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Thèse de doctorat/ PhD Thesis Citation APA:

Rebreanu, L. (2009). Study of the Si biogeochemical cycle in the sediments of the Scheldt continuum, Belgium/The Netherlands (Unpublished doctoral dissertation). Université libre de Bruxelles, Faculté des Sciences – Sciences de la Terre et de l'Environnement, Bruxelles.

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FACULTÉ DES SCIENCES

D

epartement des

S

ciences de la

T

erre et de l

'E

nvironnement

L

aboratoire d

'O

ceanographie

C

himique et

G

eochimie des

E

aux

Study of the Si Biogeochemical Cycle in the Sédiments of the Scheldt

Continuum (Belgium/The Netherlands)

Promoteur : Prof. Lei CHOU

en vue de l'obtention du grade de Docteur en Sciences

Thèse présentée par Laura REBREANU

Université L bre de Bruxelles

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Mes premiers remerciements s'adressent au Professeur Lei Chou, qui m'a accueillie dans son laboratoire et m'a permis de réaliser ce rêve un peu fou: le doctorat. Ses conseils précieux et son support m'ont guidée tout au long de ces années.

Les débuts de cette thèse furent accompagnés par feu le Professeur Roland Wollast dont la passion pour la recherche fut extrêmement stimulante; ses conseils, ses encouragements, et son soutien furent d'une grande importance dans le développement de ce projet.

Je souhaite également remercier chaleureusement Jean-Pierre Vanderborght pour son aide précieuse sur le plan scientifique et technique, ses nombreux commentaires et suggestions.

Beaucoup de résultats dans ce travail n'auraient pas pu être obtenus sans les divers ingénieux appareils et systèmes réalisés par Didier Bajura.

Martine Leermakers m'a initié à l'échantillonnage des sédiments et m'a offert beaucoup de conseils concernant le traitement des échantillons.

Je remercie de tout cœur également Nathalie Roevros, qui m'a si gentiment accueillie dans son "bureau" et a répondu à mes très nombreuses questions durant toutes ces années.

Jérôme Petit et Aurélien Taillez m'ont également beaucoup aidé lors des diverses campagnes de mesures, non seulement au niveau de l'échantillonnage, mais également par leur humour à tout épreuve, que j'ai pu aussi apprécier en dehors du travail.

Je remercie tous mes collègues au sein de ce laboratoire et d'ailleurs, présents et passés, pour avoir transformé le travail en un plaisir de tous les jours.

Mes amis en dehors de l'université ont toujours accueilli mon enthousiasme à l'égard de la thèse avec le sourire, m'ont encouragé dans les moments difficiles et c'est grâce à eux, entre autres, que ces années m'ont paru passer si vite.

Last but not least, rien de toute cette aventure n'aurait été possible sans le soutien inconditionnel et les encouragements constants de mes merveilleux parents et sœur. Je ne saurais jamais leur exprimer ma reconnaissance pour

m'avoir toujours épaulée, dans les hauts et dans les bas.

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Le continuum de l'Escaut (estuaire - zone côtière) est une des zones ies plus polluées d'Europe. Les apports excessifs de N et P ont modifié les ratios des différents nutriments, entraînant un changement dans la spéciation du phytoplancton qui affecte toute la chaîne trophique marine dans la zone côtière belge. Le phénomène est aggravé par le faible taux de recyclage de la silice dissoute (DSi) dans la colonne d'eau. Le recyclage et la rétention de la silice restent encore peu connus et les données existantes sur la silice biogénique (BSi02) dans les sédiments du continuum sont très peu nombreuses et parcellaires. Le but général de ce travail est donc de quantifier la BSi02 dans les sédiments, ainsi que son taux de recyclage / rétention.

Le coefficient de diffusion moléculaire de la DSi a été déterminé pour des valeurs de température variant entre 2 et 30°C et deux salinités différentes, à savoir 0 et 36. A partir de ces résultats, une relation empirique reliant le coefficient de diffusion à la température et à la viscosité de la solution a été établie.

La BSI02 dans la phase particulaire et la DSi dans l'eau interstitielle des sédiments ont été estimées de façon saisonnière durant deux années (2004 et 2005). La BSi02 a été également déterminée dans les sédiments de surface du continuum de l'Escaut échantillonnés en 2004. Les concentrations en BSi02 et DSi apparaissent être essentiellement liées à la granulométrie du sédiment. Les concentrations en BSi02 varient de moins de 0.1% à 4.0%. Les concentrations en DSi varient quant à elles entre 2 et 744 pmol L ^

Les flux de DSi à l'interface eau - sédiments ont été estimés sur base des expériences d'incubations de carottes de sédiments et à partir des profils verticaux de DSi dans l'eau interstitielle. Le flux expérimental de DSi vaut en moyenne 0.30 ± 0.22 mmol m'^ d'^ dans l'estuaire et 1.49 ± 1.56 mmol m'^ d'^

dans la zone côtière. Le flux estuarien calculé est de 0.20 ± 0.15 mmol m'^ d'^

et le flux marin calculé est de 0.02 ± 0.01 mmol m'^ d ^ Tout comme pour les concentrations en BSi02 et en DSi dans les sédiments, les flux varient fortement entre les différents sites d'échantillonnage et les différentes saisons.

Des expériences de dissolution des sédiments ont été également réalisées et modélisées. Les constantes de vitesse varient de 1.4 x 10'^ à 14 x 10'^ h'^. La solubilité apparente est faible et varie entre 5 et 185 pmol L"^ Les résultats suggèrent la présence de réactions secondaires de précipitation.

L'existence de réactions secondaires est également mise en évidence par les concentrations de différents ions dans l'eau interstitielle.

Le taux de recyclage de la BSi02 varie entre 8 et 92%. Ceci correspond à une rétention moyenne de la BSi02 dans les sédiments de l'estuaire de l'Escaut supérieure à 60%, ce qui souligne l'efficacité du filtre estuarien par rapport à la silice. En revanche, le recyclage de BSi02 dans la zone côtière apparaît comme très intensif, avec des valeurs souvent supérieures à 40%. Ces résultats montrent l'importance d'inclure les estuaires dans le calcul des budgets globaux de Si.

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Remerciements i

Résumé ii

Chapter 1 General introduction 1

1.1 The biogeochemical Silicon cycle 1

1.1.1 The terrestrial Silicon cycle 3

1.1.2 The aquatic Silicon cycle 6

1.1.2.1 Diatonns 6

1.1.2.2 Recycling of biogenic silica in the water column 8

1.1.2.3 The marine Silicon cycle 12

1.1.2.4 Biogenic silica transformation along the land-ocean continuum 13 1.1.2.5 Anthropogenic perturbations of the Si cycle 15

1.2 The Scheldt estuary 16

1.2.1 Estuaries 16

1.2.2 General characteristics 18

1.2.3 Morphological and hydrological characteristics 18

1.2.4 Chemical characteristics 21

1.2.5 Biological characteristics 22

1.2.6 The maximum turbidity zone and suspended particulate matter 23

1.2.7 Sédiment characteristics 25

1.3 Early diagenesis 28

1.3.1 Basic concepts 29

1.3.2 Diagenetic processes 31

1.3.2.1 Advection 31

1.3.2.2 Diffusion 32

1.3.2.2 The general diagenetic équation 36

1.4 Thesis objectives and outline 37

References 39

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Chapter 2 The diffusion coefficient of dissolved silica revisited 47

