root zone processes, groundwater storage, riparian processes as well as
sewage treatment, benthic denitrification and sediment storage. Table 3 illustrates the results (in kg N
km-2 y-1) obtained by the Seneque/Riverstrahler model for the river Seine (Billen et al., 2009).
Table 3: Budget of nitrogen transfers in the river Seine, according to the Seneque/Riverstrahler model
for dry (1996) and wet (2001) conditions (in kg N km-2 y-1).
Process
Root zone
Groundwater storage
Riparian
Benthic
Sediment storage
River export
Total retained in Seine
Dry conditions
2875
360
525
135
65
1380
3960
Wet conditions
885
710
1160
105
2310
2925
Another, more simplified model is NANI (= net sum of anthropogenic nitrogen inputs) (in kg N km-2 y-1
(1 km2 = 100 ha)). Howarth and colleagues (1996) proposed the following relationship: Nflx = 0,26 x
NANI + 107 and retention = 1 – Nflx/NANI = 1 – (0,00087 x Q – 0,096) and whereby Nflx is
the flux of riverine nitrogen exported at the outlet, a fraction (0,26) of the anthropological nitrogen
input that is transferred by the river at the outlet and an independent term (107), exported by the
river and Q as precipitation or discharge.
Billen and colleagues (2009) found a total N retention in the river Seine of 4015 kg N km-2 y-1, which is
comparable to the Riverstrahler model. This is illustrated in the following table 4.
Table 4:
Billen and colleagues (2009) used the available data for the Seine, Somme and Scheldt to test the
NANI model and found, in comparison with the Riverstrahler, quite applicable results (table 3).
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WETLAND ECOSYSTEMS FOR DENITRIFICATION PROCESSES IN RIVERS
1.4.2.3. Nitrate removal
In order to compare results from studies with denitrification rates, several nitrate removal processes
need to be discussed. A difference has to be to be made between accumulation or retention and
conversion or transformation of nitrates.
Accumulation or retention means that nitrates in the wetland ecosystem will be stored and hold in the
biomass of the vegetation or absorbed in sediments or soils, in contrary to the conversion or
transformation to N2O and/or N2. Transformation, immobilization or elimination of nutrients is one the
most important functions of ecosystems (Meire et al., 1998).
Vegetation can take up nitrates of which a part will be stored in the roots and another part will be
liberated due to leaching. Only the removal of the upper part of the plants decreases the nutrient loads
in the wetlands (de Vries, 2002).
Finally, nitrates can also be stored in groundwater or aquifers.
Within the framework of this study, a broader term, namely ‘retention’ will be used. It is according to
Billen and colleagues (2009) defined as ‘immobilising reactive nitrogen by storing into a long residence
time compartment or to eliminate it by conversion into the non-reactive atmosphere form’.
1.4.2.4. Vegetation
Vegetation is an important factor for wetlands, especially for the storage of nutrients in biomass
(Hogan et al., 2000). Figure 5 shows the difference between emergent, floating and submerged plants.
Fig.5: Plants in wetlands in Dutch, from top to bottom: emergent, floating and submerged (Van Ballaer
et al., 2006).
Emergent aquatic macrophytes or helophytes are characterized with their roots in the sediment, such
as reed (Phragmites australis), broadleaf cattail (Typha latifolia), reed sweetgrass (Glyceria maxima).
Marsh or swamp vegetation comprises aquatic floating plants, divided in macrophytes with floating
roots or rooted plants, such as common duckweed (Lemna minor), water hyacinth (Eichlornia
crassipes), white waterlilly (Nympaea alba).
At last, submerged plants depend on oxygen-rich environments with their roots in the sediment, like
curly-leaved pondweed (Potamotegon crispus) or American shoreweed (Littorella uniflora).
The following table 5 shows the results from 4 studies with nitrate removal or retention and
denitrification results (in kg N ha-1 y-1) with plants for some rivers or laboratory conditions
(microcosms). For some, a comparison is made with accumulation in bare sediments or soils with
floodplains. The results of these studies illustrate that retention with vegetation do not always give the
best performance rates. It depends on the kind of vegetation and the environmental conditions.
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WETLAND ECOSYSTEMS FOR DENITRIFICATION PROCESSES IN RIVERS
Table 5: 4 studies with nitrate accumulation/retention in vegetation (in kg N ha-1 y-1).
river/lake or
microcosms
Mississippi
backwater
lake (US)
Vegetation
Retention
/denitrification
(with
vegetation)
108
Floodplains/
Sediments
(without
vegetation)
1067
Ceratophyllum
demersum
(submerged
macrophyte)
Rhine river
Flooded
55
(DE)
grasslands and
reedbeds
(emergent
macrophyte)
Mincio river
V. Spiralis
389
304
(IT)
(submerged
meadows)
18
Floating:
24529*
4905-8585*
microcosms
Lemna sp.
(duckweed)
3679-12264*
Submerged:
Elodea nuttallii
(waterweed)
* μmol N m-2 h-1 has been converted to kg N ha-1 y-1 (extrapolation to
accumulation/denitrification)
reference
Kreiling et
al., 2010
Venterink
et al.,
2003
Pinardi et
al., 2009
Veraart et
al., 2010
annual
1.4.3. The river Seine estuary or basin
For comparing a number of rivers with wetlands for nitrate removal, similar characteristics of the river
Seine have to be selected.
Downstream of Paris, the Seine river is a 7th order river (see figure 6), with a catchment of 44.000 km2
(Garnier et al., 2006) and receives, downstream Paris, a number of rivers, like Marne, Oise and Eure.
Fig.6: river order (Strahler, 1957) - http://www.marietta.edu/~biol/biomes/streams.htm (05/12/10)
Regarding the target of this study (see point 1.3), the estuarine part of the river Seine in particular
two zones with wetlands between Caudebec and Honfleur, is selected to be compared with some
international examples. These wetlands are influenced by 1) natural processes and 2) human activities
(Alard et al., 2002).
The first zone is located in the estuary nearby Honfleur and consists of ‘Marais du Hode’ and ‘Grande
Vasière Nord’, illustrated by figure 7. This zone is characterized by wetlands, connected with the Seine.
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Both marshes are characterized by improved denitrification due to subhalophilous grasslands (1) and
lesser denitrification in the reed soils (Chabrerie et al., 2001; Alard et al., 2002). They are partially
connected to the Seine via a network of ditches, a sluice (2) and openings in a dam (3) in order to
drain Seine water. Furthermore a number of creeks connect the river Seine with reed (Phragmites
australis), mudflats (4) and an artificial meander (5). Marais du Hode covers an area of 58 km2 (=
5800 ha).
1
2
5
4
3
Fig. 7 : Marais du Hode and Grande Vasière Nord http://seine-aval.fr/ (cd-rom Seine-Aval).
A second zone is located closer to Caudebec and comprises ‘Grande-Mare’, ‘Marais Vernier’ and ‘Marais
de la Risle’, shown in figures 8 and 10. Grande-Mare is a pool of the Marnier Vernier, connected with
the river Seine via the Canal St-Aubin and further with Petit-Mare (figure 9).
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WETLAND ECOSYSTEMS FOR DENITRIFICATION PROCESSES IN RIVERS
6
7
8
Fig. 8: Marais Vernier http://seine-aval.fr/ (cd-rom Seine-Aval).
