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Journal ArticleDOI

Spatiotemporal aspects of silica buffering in restored tidal marshes

20 Oct 2008-Estuarine Coastal and Shelf Science (Elsevier)-Vol. 80, Iss: 1, pp 42-52

AbstractLosses of pelagic diatom production resulting from silica limitation have not only been blamed for toxic algal blooms, but for the reduction in ability of coastal food webs to support higher trophic levels. Recent research has shown the importance of advective seepage water fluxes of dissolved silica (DSi) from freshwater marshes to pelagic waters during moments of riverine Si-limitation. In this study, we investigated the potential impact of recently installed new tidal areas along the Schelde estuary, located in former polder areas and characterized by so-called controlled reduced tidal regimes (CRT). Nine mass-balance studies were conducted in a newly constructed CRT in the freshwater Schelde estuary. During complete tidal cycles both DSi and amorphous silica (ASi) concentrations were monitored at the entrance culverts and in different habitats in the marsh. A swift DSi-delivery capacity was observed despite the shifted spatiotemporal frame of exchange processes compared to reference marshes. As silica-accumulating vegetation is not yet present, and difference with reference marshes' deliveries is surprisingly small, we indicate diatomaceous debris and phytoliths to be the main silica source. Although further research is necessary on the driving forces of the different processes involved, restoration of former agricultural areas under CRT-regime provide the potential to buffer silica in the estuary.

Topics: Dissolved silica (60%), Marsh (55%), Salt marsh (54%), Estuary (52%), Wetland (50%)

Summary (4 min read)

1. Introduction

  • Estuaries are biogeochemical hot-spots and are amongst the most productive ecosystems of the world (Costanza et al., 1993).
  • Within the estuarine ecosystem, fringing tidal marshes act as a biogeochemical filter, removing inorganic and organic substances from the floodwaters and changing substance speciation (e.g. Gribsholt et al., 2005).
  • Tidal freshwater marshes contain large amorphous silica stocks in marsh soils, built up through sedimentation of diatom shells and incorporation of silica in marsh vegetation (Struyf et al., 2005).
  • In the first implemented CRT, an intensive spatiotemporal sampling scheme was carried out during the first 16 months of development.

2.1. Study area

  • The Schelde estuary is one of the last European estuaries with a complete fresh- to saltwater tidal gradient, located in the Netherlands and Belgium.
  • The rotation system was abandoned in 2003.
  • During the two-year construction phase (2003–2005), crops were replaced by pioneer vegetation (mainly Epilobium hirsutum and Urtica dioica) (Fig. 1B).
  • Because site elevation is several metres under mean high water level, reconstruction of spring-neap tide flooding variation required the construction of separate inlet culvert and outlet culvert (Maris et al., 2007).
  • This results in a controlled reduced tidal area (CRT) with unique tidal features, such as a pronounced spring–neap variation and a prolonged stagnant phase (Fig. 3, for details see Cox et al., 2006; Maris et al., 2007).

2.2. Sampling

  • A total of 796 data points were obtained during the nine mass balance studies.
  • This intensity was necessary to explore the spatial patterns in the marsh; however this exhaustive scheme was not entirely repeated during all campaigns.
  • Samples were taken approximately 10 cm below the surface, and stored in dark incubators at 5 C for a maximum of 24 h. at typical neap (–), mean ($ $ $) and spring (–) tide outside and inside Lippenbroek CRT agnant phase, (220) start outstream ¼ stop stagnant phase and (580) stop outstream.
  • For each of the samples analysed for ASi (Table 2), three sub samples of 25 ml each were filtered over 0.45-mm filters, from a well-mixed total sample of 150 ml.
  • [ASi] (mg l 1) was then calculated by extrapolating the linear line through the three extraction points in a time-extracted silica plot (DeMaster, 1991).

2.3. Water and silica mass balances

  • All calculations and statistical analyses were performed in R (R, 2006).
  • Inlet and outlet culverts are the only exchange points with the river.
  • Flow velocity was measured acoustically (Sontek ‘‘Argonaut’’).
  • Water mass balances were calculated with an averaged discharge value throughout the water column for every 2 min, assuring accurate volume-weighing of concentration values during all tidal phases.
  • General patterns were not seriously influenced by this effect (Figs. 5 and 6).

3.1. Concentration profiles

  • DSi concentration profiles show different seasonal patterns (Fig. 4).
  • Instream phases (see Fig. 3) are marked by steep concentration changes, whilst the fluctuations during stagnant phase do not exceed 0.2 mg l 1.
  • Concentration profiles of ASi present a more variable pattern over a smaller concentration range (not shown).
  • And differences are generally lower for ASi compared to DSi, there is a general evolution of increase or status quo (May, July, September 2006 and October 2006, not shown) towards strong ASi decrease in ASi- concentration profiles during later tidal cycle (March and June 2007b, not shown), with the exception of June 2007a.

3.2. Mass balances

  • Calculated DSi mass balances indicate enrichment of exported water in summer months (July 2006 and June 2007, Fig. 5, upper left), but also in late autumn (October 2006) and during one spring campaign (March 2007).
  • Absolute numbers (Fig. 5 upper right) are lower due to small water mass at neap tide, while the opposite is true for the 2007 campaigns.
  • ASi mass balances confirm the transition from slight ASi delivery or status quo towards ASi caption by the marsh (Fig. 5, lower graphs).
  • Epresented as percent, (out(g) in(g))/in(g)) (left graphs); and in absolute numbers (kg alance, represented as percent in both directions.
  • Yet, despite large ASi retention in the March 2007b and June 2007a campaigns, DSi delivery can still be sufficient (Fig. 7, upper graphs) to provide the estuary with net silica (Fig. 6).

3.3. Ephemeral and diurnal aspects

  • Non-numerical variable classes, being ‘‘day/night’’ show opposite differences in March vs. June 2007 campaigns.
  • Variation of obtained numeric variables was maximized along two components (PCA correlation circle, see Section 2, Fig. 7).
  • Parallel vectors indicate high correlation along the two components, whilst squared vectors are not correlated.
  • Explicit Rsquared and p-values depend on the amount of variation explained by the components.
  • Percent DSi delivery was negatively correlated with the logarithm of riverine DSi concentration (R2 ¼ 0.6397, p-value 0.0096), and the logarithm of mean water depth (R2 ¼ 0.52, p-value 0.02), and not correlated with temperature of outstream water, percent ASi delivery, or riverine ASi concentrations.