Abstract 48

2.1 Introduction 48

2.2 Material and methods 48

2.3 Results and discussion 52

2.4 Conclusions 55

Acknowledgements 56

References 57

Chapter 3 Détermination of biogenic silica and possible interférences 59

3.1 Methodology 60

3.1.1 Alkaline digestion 60

3.1.2 Acid leach 62

3.1.3 Analytical methods 62

3.2 Choice of a correction technique for the lithogenic fraction 64

3.3 Acid leach of surface sédiments 65

3.3.1 Extractable Silicon and other éléments by acid pre-treatment 65 3.3.2 Applicability and suitability of acid pre-treatment 69

3.4 Conclusions 73

References 74

Chapter 4 Biogenic and dissolved silica distribution in the sédiments of the Scheldt continuum and fluxes of dissolved silica across the sediment-

water interface 77

4.1 Introduction 78

4.2 Study area 79

4.3 Material and methods 82

4.3.1 Sample collection and treatment 82

4.3.2 Incubation experiments 82

4.3.3 Analytical methods 83

4.4 Calculation of the DSi fluxes across the sédiment - water interface and

modelling of the pore water DSi profiles 84

4.4.1 Model 1 85

4.4.2 Model 2 86

4.4.3 Model 3 88

4.5 Results 92

4.5.1 BSi02 longitudinal profiles in surface sédiments 92

4.5.2 Vertical profiles of BSi02 and DSi 94

4.5.3 Incubation experiments and DSi fluxes 98

4.5.3.1 DSi and alkalinity release 98

4.5.3.2 DSi fluxes 100

4.5.4 Model calculations 101

4.6 Discussion 107

4.6.1 BSi02 distribution in sédiments 107

4.6.2 DSi profiles in pore waters 108

4.6.3 DSi fluxes across the sediment-water interface 109

4.6.4 Modelling of pore water profiles 112

4.6.5 Silica dissolution efficiency 115

4.7 Conclusions 117

Acknowledgments 118

References 119

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Chapter 5 Dissolution of biogenic silica in the sédiments of the

Scheldt continuum 125

Abstract 125

5.1 Introduction 126

5.2 Study area 128

5.3 Material and methods 130

5.3.1 Material 130

5.3.2 Dissolution experiments 131

5.3.3 Solid phase characterisation 131

5.4 Results 132

5.4.1 Chemical characterisation of the solid phase 132 5.4.2 Dissolution experiments of a reference material 135 5.4.3 Dissolution experiments of natural samples 135

5.5 Discussion 139

5.5.1 Modelling of diatomaceous earth 139

5.5.2 Modelling - Natural samples 141

5.5.3 Factors influencing silica dissolution 148 5.5.4 Effect of salinity on biogenic silica dissolution kinetics 150 5.5.5 Silicic acid released as a fonction of time - Variation with depth 152

5.6 Conclusions 154

Acknowledgments 156

References 156

Chapter 6 Geochemical characterisation of interstitial waters in the Scheldt Continuum: a preliminary évaluation of the diagenetic

processes 161

6.1 Introduction 161

6.2 Materials and methods 163

6.2.1 Study area and samples treatment 163

6.2.2 Analytical methods 163

6.3 Results and discussion 164

6.3.1 Chlorinity 164

6.3.2 pH, alkalinity and sulphate 173

6.3.3 Ca and Mg 176

6.3.4 Fe and Mn 178

6.3.5 Al 182

6.3.6 Cd and Cu 184

6.4 Conclusions 185

Acknowledgments 185

References 186

Annex 6A 189

Chapter 7 General discussion and conclusions 191

7.1 Background 191

7.2 DSi molecular diffusion 192

7.3 Biogenic and dissolved silica inventory 193

7.4 Biogenic silica dissolution 196

7.5 Geochemical processes in sédiments 199

7.6 Conclusions 200

References 203

References cited in the entire thesis 207

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Chapter 1

General introduction

1.1 The biogeochemical Silicon cycle

Silicon is the second most abondant element in the Earth's crust after oxygen, comprising about 28% of the lithosphère. It is mainly présent as silicates, which represent about 90% of the rocks présent at the Earth's surface.

They consist essentially of Si04 tetrahedra that are linked together by covalent

=Si-0-Si= (siloxane) bonds, where = represents the silica lattice; in aluminosilicates, one of the Si atoms may be replaced by Al: =Si-0-AI=. Silicate dissolution involves the hydrolysis of the siloxane (and of the =Si-0-AI=) bonds, which is the rate-controlling step. Most of the silicate minerais are thermodynamically unstable at the surface température and pressure and therefore are subject to weathering (White, 2003), although on biological time scales their dissolution is relatively slow (Mackenzie and Garrels, 1965). Quartz, the pure Silicon dioxide (SiOa) form representing about 12% of the Earth's crust minerais, is an extremely stable phase under Earth surface conditions, a characteristic that makes it one of the most résistant Si forms to weathering (Dove and Rimstidt, 1994). Generally speaking, silicate weathering implies consomption of atmospheric CO2, which is transformed into dissolved bicarbonate (HCOa') and can therefore be trapped in sédiments by carbonate

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précipitation; thus, this process plays a key rôle in controliing atmospheric CO2 concentrations over millions of years.

Figure 1.1 Biogeochemical cycle of Si. D: dissolution; P; précipitation; T: transport;

Dp: uptake; De: death; 1 teramole (Tmol) = 10^^ moles. Adapted from Basile- Doelsch (2006) with values from: (1) Conley et al. (2002); (2) Tréguer et al. (1995).

(3) "Other inputs" comprise the eolian input (0.5 Tmol Si yr'^), seafloor weathering (0.4 Tmol Si yr'^) and hydrothermal sources (0.2 Tmol Si yr'^). Burial in marine sédiments comprises ~ 2 Tmol Si yr'^ removed at continental margins and 4.1 Tmol Si yr"^ in deep-ocean sédiments (DeMaster, 2002); DeMaster (2002) gave actually higher values for Si rétention at continental margins (2.4 to 3.1 Tmol Si yr'^), but considered estuarine removal to be less than 0.6 Tmol Si yr ^

Silicon is aiso a major biogenic element, used as dissolved silica (DSi) by many terrestrial and aquatic organisms to build structural éléments. Biological assimilation of DSi, mainly as silicic acid (H4Si04), leads to the formation of biogenic silica (BSi02), a form of amorphous hydrated silica, Si02.nH20, or opal-A (often simpiy referred to as opal). Opal is characterised by relatively low thermodynamic stability at Earth's surface températures and pressures, and its dissolution rate is five orders of magnitude higher than that of lithogenic silicates, which makes it relevant for biological processes (Conley, 1997). The process through which organisms precipitate minerais is known as biomineralisation (Skinner, 2003). The overall reaction describing Si biomineralisation is (Van Cappellen, 2003):

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H4Si04 SiOz.nHjO + (2 - n)H20 (1.1)

As an example, Silicon constitutes an absolute requirement for diatoms, a phytoplankton group responsible for as much as 50% of the oceanic primary production (Nelson et al., 1995; Tréguer et al., 1995), and plays a key rôle in the biological carbon pump (Smetacek, 1999). Thus, through the CO2

consumption by silicate weathering and diatom production, study of the global biogeochemical Si cycle is of great interest in the context of global change, although the two processes take place on different time scales. However, several aspects are still relatively little understood, as for example the real importance of the terrestrial biogenic Si cycle or the importance of Si benthic recycling in estuarine and Coastal environments. In the following, we will briefly présent the main points of the terrestrial and aquatic Si cycling, which are schematically illustrated in Figure 1.1.