A number of local reedbeds (6) is located close to the riverside and for some spots; the draining of the
river water is regulated by a valve (7).
Marais Vernier (8) is the largest remaining wetland system in the lower Seine Valley and is about 4500
ha (Frouin et al., 2006).
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WETLAND ECOSYSTEMS FOR DENITRIFICATION PROCESSES IN RIVERS
Fig.9: Grande Mare, connected with the Seine via St-Aubin Canal (Alard et al., 2000).
9
10
Fig.10: Marais de la Risle (cd-rom Seine-Aval).
350 ha of floodplains (mudflats) in the Risle valley are frequently subject to tidal flooding (Bessineton,
2007). This zone is further characterised by isolated wetlands, like ponds or pools (9,10).
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WETLAND ECOSYSTEMS FOR DENITRIFICATION PROCESSES IN RIVERS
1.5. Conditions
Besides the important ecological functions of wetlands, legal, economic and financial aspects for
realizing projects have to be taken in consideration. When examining the opportunities for
implementing or restoring wetland ecosystems for the river Seine, these aspects have to be calculated
within strategic environmental projects.
1.5.1 Legal framework
The control of nitrate inputs into the environment, including wetlands, is regulated by the EC Nitrate
Directive (91/676/ECC) (Kennedy & Murphy, 2004). An example is the aquifer of Vitoria-Gasteiz
(Spain), of which the wetlands (Zurbano wetland) have been restored through the closure of main
drainage ditches due to the 91/676/CEE European directive (Garcia-Linares et al., 2003).
In the Water Framework Directive, wetlands are not recognized as water bodies per se, but as
groundwater bodies or surface water bodies (Merot et al., 2006).
The following table 6 gives a summary of the legal framework and non-legal initiatives or projects to
assess the role of nutrients in surface waters primarily and the protection of wetlands otherwise.
Table 6: Overview legal framework or non-legal initiatives
Legal framework
Ramsar Convention
(1971)
Nitrate Directive
Second Water Law
(1992)
EVALUWET (EU
project)
Water Framework
Directive (WFD) 2000
Performance ranges
denitrification (Janssen
et al., 2005)
nutrients
>50 mg/l nitrate
Water agencies SDAGE
•
•
•
wetlands
Protection
Impact
International
91/676/ECC
France
Ecological
functions
(WEDSS)
Protection
High:
10<x<300 kg
N ha-1 y-1
Medium:
10<x<80 kg N
ha-1 y-1
Low: 5<x<15
kg N ha-1 y-1
EU
2006/60/EU Into new
France law
With respect to table 6, in this work, only the performance ranges of denitrification, according to
Janssen et al. (2005), will be used to evaluate denitrification rates.
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WETLAND ECOSYSTEMS FOR DENITRIFICATION PROCESSES IN RIVERS
1.5.2 Economical/financial
The following figure 11 shows the relationship between the ecological and societal context. Several
stakeholders calculate, due to the multiple functions of wetlands the benefits for a future strategy.
Fig.11: ecosystem services (Maltby et al., 1996; de Groot et al., 2006; Haines et al., 2006, McInnes,
2007).
Identifying stakeholders is important to define the functions and the economic value of wetlands. They
can be divided in direct extensive and intensive users, agriculture producers, water abstractors, human
settlements, indirect users, nature conservation groups and nonusers (Turner et al., 2000).
According to EVALUWET, economic analysis of a wetland is an assessment of the functions (Maltby et
al., 2005). Tuner and colleagues (2000) refer to the lack of a market for wetland functions and
therefore limit the incentive to maintain the wetland. Nevertheless, some wetlands offer some
incentives for human consumption, like potential energy by peatlands (Oquist & Svensson, 1996).
Schuyt & Brander (2004) calculated the value for a number of wetlands, based on 89 wetland
valuation case-studies (Maltby et al., 2005). Unvegetated sediments and freshwater wooded wetlands
are identified as having the highest values per unit area, due to their goods and services, like
recreation and the wood industry.
Merot and colleagues (2006) refers to the PEEW approach (potential, existing and efficient wetland).
Potential wetlands are defined as areas which retain the geomorphologic and soil features of wetlands,
but are isolated from the river due to artificial drainage and river straightening. Wetlands existing at a
given time (soil moisture ascertained) can be defined as existing wetlands. According to a given
function (runoff control, water quality, biodiversity,…) wetlands can be defined as efficient wetlands.
The foregoing is important to construct or restore wetlands in close relationship to polluted rivers.
Four kinds of human implemented wetlands have been identified by Maltby and colleagues (2005):
o Constructed wetlands
o Created wetlands
o Degraded wetlands
o Restored wetlands
Constructed wetlands are created in locations where no wetland existed before. Low cost, low energy
and require no chemical additions for the treatment of water.
Contrarily, created wetlands have the objective to simulate all aspects of natural wetlands.
Modifications of water bodies, like straightening, embanking and widening result in degradation of
riparian wetlands. They’re called degraded wetlands.
At last, repairing wetlands to their natural condition or an enhanced situation relative to their degraded
condition are called restored wetlands.
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WETLAND ECOSYSTEMS FOR DENITRIFICATION PROCESSES IN RIVERS
2. Methods
In a first step, a literature search on the internet has been executed to identify the different kinds of
wetlands and further a selection has been made to identify by a few examples of rivers, comparable to
the river Seine. These examples are assessed by comparing a number of parameters as identified in
point 1.5. Finally, the results are reflected in relation to the characteristics of the river Seine (France).
2.1 Literature search
With the objective to give 1) an overview of the different types of wetlands 2) a few examples of
denitrification results with wetlands in rivers and 3) an overview of influencing environmental factors,
in a first step, a keyword search on the internet was undertaken to identify scientific articles.
The following keywords have been used separately or combined: ‘nitrate’, ‘nitrification’,
‘denitrification’, ‘ecosystem’, ‘river’, ‘wetland’, ‘estuarine’, ‘floodplain’.
A second step consists of identifying the opportunities for the implementation of wetlands near the
river Seine (France). In this literature, a number of rivers with comparable conditions of the Seine are
selected. The articles have been selected from the following databases ‘PubMed’, ‘ScienceDirect’, ‘Wiley
Online Library’, ‘ESA Online Journals’, ‘SpringerLink’ and ‘Hydrology and Earth Systems Sciences
(HESS)’. Full-text articles are ordered in our internal library and examined. CNRS offered a number of
full-text journals and a CD-rom of the Seine.
2.2 Analysis
2.2.1 Study area
In this work, 8 different studies with river-wetland relationships are compared. Different types of
wetlands (3 in EU and 5 in the US) will be evaluated in relation to the criteria and their relevance to
the river Seine. The following references are used: country, name and river order, author(s), surface,
type and origin of wetland (see 1.4.1), climate, hydrological, location and N fluxes, duration and/or
period, measuring (see 1.4.2.1), vegetation (see 1.4.2.4).
For Europe, 3 studies: the Great Ouse River (UK) (Machefert & Dise, 2004), the Neva estuary (FI)
(Gran & Pitkänen, 1999) and the rivers Po/Mincio/Oglio (IT) (Racchetti et al., 2010) were selected.
