Download Biogeochemical cycles in sediment and water column of the

Ocean & Coastal Management 68 (2012) 102e113
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Ocean & Coastal Management
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Biogeochemical cycles in sediment and water column of the Wadden Sea:
The example Spiekeroog Island in a regional context
Melanie Beck*, Hans-Jürgen Brumsack
Institute for Chemistry and Biology of the Marine Environment (ICBM), Carl von Ossietzky University, Carl-von-Ossietzky-Str. 9-11, D-26129 Oldenburg, Germany
a r t i c l e i n f o
a b s t r a c t
Article history:
Available online 17 June 2012
Tidal flats like the Wadden Sea are areas of high primary production and organic matter remineralization
rates. This paper provides an overview of benthic remineralization pathways and the recycling of various
metabolic products, exemplified by interdisciplinary studies around Spiekeroog Island (Germany).
Organic matter produced in the Wadden Sea area as well as material imported from the North Sea is
remineralized in tidal flat sediments. Wadden Sea sediments may thus be regarded as biogeochemical
reactors promoting or accelerating organic matter remineralization. Due to advective flow, which is of
special importance in permeable sandy sediments, pore waters enriched in remineralized nutrients and
methane are actively released from sediments into the overlying water column. This biogeochemical
recycling forms the prerequisite for continuously high primary production in the Wadden Sea, and
proves a tight coupling between benthic and pelagic dynamics. Additionally, the export of excess
nutrients from the Wadden Sea further offshore may trigger biological activity in coastal waters of the
North Sea. In this contribution, we will also summarize open questions which need to be answered for
a thorough understanding, management and protection of the unique Wadden Sea ecosystem. In
particular, the currently understudied, but potentially significant effects of climate change (e.g., rising sea
level and increase in storm surge extremes) on biogeochemical cycles in sediments and open waters of
the Wadden Sea are discussed.
Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction
The tidal flats of the Wadden Sea form the largest continuous
area of sand- and mudflats worldwide, accounting for 60% of all
tidal areas in Europe and North Africa (Marencic, 2009). The region
sustains a rich and diverse flora and fauna and is of outstanding
international importance as staging and wintering area for migratory birds (Marencic, 2009). Due to its unique nature, the Wadden
Sea has been inscribed on the World Heritage List in 2009 and often
serves as a worldwide reference for comparisons with other tidal
flat systems. This highlights the importance to attain the best
possible understanding of biological, chemical and physical
processes sustaining this ecosystem. The intense biogeochemical
cycling of carbon and nutrients has been identified as crucial for
controlling life and ecosystem dynamics in the Wadden Sea.
Organisms living in tidal flat ecosystems have to tolerate
extreme environmental gradients in salinity, incident light, oxygen
availability and temperature. Nevertheless, this type of landscape
* Corresponding author. Tel.: þ49 441 7983627; fax: þ49 441 7983404.
E-mail address: [email protected] (M. Beck).
0964-5691/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.ocecoaman.2012.05.026
exhibits high rates of primary production (Cadée and Hegeman,
2002; Loebl et al., 2007; Poremba et al., 1999). These high rates
are not only supported by the presence of well-adapted phytoplankton and microorganism consortia, but also by enhanced
nutrient availability due to rapid organic matter (OM) remineralization. In intertidal areas, aerobic and anaerobic OM degradation
processes are fuelled by filtration of suspended particles and dissolved OM from the water column within permeable sediments.
Continuous supply of organic substrate supports enhanced microbial activity, and ultimately the release of metabolic products such
as nutrients and methane (CH4) to the pore waters. Tidal pumping
induces advective flushing of permeable sediments and the transport of remineralization products to the open water column, where
they can once again support primary production. Furthermore, the
Wadden Sea is an open system where water exchange with the
North Sea occurs through tidal inlets. Thus, the quality of water,
sediment and marine habitats is to a large degree influenced by
processes occurring in the North Sea and vice versa.