1.1.1 The terrestrial Silicon cycle

Chemical and physical weathering of rocks produces dissolved and particulate materials, part of which is transported to océans via rivers.

Examples of silicate weathering reactions consuming primary minerais and leading to the formation of soluté species and secondary minerais are (Wollast and Mackenzie, 1983; Berner étal., 2003):

CaSiOa + 2CO2 + H2O ^ Ca^-' + 2HC03" + Si02 (1.2) 2KAISi30s + 2CO2 + IIH2O ^ Al2Si20s(0H)4 + 2K^ + 2HC03' + 4H4SI04 (1.3) where CaSi03 represents calcium containing silicate minerais. A Chemical reaction similar to reaction (1.2) aiso stands for magnésium silicate minerais.

These relations both show an important feature of Chemical weathering, which is the consumption of atmospheric CO2. The cations and bicarbonate released by weathering are carried to océans where they are precipitated as Ca (and Mg) carbonates:

Ca^-" + 2HC03' -> CaC03 + CO2 + H2O (1.4) The overall reaction resulting from combining équations (1.2) and (1.4) shows that weathering of calcium (and magnésium) silicates leads to net removal of atmospheric CO2:

CaSi03 + CO2 —> CaC03 + Si02 (1.5)

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Over geological time reactions such as (1.2) and (1.4), along with the burial of organic matter, control the atmospheric levels of CO2 (Berner et al., 2003 and references therein).

Chemical weathering of silicates aiso releases dissolved silica, mostly as silicic acid (Eq. 1.3), which is an essential nutrient for aquatic organisms and a bénéficiai element for terrestrial plants. On land, vascular plants take up DSi, which they deposit in their tissues as phytoliths (Figure 1.2), a form of BSi02 (Epstein, 1994; Alexandre et al., 1997). The Si content in plants, which is dépendent of plant species and soil type, varies widely ranging from 0.1% up to 10% of dry weight of plant biomass (Epstein, 1994). The accumulation of silica in plants is thought to play an important rôle in their wellbeing and résistance to biological, Chemical and physical stress (Epstein, 1994). Phytoliths are released to soils by the decaying plant biomass, where their dissolution is the main contributor to the DSi présent in soil solution as their solubility largely exceeds that of other silicate minerais (Farmer et al., 2005). Part of this DSi enters the surface aquatic Systems either via run-off or ground water flow.

Flowever, if phytoliths production exceeds their dissolution, they may accumulate in soils. For example, Alexandre et al. (1997) showed that 74% of the DSi présent in the soil of a tropical forest in Congo (Brazaville) was due to phytolith dissolution. Mean résidence times for the opal particles are about 6 months, indicating that BSi02 dissolution is much faster than that of silicate minerais, and only ~7.5% of the opal produced by the plants was preserved and accumulated in soils, while most of the DSi is recycled by the forest or consumed by neo-minerals formation. In addition to secondary clay formation, DSi may aIso interact with dissolved Al to build short-range ordered minerais, or be adsorbed onto carbonates. Al hydroxides and Fe oxy-hydroxides (Conley et al., 2006).

Figure 1.2 Picture of siliceous phytoliths in soil (adapted from Farmer et al., 2005).

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Until recently weathering was most often considered uniquely as inorganic rock - water interaction, and the rôle of plants and microbiota was generally ignored (Berner et al., 2003). However, plants and associated micro- organisms can enhance the weathering rates by a factor of 2 to 10 (Berner et al., 2003) via a variety of mechanisms. The chemistry of soil solutions is altered around plant roots and below the litter, with an increase in acidity favouring the hydrolysis of silicates and the release of associated éléments in soils. The organic acids produced by micro-organisms increase the availability of some nutrients and form soluble complexes with Al and Fe, reducing their toxicity for the plants. Plant roots increase the rétention of water in soils and soil porosity by fracturing minerai particles, which aiso increases minerai surface exposed to solution; these phenomena resuit in an increase in the weathering rates due to greater and prolonged contact between soil solutions and minerais (Berner et al., 2003).

Both of these processes - Si accumulation in plants and enhancement of weathering due to the végétation cover - affect strongly the continental runoff, which is responsible of more than 80% of the input to the marine Si budget (Tréguer et al., 1995). Conley (2002) estimated the production of BSi02 by plants to be 60 - 200 Tmol yr'S which is to be compared with the 240 Tmol yr‘^

of opal produced by diatoms in océans (Tréguer et al., 1995). AIso, storage capacity of terrestrial ecosystems is often assumed to be smaller than that of the océans and therefore turnover times are supposed to be smaller. For instance, Conley (2002) showed that the turnover time of reactive Si in a forest is of a few thousand years, almost one order of magnitude lower that that in océan, which is 1 - 2 x lO'* years. Although these values are based only on the very few data available, they indicate that the terrestrial Si cycle is far from negligible and should not be ignored when considering the global Si cycle and when establishing the global budgets. Equations (1.2) and (1.3) show that during weathering of silicates, atmospheric CO2 is taken up and converted to dissolved HCOs" in natural waters, resulting in a net loss of atmospheric CO2

(Van Cappellen et al., 2002; Berner et al., 2003). The impact of végétation on weathering rates implies that modifications in the soil cover may not only resuit in changes of the dissolved and particulate load to aquatic Systems, but aiso affect the net removal of atmospheric CO2. Modifications of the terrestrial input to freshwater and marine Systems can hâve an impact on the long term on various biological processes, such as diatoms growth, key producers in aquatic Systems, as it will be shown below.

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1.1.2 The aquatic Silicon cycle

Continental weathering is thought to be the major source of DSi to aquatic Systems (Tréguer et al., 1995). Dissolved silicate is carried to marine environments via rivers, and the riverine input is estimated to be approximately 80% of the total DSi input to océans (Tréguer et al., 1995). Particulate inputs, mainly clays and rock fragments resulting from Chemical and physical weathering, are generally not taken into account as their dissolution is slow compared to the time scales of biological processes, and therefore do not contribute significantly to the DSi input. Dissolved silica is mainly présent as monomeric silicic acid, H4Si04, with about 2 - 5% (depending on the solution pH) as the dissociated anion H3Si04‘. Uniike other major nutrients such as phosphate and nitrate, dissolved silicate is essentiel only for certain biota.

These aquatic organisms - diatoms, radiolarian, sponges, silicoflagellates and chrysophytes - use H4Si04, most often the monomeric, non-dissociated species but not exclusively, to build structural skeletons which ensure protection from predators, buoyancy, and increase their metabolic efficiency in varions ways (Skinner, 2003; Sarmiento and Gruber, 2006).

From a structural point of view, biogenic silica consists mostly of globular spheres of colloïdal silica, with a diameter of ~ 100 nm; spaces between spheres are often filled with silica (DeMaster, 2002). Opal density is approximately 2.0 g cm'^, and water content varies from 8 to 17% depending on the species and the âge of particles (DeMaster, 2002). The spécifie surface area of frustules of cultured diatoms and natural assemblages of siliceous phytoplankton ranges between 20 and 260 m^ g'^ (Dixit et al., 2001; Van Cappellen and Dixit, 2002).