For the US, the following 5 studies were selected: Mississippi (Davis Pond-Louisiana) (Yu et al., 2006),
7 rivers of the Chesapeake Bay watershed (Noe & Hupp, 2009), a comparison of Olentangy river
(Ohio) and Caernarvon wetland (Mississippi/Louisiana) (Mitsch et al., 2005), Delaware river estuary
(Patrick Center for Environmental Reasearch et al., 1998) and Mississippi (Finger lakes) (James
(2010)).
These rivers were primarily selected on their different types of wetlands and secondly on their
relevance for implementing in the estuary of the Seine river.
A detailed table (enclosed as annex A), gives an overview of the several characteristics of the rivers
and their wetlands. Hereafter, their relevance in relation to the estuary of the Seine river will be
shortly explained.
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WETLAND ECOSYSTEMS FOR DENITRIFICATION PROCESSES IN RIVERS
•
Estuary denitrification rates and the influence of tidal flooding are characteristics of The Neva
estuary and the estuary of the Delaware river, with an upper and lower zone. Furthermore
the impact of emergent aquatic vegetation will be examined for the latter (see figure 12)
Fig.12: Delaware (Patrick Center for Environmental Reasearch et al., 1998).
Mantua Creek is the lower zone (619 ha marsh) and Rancocas Creek forms the upper zone (793 ha
marsh).
•
•
Floodplains, their surface and position for N retention is important in the Great Ouse river
and the 7 rivers of the Chesapeake Bay watershed. The % water filled pore space (WFPS) is
also important in the former.
Connected wetlands with a river are described in diversion wetlands of the Mississippi river like
Davis Pond (figure 13), Olentangy/Caernarvon (figure 14) and the backwater of Finger
Lakes. Experimental connected and isolated wetlands are compared in the rivers Po-MincioOglio. Hydraulic connectivity, vegetation and residence time are compared.
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WETLAND ECOSYSTEMS FOR DENITRIFICATION PROCESSES IN RIVERS
Fig.13: Davis Pond, Mississippi river (Yu et al., 2006).
Davis Pond is a freshwater marsh, connected by an inflow channel with the Mississippi river and
outflow via Lake Cataouatche.
Fig.14: left: Olentangy River; right: Mississippi river, Caernarvon (Mitsch et al., 2005)
For the Olentangy river, a diversion wetland basin has been created, while for the Mississippi river, the
diversion wetland Caernarvon has been restored.
2.2.2 Evaluation and models
The nitrogen removal/retention will primarily be evaluated by using the performance ranges
denitrification scale by Janssen et al.(2005), mentioned in point 1.5.1. The results of the
denitrification/retention/removal of the several studies were converted to kg N ha-1 y-1 for comparison.
Furthermore in 6 of the 8 studies with river/wetlands, a denitrification model was used and in all
studies, denitrification rates were based on estimations.
Next, the NANI model (see point 1.4.2.2) will be used to compare the calculated ‘theoretical’ retention
and the results obtained in the studies.
The NANI model uses kg N km-2 y-1 (Howarth et al., 1996; Billen et al., 2009). In order to compare
results: μmol N m-2 h-1 need to be converted, 1 μmol N = 1.10-6 mol N = 1.10-6 x 14 g/mol =
0,000014 g = 0,000000014 kg; km2 = 1.106 m2; 1 y = 365 d = 8760 h.
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WETLAND ECOSYSTEMS FOR DENITRIFICATION PROCESSES IN RIVERS
The nitrate removal results of the different studies will be converted into kg N ha-1 y-1 or
kg N km-2 y-1.
Based on the results, environmental conditions will be discussed and evaluated in the framework of the
results found in the river Seine (see table 1: Garnier et al., 2006)
3.Results
3.1 Inventory of types of wetlands
In this study, five ways of typing the kinds of wetlands (categories) (I→V) have been identified. Each
category has a number of types and subtypes, as shown in table 7.
Table 7: Overview inventory wetlands
Category
Origin (I)
Habitats (II)
Types
-Natural (a)
Subtypes
-Inland (non-tidal)(1)
-Coastal (tidal)(2)
-Artificial (b)
-Constructed (1)
-Created (2)
-Degraded (3)
-Restored (4)
-Grass (1)
-Woodland (2)
-Scrub (3)
-Fen (4)
-Ecological (a)
-Vegetation type (1’)
-Primary production
(2’)
-C cycling (3’)
-Nutrient cycling (4’)
Landscape (III)
-floodplain (a)
-valley slopes (b)
-flat area (c)
-slope (d)
-depression (e)
-valley bottom (f)
-Unvegetated (1’’)
-Freshwater wood
(2’’)
-Salt/brackish marsh
(3’’)
-Freshwater marsh
(4’’)
-Mangrove (5’’)
-depressions
(+ditches) (1)
-backland (+ditches)
(1)
-flat upland (1)
-flat lowland (2)
-surface water (1)
-ground water (2)
-surface water (1)
-groundwater (2)
-surface water (1)
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Reference(s)
Boudreau et al., 2004
Bruland et al., 2006
Commonwealth
(2001)
Maltby et al., 2005
Hogan et al., 2000
Wheeler & Shaw,
(2000) ; Maltby et al.
2005
Gosselink & Turner
(1978) ; Maltby et al.
2005
Maltby et al., 2005 ;
Schuyt & Brander
(2004)
Hogan et al., 2000
Maltby et al., 2005
Bullock & Acreman,
2003
WETLAND ECOSYSTEMS FOR DENITRIFICATION PROCESSES IN RIVERS
-coastal (g)
-underground (h)
Hydrology (IV)
-over-bank flooding
from river (a)
-high levels of
groundwater (b)
-groundwater (2)
-surface water (1)
-groundwater (2)
-groundwater (1)
-slope (outlet contact
with river) (1)
-depression (outlet
no contact with river)
(2)
-precipitation (1’)
-evapotranspiration
(2’)
-runoff (3’)
-lateral inflow (4’)
-drainage (5’)
-over bank flow (6’)
-out flow (7’)
-pumping (8’)
-tidal inflow (9’)
-tidal outflow (10’)
-spring (11’)
-slope (outlet contact
with river) (1)
-depression (outlet
no contact with river
(2)
-groundwater
discharge (1’)
-groundwater
recharge (2’)
-groundwater
seepage (3’)
Regional (V)
(biogeochemical)
-seasonal (a)
-flooding (b)
-temperature (1)
-water table height
(2)
-rainfall (3)
-sunlight (4)
-plant activity (5)
-sediment (1)
deposition
-trace elements (2)
storage in floodplains
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Bullock & Acreman,
2003; Hogan et al.
2000
Acreman (2004);
Maltby et al., 2005
Bullock & Acreman,
2003; Hogan et al.
2000
Acreman (2004);
Maltby et al., 2005
Maltby et al., 2005
WETLAND ECOSYSTEMS FOR DENITRIFICATION PROCESSES IN RIVERS
3.2 Denitrification results and wetlands in rivers
The following table 8 gives an overview of the denitrification removal rates in the 8 studies with
wetlands (mentioned in 2.2.1) and associated with the codes of these corresponding wetland types
according table 7.