In this paper we will summarize benthic remineralization
pathways, with a focus on metabolic products including nutrients
and CH4. Benthic OM remineralization and the subsequent nutrient
recycling are essential mechanisms providing important building
M. Beck, H.-J. Brumsack / Ocean & Coastal Management 68 (2012) 102e113
blocks of life. The trace gas CH4, which is produced as metabolic
end-member in deeper sediments, may contribute to global
warming when released to the atmosphere. However, the assessment of CH4 sources and sinks is still not fully explored in intertidal
areas such as the Wadden Sea. Most of the biogeochemical cycles
are exemplarily presented using interdisciplinary studies around
Spiekeroog Island (Germany). Our paper further addresses open
scientific questions and future challenges for the protection of the
Wadden Sea. A thorough understanding of biogeochemical
processes in the Wadden Sea will be a requirement for its
sustainable future management, as biogeochemical cycles form an
essential mechanism controlling chemical reactions and biological
growth in this ecosystem. Gaps in knowledge, e.g. regarding the
effects of climate change on the Wadden Sea, need to be recognized
and addressed by future research plans.
103
acceptor in the cascade will occur when the pool of an electron
acceptor with a higher energy yield is depleted. Nevertheless,
microorganisms may create their niche by using substrates not
consumed by the community living in the adjacent zone (van der
Maarel and Hansen, 1997; Wilms et al., 2006). Furthermore, pore
water exchange may blur the gradients by a continuous resupply of
electron acceptors (Beck et al., 2008c; Jansen et al., 2009). Therefore, Fig. 1 does not exhibit the conventional redox sequence in
mineralization pathways, but shows that in the highly dynamic
intertidal sediments, the classical redox cascade may be disturbed
by pore water flow, especially in the strongly irrigated top layer.
Finally, processes of OM remineralization lead to the accumulation
of dissolved organic carbon (DOC), alkalinity, and nutrients with
increasing depth in pore waters (Fig. 1).
2.1. Aerobic oxidation, reduction of nitrate and Mn/Fe oxides
2. OM remineralization in Wadden Sea sediments
In the uppermost sediment layer, aerobic respiration dominates
OM degradation. The depth where oxygen is still available for
carbon (C) mineralization depends on sediment permeability. The
high permeability of sand facilitates advective pore water transport, in contrast to diffusion-controlled muddy sediments.
Furthermore, pore water flow is enhanced in surface sediments
with ripple structures due to pressure gradients generated by the
interaction of bottom currents with sediment topography (Huettel
and Gust, 1992; Huettel et al., 2003). This pore water flow at the
sediment surface is an effective mechanism for rapid exchange of
oxygen (Precht et al., 2004; Ziebis et al., 1996). Consequently,
Bioturbation, bioirrigation, and/or diffusive and advective
processes introduce OM into surface sediments of the Wadden Sea
(Meysman et al., 2005, 2007; Rusch et al., 2001; Volkenborn et al.,
2007; see chapter 3) where it is degraded by a cascade of redox
processes. Aerobic respiration is followed by nitrate reduction,
reduction of manganese (Mn) and iron (Fe) oxides, sulfate reduction and finally methanogenesis (Froelich et al., 1979; Jørgensen,
2006). Theoretically, the sediments are divided into different
zones, each characterized by a microbial community using
a specific electron acceptor. In general, a shift to the next electron
-
Mn [µM]
NO [µM]
3
0
0
3
6
9
12
0
8
16
SO
Fe [µM]
24
32
0
4
8
12
16
0
8
24
[mM]
16
24
32
Depth [m]
1
2
3
4
5
0
0
1
2
3
+
4
0
12
24
36
3-
NH4 [mM]
Alkalinity [mM]
DOC [mM]
48
0
3
6
9
PO4 [µM]
12
0
300
600
900 1200
Si(OH)4 [µM]
0
300
600
900 1200
Depth [m]
1
2
3
4
5
Fig. 1. The redox cascade in Wadden Sea sediments: pore water profiles of nitrate, manganese (Mn), iron (Fe), and sulfate. The compounds are either used as electron acceptors or
3
produced during OM remineralization. Dissolved organic carbon (DOC), alkalinity, NHþ
4 , PO4 , and Si(OH)4 accumulate with depth as they are released when OM or diatom frustules
are degraded. The sampling site is located in an intertidal sand flat close to a creek bank in the backbarrier area of Spiekeroog Island. Permanently installed samplers were used to
extract pore waters from 20 different depths (Beck et al., 2007). Data are derived from a sampling campaign in April 2006. The figure was modified after Beck et al. (2008a,b).
M. Beck, H.-J. Brumsack / Ocean & Coastal Management 68 (2012) 102e113
dsrA targets [g-1 sediment]
Sulfate [mM]
0
10
20
30
1x106
2x106
0
1
Depth [m]
oxygen is depleted within the uppermost millimeters in clay-rich
sediments (Böttcher et al., 2000), whereas oxic conditions persist
down to some centimeters depth in permeable sediments (Jansen
et al., 2009).