1.1.2.1 Diatoms

Diatoms (Bacillariophyceae) are responsible for 30 - 50% of the primary production occurring in the océan (Tréguer et al., 1995; Nelson et al., 1995) and are often the dominant phytoplankton community in many fresh- and brackish water Systems. They aiso play a major rôle in the export of organic matter from the surface océan (Tréguer et al., 1995). There are more than 10,000 of identified diatom species and probably many more others (Martin- Jézéquel et al., 2000).

Diatoms use DSi mainly to build an external skeleton of opal, called frustule, but aIso to synthesise their DNA and transcriptional proteins (Brzezinski et al., 1990; Skinner, 2003). The frustule, externally protected by

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an organic coating, is made of two valves fitting together, with girdle bands deposited during asexual reproduction, after production of new valves. During this process (cell division), the diatom forms two silica déposition vesicles (SDV) which expand to form two new inner valves contained in the original cell.

Thus, the daughter-cell is smaller and a decrease in cell size is observed with each new reproductive cycle. The original cell size is re-established only through sexual reproduction (Sarmiento and Gruber, 2006). Dissolved silica is transported by protein transportées to the SDV, where it is concentrated to values at least equal to saturation with respect to opal (1000 - 2000 pmol L'^

depending on température and salinity). Due to the high concentrations, the monomeric H4Si04 polymerises and finally précipitâtes outside the SDV, on the vesicle membranes, forming the new cell walls. Précipitation is mediated by organic molécules and dépends on pH, sait content and presence of trace metals such as Fe, Zn, or Al (Martin-Jézéquel et al., 2000).

Figure 1.3 Picture of the diatom Cyciostephanos invisitatus (adapted from Conley, 2002).

Biogenic silica content in cells varies among species and dépends mainly on the cellular growth rate and the ambient DSi available during biomineralisation (Martin-Jézéquel et al., 2000). The growth rate varies with nutrient availability (N and P), trace métal ions (Fe^VFe^^, Zn^"^), light and température, but aiso with cell size, with smaller cells growing faster even within the same species (Martin-Jézéquel et al., 2000). Energy for silicification is provided by respiration rather than photosynthesis, which means that Silicon metabolism is decoupled from the C and N metabolism. This uncoupling probably explains why silicification continues to occur even in the dark and in case of limitation of major nutrients (N, P) or iron (Martin-Jézéquel et al., 2000). Under non-limiting DSi conditions, biomineralisation is generally inversely correlated to the cell growth rate, with the duration of the cell wall synthesis controlling the degree of Si incorporation (Martin-Jézéquel et al..

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2000). Diatoms suffering from stress due to nutrient or light limitations hâve slower growth rates and tend to hâve more heavily silicified cell walls, which means higher Si/C and Si/N ratios (Sarmiento and Gruber, 2006 and references therein). When DSi is the limiting nutrient, the frustules are thinner and the silicification varies directiy v\/ith the growth rate (Martin-Jézéquel et al., 2000).

A notable différence is observed for instance between freshwater and marine diatoms, with the latter ones being less silicified, probably mainly due to lower levels of available DSi in marine environments compared to freshwater Systems, although salinity and different sinking strategies may aiso play a rôle (Martin- Jézéquel et al., 2000).

1.1.2.2 Recycling of biogenic silica in the water column

Biological processes

Living diatoms are efficiently protected against dissolution by an organic coating (Lewin, 1961), which has to be removed before significant dissolution of the frustule is observed (Lewin, 1961; Kamatani and Riley, 1979; Bidie and Azam, 1999). At the end of the life cycle of diatoms, colonisation of the frustule by bacterial assemblages leads to its dénudation through consomption of the organic casing. The intensity of bacterial activity Controls therefore the initial dissolution rate of frustules and it is the limiting kinetic step (Bidie and Azam,

1999 and 2001).

Another biological processes affecting biogenic silica dissolution is zooplankton grazing, which has been shown to preferentially preserve opal over organic matter in faecal pellets (Ragueneau et al., 2006). Export of BSi02 trapped in faecal pellets may be enhanced due to higher sinking rates and protection from direct contact with the surrounding unsaturated water.

However, breakage of frustules can enhance dissolution and although limited, dissolution in the pellets has been observed (Ragueneau et al., 2006). The rôle of grazing on opal dissolution is thus complex and difficult to assess.

Diatom blooms often terminate with the formation of aggregates that sink rapidiy out of the euphotic zone (Sarmiento and Gruber, 2006). The formation of such aggregates brings diatoms together, and although bacterial activity is enhanced, so is the DSi concentration inside the aggregates and the viability of the entrapped cells (Passow et al., 2003). As dissolution of living diatoms is negligible and relatively high levels of DSi further slow down dissolution, aggregate formation may be overall considered to increase opal préservation and export from the surface.

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Surface properties

After the disappearance of the organic matrix, dissolution of BSiOa dépends on intrinsic physicochemical properties of the frustule, silica surface and the bulk solid, in addition to those of the solvent. As mentioned above diatoms exhibit a large variation in Si content, frustule morphology and spécifie surface area. The degree of silicification may aiso vary within a single specles depending on environmental conditions, such as ambient nutrient and trace métal availability, light and température. Therefore, these factors Indirectiy lead to highiy variable dissolution rates among species (several orders of magnitude), but aIso intra-species (Kamatani, 1982; Ryves et al., 2001).

Interactions between the water and the exposed frustules may aiso influence the dissolution of BSi02 through changes in the Chemical structure of the surface (Dixit and Van Cappellen, 2002). Water molécules break the siloxane bonds at the silica surface - solution interface producing hydroxyl groups, =S\- OH (silanols), responsible for the silica acid-base properties. The decrease in surface density of these ionisable groups appears to be one of the mechanisms resulting in progressive loss in the surface reactivity (ageing) of opal during sinking and burial into the sédiments (Van Cappellen and Qiu, 1997b; Dixit and Van Cappellen, 2002).

Température^ pressure and pH

Opal solubility and dissolution kinetics are influenced by a wide range of abiotic parameters, including température, pressure, pH, salinity, metallic ion contaminants (Al, Fe, Mg, K, etc) and authigenic minerai coatings. Température affects strongly both solubility and dissolution kinetics (Kamatani and Riley, 1979, Kamatani, 1982; Van Cappellen and Qiu, 1997a and b). Kamatani (1982) determined the values of experimental activation energy for the dissolution of cultured diatoms and natural assemblages to be 57 - 58 kJ mol'^ which implies that the dissolution rate at 25°C is more than 6 times higher than that at 3°C.

Solubility values aiso vary with température, from ~ 1000 pmol L'^ at 3°C to ~ 1500 pmol L'^at 23°C (DeMaster, 2003).

The effect of pressure on solubility was studied by Wiley (1974) in laboratory experiments using synthetic amorphous silica. The author showed that dissolution increases with increasing depth, but the effect is small (maximum 20% over the océans depth range - Wiley, 1974).

Enhanced dissolution of silica polymorphs at pH values above 4 is generally attributed to an increase in deprotonated silanol groups =Si-0' at the surface - solution interface (Dove and Rimstidt, 1994). The influence of pH (0- 7) on the dissolution rate of amorphous silica was aiso investigated by Plettinck et al. (1994) who found a minimum rate at pH close to 3. Van Cappellen and

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Qiu (1997b) reported an increase in the dissolution rates of sédiments of the Southern Océan with pH increasing from 6 to 9. The authors hâve aiso shown that the solubility of the same siliceous ooze increased by 10% with pH increasing from 6 to 8 (Van Cappellen and Qiu, 1997a). Although the pH of marine surface waters is almost constant, it can vary significantly in estuarine and shallow-water environments, as well as in interstitial waters.