Table 8: Overview N retention/removal rates in 8 wetland/river relationships
River
Wetland
type
(3.1)
Great Ouse
(UK)
Neva estuary
(FI)
Po/Mincio/Oglio
(IT)
IIId1
Mississippi
(Davis Pond)
(US)
Chesapeake
Bay watershed
(US) – 7 rivers
Ia2
IVa1
(C)
IVa1’(I)
IVa3’(I)
IVb1’(I)
IVa8’
IIa4’’
IIIa
IIa2
Olentangy (US)
Ib2
IVa8’
Mississippi (US)
Ib4
IVa8’
Delaware river
(US)
IIa4’’
IVa9’
Mississippi
(Finger Lakes)
(US)
IIIb1
N retention/
removal/denit.
(kg N ha-1 y-1)
(s)* (m)**
Target
Important
5 (mid/top)
71,7 (bottom)
0,4 (inner)
64,2 (outer)
43-2315 (C)
2,4-283,3 (I)
N2/N2O
WFPS (>70%)
Slope/River
Sediment
147
1,5 %
N2O
emission
retention
4749 ha – 93 (s)
307 ha – 36 (m)
1707 ha – 70 (s)
2132 ha – 76 (m)
1668 ha – 44 (m)
2856 ha – 44 (m)
3674 ha – 54 (s)
1 ha - 390
(average)
NH4+
Denitr. Summer >
winter
Surface wetland
2,6 ha - 460
(average)
Lower: 1654
(Mantua Creek –
256 ha/4,3
RM***)
Upper: 28710
(Rancocas Creek
– 466 ha/15,3
RM)
500
-Water retention
time
-Temp.
Surface floodplain
Emergent aquatic
vegetation (EAV)
retention
NH4+
* (s) SPARROW watershed model ** (m) measured *** RM river miles
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WETLAND ECOSYSTEMS FOR DENITRIFICATION PROCESSES IN RIVERS
With the nitrate removal results from the 8 studies, I constructed the following graph (figure 15). One
result (Rancocas Creek) is not included in this graph, due to the extreme high value.
Denitrification results in 8 rivers with wetlands (EU/US)
kg N /ha/y
2500
2000
1500
1000
500
7,5ha
Lower
Connected
1ha
2,6ha
3800ha
Isolated
4749ha
2132ha
Bottom
1707ha
Outer estuary
1668ha
2856ha
307ha
3674ha
Top
Mid
Inner estuary
Finger Lakes
Po/Mincio/Oglio
Neva
Delaware
Great Ouse
Chesapeake
Davis Pond
Olentangy
Caernarvon
0
Fig.15: Graph with comparison of denitrification/retention rates in 8 studies (Rancocas Creek
(Delaware river) is not mentioned in this graph).
The precedent graph (fig.15) shows on the right hand side, the best nitrate removal results, observed
in the 8 studies. Connected wetlands (Racchetti et al., 2010) and the lower marsh of the river
Delaware (Patrick Center for Environmental Reasearch et al., 1998) gave the best nitrate removal
results, respectively followed by the 7,5 ha backwater of Finger Lakes (Mississippi river) (James
(2010)), diversion wetland Caernarvon (Mississippi river) and pumping water from Olentangy river
(Mitsch et al., 2005), according to the performance ranges of Janssen et al., 2005 (see point 1.6.1).
Hence, the N removal results were far above the maximal performance range, namely
300 kg N ha-1 y-1.
The Chesapeake Bay floodplains gave denitrification results between 40 and 100 kg N ha-1 y-1 which is
still in the high performance range of Janssen et al. (2005).
The inner Neva estuary showed the lowest nitrate removal results (Gran and Pitkänen, 1999).
Next, a more detailed analysis of the nitrate removal results will be illustrated.
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WETLAND ECOSYSTEMS FOR DENITRIFICATION PROCESSES IN RIVERS
Connected and isolated wetlands (cat. IV (table 7))
When comparing the nitrate removal results (fig. 15) of connected (Davis Pond, Olentangy,
Caernarvon, Finger Lakes, Po/Mincio/Oglio) and isolated (Po/Mincio/Oglio) wetlands, a more favorable
position is found for the first category.
The connected wetlands, related to the Po/Mincio/Oglio (Racchetti et al., 2010) are identified as ‘over
bank flooding with slope, connected with the outlet to the river’. The nitrate removal results are two
orders higher than with the isolated wetlands, categorized as ‘over bank flooding with precipitation and
run-off’ or as ‘high groundwater level and discharge’.
Furthermore Racchetti and colleagues (2010) found that during summertime (2007), the anoxic
sediment denitrification was higher than coupled denitrification/nitrification.
Mitsch and colleagues (2005) calculated better nitrate removal results for connected wetlands of the
Mississippi, a 9th order river (Caernarvon), than for the Olentangy river (4th order river), knowing that
the surface area of the first is at least twice the surface of the latter but with a higher N load for
Olentangy. They found respectively a retention of 55 and 35 %.
At Davis Pond (Mississippi), Yu and colleagues (2006) determined a denitrification rate in a forested
marsh, connected to Lake Cataouatche (fig.13) and calculated at a concentration of 1 mg N l-1, 147 kg
N ha-1 y-1 removal and 1,5 % N2O production. Furthermore respectively, 96 and 48 % nitrate reduction
was calculated when water passed through the marsh at 25 m3 s-1 and 78 m3 s-1.
Figure 16 illustrates the pumping of river water in the wetlands, related to Mitsch and colleagues
(2005) and Yu and colleagues (2010).
Fig.16: connected wetlands, pumping (Maltby et al., 2005)
Finally, James (2010) measured a denitrification rate of 500 kg N ha-1 y-1, with a nitrate load of 931 kg
N ha-1 y-1 and an output of 431 kg N ha-1 y-1. This results in an efficiency of 54%.
86% was covered by floating Lemna sp., 21% by N. odorata and between 46 – 65% by C. demersum.
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WETLAND ECOSYSTEMS FOR DENITRIFICATION PROCESSES IN RIVERS
Floodplains (cat. III (table 7))
Figure 17 shows a number of floodplains, according to the Great Ouse river and the Chesapeake Bay.
Fig.17: Backland depression and floodplain (Maltby et al., 2005).
Machefert and Dise (2004) found that annual denitrification was about 15 times higher near-stream (=
bottom) (Great Ouse river) than higher up the slope (= top) of the floodplains. Generally, they
concluded that for high denitrification rates, a threshold moisture of about 65 – 70% WFPS has to be
reached.
Noe and Hupp (2009) found a positive correlation between the surface area of the forested floodplains
and the nitrate accumulation rates. Figure 18 was constructed with the results of table 8 to show a
relation between surface area and nitrate removal rates.
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Nitrate removal results: Chesapeake floodplains
5000
4500
4000
surface area (ha)
3500
3000
2500
2000
1500
1000
500
0
0
10
20
30
40
50
60
70
80
90
100
nitrate removal (kg N /ha/y)
Fig 18: relationship between nitrate removal (kg N ha-1 y-1) and floodplain surface area (ha) (data
source: Noe & Hupp (2009))
Estuaries (cat. I (table 5))
Mass balance calculations indicate a denitrification rate of 50 kg N ha-1 y-1 for the Neva estuary. When
comparing inner and outer estuary, Gran and Pitkänen (1999) found respectively, 0,4 kg N ha-1 y-1
(redox 26 mV) denitrified and 44,9 kg NH4+ ha-1 y-1 outflux and 64,2 kg N ha-1 y-1 (redox 457 mV)
denitrified and 5,4 NH4+ ha-1 y-1 outflux.