The oxygen penetration in permeable surface sediments leads to
enhanced aerobic mineralization. Highest areal oxygen consumption rates (OCR) reaching up to 190 mmol m2 day1 were
measured, for example, in tidal flat margin sediments flushed with
seawater each tidal cycle (Billerbeck et al., 2006b; de Beer et al.,
2005; Werner et al., 2006). To obtain these areal OCRs, the potential OCRs measured were integrated over the oxygen penetration
depths. Aerobic mineralization rates were calculated by subtracting
depth-integrated sulphate reduction rates from OCRs (assuming
that sulphate reduction is the most important anaerobic OM
degradation process) and reached up to 150 mmol C m2 day1
(Werner et al., 2006). The OCRs determined in the permeable
top layer of Wadden Sea sediments are about 5 times higher
than in open North Sea sediments (Osinga et al., 1996; Upton et al.,
1993).
The importance of denitrification as well as of Mn/Fe oxide
reduction for OM remineralization is not as well-constrained as the
contribution of aerobic respiration. In general, it was estimated that
denitrification accounts only for a small percentage to carbon
oxidation in coastal North Sea sediments (Sørensen et al., 1979;
Werner et al., 2006). Estuarine intertidal mudflat sediments
exhibit decreasing nitrous oxide emissions with increasing salinity,
with emissions of almost zero at a salinity of 15 (Middelburg et al.,
1995). A whole ecosystem 15N labeling experiment in a tidal
freshwater marsh fringing the nutrient-rich Scheldt River, Belgium,
indicated that only 2.4 and 0.02% of the watershed derived
ammonium was transformed to N2, N2O, respectively (Gribsholt
et al., 2006). However, permeable Wadden Sea sediments show
potential denitrification rates up to 0.19 mmol N m2 h1, one of the
highest rates determined in the marine environment so far (Gao
et al., 2010). The denitrifying bacteria appear well-adapted to
tidally induced redox oscillations in these permeable sediments
because denitrification is not entirely inhibited even when oxygen
is available. The contributions of Mn and Fe reduction to OM
remineralization are minor, but gain importance in Mn- or Fe-rich
sediment layers (Canfield et al., 1993; Thamdrup et al., 1994). In
Wadden Sea sediments, a Fe(III)-reducing bacterial strain accounted for up to 6% of total cell numbers and even exceeded the
numbers of sulfate-reducing bacteria in the upper sediment layers
(Mussmann et al., 2005). Thus, these bacteria may substantially
contribute to carbon degradation via dissimilatory reduction of Fe
oxides in surface sediments of tidal flats.
2
3
4
0
50
150
100
Methane [nmol cm-3 ]
10
20
1x105
2x105
mcrA targets [g-1 sediment]
dsrA targets [g-1 sediment]
Sulfate [mM]
0
30 0
2x106
4x106
6x106
0
5
Depth [m ]
104
10
15
20
0
2x106
4x106
6x106
100
200
300
400 0
Methane [nmol g -1 ]
mcrA targets [g-1 sediment]
Fig. 2. Sulfate and CH4 show inverse depth profiles, with CH4 maxima in the sulfatedepleted zone. The key-genes for sulfate reduction and methanogenesis, the a-units of
the dissimilatory sulfite reductase (dsrA) and the methyl coenzyme-M reductase
(mcrA), respectively, correspond well with the vertical sulfateemethane profiles. The
grey bars highlight the sulfateemethane interfaces. The figure was modified after
Engelen and Cypionka (2009) and Beck et al. (2011).
2.2. Sulfate reduction and methanogenesis
Sulfate reduction and methanogenesis are the main terminal
pathways of anaerobic OM remineralization in sediments of the
Wadden Sea, and have been intensively studied down to 5 m depth
in the tidal flats of Spiekeroog Island. Sulfate-reducing bacteria
form highly abundant and active populations in anoxic sediments
(Gittel et al., 2008; Ishii et al., 2004; Llobet-Brossa et al., 2002). In
general, sulfate and CH4 show inverse depth profiles, with a CH4
maximum in the sulfate-depleted zone (Fig. 2; Beck et al., 2009;
Wilms et al., 2007). The population sizes of Bacteria, Archaea,
sulfate reducers, and methanogens correspond well to the vertical
sulfateemethane profiles (Fig. 2; Wilms et al., 2007). Although
sulfate reducers and methanogens may compete for the same
substrates, methanogens can avoid competition by utilizing noncompetitive substrates such as methylated compounds (Wilms
et al., 2006). This may be one reason why methanogens are not
restricted to the sulfate-depleted sediment layers.