Sait content

Dissolution rates of quartz are enhanced by the presence of salts of alkali and alkaline earth ions (Plettinck et al., 1994; Dove, 1999). Similar effects were observed for biogenic silica (Barker et al., 1994), but the mechanism is not yet fully understood. Opal solubility increases aIso in marine environments compared to freshwaters due to the increase in sait content, but at high salinities this effect appears to be reduced by the presence of alkaline earth ions such as Ca^"^ and Mg^^. Varions individual salts hâve indeed contradictory effects on the dissolution of amorphous silica of either artificial or natural origin (Barker et al., 1994). Plettinck et al. (1994) found that the rate of dissolution of amorphous silica increased with the ionic radius of the cations investigated including U, Mg, Na, K, Sr, Ba and Cs. The ionic strength of the solution is thought to control silica solubility, by reducing the activity coefficient of free ions to a minimum between values of 0.5 and 1 and increasing it at higher values (Barker et al., 1994). The influence of salinity on opal dissolution is important as it may explain the generally more efficient recycling of BSi02 in marine environments compared to fresh- and brackish water ones.

Interactions between opal - aluminium and detrital fraction

A métal ion that we hâve not yet mentioned but that affects diatoms and opal solubility is aluminium. Mackenzie et al. (1978) suggested that diatoms might provide the link between marine Si and Al cycles by controlling the availability of dissolved Al in the surface waters. Al concentrations in surface waters were shown to resuit from a balance between the magnitude of aeolian input to an area and the local biological productivity, although not specifically linked to diatoms (Hydes et al., 1988; Hydes, 1989). Chou and Wollast (1997) found that dissolved Al and DSi co-varied with an Al/Si ratio of 0.011 in the upper 1000 m water column of the Mediterranean sea and concluded that the distribution of these two éléments were closely associated with diatom production. The authors aiso showed that the amount of dissolved Al and Si released to the pore waters in the first two centimètres of the sédiments had a mean Al/Si ratio of 2.8 x 10'^ resulting from the dissolution of biogenic silica during early diagenesis. Incorporation of Al (under the form of AP"^) can occur

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during the biosynthesis of the silica walls (Gehlen et al., 2002) and this seems to affect both the development of individuel cells and diatom communities, with Al limiting the DSi assimilation (Stoffyn, 1979). This primary uptake is species dépendent, it is correlated with the ambient dissolved Al concentrations and it is characterised by a maximum value beyond which no further incorporation takes place (Dixit et al., 2001 and references therein). This variable uptake is one of the mechanisms explaining the variations in the solubility of biogenic opal of different origins.

Aluminium uptake by the frustules can aiso occur during early diagenesis (secondary uptake) and this appears to be by far the dominant process. In the water column, the atomic Al/Si ratio of diatom frustules ranges between 10‘^

and 10 “^ due to the low concentrations of dissolved Al; however, much higher Al/Si ratios (in the range of ~0.07) can be observed for diatoms buried in the sédiments (Van Bennekom et al., 1989; Dixit et al. 2001; Van Cappellen and Dixit, 2002; Sarmiento and Gruber, 2006). This is mainly explained by higher levels of dissolved Al in pore waters due to the dissolution of lithogenic aluminosilicates. Values as low as 0.1% of Al in the diatom frustules can aiready significantly reduce BSi02 solubility and dissolution rates (Van Bennekom et al., 1989; Van Cappellen et al., 2002; Van Cappellen and Dixit, 2002), Dixit et al.

(2001) reported a 15% decrease in opal solubility with a seven-fold increase in the Al/Si ratio.

Biogenic silica that reaches the sédiments continues to dissolve while being further buried. This leads to a build-up of dissolved silica in the interstitiel waters, with concentrations often increasing downward until they reach a quasi- constant value, referred to as "asymptotic concentration" or "apparent silica solubility". Several experimental studies showed the existence of an inverse relationship between the asymptotic DSi concentration in pore waters and the abondance of the lithogenic (detrital) fraction (Van Cappellen and Qiu, 1997a;

Dixit et al., 2001). The dissolution of aluminosilicates is much slower than that of BSi02 and therefore does not contribute significantly to the build-up of DSi, as long as the mass of lithogenic material does not exceed that of opal by at least a factor of 10 (Van Cappellen et al., 2002). However, the release of Al by the lithogenic fraction can resuit in structural uptake of Al by the frustules leading to a decrease in the apparent solubility, as described above. The Al released may aIso be adsorbed on the reactive surface sites of the opal particles, leading to a decrease in their dissolution rates (Dixit et al., 2001).

Dixit et al. (2001) aiso found evidence of aluminosilicate précipitation even at low levels (nmol L'^) of dissolved Al, explaining (at least partiy) why dissolved silica concentrations remain below the solubility of fresh opal.

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Reverse weathering reactions

The main sink of biogenic silica is burial into sédiments, with about 40%

of it occurring in near-shore and continental shelf sédiments (DeMaster, 2002).

Mackenzie and Garrels (1966) suggested that early diagenetic processes such as reverse weathering reactions resulting in cation-rich aluminosilicates may exert a major control on the biogeochemical Si cycle in océans. Implication of Al in such reactions was shown in a number of studies (e.g. Willey, 1975a and b;

Mackin and Aller, 1984: Mackin, 1986; Dixit et al., 2001). Studies on Amazon delta sédiments reported the presence of diatom frustules partiy or fully converted into authigenic K-rich and Fe-rich aluminosilicates and laboratory incubation experiments showed that conversion of frustules into clays could be completed in less than 2 years (Michalopoulos and Aller, 1995; Michalopoulos et al., 2000). These investigations hâve demonstrated the existence of rapid reverse weathering reactions in areas with high sédimentation rates and indicated that formation of authigenic aluminosilicates in sédiments with high content of métal oxides can be limited by the available biogenic (reactive) silica (Michalopoulos and Aller, 1995; Michalopoulos et al., 2000 and 2004).

Ageing of biogenic silica surfaces. Al uptake by the frustules and précipitation of aluminosilicates on particles are processes that can occur rapidiy, resulting in decreased silica solubility. The usual alkaline leaching techniques used to détermine BSi02 concentrations in sédiments appear to fail to take into account the diagenetically altered opal, resulting in an under- estimation of the estuarine and continental shelf areas as sinks of BSi02

(Michalopoulos et al., 2004; Presti and Michalopoulos, 2008). Although uptake by biogenic silica and précipitation of secondary minerais may aiso hâve an impact on the biogeochemical cycles of other éléments such as Al, Fe, K, Mg, P, these processes are yet rarely considered, particularly in the calculus of budgets and this is mostly due to the difficulties to quantify them.

1.1.2.3 The marine Silicon cycle

The marine Silicon cycle (Figure 1.1) is dominated by the production and subséquent dissoiution of biogenic silica in the water column. The external DSi flux is estimated by Tréguer et al. (1995) to be approximately 6.7 Tmol Si yr \ mainly supplied by rivers (~84%), with the other sources being aeolian input, basait weathering and hydrothermal inputs. The DSi cycle is in steady State, with annual external input balanced by the burial of opal in sédiments as indicated in Figure 1.1.