Very ‘important’ denitrification results were found in the upper zone of the Delaware river, the
Rancocas Creek. 20,4 mg N g-1 dw has been measured in low marsh (Nuphar advena), and 12 mg N g1
dw in high marsh (Zizania aquatica, Typha sp., Phragmites australis), while similar results were
measured in the lower zone (Mantua Creek).
Generally, Patrick Center for Environmental Reasearch et al., 1998 measured between 0,5 and 131 kg
N RM-1 d-1 (av.65 kg N RM-1 d-1) incorporated in biomass. Respectively for Rancocas, 51 kg N ha-1 y-1
(466 ha = 15,3 RM) and for Mantua, 91 kg N ha-1 y-1 (256 ha = 4,3 RM).
NANI model
I calculated the river nitrate flux (Nflx) (see 1.4.2.2) by using the NANI model in order to compare the
nitrate removal results for the rivers Olentangy and Mississippi (Mitsch et al., 2005) and for Finger
Lakes (James (2010)). I only used the nitrate input loads, cited by the authors, which is not
necessarily the total anthropological input.
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WETLAND ECOSYSTEMS FOR DENITRIFICATION PROCESSES IN RIVERS
For the Olentangy river, Nflx = 0,26 X 108000 + 107 = 28187 kg N km-2 y-1 is calculated with the
NANI model, while Mitsch et al., 2005 calculated a river output of 70200 kg N km-2 y-1. For the
Mississippi river, Nflx = 0,26 X 84000 + 107 = 21947 kg N km-2 y-1 but a river output of 37800 kg N
km-2 y-1 determined by Mitsch et al., 2005.
For Finger Lakes, with the NANI model, Nflx = 0,26 X 93075 + 107 = 24307 kg N km-2 y-1 and James
(2010) calculated a river output of 43070 kg N km-2 y-1.
The Nflx results obtained with the NANI model were much lower than the observed results by the
authors, which means that the calculated results by the NANI model overestimate the retention rates.
3.3 Influencing factors for denitrification:
Before listing the potential factors that could influence nitrate removal, regarding this literature study,
I compare the nitrate budget results from Kreiling and colleagues (2010) for Third Lake (Finger Lakes,
US) with the nitrate budgets of the Riverstrahler model for the Seine River (Billen et al., 2009). The
following graphs (fig.19) show that for Finger Lakes (left graph), the plant uptake (=root zone) and
the denitrification/nitrification coupling are of minor importance for nitrate removal in relation to
discharge and sediment uptake, compared to the right graph, based on calculations by the
Riverstrahler model for the Seine.
Budget of nitrogen transfers in the Seine (FR) according to the Seneque/Riverstrahler model
Nitrate mass balance estimation in a Mississippi backwater lake: Finger Lakes (US)
17%
2%
39%
45%
53%
27%
10%
SEDIMENT
1%
6%
ROOT ZONE
NITR/DENITR
DISCHARGE
AQUIFERS
IN STREAM
ROOT ZONE
SEDIMENT
DISCHARGE
Fig.19: left graph shows the nitrate mass balance for Finger Lakes (US), a Mississippi backwater lake
(data source: Kreiling et al., 2010); in this model, aquifers and in stream nitrate removal are not
explicitly considered; right graph illustrates the Seneque/Riverstrahler model for the Seine (FR) (data
source: Billen et al., 2009); nitrification-denitrification coupling is not independently considered.
a) climate, season and temperature:
Mitsch and colleagues (2005) explain the higher denitrification rate of the Mississippi river compared to
the Olentangy river due to the subtropical climate with higher temperatures and a longer growing
season for vegetation. Racchetti and colleagues (2010) also found significantly higher denitrification
rates in summer than in winter. Yu and colleagues (2006) mentioned also the importance of the
temperature in relation to the biological reaction rate.
Sirivedhin & Gray (2006) found a higher sediment denitrification potential at 25°C than at 4°C, but in
relation to a higher nitrate concentration. In general, denitrification rates increase with higher
temperatures.
b) wetland surface area:
Three determinants have been identified, namely, the distance from the river and the soil volumetric
water content (Machefert and Dise, 2004), as well as the floodplain surface (see fig.18) (Noe and
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WETLAND ECOSYSTEMS FOR DENITRIFICATION PROCESSES IN RIVERS
Hupp, 2009). Furthermore, they found that for larger riverine loads a smaller proportion of that load
was retained on the floodplains. Mitsch and colleagues (2005) indicated that a removal of 40% of total
nitrate, requires the creation or restoration of 22000 km2. Mainly, a larger wetland surface area
enhances nitrate retention, but is dependent of the N load.
c) vegetation:
James (2010) suggested that biotic uptake (C. demersum and Lemna sp.) was responsible for
retaining nitrate, because denitrification was only responsible for 57 %.
Fig.20: Comparison of denitrification in different vegetation conditions (Veraart et al., 2010)
Another study showed that the highest denitrification rates were observed under a closed mat of
floating macrophytes (upper graph in fig.20), with low oxygen conditions and nitrate removal was
dominated by plant uptake (Veraart et al., 2010). Nitrate retention can thus be directly related to
vegetation by uptake or indirectly by the creation of favorable conditions by the plants. Vegetation will
preferentially assimilate ammonium over nitrate (Kreiling et al., 2010).
d) water velocity and retention time:
A lower water velocity (25 m3 s-1) in the wetland resulted in a 96 % nitrate reduction, while at
78 m3 s-1 only a 48 % reduction was measured. For a retention time of 1 to 5 days in the wetlands,
respectively 42, 68, 82, 90 and 95 % of the nitrate was removed (Yu et al., 2006).
Venterink et al. (2003) suggested that a low N retention in the deep and fast flowing River Rhine may
be explained by a very low contact time between river nitrate and sediment.
Sirivedhin & Gray (2006) observed a greater nitrate removal in a shallow, young and well-mixed
constructed wetland. Apparently, a strong correlation between a lower water velocity and a longer
water retention time with increasing N retention capacity was found.
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e) hydraulic connectivity:
Racchetti and colleagues (2010) showed that within connective wetlands, denitrification was mainly
determined by nitrate diffusing into anoxic sediments from the water column.
Nitrate influx to sediment were greater in anoxic versus oxic conditions (James (2010)).
Fig.21: Comparison of denitrification between sediment and periphyton (Venterink et al., 2003)
Denitrification reached maximal rates in oxygen-poor flooded sediment’, but no great differences were
found between reedbeds and grasslands, according to fig.21 (Venterink et al., 2003). Improved
denitrification is related to hydraulic connectivity by oxygen-poor flooded sediments.
f) redox potential:
In the outer Neva estuary, higher denitrification rates were determined at higher redox potentials and
lower ammonium fluxes (Gran & Pitkänen, 1999). Based on the results of Gran & Pitkänen (1999), a
relation can be observed between the location in the estuary (inner/outer), the redox potential and the
denitrification rate (fig. 22).