Within tidal flats, large spatial differences in sulfate depth
profiles point towards changes in sulfate reduction rates. In central
parts of tidal flats, seawater sulfate concentrations persist from the
surface down to 5 m depth (Beck et al., 2008c, 2009). In contrast, in
creek bank sediments, concentrations decrease strongly with depth
and reach values close to zero within a few decimeters (Al-Raei
et al., 2009; Beck et al., 2009; Riedel et al., 2011). In these sediments, microbial activity is enhanced by two processes supplying
substrate and/or sulfate. First, rapid sedimentation leads to
enhanced OM sequestration at prograding tidal flat margins (Beck
et al., 2009; Oenema, 1990). Second, surface and deep pore water
advection replenishes the dissolved organic matter and sulfate
pools (Billerbeck et al., 2006b; Huettel et al., 2007; Huettel and
Rusch, 2000; Riedel et al., 2010). The increase in microbial
activity across creek bank sediments is not only reflected in sulfate,
but also in CH4 profiles as well as in total cell numbers, sulfate
reduction rates, and concentrations of remineralization products
M. Beck, H.-J. Brumsack / Ocean & Coastal Management 68 (2012) 102e113
(Al-Raei et al., 2009; Beck et al., 2009; Røy et al., 2008; Werner et al.,
2006). In contrast to creek bank sediments, low sedimentation
rates and pore water flow velocities limit the microbial activity in
central parts of the tidal flats despite the high availability of electron acceptors (such as sulfate) for microbial respiration.
On average 78 g C m2 year1 are mineralized in surface sediments of the Wadden Sea via sulfate reduction (Al-Raei et al., 2009).
This estimate is based on a modeling approach for the 154 km2
large tidal area of Spiekeroog Island using sulfate reduction rates of
the top 15 cm sediments, a mapping of surface sediment distribution (mudflats, mussel banks, mixed flats dark, sand flat, and light
sand flat), and empirical site-specific temperature relations of
sulfate reduction rates. The importance of sulfate reduction for OM
remineralization in these surface sediments is, however, constrained by the finding that sulfate reduction contributes only up to
25% to total OM mineralization (Billerbeck et al., 2006b; Werner
et al., 2006). In North Sea sediments, the relative contribution of
sulfate reduction to total OM remineralization is similar, but rates
are a factor 5e10 lower than in Wadden Sea sediments (Upton et al.,
1993). Overall, these data can be compared to estimates for pelagic
and benthic primary production in the Wadden Sea. A total
production between 30 and 950 g C m2 year1 was reported
(Billerbeck et al., 2006a; Tillmann et al., 2000).
3. Benthicepelagic coupling
Biogeochemical processes in the sediment are closely coupled to
dynamics in the open water and vice versa. Especially in areas
dominated by permeable sandy sediments, the exchange of dissolved and particulate compounds between both ecosystems is fast.
Billerbeck et al. (2006b) proposed a concept consisting of two pore
water circulation processes for permeable sediments, leading to
a tight coupling of sediment and water column biogeochemistry.
‘Skin circulation’ forms an effective mechanism for rapid water
exchange in surface sediments. It results from pressure gradients
generated when bottom currents are deflected by small sediment
structures of hydrodynamic or biological origin. This type of
circulation exerts a major control on the exchange of dissolved and
particulate organic matter across the sedimentewater interface
(Ehrenhauss et al., 2004; Huettel and Rusch, 2000; Huettel et al.,
1996; Rusch et al., 2001; Rusch and Huettel, 2000), microbial and
benthos ecology (de Beer et al., 2005; Evrard et al., 2010;
Middelburg et al., 2000), and sediment biogeochemistry (Evrard
et al., 2008; Huettel et al., 1998). The close coupling of algal cell
concentrations in the boundary layer and those in the uppermost
sediment layer even suggest that permeable sediments may act as
short-term storage buffer for phytoplankton (Huettel et al., 2007).