3.4. Spatial aspects

  • Concentration profiles observed at other locations were not used for local mass balances, because water volumes and tidal phases were not measured separately for these locations.
  • Concentrations appear unchanged during stagnant phase (minute 150 until 250 after instream in Fig. 8), but an hourglass pattern appears during outstream, when concentrations from different locations diverge strongly (Minute 300; Fig. 8).
  • Typical retention habitat during the May 2006 campaign was the tidal pool (site 2 in Figs. 8 and 1C), whilst opposing behaviour is observed at the lower mudflat (site 3 in Figs. 8 and 1C).
  • The hourglass structure returns, albeit less complete, in the other spatially sampled campaigns in the summer and autumn with net DSi delivery (not shown).
  • A similar spatial sampling took place for ASi (Table 2).

4. Discussion

  • Numerous processes are involved in the silica exchange between tidal marsh and flooding water (Scheme 1).
  • In the following overview, these processes are described and linked to the obtained data.

4.1. Diffusive and advective transport

  • In several systems, the importance of advective ground water fluxes to the estuarine nutrient balance has been emphasized (i.e. Herrera-Silveira, 1998; Hays and Ullman, 2007; Niencheski et al., 2007).
  • As previously explained, the key process in silica buffering by tidal marshes is the swift replenishing of dissolved silica (DSi) in the flooding water during seepage (e.g. Struyf et al., 2006).
  • Other authors, however, have suggested the importance of bioturbation (Berner, 1980; Meile et al., 2005), resuspension (Mortimer et al., 1998), bioirrigation (Aller, 1965; Mortimer et al., 1998), advection and subsurface circulation patterns (Vanderborght et al., 1977) in diagenetic equations.
  • In fact, the end of overmarsh tide and start of seepage coincides with a sudden increase in DSi concentrations in outstream water for almost all campaigns.
  • The often observed decline in concentrations at the very end of the outstream phase is probably due to riverine water slowly entering the exit culvert.

4.2. Sedimentation and resuspension

  • Sedimentation and recycling of suspended particulate silica are key processes in the marine and lacustrine silica cycle (Treguer et al., 1995; Bidle and Azam, 2001).
  • In estuaries it has been shown that regeneration of silicic acid from particulate silica does not account for observed summer increases in silicic acid concentration (van Bennekom et al., 1974; Yamada and Delia, 1984).
  • DSi delivery occurred equally from location 4 during the initial Urticadominated period, over the period of gradual decay of the Urtica vegetation, towards a rather bare mudflat stage.
  • When discussing patterns of ASi delivery, it is necessary to emphasize that ASi concentrations are probably underestimated, as only surface samples were taken, and suspended solids concentrations are expected to be higher near the bottom.
  • Throughout the first summer small creeks developed, and vegetation and algal layers began to consolidate marsh soil while sedimentation and erosion zones became apparent, as in reference marshes.

4.3. Diatom die-back and frustule dissolution

  • PH, temperature, etc.) on the solubility of silicic acid have been well studied (Greenberg and Price, 1975; Hurd and Theyer, 1975; Kamatani and Riley, 1979), silicic acid is likely to be found at concentrations considerably below saturation in the water column and surface sediments of most estuaries (Yamada and Delia, 1984).
  • Not only does increased temperature directly boost silica efflux from sediments (Yamada and Delia, 1984),it also enhances efficiency of bacterial removal of the organic carbon matrix from diatoms, increasing the surface area of naked silica exposed to dissolution and fastening chemical dissolution rates (Bidle and Azam, 2001).
  • Also, the relationship could be magnified by temporal concurrence of low riverine DSi concentrations with periods of high temperature in summer.
  • This suggests that day delivery might be lower, due to uptake of DSi by autochthonous diatoms.
  • This diurnal aspect certainly deserves more attention.

4.4. Si uptake by diatoms

  • The DSi retention in the tidal marsh, until now not observed in important quantities (Struyf et al., 2006), may be the most striking difference of this study with earlier findings.
  • The cause is almost certainly diatom uptake.
  • In later campaigns benthic diatom populations were observed throughout the marsh (Jacobs, personal observation), as were decreasing DSi profiles during stagnant and bulk phase (not shown).
  • Settling of the sediments and very low depth compared to the pelagic optimizes light conditions and surface aeration.
  • The uptake process is dominant in spring, as low ambient DSi concentrations disadvantage production and promote relative export numbers in summer campaigns.

4.5. Interactions

  • In contrast to other systems where silica fluxes are studied, processes influencing silica cycling from tidal marshes are mostly separated in space and/or time (Scheme 2).
  • Specific tidal patterns in CRT’s influence the observed processes and their temporal distribution, most likely in favour of import processes (Scheme 2).
  • When DSi deliveries are compared with earlier measured reference tidal marshes, the difference is surprisingly small (Fig. 9).
  • The above-mentioned difference (Fig. 9) is mainly governed by two DSi-retention events, which are caused by DSi uptake by diatoms, a process that occurs throughout the marsh at stagnant phase and in the tidal pool throughout the whole tidal cycle.

4.6. Conclusions

  • The typical tidal features of CRT areas influence their silica cycling, both through increased potential for uptake of DSi and enhanced sedimentation of ASi during the stagnant phase and in tidal pools.
  • When DSi deliveries are compared with earlier measured reference tidal marshes, the difference is surprisingly small.
  • Export processes do not seem strongly limited by their decreased time budget.
  • This indicates that CRT areas are capable of fast build-up of Si-recycling capacity and swift DSi delivery at limitation events.
  • The authors main conclusion is that recently constructed CRTs along estuaries are capable of a silica buffering role comparable to older, reference tidal marshes.