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WETLAND ECOSYSTEMS FOR DENITRIFICATION PROCESSES IN RIVERS
Ratio redox and denitrification in the inner and outer Neva estuary
600
500
Denitrification (mg N m-2 d-1)
400
300
inner
outer
Linear (outer)
Linear (inner )
200
100
0
0
2
4
6
8
10
12
14
-100
-200
redox (mV)
Fig.22: Comparison of redox and denitrification rate in the inner (blue line) and outer (pink line) Neva
estuary (FI) (data source: Gran & Pitkänen, 1999).
NH4+ efflux from the sediment was highest in anoxic conditions (18469 kg km-2 y-1), compared to oxic
conditions (9746 kg km-2 y-1) in Finger Lakes (James (2010)). This is probably due to a nitrification
process in oxic sediments.
Brion and colleagues (2000) observed decreasing ammonium concentrations in the Seine estuary and
increasing nitrate concentration. A higher redox potential and lower ammonium concentrations
improve denitrification in the estuary due to higher nitrate availability, which is confirmed by fig.22.
4.Discussion en Conclusions
4.1 Conclusions
In this work, 8 studies with river-wetland relationships, 3 wetland types were identified with a high
nitrate removal performance, in decreasing performance order: connected, floodplains and estuary
wetlands. More specifically, according to table 7, these wetlands are identified as over bank flooding
with a slope (IVa1), freshwater marsh (IIa4’’) with tidal inflow (IVa9’), floodplain depressions (IIIb1),
woodland (IIa2) and coastal wetlands with tidal influence (Ia2).
In addition to that, nitrate removal or denitrification rates in these 8 studies were estimated by using
models and based on extrapolations. The NANI model that I used for 2 calculations showed higher
retention rates than observed in the studies.
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Within this study, a number of factors that could influence nitrate removal have been cited by a
number of authors. I scheduled (fig.23) these factors in relation to the type of wetland.
created
connected
restored
velocity
vegetation
retention
time
wetlands
surface area
floodplains
estuary
distance
anoxic
redox
oxic
Fig.23: Schematic overview of the wetland types (green) identified in this study and their
characteristics (blue), oxygen status (orange).
The connected wetlands in this study showed a high nitrate removal performance, which was less for
isolated wetlands. Related to the nitrate removal performance of these connected wetlands are water
velocity, contact time and retention. Lower water velocity in the wetland enhances the nitrate
reduction (Yu et al., 2006). A very low contact time between the river nitrate and the sediment could
reduce the nitrate retention (Venterink et al., 2003). Vegetation and biotic uptake are able to retain
nitrate (James, 2010; Veraart et al., 2010). However, Venterink and colleagues (2003) found no great
differences in nitrate removal between reedbeds and grasslands.
High nitrate removal has also been observed in floodplains, in relation to a relative great surface area
and close to the riverside (Noe and Hupp, 2009). They further observed that the larger the nitrate
load, the smaller the retention on the floodplain.
Oxygen poor flooded sediment showed the best denitrification rate (Venterink et al., 2003). This also
illustrated in the model by Kreiling et al., 2010 in Finger Lakes (fig.19) with a major impact of the
sediment nitrate removal, compared to biotic and nitrification-denitrification coupling.
In estuaries, these zones with a higher redox potential and lower ammonium fluxes showed an
important denitrification performance (Gran & Pitkänen, 1999). Low marsh (Nuphar advena) showed
better denitrification results than high marsh (Zizania aquatica, Typha sp., Phragmites australis)
(Patrick Center for Environmental Reasearch et al., 1998).
The Seine estuary from Caudebec to Honfleur has already a number of created or restored wetlands.
Nevertheless, I did not find nitrate removal performance results for this zone.
Brion and colleagues (2000) mentioned an increasing nitrate concentration and a decreasing
ammonium concentration in the Seine estuary up to Caudebec. This could indicate that in this oxygen
rich zone, ammonium conversion has been executed by nitrifying bacteria. Otherwise, in estuary
sediments, vegetation preferentially assimilates NH4+ over NO3- (Kreiling et al., 2010). Furthermore,
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WETLAND ECOSYSTEMS FOR DENITRIFICATION PROCESSES IN RIVERS
temperature plays a role in the performance of biological processes and could so influence nitrate
removal.
4.2 Discussion
Referring to the nitrogen pathways (fig.1) denitrification and nitrate retention/removal are influenced
by a number of processes like nitrification/denitrification coupling, microbial transformation, sediment
absorption and plant uptake.
Arrigoni and colleagues (2008) stated that observed nitrate removal may be due to multiple processes
and different controlling factors. Plant uptake, microbial immobilization and denitrification are all
functioning at high rates in tidal wetlands. Sirivedhin & Gray (2006) also confirm that a variety of
factors influence the process of denitrification but that it is impossible to provide a rank order of
importance.
When comparing the different nitrate removal processes in relation to the characteristics of the
wetlands that have been identified in this study and the wetlands in the estuary of the Seine River
(from Caudebec to Honfleur), a number of similarities could be found. Eventhough, the relationship
between the different factors is difficult to determine, an attempt to indicate important factor will be
given below.
Denitrification is determined by available nitrate, reduced oxygen conditions and electron donors. This
means a sufficiently nitrate load, anoxic or suboxic conditions and the availability of organic matter.
Furthermore, environmental conditions (anaerobic and aerobic) determine the coupling
denitrification/nitrification. Like already stated in the beginning of this study, Maltais-Landry and
colleagues (2009) mentioned that, due to limited oxygen diffusion in constructed wetlands, nitrification
is often the limiting step in the denitrification process.
In order to improve the balance between oxic and anoxic conditions, a number of environmental
conditions have been adapted, like in restored or created wetlands.
In the Des Plaines River Wetlands Demonstration Project (DPRWDP)(US), shallow and young, wellmixed constructed wetlands showed consistently greater nitrate removal than mature, less-mixed
wetlands (Sirivedhin & Gray, 2006). This is also mentioned by Fisher and Acreman (2004), referring in
their study to the percentage of nutrient removal, that is inversely related to loading and the number
of years that the wetland received anthropogenic nutrient loading.
This means that a good contact zone between anoxic and oxic conditions stimulates the nitrate
removal process, but also that the nitrate removal performance of older wetlands decreases with the
years. In connected wetlands, low water velocity and retention increase the contact zone. River water
can be drained into ponds or marshes in a controlled way by regulation means, like pumps, sluices or
dams. Craig and colleagues (2008) found that low velocity environments and increasing hydraulic
retention allow greater contact between the water and the benthos and can so improve N removal.
Grande-Mare, a pool of Marnier Vernier is connected to the Seine by a 5 km long canal, St-Aubin.
Furthermore, in Marais du Hode and Grande Vasière Nord, several grasslands are connected with the
river Seine by a network of ditches and controlled by draining regulating systems (sluice, opening in
dam). Marais de la Risle has a number of disconnected wetlands, like pools and ponds.
Small streams (1st to 3rd order) with considerable loads during low or moderate flows, could offer
opportunities for N removal (Craig et al., 2008). In the deep and fast flowing river Rhine, a very low N
retention was found, which could be explained by a very low contact-time between river nitrate and
sediment (Venterink et al., 2003).