In contrast, ‘body circulation’ affects pore waters located at
greater depth (up to some meters) in creek bank sediments. It is
generated by the hydraulic gradient between the seawater level in
a tidal channel and the pore water level in the sediment. The
hydraulic gradient starts to develop as soon as the tidal flat surface
falls dry and is highest during low tide (Riedel et al., 2010; Wilson
and Gardner, 2006). The induced deep pore water flow is directed
towards the tidal channel, with highest flow velocities in sediments
close to the low water line. This deep advective flow supplies
permeable sediments with electron acceptors and donors and leads
to the discharge of nutrient-rich pore waters into the water column
(Beck et al., 2008c; Billerbeck et al., 2006b; Riedel et al., 2011).
3.1. Estimates of pore water flow velocities and discharge
Knowledge about pore water flow velocities and the amount of
pore water discharged from the sediments to the open water
column is essential to budget biogeochemical cycles in the Wadden
105
Sea. Different approaches have been used to estimate exemplarily
pore water flow rates in the Janssand tidal flat located in the
backbarrier area of Spiekeroog Island. This tidal flat is covered by
approximately 1.5e2 m of water during high tide and becomes
exposed for about 6e8 h during low tide. It is almost plane,
except at the margin where the sediment surface slopes with
1.6 cm m1 over ca. 80 m (Billerbeck et al., 2006a). Pore water
flow estimates are still challenging, which is reflected by the
different measured or modeled rates. One of the challenges is
related to the extrapolation of point measurements to a larger area.
The question whether the on-site lithology or topography is
representative for the investigated tidal basin or the entire Wadden
Sea has to be addressed. By studying eight sites in the West and East
Frisian Wadden Sea, Røy et al. (2008) showed that topography and
seepage zone of the well-studied Janssand tidal flat are representative on a regional scale.
The first estimate for pore water flow velocity and discharge at
the Janssand tidal flat was presented by Billerbeck et al. (2006a).
The horizontal flux was measured by following the passage of
a fluorescent dye through the sediment at depths from 2 to 50 cm,
and ranged from 0.5 to 0.9 cm h1. Pore water discharge from the
sloping margin was quantified during exposure by measuring the
volume of water collected at the end of an open seepage meter.
Measured rates of discharge were 2.4 (March) and 4.2 l m2 d1
(July).
Finite element modeling was applied by Røy et al. (2008) to
predict pathways and ages of pore water. The calculated flow
pattern can explain the CH4 and sulfate distributions measured
along a transect across the creek bank, and predicts a residence
time of the seepage water of about 30 years. This corresponds to
a calculated pore water flow ranging from 0.5 to 7 l m2 d1,
depending on the permeability used in the model approach. In
contrast to the estimate of Billerbeck et al. (2006a), which integrates the rapid shallow flow over the entire slope, Røy et al. (2008)
considered only the CH4-rich pore water flowing deeper and
slower.
Riedel et al. (2010) applied a hydrogeological numerical simulation to investigate groundwater flow in sediment layers down to
5 m depth. Maximum simulated groundwater velocities of up to
7 cm h1 are reached during ebb tide, driving circulation of
seawater through the sediment with subsequent discharge.
Discharge averages 0.97 m3 per meter of margin length per tide, but
may vary significantly, most pronounced with the spring-neap tide
cycle. Pore water ages were calculated using the same hydrogeological simulation with some slight modifications (Riedel et al.,
2011). For example, sulfate is almost completely depleted after 200
days when it reaches the discharge zone close to the low water line.
In a fourth approach, radium isotopes were applied to quantify
the flow of pore water entering the tidal channel during low tide.
Using a flushing time of four days for the water mass within the
backbarrier basin and average activities of 224Ra, 223Ra, and 228Ra
measured in the backbarrier surface and pore waters, a balance of
these isotopes was constructed, which is sustained by a pore water
flow of 2e4 108 l per tidal cycle (Moore et al., 2011).
Depending on the applied approach, the total volume of
measured or modeled pore water discharge varies. The different
estimates are subjected to errors resulting from the necessary
approximations. Moore et al. (2011) tried to compare the different
fluid flow estimates and concluded that the average flow rate
estimated by Riedel et al. (2010) is 20e40 times larger than the flow
presented by Billerbeck et al. (2006a), whereas it represents only
about 20% of the radium-based fluid flow for the entire area. In
contrast to Riedel et al. (2010), who evaluated deep pore water
fluxes, Billerbeck et al. (2006a) only considered fluxes in the upper
sediment layers. However, the Riedel et al. (2010) estimate only
106
M. Beck, H.-J. Brumsack / Ocean & Coastal Management 68 (2012) 102e113
covers pore water input from one distinct transect located at the
Janssand margin, whereas the radium isotopes include pore water
input from other tidal flats as well as groundwater input from
terrestrial sources.