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Spatiotemporal aspects of silica buffering in restored tidal marshes
Sander Jacobs
a
,
*
, Eric Struyf
a
,
b
, Tom Maris
a
, Patrick Meire
a
a
Department of Biology, Ecosystem Management Research Group, University of Antwerp, Universiteitsplein 1, 2610 Wilrijk, Antwerp, Belgium
b
GeoBiosphere Science Centre, Department of Geology, Lund University, So
¨
lvegatan 12, 22362 Lund, Sweden
article info
Article history:
Received 23 January 2008
Accepted 8 July 2008
Available online 15 July 2008
Keywords:
wetlands
restoration
tidal flats
nutrient cycles
eutrophication
silica
Schelde estuary
Belgium
51
03
0
53
00
N
4
08
0
55
00
E
abstract
Losses of pelagic diatom production resulting from silica limitation have not only been blamed for toxic
algal blooms, but for the reduction in ability of coastal food webs to support higher trophic levels.
Recent research has shown the importance of advective seepage water fluxes of dissolved silica (DSi)
from freshwater marshes to pelagic waters during moments of riverine Si-limitation. In this study, we
investigated the potential impact of recently installed new tidal areas along the Schelde estuary, located
in former polder areas and characterized by so-called controlled reduced tidal regimes (CRT). Nine
mass-balance studie s were conducted in a newly cons tructed CRT in the freshwater Schelde estuary.
During complete tidal cycles both DSi and amorphous silica (ASi) concentrations were monitored at the
entrance culverts and in different habitats in the marsh. A swift DSi-delivery capacity was observed
despite the shifted spatiotemporal frame of exchange processes compared to reference marshes. As
silica-accumulating vegetation is not yet present, and difference with reference marshes’ deliveries is
surprisingly small, we indicate diatomaceous debris and phytoliths to be the main silica source.
Although further research is necessary on the driving forces of the different processes involved,
restoration of former agricultural areas under CRT-regime provide the potential to buffer silica in the
estuary.
Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction
Estuaries are biogeochemical hot-spots and are amongst the
most productive ecosystems of the world (Costanza et al., 1993). As
the interface between terrestrial and coastal waters, they support
processes that are central to the planet’s functioning (Costanza
et al., 1997). Estuaries are characterized by steep chemical gradients
and complex dynamics, resulting in major transformations in the
amount, the chemical nature and the timing of material fluxes.
Eutrophication is one of the most important problems that
confronts these systems. Eutrophication phenomena in estuaries
are related to the balance between N, P and Si in river loading, and
are thus dependent on the interactions between human activity
and natural processes in the watershed, which ultimately deter-
mine the riverine nutrient delivery into the marine environment
(Officer and Ryther, 1980; Billen and Garnier, 1997; Lancelot et al.,
1997; Cugier et al., 2005). Eutrophication can cause anoxia, extreme
turbidity and even toxicity in coastal areas and lakes, mostly
provoked by shifts in plankton community following excessive
inputs of N and P compared to Si. Decreases in the availability of
silica relative to N and P in estuaries may result in a shift in the
phytoplanktonic community from a dominance of diatoms to other
phytoplankton forms as cyanobacteria or toxic dinoflagellate,
affecting zooplankton and fisheries (see also Chı
´
charo et al., 2006;
Wolanski et al., 2006). Losses of diatom production, resulting from
silica limitation, have not only been blamed for these toxic algal
blooms, but for the reduction in ability of coastal food webs to
support higher trophic levels (Treguer et al., 1995; Cugier et al.,
2005; Kimmerer, 2005). Estuarine and marine foodwebs are based
essentially on diatoms (Irigoien et al., 2002; Kimmerer, 2005).
Dissolved silica concentrations have since long been known to
control diatom populations (Wang and Evans, 1969), diatom
blooms (Tessenow, 1966; Schelske and Stoermer, 1971; Davis et al.,
1978), and seasonal succession in plankton communities (Kilham,
1971). In fact, the availability of dissolved silica (DSi) has been
shown to control diatom silica production rates, at least seasonally,
in every natural system examined to date (Nelson and Brzezinski,
1990; Nelson and Treguer, 1992; Brzezinski and Nelson, 1996;
Nelson and Dortch, 1996; Brzezinski et al., 1998; Bidle and Azam,
2001).
Within the estuarine ecosystem, fringing tidal marshes act as
a biogeochemical filter, removing inorganic and organic substances
from the floodwaters and changing substance speciation (e.g.
Gribsholt et al., 2005). The interaction between tidal marshes and
estuaries or coastal zones received much attention through
*
Corresponding author.
E-mail addresses: sander.jacobs@ua.ac.be (S. Jacobs), eric.struyf@geol.lu.se
(E. Struyf).
Contents lists available at ScienceDirect
Estuarine, Coastal and Shelf Science
journal homepage: www.elsevier.com/locate/ecss
0272-7714/$ see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.ecss.2008.07.003
Estuarine, Coastal and Shelf Science 80 (20 08) 42–52