On the contrary, Montreuil and colleagues (2010) found that the reduction in nitrate fluxes was greater
in catchments of high stream orders and that the proportion of fluxes depleted during transfer with a
increasing distance. In the Scorff River basin (FR), a 6th order river, Montreuil and colleagues (2010)
cited a maximum of 91% reduction (= 16,79 kg N ha-1 y-1) at low water with an increasing residence
time. The largest part of the retention processes occurs in the downstream sectors of the drainage
network, with stream order ≥ 5 (Billen et al., 2007).
Fisher & Acreman (2004) further mentioned that the oxygen concentration of sediments in wetlands
can be determined by vegetation, more particularly in the root zone. Emergent macrophytes in low
marsh zones, that were permanently flooded, favored the organic matter supply for denitrification and
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WETLAND ECOSYSTEMS FOR DENITRIFICATION PROCESSES IN RIVERS
they stimulate water movement into the soil, which facilitates the diffusion of nitrates to the anoxic
zones for denitrification (Hernandez & Mitsch, 2007). On the other hand, submerged vegetation in tidal
channels can alter oxygen concentrations (Arrigoni et al., 2008). According to the results of the studies
with the 8 rivers and wetlands, seasonal differences and temperature were mentioned as important
factors for denitrification rates. Denitrification activity slows down significantly below 4° C (Sirivedhin &
Gray, 2006). Moreover, during autumn an increase of soluble nutrient loading has been cited in many
studies due to a lower plant uptake and so stimulates flushing and diluting of nutrient outputs.
Contrarily, vegetation decay can increase microbial activity, which could promote denitrification and
mineralization of nutrients in the wetlands (Fisher & Acreman, 2004). Yu and colleagues (2006) refer
to the importance of the temperature in biological reaction rates.
Regarding fig.19, Kreiling and colleagues (2010) showed that denitrification/nitrification coupling and
plant uptake was of minor importance for nitrate removal, which is confirmed by Venterink et al. 2003,
according to fig.21. However, in this study low marsh (Nuphar advena) showed better nitrate removal
results than high marsh (Zizania aquatica, Typha sp., Phragmites australis) (Patrick Center for
Environmental Reasearch et al., 1998). Marais du Hode, Grande Vasière Nord and Marais Vernier are
provided with reedbeds (Phragmites australis) and subhalophilus grasslands. Chabrerie and colleagues
(2001) found in Marais du Hode, three different zones, namely grasslands, mesophilous grassland and
reedbeds. They mentioned that denitrification rate was highest in the second zone. In the first zone,
low denitrification rates were explained by the soil properties, low organic C content, due to tillage and
low availability of oxygen.
Sabater and colleagues (2003) did not find clear N removal patterns between herbaceous and forested
zones nor significant seasonal differences in 14 riparian sites throughout Europe.
The presence of NH4+ for the transformation to NO3- is another condition. Brion and colleagues (2000)
examined nitrate and ammonium flux in the Seine from Paris to the estuary (Caudebec) for the period
July and September 1995 and July 1996 and found increasing nitrate and decreasing ammonium
concentrations to the estuary.
Gran and Pitkänen (1999) found that the zones in the estuary with a higher redox potential and lower
ammonium fluxes, showed an important denitrification performance.
Anammox (anaerobic ammonium oxidation) could be a plausible reason for this phenomenon. KoopJakobsen & Giblin (2009) found that the highest relative importance of anammox was found in
sediments of freshwater marshes, namely only 3%, while it was less important in saline marsh
sediments. They further found that the decrease of anammox was due denitrification.
However, aerated environments showed the best denitrification performance, according to MaltaisLandry et al. (2009) and Curie et al. (2009).
Floodplains and mudflats showed great nitrate retention close to the river and when the surface area
was large enough (Noe and Hupp, 2009). Racchetti and colleagues (2010) refer to Verhoeven et al.
(2006), who stated that wetlands have to account for at least 10% of the watershed area in order to
contribute significantly to water quality improvement.
Maltby and colleagues (2005) mentioned that when pore spaces within soil are filled with water, the
diffusion of oxygen is greatly reduced, which gives a soil with low oxygen content and low redox
potential. Venterink et al. (2003) found that oxygen poor flooded sediments showed the best
denitrification rate.
In Marais du Hode, an artificial meander has been created to reduce the water velocity and to improve
the contact between the river water and the sediment (fig.7 (5)).
The formation of N2O as greenhouse gas is related to the nitrate conversion process.
In the Great Ouse river (UK), the highest N2O concentrations were found at the upper slope, further
away from the river and so related to the soil moisture of the water-filled pore space (WFPS). With soil
volumetric water contents below 35%, gaseous N was emitted as 50% N2O and 50% N2 at the lower,
intermediate and upper zones, while for soil water contents above 35%, the near-stream zone showed
20% N2O, and 80% N2, the intermediate 10% N2O and 90% N2 and the upper zone 40% N2O and 60%
N2 (Machefert & Dise, 2004). Furthermore, Hernandez and Mitsch (2007) concluded that N2O
production was higher in high marsh and edge zones than in low marsh and increased in cold seasons
(autumn and winter) in a created riverine wetland (Olentangy River). Permanently flooded low marsh
zones had lower N2O ratios of emissions than less frequently flooded high marsh and edge zones. This
could be explained by more aerobic conditions in high marsh. However, Garnier and colleagues (2006)
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concluded that the N2O emissions in the freshwater Seine estuary was less than 0,1% of both
denitrification and nitrification fluxes.
Concerning the methodology of this study the NANI model and the performance ranges denitrification
have been used. The NANI model (Howarth et al., 1996; Billen et al., 2009) for Olentangy/Mississippi
(Mitsch et al., 2005) and Finger Lakes (James (2010)), showed a much higher retention efficiency than
measured or calculated. These differences can be explained by the simplicity of the model and
probably by the difference between the N input loads measured and the total anthropological N
sources. Furthermore, I used the denitrification performance ranges by Janssen et al., 2005 to
interpret the nitrate removal performance in the wetlands. I did not find another reference point in
order to compare these ranges.
Most of the rivers with wetlands that have been analyzed in this study were only examined for short
periods by the authors. Consequently, it is difficult to interpret the results over a long time.
Furthermore, estimations were made by models.
The wetlands of the Seine estuary (Marais du Hode, Grande Vasière Nord, Grande-Mare, Marais
Vernier, Marais de la Risle) are mainly characterized by tidal flooding with seawater, brackish and
freshwater over grasslands, reedbeds and mudflats and a connection with a backwater.
According to the results of my study with the list of different types of wetlands (table 7), the foregoing
wetlands can be indicated as Ib2 (artificial-created), IIa1 (grasslands), IIIa1 (floodplain-depression),
IVa5’ (hydrology-drainage), IVa9’ (hydrology-tidal inflow) and Vb (biogeochemical-flooding).
Table 9: Comparison wetland characteristics Seine estuary and the opportunities for implementation
Wetland
characteristics
1. Connected
wetlands
Created
Velocity
Vegetation
2. Floodplains
Surface area
Distance
3. Estuary
Redox
‘What is’
‘Could be’
Location in Seine
estuary
ditches, sluice,
openings in dam
valve
drainage
shallow, young and
well-mixed
subhalophilous
grasslands
reedbeds;
peat
low marsh;
floating macrophytes;
woodland
Marais du Hode
Grande Vasière Nord
Marais Vernier
Marais du Hode
Grande Vasière Nord
Marais du Hode
Grande Vasière Nord
Marais Vernier
mudflats;
artificial meander
close to riverbank
controlled pumping
slope; low marsh
tidal flooding;
isolated ponds and
pools
turbulence
stimulation;
vegetation
Marais du Hode
Grande Vasière Nord
Marais de la Risle
Marais du Hode
Grande Vasière Nord
Marais de la Risle
Marais Vernier
I compare the current situation (strengths and weaknesses) and the potential future risks and
opportunities for the above mentioned wetlands with the aid of table 9.