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The global rate of biogenic silica production in the océan has been estimated to be between 200 and 280 Tmol Si yr'^, with diatoms being the main opal producer (35 to 50% of the primary production in terms of POC - Nelson et al, 1995; Tréguer et al., 1995). Hydrographie conditions, nutrients availability and light intensity are the main parameters controlling the global opal production on short time scales. On longer time scales, of the order of the résidence time of Si in the océan (~1.5 xlO "* yr - Tréguer et al., 1995), the BSi02 is controlled by oceanic sources and sinks (Van Cappellen, 2003).

Because marine waters are strongly under-saturated with respect to BSiOa, about 90% of the global opal production is recycled within the water column, with approximately 50% dissolving in the upper 100 meters. The remaining 10% reaches the seafloor, where dissolution continues and ultimately only ~3%

of the biogenic silica produced in the euphotic zone is permanently buried in sédiments. Tréguer et al. (1995) estimated that opal dissolution in surficial sédiments (the upper 10-20 cm) would suppiy as much as 23 Tmol Si yr'^ to bottom waters and only accumulation of biogenic silica below this upper zone should be considered as the ultimate burial term (DeMaster, 2002). Besides BSi02 dissolution, transport of dissolved silica through the sediment-water interface and sédiment burial and mixing are important processes in the opal régénération/ rétention. The efficient recycling of BSiÛ2 in marine environments is an important source of dissolved silica for the siliceous primary production and directiy affects the carbon biological pump, as the export of particulate organic carbon from the euphotic zone is highiy correlated with the flux of particulate silicate (Faikowski, 2003).

1.1.2.4 Biogenic silica transformation along the land-ocean continuum

Riverine input accounts for much of the nutrients (N, P, Si) présent in marine environments (Chou and Wollast, 2006). Before reaching the sea these éléments are involved in a variety of biological and physico-chemical processes.

Dissolved silica concentrations in riverine waters are about 200 pmol L'\ while in marine waters they remain below 100 pmol L'^ (Chou and Wollast, 2006).

The différence in concentrations has been explained mainly by biological uptake, essentially by diatoms (Wollast and De Broeu, 1971), although reactions between silicate minerais and DSi can aiso contribute to the DSi removal (Mackenzie and Garrels, 1966). A major différence exists between N, P and Si: while recycling of the first two éléments is a rapid, bacterially mediated process, remineralisation of Si requires the slow dissolution of diatom frustules and other biosiliceous products (phytoliths, sponge spiculés). Therefore Si rétention in freshwater Systems and estuaries can be expected to be higher

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than that of N and P. This is particularly the case in estuaries, where high inputs of nutrients resuit in high biological activity. Phytoplankton communities in estuaries are often dominated by diatoms, due to their tolérance to salinity changes and to low light (Help et al., 1995). These characteristics make estuaries as potential sites for silica rétention, but in reality their rôle as filters for Si is difficult to assess due to the scarcity of data and high variability among the few studied sites (DeMaster, 2002). The removal rate of opal in estuaries was estimated in a few studies ranging from 10 to 40% of the DSi riverine flux (DeMaster, 1981; Conley, 1997; DeMaster, 2002).

Minerai dissolution reactions control ultinnately the availability of DSi in the environment, but plants and continental aquatic organisms hâve a strong biological control on Si cycling and export (Conley, 2002; Derry et al., 2005).

Conley (2002) suggested that production of BSi02 by plants and freshwater organisms is of similar magnitude compared to that by marine producers. Given the efficient recycling of this terrestrial opal, particularly when compared with weathering of silicate minerais, most of the reactive silica (either dissolved or particulate) reaching the océans has aiready undergone intensive biological cycling in terrestrial environments. Dissolution of opal phytoliths and freshwater diatoms may account for a significant fraction of the DSi load in rivers (Derry et al., 2005) and Conley (1997) estimated that up to 16% of the gross riverine Si load was delivered to the world océan as BSiOa. Dissolution of this BSi02 may constitute an important source of DSi for near-shore environments, particularly as salinity enhances opal dissolution rates, with rates approximately 4 times higher at salinities typical of marine waters than in freshwater Systems (Hurd, 1983). Estuarine, Coastal and shelf environments represent a dynamic interface between the continents and the open océan, generally characterised by relatively high productivity.

Near-shore sédiments are the main récipients of particulate matter originating from the continents, especially terrigenous clays. Early diagenetic processes (reverse weathering reactions) involving lithogenic clays and reactive silica of continental origin (dissolved or particulate) may resuit in the production of new authigenic minerais, which in turn constitutes a sink for reactive Si, but aiso for a number of other éléments (Mackenzie and Garrels, 1966). In these reactions, the rôle of biogenic silica can be double; it provides through dissolution the reactant, i.e. the dissolved silica (or silicic acid), and can act as the substrate for the formation/precipitation of the product (the new minerai).

Almost 40% of ail marine opal burial takes place in near-shore and shelf sédiments (DeMaster, 2002). Laboratory and field studies reported the occurrence of such reverse weathering reactions, mostly, but not exclusively, in sédiments of the Amazon and Mississippi Deltas (Michalopoulos and Aller, 1995

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and 2004; Michalopoulos et al., 2000; Dixit et al., 2001; Presti and Michalopoulos, 2008). Dissolved silica pore water profiles and benthic DSi fluxes are aiso often explained as the resuit of a variety of early diagenetic processes altering BSi02 and consuming the DSi along with other éléments such as Al, K, Mg, Fe, etc. (e.g. McManus et al., 1995; Ragueneau et al., 2001; Dixit and Van Cappellen, 2003).

1.1.2.5 Anthropogenic perturbations of the Si cycle

The terrestrial and aquatic biogeochemical Si cycles hâve been deepiy altered by anthropogenic activities. Déforestation increases soil érosion and transport to océans by rivers and runoff. It aIso leads first to an increase in the DSi transport, most probably due to the dissolution of phytoliths, followed by a progressive decrease over the years (Conley, 2002) as the biogenic Si stock decreases. Changes in the végétation cover alter the biological, Chemical and mineralogical properties of the soils and modify the partition of Si in various réservoirs. For example, opal phytolith production per unit weight of soil is more important for grasses than for trees, and hence, a greater proportion of the soil silica will be retained and recycled within a grassland ecosystem (Kelly et al., 1998). Déforestation, agricultural practices and urbanisation hâve therefore profoundly altered the weathering of soils and the terrestrial biogeochemical Si cycle.

Increased delivery of the essentiel plant nutrients such as N and P due to anthropogenic activities has induced the eutrophication of aquatic ecosystems (Conley et al., 1993), which in turn has greatly altered the biogeochemical Si cycle in aquatic Systems. Dissolved silica is not added with this nutrient enrichment, and its concentrations hâve even decreased in some cases. Hence, the Si:N and Si:P ratios hâve been modified in many aquatic ecosystems.

Higher N and P levels stimulate the growth of algae, including that of diatoms.

Subséquent settling and burial of diatom frustules in sédiments can remove significant quantities of DSi from the biogeochemical cycle, leading to a long- term décliné in DSi concentrations in the water column, especially in Systems with long résidence times. Diatom growth can become limited by the available DSi, allowing for the development of algae that do not require DSi (Officer and Ryther, 1980). This can resuit in excessive blooms of non-diatom phytoplankton species (Struyf et al., 2006) and the shift in the composition of the phytoplankton communities can hâve harmfui conséquences for the entire ecosystem (Rousseau et al., 2002).