numerous exchange studies (e.g. Valiela et al., 2007; Spurrier and
Kjerfve, 1988; Whiting et al., 1989; Childers et al., 1993), with
emphasis on C, P and N. Dominant questions were whether
marshes were importing or exporting N, P, C or particulate matter,
often testing the ‘‘outwelling’’ hypothesis (e.g. Dame et al., 1986),
which states that a large part of the organic matter produced in the
intertidal marshes is not used in internal trophic chains but is
transported into the adjacent sea areas and increases their
productivity. Only a limited number of mass balance studies have
targeted freshwater tidal marshes (e.g. Childers and Day, 1988;
Gribsholt et al., 2005; Struyf et al., 200 6). The freshwater systems
are characterized by botanical properties resembling inland fresh-
water wetlands and by more direct contact with human-impacted
river water. These characteristics make freshwater tidal marshes
potentially important process interfaces. Struyf et al. (2006) have
shown the importance of advective seepage water fluxes of
dissolved silica (DSi) from freshwater marshes to pelagic waters
during moments of riverine Si-limitation. Tidal freshwater marshes
contain large amorphous silica stocks in marsh soils, built up
through sedimentation of diatom shells and incorporation of silica
in marsh vegetation (Struyf et al., 2005). Export is the result of
consequent dissolution of this amorphous silica (ASi) in marsh pore
water from litter and sediments, and advective export of marsh
pore- and puddle water between tidal flooding events (Struyf et al.,
2007a,b). Silica limitation of diatoms (Conley et al., 1993; Smayda
1997) and the consequent negative effects on food web structure
may be avoided. However, data are available only from few tidal
freshwater wetlands and conclusions are presently only applicable
on a local scale. Furthermore, a recent review stresses the need for
more research on silica cycling in wetlands, as it rivals their impact
on other biogeochemical cycles and, to date, this topic has not
received sufficient attention (Struyf and Conley, in press).
In this study, we investigated the potential impact of recently
installed new tidal areas along the Schelde estuary, located in
former polder areas and characterized by so-called controlled
reduced tidal regimes (CRTs) (Cox et al., 2006; Maris et al., 2007).
Along the Schelde estuary, more than 50% of marsh area will
eventually be located in such areas, and may result in international
application. This article focuses on the silica biogeochemistry
within these new systems and aims to explore spatiotemporal
patterns of deposition and dissolution in recently flooded formerly
agricultural polder areas. In the first implemented CRT, an intensive
spatiotemporal sampling scheme was carried out during the first
16 months of development. This research expands the growing
awareness that ecosystems and associated biogenically fixed
amorphous Si rather than geological weathering control silica
availability in the aquatic environment on a shorter, biological
timescale (Conley, 2002; Humborg et al., 2004; Derry et al., 2005).
2. Materials and methods
Nine mass-balance studies were conducted in a newly
constructed CRT in the freshwater Schelde estuary: on May 16, July
3, September 10 and 11 and October 10, 2006, and on March 20 and
21 and June 4 and 5, 2007. During nine complete tidal cycles both
DSi and ASi concentrations were monitored at the entrance culverts
as well as in different habitats in the marsh.
2.1. Study area
The Schelde estuary is one of the last European estuaries with
a complete fresh- to saltwater tidal gradient, located in the Neth-
erlands and Belgium. Maps and extensive descriptions of hydrology
and ecology can be found in several recent papers (Temmerman
et al., 2003; Meire et al., 2005; Van Damme et al., 2005; Soetaert
et al., 2004). The studied CRT area is a newly constructed inunda-
tion area, the ‘‘Lippenbroek’’ (surface approximately 80,000 m
2
),
situated at Moerzeke (51
03
0
53
00
N; 4
08
0
55
00
E). Maximal tidal
amplitude in the Schelde at this point is approximately 6 m. The
area was mostly used as cropland (rotation system with Zea mays
and Solanum tuberosum; the lower parts were planted with Populus
sp. trees or over-grown with Salix sp. trees (Fig. 1A). The rotation
system was abandoned in 2003.
During the two-year construction phase (2003–2005), crops
were replaced by pioneer vegetation (mainly Epilobium hirsutum
and Urtica dioica)(Fig. 1B). Part of the polder was devegetated due
to building construction work (Fig. 1B). Tidal inundation was initi-
ated in March 2006. Since the first inundation, vegetation has been
progressively replaced by flood-tolerant species (mainly Lythrum
salicaria, Lycopus europaeus and Phragmites australis)(Fig. 2).
Because site elevation is several metres under mean high water
level, reconstruction of spring-neap tide flooding variation required
the construction of separate inlet culvert and outlet culvert (Maris
et al., 2007). At the riverside, an inlet culvert permits flooding from
4.80 m TAW and higher, whilst a valved outlet culvert guarantees
one-way emptying from 1.5 m TAW and lower (TAW is the Belgian
Ordnance Level, which is approx. 2.3 m below mean sea level at the
Belgian coast). Consequently, only the top of the tidal cycle is
permitted to flood the polder surface. This results in a controlled
reduced tidal area (CRT) with unique tidal features, such as
a pronounced spring–neap variation and a prolonged stagnant
phase (Fig. 3, for details see Cox et al., 2006; Maris et al., 2007). The
marsh is surrounded by a dike at 8 m TAW. Because of the deep
artificial dike bases and thick riverine clay deposit in the CRT,
ground water fluxes were assumed to be small compared to
observed tidal surface water fluxes.
: drainage structures
: Salix wood
: Populus plantage
: bare ground
: pioneer vegetation
: sampling point
3
1
5
8
6
2
3
4
10
11
12
15
16
Schelde
N 50m
: drainage structures
: Salix wood
: Populus plantage
: agricultural fields
: pre-building contours
Schelde
N
ABC
50m
: drainage structures
: Salix wood
: Populu
s plantage
: bare ground
: pioneer vegetation
Schelde
N50m
9
7
13
14
Fig. 1. Schematic overview of study site before (A) and after building works (B). (C) Sampling locations in the CRT. Sampling intensity at locations is given in Tables 1 and 2.
S. Jacobs et al. / Estuarine, Coastal and Shelf Science 80 (2008) 42–52 43

2.2. Sampling
A total of 796 data points were obtained during the nine mass
balance studies. Surface water samples were collected at the
entrance and outlet culvert (1 in Fig. 1C) and in selected habitats
throughout the marsh (2–16 in Fig. 1C). Sampling covered the full
13 h of the tidal cycle for May, July and October 2006 campaigns,
and double cycles of one night (‘‘a’’ in text) plus day (‘‘b’’ in text) of
26 h for September 200 6 and March and June 2007 campaigns.
Sampling intensity was highest during the first campaign
(Tables 1 and 2). This intensity was necessary to explore the spatial
patterns in the marsh; however this exhaustive scheme was not
entirely repeated during all campaigns. The selection of habitats
during subsequent campaigns was based on maximal cover of
different habitat features. A selection of samples was analysed for
ASi (Table 2), also covering different habitat features.
Samples were taken approximately 10 cm below the surface,
and stored in dark incubators at 5
C for a maximum of 24 h.
Fig. 2. Vegetation development in devegetated zones. A and B (upper) show overview; C and D (lower) detail. A and C are taken in spring 2006 (1 month after first inundations),
B in summer 2006, and D in summer 2007.
0 100 200 300 400 500 600 700
0
5000
10000
15000
20000
Time
(
minutes
)
Volume (m
3
)
Instream Stagnant Bulk outstream Seepage outstream
Fig. 3. Water mass balance during a typical tide in Lippenbroek. Inset illustrates tidal curves at typical neap (–), mean ($$$) and spring (–) tide outside and inside Lippenbroek CRT
(from Cox et al., 2006). Grey lines indicate (0) start instream, (110) stop instream ¼ start stagnant phase, (220) start outstream ¼ stop stagnant phase and (580) stop outstream.
Outstream consists of a bulk outstream (overmarsh tidal frame) and a seepage phase (here at approx. 340 min. Phase lines are indicated in relevant figures throughout the MS.
S. Jacobs et al. / Estuarine, Coastal and Shelf Science 80 (2008) 42–5244