Strengths:
Marais du Hode, Grande Vasière Nord and Marais Vernier are all characterized by wetlands that are
connected with the Seine River by ditches or creeks. The draining is regulated by a sluice or openings
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WETLAND ECOSYSTEMS FOR DENITRIFICATION PROCESSES IN RIVERS
in a dam. This enables to control the water velocity and retention into the wetlands. The river Seine is
in Marais Vernier connected to Grand-Mare, a pool for delivering water in dry periods. This is
comparable with the connected wetlands (Mitsch and colleagues (2005), Racchetti et al., 2010, Yu et
al., 2006).
In Marais du Hode, the highest denitrification rate is found in dry subhalophilous grasslands and the
lowest rate in reedbeds (Alard et al. 2002).
Marais du Hode is further provided with mudflats and an artificial meander. In 2002, a surface of 47 ha
of Marais de la Risle valley was reconnected within the Seine estuary, as well as the grasslands of
Marais du Hode in 2004 and 2005. The cost was respectively € 6800 and € 99000 (Bessinton, 2007).
In the framework of a cost-benefit approach, it is important to have a reference fee for future
developments.
The French water management system encouraged public-private partnerships by setting up contracts.
The role of the Seine-Normandy Water Agency (AESN, Agence de l’Eau Seine-Normandie) at the river
basin level is to ensure a balance between the water quality and the civil needs. The AESN has
encouraged efforts to restore wetlands by awarding prizes. They invested 1.6 million Euro in wetlands
in 2000 (AESN, 2002).
Weaknesses:
Marais de la Risle is characterized by a number of isolated wetlands. In this study, isolated wetlands
showed a lesser nitrate removal performance than connected wetlands. Marais du Hode and Marais
Vernier are provided with reedbeds. Higher denitrification was found in grasslands than in reedbeds
(Chabrerie et al., 2001; Alard et al., 2002). Unfortunately, no comparable data of nitrate removal for
these zones of the Seine estuary, in relation to the effects of floodplains or mudflats, has been found in
the literature.
Opportunities:
The Seine is fed by the Oise, Marne and Eure rivers. These smaller streams with a lower stream order
(< 7th order river) could be examined for implementing a wetland strategy, according to Craig and
colleagues (2008). This has already been done with valley bottom wetlands in agricultural landscapes,
located in headwater catchments (1st to 3rd order streams) (Merot et al., 2006). A combination of
grasslands (wet meadows) as wider buffer zones (10 - 50 m) and forest communities as buffer strips
(5 – 10 m) on stream banks can be useful (Mander et al., 2005).
Nuphar advena, low vegetation marsh, gave favorable denitrification results in a tidal freshwater
estuarine of the Delaware River (Patrick Center for Environmental Reasearch et al., 1998).
Craig and colleagues (2008) mentioned that low velocity environments and increasing hydraulic
retention can improve nitrate removal processes. They also refer to the creation of denitrification
hotspots by providing energy for denitrifying energy, stimulation anoxia and slowing water velocities
for improving the contact time with denitrifiers. This could be tested in the creeks or ditches.
Societal importance indicates that unvegetated sediment wetlands are found to have the highest
economical values ($ 374 ha-1 y-1), because of their storm protection, recreation and commercial
impact. Only freshwater marshes, present in riparian floodplains provide fresh drinking, watering or
irrigation water (Malty et al., 2005).
The PEEW approach of Merot and colleagues (2006), already mentioned in point 1.5.2 could help to
determine ‘efficient’ wetlands with the goal to focus investments on ‘efficient wetlands’. For wetland
conservation decisions, cost-benefit analysis (CBA) based on economic efficiency criteria offers a
method for decision makers. This requests improved information that can be obtained from integrated
ecological-economic models (Turner et al., 2000).
Threats:
Due to the complexity and the uncertainty of the relationship between the different factors that could
influence denitrification, precaution has to be taken in account when implementing great changes.
Furthermore, interactions that are made upstream could have consequences lower in the river estuary.
Lack of a market for wetland functions could limit the incentives for stakeholders to improve wetland
quality. Maltby and colleagues (2005) concluded that unvegetated sediments and wooded wetlands
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WETLAND ECOSYSTEMS FOR DENITRIFICATION PROCESSES IN RIVERS
showed the highest value per unit area. Dependent from the perspective of the different stakeholders,
opposite visions could lead to a standby situation. Chabrerie and colleagues (2001) found in Marais du
Hode a lower denitrification rate in grasslands due to tillage. Contrary, nitrates could be removed by
harvesting the vegetation biomass.
5.Suggestions
Most of the research to denitrification effects in the Seine River is limited to the region between Poses
and the estuary at Caudebec. In order to determine the nitrate removal performance of the existing
wetlands from Caudebec to Honfleur, multiple studies for each kind of wetland should be set-up.
Furthermore, a number of studies should be focused on lower order rivers like the Marne, Oise and
Eure in order to examine the impact of the nitrate removal effects downstream in the estuary of the
Seine River. The collected data can be used to estimate or extrapolate nitrate removal by a model,
preferably the Riverstrahler model.
Besides the denitrification performance range according to Janssen et al., 2005, another reference
framework would be useful to compare nitrate removal results. This could be discussed in a legal
context.
A better understanding of the relation between the different factors that influence nitrification and
denitrification, as well as the importance of the interaction between the processes is the next
challenge. This insight in the interaction could help to influence a number of parameters. One example
is to study the influence of harvesting vegetation in relation to the removal of the carbon source for
denitrification.
Due to the importance of the greenhouse gas effect of N2O, the influence of the denitrification or
dissimilatory nitrate reduction in the production of N2O in the estuary of the Seine is not clear (Garnier
et al., 2006). Further research of N2O production in the estuarine wetlands would be useful in order to
estimate the effects. This enables to adapt the characteristics of the wetlands, like the vegetation or
the floodplain geometry.
In order to determine efficient wetlands, according to the PEEW approach (Merot et al., 2006), a
number of indicators have to be defined. Directly related to this approach, a cost-benefit analysis
needs to be set up, indicating the different services with the quality parameters of the river water.
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WETLAND ECOSYSTEMS FOR DENITRIFICATION PROCESSES IN RIVERS
Annex A: Description 8 rivers and their
wetlands
VMAB2010nj-M105-DENITRIFICATIE-PWADeconinck-20110526.doc
Pagina 47 van 50
WETLAND ECOSYSTEMS FOR DENITRIFICATION PROCESSES IN RIVERS
VMAB2010nj-M105-DENITRIFICATIE-PWADeconinck-20110526.doc
Pagina 48 van 50
faculteit Natuurwetenschappen/School of Science
Open Universiteit Nederland
Postbus 2960
6401 DL Heerlen, NL
tel. +31 45 576 2877
[email protected]
www.ou.nl/nw
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