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The construction of dams and river reguiation has resulted in the réduction of DSi transport by rivers to the Coastal marine environment (Humborg et al., 2000, 2002 and 2006). Dam réservoirs and artificial lakes act as nutrient traps and retain naturally part of the nutrients transported along the aquatic continuum. It is generally thought that this is due to increased résidence times of freshwater favouring the growth of diatoms and their subséquent sédimentation, which results in a decrease of the DSi load transported downstream (Conley, 2002). This mechanism has been questioned as résidence times of water in réservoirs are often too short to allow for the full development of diatom blooms, and some results showed the absence of any significant removal of DSi by autochthonous diatom blooms (Humborg et al., 2006 and references therein). However, dam réservoirs are known to trap efficiently the suspended sédiments and therefore aiso the BSi02 produced upstream. Given the important burial rates, estimated to be 6 to 80 times higher than in natural lakes (Humborg et al., 2006), opal dissolution is low, while diagenetic alteration is probably extremely rapid. In eutrophied and regulated rivers, the BSi02 production is enhanced by the high nutrient levels and calmer waters; the opal produced can thus sédiment in réservoirs, before reaching the sea, thus resulting in a decrease in the total DSi input to Coastal areas. In addition, the contribution of BSi02, carried by rivers in suspension, is a significant component of the world's océan Si budget (Conley, 1997). In undammed rivers, the BSi02 reaching the estuarine environment is quickly dissolved as freshwater diatoms die, and the higher salinity enhances dissolution rates. Dams aiso appear to affect the groundwater - surface waters interactions, lowering the Si weathering fluxes, which is probably an additional mechanism explaining the decrease in the DSi loads (Humborg et al., 2002).

1.2 The Scheldt estuary

1.2.1 Estuaries

From a physical and Chemical standpoint, estuaries are transition zones between freshwater and saltwater environments that are affected by tidal oscillations. There are many définitions, but none comprises ail types of estuarine Systems. The most widely used is that proposed by Pritchard (1967):

"An estuary is a semi-enclosed Coastal body of water which has a free connection with the open sea and within which sea water is measurably diluted with fresh water derived from land drainage". There are aiso different classifications, generally based on the geomorphological characteristics and the types of water flow and mixing. A few examples of geomorphological categories

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are Coastal plain estuaries (or drowned river valley), bar-built estuaries and fjords (Bianchi, 2006 and references therein). Coastal plain estuaries, such as the Scheldt, are probably amongst the most studied ones in the world. They are generally characterised by unconsolidated sédiments of fluvial (land-derived) and marine origin (Bianchi, 2006). Land-derived matériels often form muddy deposits in the upper zones, while marine sands dominate towards the estuarine mouth.

The passage from the tidal conditions seaward to the freshwater flows from land not only involves a transition from saline to freshwater conditions, but aiso a change from the reversing tidal flow to the unidirectional river flow upstream. As saline and freshwater bodies meet, mixing takes place, to a greater or lesser degree, and can give rise to a marked interface between the two bodies. The salinity gradient resulting from the mixing of freshwater and saltwater is a unique hydrological feature of estuarine Systems. The estuarine mixing zone is the area where a graduai increase of salinity towards the sea takes place with its upper limit depending on the mixing conditions in the estuary. Based on these conditions, estuaries can be classified in salt-wedge, partially mixed and well mixed estuaries depending on the extent of the water column stratification. Another possible classification is based on the tidal range:

microtidal (tidal range < 2 m), mesotidal (tidal range between 2 and 4 m) and macrotidal (tidal range > 4 m) (Bianchi, 2006).

The different types of classifications cited above provide a broad description of estuarine Systems, but estuaries aIso contain a number of other distinct features, which distinguish them from marine and terrestrial habitats.

They generally contain wetlands that form at the margins of the land and the sea; on the seaward side are banks, shoals, sand flats, mud flats and sait marsh habitats, which link to progressively less saline terrestrial habitats, such as freshwater marshes. Some characteristic features of estuaries include: semi- diurnal or diurnal tidal régime, salinity gradients, high levels and rapid exchange of nutrients, high productivity, sédiment suspension and transport, extensive intertidal areas including sait marshes, mudflats and sand flats, etc.

Water movements in an estuary are extremely complex, resulting from the tidal and the river flows, the presence of surface waves and propagation of externally generated waves. Hydrodynamics affect the sédiment transport within the System. Sédiments can be supplied from marine or freshwater sources. Sédiment reworking within an estuary can be high, as well as érosion and déposition sites, which can co-exist in close proximity. Intertidal mudflats and sand flats are relatively dynamic areas, where déposition and érosion often take place at comparable rates. Sédiments can be cycled on a variety of timescales and, for example, changes in the configuration of channels and bed

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forms can occur over periods as short as days. The variety of environments and the variable conditions in different areas in the same estuary, but aiso their economical vaiue, make estuaries some of the most studied areas.

1.2.2 General charactehstics

The Scheidt estuary (Figure 1.4), a heavily industrialised and denseiy populated area situated in Northern Belgium (Fianders) and Southwest of the Netheriands, is the downstream part of the Scheidt river basin. From its source in Northern France (St. Quentin) to its mouth in the North Sea (Vlissingen), the river has a length of 355 km. It is a rain-fed, low-land river (Meire et ai., 2005), as are its tributaries. In Belgium, it receives water mainly from the Leie, the Durme, the Dender and the Rupei. The total basin area amounts to 21,863 km^

and is extended over France (31%), Belgium (61%) and the Netheriands (8% - Soetaert et al., 2006). The total population of the catchment area exceeds 10 miliion people, with an average density of 477 inhabitants km'^ (Meire et ai., 2005).

1.2.3 Morphological and hydrological characteristics

The part of the river influenced by the tide (160 km long) is referred to as the Scheidt estuary (Figure 1.4) and extends from the estuarine mouth at Vlissingen to Ghent, where the tidal wave is biocked by the presence of sluices.

It is a shallow, well-mixed, and relatively turbid macrotidal estuary. Mean depth varies from about 14 m at the mouth to 7 m near Ghent (Soetaert et al., 2006).

The tidal wave is semidiurnai and propagates within a complex network of tributaries inciuding Durme, Rupei, Dijie, Zenne and Nete (Meire et al., 2005).

The mean vertical tidal range is 2.7 to 4.5 m at the mouth of the estuary (Soetaert et al., 2006). The tidal amplitude increases as the tidai wave progresses upstream with the narrowing of the channel to reach maximal values between Antwerp and Rupelmonde (Meire et ai., 2005), and decreases again afterwards by ioss of energy due to friction (Chen et ai., 2005).