Dissolved silica (DSi) was analysed on a Thermo IRIS ICP (Induc-
tively Coupled Plasmaspectrophotometer) (Iris
Ò
). For each of the
samples analysed for ASi (Table 2), three sub samples of 25 ml each
were filtered over 0.45-
m
m filters, from a well-mixed total sample
of 150 ml. After drying at 20
C, ASi was extracted from the filters in
a0.1MNa
2
CO
3
solution at 80
C in a shaker bath. Sub samples were
taken at 60, 120 and 180 min. Blank extractions revealed insignifi-
cant DSi release from filters, recipients or chemicals. [ASi] (mg l
1
)
was then calculated by extrapolating the linear line through the
three extraction points in a time-extracted silica plot (DeMaster,
1991). This approach corrects for the additional release of Si from
mineral silicates. The ASi wet-alkaline extraction is prone to addi-
tional release of DSi from amorphous mineral silicates. Despite its
flaws, ASi wet-alkaline extraction is for the moment still the most
representative method to analyse for ASi (Saccone et al., 2007).
2.3. Water and silica mass balances
All calculations and statistical analyses were performed in R (R,
2006). Inlet and outlet culverts are the only exchange points with
the river. Their dimensions are exactly known. Flow velocity was
measured acoustically (Sontek ‘‘Argonaut’’). Water mass balances
were calculated with an averaged discharge value throughout the
water column for every 2 min, assuring accurate volume-weighing
of concentration values during all tidal phases. Measurements,
calibration and operation of the flowmeters were performed by
Flanders hydraulics research laboratory (W&L) experts. Concen-
tration profiles as well as nutrient discharges were calculated as
6004002000
3.0
3.5
4.0
May 06
600400
200
0
0
1
2
3
4
5
6
Jul 06
6004002000
4.0
4.5
5.0
5.5
6.0
Sep 06a
6004002000
4.0
4.5
5.0
Sep 06b
6004002000
3.8
4.0
4.2
4.4
4.6
4.8
5.0
Oct 06
6004002000
6.3
6.4
6.5
6.6
Mar 07a
6004002000
6.2
6.3
6.4
6.5
6.6
6.7
Mar 07b
600400
2000
1.0
1.5
2.0
2.5
3.0
3.5
4.0
Jun 07a
6004002000
1.0
1.5
2.0
2.5
3.0
3.5
Jun 07b
Time
(
min
)
DSi-concentration (g.L
-1
)
Fig. 4. DSi concentration profiles at in- and outstream location for all campaigns (tidal phases as in Fig. 3). Campaign month is indicated above each subpanel (dots are measured
concentrations, lines represent linear extrapolation).
Table 1
Sampling intensity of DSi samples at different locations (see Fig. 1C) and during
different campaigns
Location May
06
Jul
06
Sep
06a
Sep
06b
Oct
06
Mar
07a
Mar
07b
Jun
07a
Jun
07b
Total
1 53 8 15 17 24 16 1 1 15 15 174
21623571112 55129
8 7 12 12 12 43
61010 12 5542
5108 12 30
388 12 28
1 7 21 21
4109 19
15 16 16
16 14 14
12 12 12
98 8
11 7 7
10 5 5
14 5 5
13 3 3
73 3
Total 179 95 84 40 84 16 11 25 25 559
Table 2
Selection of samples analysed for ASi at different locations (see Fig. 1C) and during
different campaigns
Location May
06
Jul
06
Sep
06a
Sep
06b
Oct
06
Mar
07a
Mar
07b
Jun
07a
Jun
07b
Total
11216451286151593
297712 5545
665 12 5533
84612 22
33 12 15
1 7 12 12
56 6
46 6
11 5 5
Total 35 45 15 18 60 8 6 25 25 237
S. Jacobs et al. / Estuarine, Coastal and Shelf Science 80 (2008) 42–52 45

linear interpolations (Becker et al., 1988). ASi concentrations were
measured on average 10 times (range 4–16) and DSi concentrations
19 times (8–53) along each tidal cycle at the culverts. Interpolation
provided 700 values/tidal cycle, for discharge (D)aswellas
concentrations. These values were used to calculate absolute mass
balance by cumulative summing of (D (m
3
) [Si] (mg l
1
)) along
the instream and outstream phase separately. Stagnant phase and
volumes entering through small leaks in the outstream culvert
were not taken into consideration.
Total mass balances were first calculated as percentages
((
P
(out)
P
(in))/
P
(in)) in order to compare between different
tidal volumes, and then as absolute masses (
P
(out)
P
(in)). In
a conservative mass balance, it is assumed that there is no net
import or export of water. However, due to inter-tide variations,
stocking or surplus release of water volumes takes place. To
compare between tides, this conservative correction was calculated
as a percentage for each campaign, recalculated on the final mass
balance and shown as a range. However, general patterns were not
seriously influenced by this effect (Figs. 5 and 6).
Additional tidal features were measured in order to compare
between tides: average flooding height was calculated from total
volume of each entering tide and total surface of the study area, water
temperature was continuously monitored at culverts, and DSi and ASi
concentrations were monitored in adjacent river water. General
relationships between silica delivery and these tidal features were
explored through PCA and ANOVA analysis (Chevenet et al., 1994).
3. Results
3.1. Concentration profiles
DSi concentration profiles show different seasonal patterns
(Fig. 4). Instream phases (see Fig. 3) are marked by steep
concentration changes, whilst the fluctuations during stagnant
phase do not exceed 0.2 mg l
1
. Outstream concentration profiles
are highly variable and show increases, decreases or both: at
starting concentrations below 2 mg DSi l
1
, concentrations increase
with 125.0% and 126.6% (June 2007 in Fig. 4) or even with a factor 17
(July 2006 in Fig. 4) at final concentration. For instream concen-
trations higher than 2 mg l
1
, profiles show slight increases (10.5%
to 31.9%, May, September and October 2006 in Fig. 4). When
instream concentrations become higher than 6 mg l
1
, profiles
show a status quo or slight decrease (6%, March 2007 in Fig. 4)
towards final concentrations.
Concentration profiles of ASi present a more variable pattern
over a smaller concentration range (not shown). Although for
September 2006 and March 2007 only a limited number of samples
was analysed for ASi, and differences are generally lower for ASi
compared to DSi, there is a general evolution of increase or status
quo (May, July, September 2006 and October 2006, not shown)
towards strong ASi decrease in ASi- concentration profiles during
later tidal cycle (March and June 2007b, not shown), with the
exception of June 2007a.
3.2. Mass balances
Calculated DSi mass balances indicate enrichment of exported
water in summer months (July 2006 and June 2007, Fig. 5, upper
left), but also in late autumn (October 2006) and during one spring
campaign (March 2007). Although July 2006 shows spectacular
enrichment in percentage, absolute numbers (Fig. 5 upper right)
are lower due to small water mass at neap tide, while the opposite
is true for the 2007 campaigns. ASi mass balances confirm the
transition from slight ASi delivery or status quo towards ASi caption
by the marsh (Fig. 5, lower graphs).
May 06
Jul 06
Sep 06a
Sep 06b
Oct 06
Mar 07a
Mar 07b
Jun 07a
Jun 07b
relative ASi delivery (%)
-150
-100
-50
0
50
100
150
May 06
Jul 06
Sep 06a
Sep 06b
Oct 06
Mar 07a
Mar 07b
Jun 07a
Jun 07b
absolute ASi delivery (kg)
-200
-100
0
100
200
May 06
Jul 06
Sep 06a
Sep 06b
Oct 06
Mar 07a
Mar 07b
Jun 07a
Jun 07b
relative DSi delivery (%)
-100
-50
0
50
100
May 06
Jul 06
Sep 06a
Sep 06b
Oct 06
Mar 07a
Mar 07b
Jun 07a
Jun 07b
absolute DSi delivery (kg)
-150
-100
-50
0
50
100
150
Fig. 5. DSi (upper graphs) and ASi (lower graphs) mass balance of all campaigns. Balance is represented as percent, (out(g) in(g))/in(g)) (left graphs); and in absolute numbers (kg
delivered/retained) (right graphs). Error bars represent deviation from conservative mass balance, represented as percent in both directions.
S. Jacobs et al. / Estuarine, Coastal and Shelf Science 80 (2008) 42–5246

Figures (11)
Citations
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Journal ArticleDOI
Abstract: [1] Low-elevation bare intertidal flats and high-lying vegetated marshes are the main components of intertidal areas of estuaries, deltas and coastal embayments. Large-scale transitions between them have been reported worldwide. Because vegetated marshes provide significant services to coastal societies, predicting transitions between vegetated and unvegetated states is of widespread importance. Previous theoretical and modeling work highlighted the potential bistable nature of intertidal elevations, with low-elevation bare flats and high-elevation vegetated marshes being two alternative stable states. However, empirical evidence of this bistable condition is limited. In this study, we tested empirically the hypothesis that bare flats and vegetated marshes can be considered as alternative stable landscape states with the occurrence of rapid catastrophic shifts between them. We analyzed historical records of intertidal elevation surveys and aerial pictures from the macrotidal current-dominated Western Scheldt estuary (SW Netherlands). We found (1) a bimodal distribution of intertidal elevations corresponding to either a completely bare state or a densely vegetated state. (2) The shift from bare to vegetated state is accompanied with a relatively rapid shift in elevation, i.e., the mean accretion rate during the shift is 2 to 8 times larger than during the equilibrium state. (3) A threshold elevation could be identified above which the shift from bare to vegetated state has a high chance to occur. Hence, our results demonstrate the abrupt nonlinear shift between low-lying bare flats and high-elevation vegetated marshes, suggesting that the occurrence of catastrophic shifts between alternative stable states is indeed a potential mechanism in intertidal systems.

88 citations


Cites background from "Spatiotemporal aspects of silica bu..."

  • ...…[Wamsley et al., 2010], the biogeochemical filtering of sediments, nutrients and contaminants [e.g., Almeida et al., 2011; Gribsholt et al., 2005; Jacobs et al., 2008; Struyf et al., 2006; Temmerman et al., 2004], and the provision of nursery areas to commercially important fish and shellfish…...

    [...]


Journal ArticleDOI
Abstract: Tidal oscillations systematically flood salt marshes, transporting water, sediments, organic matter, and biogeochemical elements such as silica. Here we present a review of recent studies on these fluxes and their effects on both ecosystem functioning and morphological evolution of salt marshes. We reexamine a simplified model for the computation of water fluxes in salt marshes that captures the asymmetry in discharge between flood and ebb. We discuss the role of storm conditions on sediment fluxes both in tidal channels and on the marsh platform. We present recent methods and field instruments for the measurement of fluxes of organic matter. These methods will provide long-term data sets with fine temporal resolution that will help scientists to close the carbon budget in salt marshes. Finally, the main processes controlling fluxes of biogenic and dissolved silica in salt marshes are explained, with particular emphasis on the uptake by marsh macrophytes and diatoms.

76 citations


Cites background from "Spatiotemporal aspects of silica bu..."

  • ...…and autochtonous BSi production in tidal marshes result in high concentrations of DSi in marsh pore water and tidal pools (100–600 μM; e.g., Norris and Hackney 1999; Jacobs et al. 2008; Struyf et al. 2005b; Querné et al. 2012) Tidal pools can show a strong decrease in DSi due to diatom production....