The estuary is funnei-shaped and the cross-sectionai area increases smoothly from the river to the mouth (Figure 1.4). The Belgian part of the estuary, the Sea Scheidt (Zeeschelde - 44 km^), comprises the freshwater tidal river (the Upper Sea Scheidt) which extends roughiy from the Rupei mouth (Rupelmonde) to Ghent. The Lower Sea Scheidt, from Rupelmonde to the Belgian-Dutch border corresponds to the upper brackish estuary (Baeyens et

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al., 1998). The Sea Scheldt is characterised by a single ebb/flood channel, bordered by relatively small mudflats and marshes amounting to 28% of the total surface. (Meire et al., 2005). The intertidal zone is often absent or very narrow. Industrial activities and agglomérations are mainly concentrated in this area. The Dutch part of the estuary called the Western Scheldt corresponds to the lower, brackish and marine estuary and stretches from Doel (the Belgian- Dutch border) to the mouth. It is composed of extended sand banks along the axis forming well defined flood and ebb channels with an artificial navigation channel approximately 15 - 20 m deep (Winterwerp et al., 2001; Meire et al., 2005). This complex network of flood and ebb channels is surrounded by several large intertidal flats and sait marshes (Figure 1.5), covering about 35%

of the total Western Scheldt area (310 km^ - Meire et al., 2005). The Canal Ghent-Terneuzen merges with the Scheldt near the mouth at Vlissingen.

Figure 1.4 Map of the Scheldt estuary, and the main tributaries Durme (tidal), Rupel (tidal) and Dender (non-tidal). Numbers on the map give the distance to the river mouth (in km).The Belgian-Dutch border is located at km 58 (adapted from Chen et al., 2005).

The brackish water tidal areas, the marshiands and the fresh tidal water areas in the upper estuary are unique and belong to the largest brackish marshes of Western Europe. However, they hâve undergone substantiel hydrological and geomorphological modifications mainly as a resuit of human activities, but aiso due to naturel process such as land formation from marshes.

The tidal river is aIso almost completely canalised upstream the Dender mouth (Meire et al., 2005). Intertidal habitat was lost since the Middie Ages by embankment, poldering and building of protection dykes (Winterwerp et al..

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2001). Since 1970, large dredging activities took place in the estuary to deepen the navigation channei (Figure 1.6). The annual sédiment import into the estuary is about 1.3 x 10^ kg, whiie 9.0 to 13.0 x 10^ kg of sédiment are dredged and dumped yeariy at varions locations in the estuary and 3.5 x 10® kg of sand are mined annualiy for commerciai purposes (Winterwerp et al., 2001).

The tidal amplitude has aiso increased in the upper estuary, with values at Antwerp twice of that at the mouth.

Figure 1.5 Morphological units in the Scheldt estuary (adapted from Van Maldegem et al., 1993).

Figure 1.6 Volumes of dredged matériels during the period 1990-2003; B - Belgian part of the estuary; NL - Dutch part of the estuary (from Meire et al., 2005).

The Scheldt estuary is one of the few remaining European estuaries that include the entire gradient from freshwater to saltwater tidal areas, with salinity reaching ~30 at the estuarine mouth. Along the estuarine gradient, salinity decreases upstream and drops to values below 1 near Rupelmonde. The salinity intrusion dépends mostly on the freshwater discharge and to a lesser degree on the phase of the tide, and may migrate over a distance of 40 km (Baeyens et

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al., 1998). Due to seasonal variations in discharge, the lower riverine part from Ghent to Antwerp is a tidal freshwater river during winter and spring, but becomes brackish between Antwerp and the Rupel mouth in summer and autumn when the discharge of the river is reduced (Baeyens et al., 1998). The average annual freshwater input at Rupelmonde is about 100 s'^ with minima of 20 s'^ reached during summers, and maxima of 400 s‘^ in winters. During normal river flows, the estuary is well-mixed, although a small vertical stratification may occur in the upper brackish area. Given the dominant tidal influence and the small river discharge, the freshwater résidence time in the brackish/marine estuary is high, from 2 to 3 months, depending on river flow (Soetaert and Herman, 1995a).

1.2.4 Chemical characteristics

The Scheldt is one of the most polluted rivers in Western Europe, although efforts are made to improve its water quality. Besides extensive urbanisation (coupled with insufficient wastewater treatment), agriculture in the catchment area is intensive and is responsible for a substantial part of the nutrient load. Large industrial areas are concentrated in the estuary, mainly near Ghent, Antwerp (one of the largest harbours in Europe) and Vlissingen.

High amounts of organic matter and nutrients in urban and industrial wastes, as well as from leaching of agricultural land, induce extensive microbial and bacterial activity resulting in an important consomption of oxygen, and even déplétion (Meire et al., 1995). Most of the Sea Scheldt has severely low oxygen concentrations and in the past, part of the water column could even become anoxie under certain conditions; anoxia can still be observed in the Rupel (Billen et al., 2005). Increased wastewater treatments resulted in a general increase of the oxygen levels throughout the estuary. However, the oxygen concentrations can still decrease dramatically during summer, particularly during periods of low discharge. In contrast, the situation has improved from the Belgian-Dutch border towards the mouth, and the Western Scheldt is almost fully oxygenated (Meire et al., 2005).

Nitrogen and phosphorous inputs remain high in spite of important réduction efforts and are never limiting factors for the phytoplankton community (Bayens et al., 1998; Van Damme et al. 2005; Soetaert et al.

2006), which is dominated by diatoms (Muylaert et al., 2000). Ammonium and phosphate hâve decreased continuousiy since the 1970s; in contrast, nitrate concentrations hâve generally increased, but this may aiso illustrate the improvement of the oxygen levels, which limit nitrate removal through

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dénitrification (Billen et al., 2005; Soetaert et al., 2006). Dissolved silica concentrations in the riverine part aiso decreased constantly since the 1970s, while they showed a very small increase seaward. The decrease in DSi is probably mostly due to higher diatomaceous production in the upper reaches of the Scheldt, although modification of the land use and river régulation probably aIso play a rôle. DSi limitation for diatoms can be observed during summer, when the productivity is high and the water discharge low (Muylaert et al., 2000; Rousseau et al. 2002).

1.2.5 Biological characteristics

Different phytoplankton communities can be observed in the freshwater reaches and in the estuary. A first phytoplankton bloom can be observed between March and May in the uppermost reaches. It is generally characterised by relatively low phytoplankton biomass, dominated by riverine taxa (Stephanodiscus hantzschii, Euglena proxima and ultraplanktonic taxa) (Muylaert et al., 2000). A second more intense phytoplankton bloom in the freshwater zone occur during summer, due to higher water température and irradiance, generally dominated by typical estuarine taxa, such as centric diatoms (Cyclotella spp) (Muylaert and Sabbe, 1999). In autumn, phytoplankton biomass decreases rapidiy and remains low during winter.

In the estuary, the phytoplankton bloom develops in spring (Van Damme et al., 2005). Riverine phytoplankton entering the more turbid estuary undergoes a shift in composition from green algae to diatoms, which are better adapted to low light conditions (Muylaert et al., 2000). However, green algae remain an important component of the phytoplankton community, particularly in the maximum turbidity zone. The community in the freshwater estuary is generally dominated by species from the tidal river, while marine diatoms are the main species found in the brackish estuary, although a few typical estuarine species are aiso présent. Another marine species encountered in the brackish estuary is Phaeocystis (Lancelot, 1995). This alga develops during spring (April - May) in the Coastal area, particularly when DSi becomes a limiting factor for diatoms (Lancelot, 1995; Rousseau et al., 2002). It is transported into the estuary up to salinity 10, and it can contribute to about half of the phytoplankton biomass in the marine part of the estuary (Rousseau et al., 2002

).

The zooplankton in the freshwater reaches is dominated by rotifers (Tackx et al., 2004 and 2005), which graze preferentially on small algae, and hâve little effect on large diatoms (Muylaert et al., 2000), as shown by the

Figure

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