    [...]

  • ...The general importance of BSi accumulation and DSi delivery by tidal marshes has been confirmed in a growing number of studies (e.g., Struyf et al. 2005a, 2006; Jacobs et al. 2008; Vieillard et al. 2011)....

    [...]

  • ...The high BSi import and autochtonous BSi production in tidal marshes result in high concentrations of DSi in marsh pore water and tidal pools (100–600 μM; e.g., Norris and Hackney 1999; Jacobs et al. 2008; Struyf et al. 2005b; Querné et al. 2012) Tidal pools can show a strong decrease in DSi due to diatom production....

    [...]


Journal ArticleDOI
Abstract: Along coasts and estuaries, formerly embanked land is increasingly restored into tidal marshes in order to re-establish valuable ecosystem services, such as buffering against flooding. Along the Scheldt estuary (Belgium), tidal marshes are restored on embanked land by allowing a controlled reduced tide (CRT) into a constructed basin, through a culvert in the embankment. In this way tidal water levels are significantly lowered (ca. 3 m) so that a CRT marsh can develop on formerly embanked land with a ca. 3 m lower elevation than the natural tidal marshes. In this study we compared the long-term change in elevation (ΔE) within a CRT marsh and adjacent natural tidal marsh. Over a period of 4 years, the observed spatio-temporal variations in ΔE rate were related to variations in inundation depth, and this relationship was not significantly different for the CRT marsh and natural tidal marsh. A model was developed to simulate the ΔE over the next century. (1) Under a scenario without mean high water level (MHWL) rise in the estuary, the model shows that the marsh elevation-ΔE feedback that is typical for a natural tidal marsh (i.e. rising marsh elevation results in decreasing inundation depth and therefore a decreasing increase in elevation) is absent in the basin of the CRT marsh. This is because tidal exchange of water volumes between the estuary and CRT marsh are independent from the CRT marsh elevation but dependent on the culvert dimensions. Thus the volume of water entering the CRT remains constant regardless of the marsh elevation. Consequently the CRT MHWL follows the increase in CRT surface elevation, resulting after 75 years in a 2–2.5 times larger elevation gain in the CRT marsh, and a faster reduction of spatial elevation differences. (2) Under a scenario of constant MHWL rise (historical rate of 1.5 cm a− 1), the equilibrium elevation (relative to MHWL) is 0.13 m lower in the CRT marsh and is reached almost 2 times faster. (3) Under a scenario of accelerated MHWL rise (acceleration of 0.02 cm a− 1), the CRT marsh is much less able to keep up with the MHWL rise; after 75 years the CRT elevation is already 0.21 m lower than for the natural marsh. In conclusion, this study demonstrates that although short-term (4 years) ΔE rates are similar in a restored CRT marsh and natural tidal marsh, these ecosystems may evolve differently in response to sea-level rise in the longer term (10–100 years).

64 citations


Cites background from "Spatiotemporal aspects of silica bu..."

  • ...Besides their safety function, tidal marshes play an important role in the cycling of nutrients (e.g., Gribsholt et al., 2005; Jacobs et al., 2008; Struyf et al., 2006) and they are characterized by high habitat diversity....

    [...]


Journal ArticleDOI
Abstract: Throughout the world, tidal marshes are being restored to obtain natural protection against recurring storm surges and sea level rise and to preserve the resources these habitats provide. Managed realignment, or breaching of the seaward dikes, is a restoration technique increasingly used; yet unsuitable site elevation has been a major constraint to habitat development. Controlled reduced tide (CRT), a new technique to implement tidal regime on low elevated sites, could offer solutions for several problems associated with reconstruction or mitigation of tidal marshes. We followed vegetation development at 3-month intervals during two years after implementation of the first pilot CRT treatment, which aimed to restore tidal freshwater marsh habitat in an agricultural site where elevation relative to the estuarine marshes has lowered. The development of the plant community and its driving forces (flooding, remnant vegetation) were studied. Arrival and establishment of typical tidal freshwater marsh vegetation was rapid, and a clear elevation gradient developed by eradication of terrestrial species and colonisation by estuarine species. Remnant vegetation substantially reduced the number of new established species. General development trajectory as well as the changing effect of driving forces and their interaction point to a swift ecosystem shift during the first two surveys, and a gradual species saturation during the seven following surveys. Vegetation communities seem to deviate from the nearby tidal freshwater marshes, but show similarities with tidal freshwater marshes described for the beginning of the 20th century. The CRT-technique provides strong potential for durable, adaptive restoration of tidal marshes on sites with low elevation. Wider implementation of the CRT technique could increase the total surface of tidal freshwater marshes with fully developed vegetation gradients.

47 citations


Additional excerpts

  • ...2009; Jacobs et al., 2008)....

    [...]


References
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Journal ArticleDOI
15 May 1997-Nature
Abstract: The services of ecological systems and the natural capital stocks that produce them are critical to the functioning of the Earth's life-support system. They contribute to human welfare, both directly and indirectly, and therefore represent part of the total economic value of the planet. We have estimated the current economic value of 17 ecosystem services for 16 biomes, based on published studies and a few original calculations. For the entire biosphere, the value (most of which is outside the market) is estimated to be in the range of US$16-54 trillion (10^(12)) per year, with an average of US$33 trillion per year. Because of the nature of the uncertainties, this must be considered a minimum estimate. Global gross national product total is around US$18 trillion per year.

15,867 citations


Additional excerpts

  • ...Estuaries are biogeochemical hot-spots and are amongst the most productive ecosystems of the world ( Costanza et al., 1993 )....

    [...]


Journal ArticleDOI
Abstract: This article provides a crude initial estimate of the value of ecosystem services to the economy. Using data from previous published studies and a few original calculations the current economic value of 17 ecosystem services for 16 biomes was estimated. The services of ecological systems and the natural capital stocks that produce them are critical to the functioning of the Earths life-support system. They contribute to human welfare both directly and indirectly and therefore represent part of the total economic value of the planet. It was estimated that for the entire biosphere the value (most of which is outside the market) ranges US$16-54 trillion/year with an average of US$33 trillion/year. Due to the nature of uncertainties this must be considered a minimum estimate. In addition the global gross national product total is around US$18 trillion/year.

2,453 citations


Journal ArticleDOI
21 Apr 1995-Science
TL;DR: The net inputs of silicic acid (dissolved silica) to the world ocean have been revised to 6.1 � 2.0 teramoles of silicon per year (1 teramole = 1012 moles).
Abstract: The net inputs of silicic acid (dissolved silica) to the world ocean have been revised to 6.1 +/- 2.0 teramoles of silicon per year (1 teramole = 10(12) moles). The major contribution (about 80 percent) comes from rivers, whose world average silicic acid concentration is 150 micromolar. These inputs are reasonably balanced by the net ouputs of biogenic silica of 7.1 +/- 1.8 teramoles of silicon per year in modern marine sediments. The gross production of biogenic silica (the transformation of dissolved silicate to particulate skeletal material) in surface waters was estimated to be 240 +/- 40 teramoles of silicon per year, and the preservation ratio (opal accumulation in sediment/gross production in surface waters) averages 3 percent. In the world ocean the residence time of silicon, relative to total biological uptake in surface waters, is about 400 years.

1,272 citations


Journal Article
Abstract: We estimate the global rate of biogenic silica production in the ocean to be between 200 and 280 × 1012 mol Si yr−1. The upper limit is derived from information on the primary productivity of the oceans, the relative contribution of diatoms to primary production and diatom Si/C ratios. The lower limit is derived independently using a multi-compartment model of nutrient transport and biogenic particle flux, and field data on the balance between silica production and dissolution in the upper ocean. Our upper limit is 30–50% lower than several previous estimates, due to new data indicating lower values for both the relative contribution of diatoms to primary productivity and their Si/C ratios. Globally, at least 50% of the silica produced by diatoms in the euphotic zone dissolves in the upper 100 m, resulting in an estimated export of 100–140 × 1012 mol Si yr−l to the deep ocean. Our estimates correspond to a global mean rate of biogenic silica production between 0.6 and 0.8 mol Si m−2 yr−1. Incubation experiments indicate that silica production rates exceed that mean by a factor of 3–12 in coastal areas and are 2–4 times less than the global average in the oligotrophic mid-ocean gyres. The mean silica production rate in waters overlying diatomaceous sediments (approximately 10–12% of the surface area of the oceans) is 0.7–1.2 mol Si m−2 yr−1. That rate is only slightly higher than the global average, indicating that the silica produced in those regions is only 10–25% of the global total. The estimated production of biogenic silica in surface waters of the mid-ocean gyres is approximately equal to that for all major areas of opal sediment accumulation combined. Regional comparison of silica production and accumulation rates suggests a strongly bimodal character in the efficiency of opal preservation in the sea. In waters overlying diatom-rich sediments 15–25% of the silica produced in the surface layer accumulates in the seabed, while virtually none of the silica produced in other areas is preserved. The global burial/production ratio of ˜ 3% is a composite of those two very different systems. The mechanisms leading to more efficient opal preservation in regions of silica accumulation are presently unknown, but they have no simple relationship to primary productivity. Regional differences in opal preservation appear to be controlled by factors such as low surface temperature, selective grazing and aggregate formation, which diminish the rate of silica dissolution in surface waters and/or accelerate its transport to the seafloor.

1,211 citations


Journal ArticleDOI
Abstract: We estimate the global rate of biogenic silica production in the ocean to be between 200 and 280 × 1012 mol Si yr−1. The upper limit is derived from information on the primary productivity of the oceans, the relative contribution of diatoms to primary production and diatom Si/C ratios. The lower limit is derived independently using a multi-compartment model of nutrient transport and biogenic particle flux, and field data on the balance between silica production and dissolution in the upper ocean. Our upper limit is 30–50% lower than several previous estimates, due to new data indicating lower values for both the relative contribution of diatoms to primary productivity and their Si/C ratios. Globally, at least 50% of the silica produced by diatoms in the euphotic zone dissolves in the upper 100 m, resulting in an estimated export of 100–140 × 1012 mol Si yr−l to the deep ocean. Our estimates correspond to a global mean rate of biogenic silica production between 0.6 and 0.8 mol Si m−2 yr−1. Incubation experiments indicate that silica production rates exceed that mean by a factor of 3–12 in coastal areas and are 2–4 times less than the global average in the oligotrophic mid-ocean gyres. The mean silica production rate in waters overlying diatomaceous sediments (approximately 10–12% of the surface area of the oceans) is 0.7–1.2 mol Si m−2 yr−1. That rate is only slightly higher than the global average, indicating that the silica produced in those regions is only 10–25% of the global total. The estimated production of biogenic silica in surface waters of the mid-ocean gyres is approximately equal to that for all major areas of opal sediment accumulation combined. Regional comparison of silica production and accumulation rates suggests a strongly bimodal character in the efficiency of opal preservation in the sea. In waters overlying diatom-rich sediments 15–25% of the silica produced in the surface layer accumulates in the seabed, while virtually none of the silica produced in other areas is preserved. The global burial/production ratio of ˜ 3% is a composite of those two very different systems. The mechanisms leading to more efficient opal preservation in regions of silica accumulation are presently unknown, but they have no simple relationship to primary productivity. Regional differences in opal preservation appear to be controlled by factors such as low surface temperature, selective grazing and aggregate formation, which diminish the rate of silica dissolution in surface waters and/or accelerate its transport to the seafloor.

1,170 citations


Frequently Asked Questions (1)
Q1. What are the contributions mentioned in the paper "Spatiotemporal aspects of silica buffering in restored tidal marshes" ?

In this study, the authors investigated the potential impact of recently installed new tidal areas along the Schelde estuary, located in former polder areas and characterized by so-called controlled reduced tidal regimes ( CRT ). Although further research is necessary on the driving forces of the different processes involved, restoration of former agricultural areas under CRT-regime provide the potential to buffer silica in the estuary.