3.2. Nutrient and methane release from sediments
Pore water flux estimates coupled to knowledge of pore water
concentrations permit to calculate the release of species such as
nutrients and methane from the sediment to the water column.
Seepage measurements and pore water nutrient concentrations
were used to estimate nutrient effluxes by Billerbeck et al. (2006a).
For instance, they reached 7.6 mmol m2 d1 for NHþ
4,
2 1
d for Si(OH)4 in
2.5 mmol m2 d1 for PO34 , and 1.7 mmol m
July. Furthermore, the nutrient input by pore water seepage was
estimated based on radium isotope measurements. In the entire
Spiekeroog tidal flat area, about 2e23 104 mol Si(OH)4 are
released from the sediments each tidal cycle (Moore et al., 2011).
The release of CH4 from sediments can be reduced by its aerobic
and anaerobic oxidation (Krüger et al., 2005; Treude et al., 2005).
For example, CH4 diffusing upwards into the sulfate zone is
oxidized to CO2 at the expense of sulfate, which amounts to
a sulfate loss equivalent to about 10% of the total sulfate reduction
in coastal sediments (Jørgensen et al., 2001). In the Wadden Sea
area, profiles of the key genes for dissimilatory sulfate reduction
and methanogenesis suggest anaerobic oxidation of methane
(AOM) at sulfateemethane transition zones (Wilms et al., 2007).
First measurements of ex-situ AOM rates show that rates peak in
the sulfateemethane transition zones (Beck et al., 2011).
At tidal flat margins, pore water exchange occurs on short
timescales, supporting the coexistence of sulfate and CH4.
Furthermore, the seeps on the slope of the Janssand tidal flat are
sulfidic, depleted in sulfate, and saturated with CH4 (Røy et al.,
2008). Thus, an oxic surface layer where re-oxidation may
prevent the emission of CH4 is partly missing. Røy et al. (2008) tried
to assess the importance of CH4 seepage for the carbon cycle in tidal
flats. Assuming a pore water flow rate of 0.5 l m2 d1 and CH4
saturation in pore waters, they calculated that 0.65 mmol CH4 are
transported to the open water column per square meter per day.
They estimated that on the Janssand tidal flat, the anoxic seeps
comprise 2% of the surface area, and only leak about
0.013 mmol C m2 d1 averaged over the entire sand flat. The
authors conclude that this is at least three orders of magnitude
lower than the rates of primary production and mineralization in
the Wadden Sea, and thus CH4 seepage is of limited importance for
the carbon cycle in tidal flats. Similarly, Middelburg et al. (2002)
identified tidal flats and creeks as CH4 sources to estuarine
waters, but concluded that estuaries contribute less than 9% to the
global marine CH4 emission.
4. Biogeochemical cycles in Wadden Sea waters
The release of nutrient-rich pore waters to the overlying water
column is an important mechanism triggering biological processes
in Wadden Sea waters. Pulses of elevated nutrient concentrations
are frequently observed in the water column during low tide
(Grunwald et al., 2010), when the discharge of nutrient-rich pore
waters from creek bank sediments is highest (Riedel et al., 2010)
(Fig. 3). The tight benthicepelagic coupling further prevents
extended periods of nutrient depletion that could limit phytoplankton growth. Only shortly after the spring phytoplankton
bloom, silicate and phosphate concentrations are close to zero
(Fig. 4), but the nutrient pools are quickly replenished by pore
water supply (Billerbeck et al., 2006b; Grunwald et al., 2010;
Kowalski et al., 2009). In contrast to phosphate, the effect of
resupply is less pronounced in seasonal silicate dynamics due to
assimilation by diatoms even outside the bloom periods. Only
nutrients like nitrate and nitrite, which are not enriched but
consumed in pore waters, show a long-lasting depletion in the
water column from early spring until autumn (Fig. 4).
Similar to nutrients, CH4 exhibits a tidally driven pattern in the
water column with highest concentrations during low tide
(Grunwald et al., 2009) (Fig. 3). The elevated CH4 levels during low
tide may have two major sources: pore water discharge from
sediments, and/or freshwater released to the tidal flat area via
flood-gates. Pore waters seeping from creek bank sediments during
low tide are rich in CH4 (Al-Raei et al., 2009; Røy et al., 2008). These
CH4-rich pore waters are the main source transporting CH4 to the
30
20
PO43- [µM]
Si(OH)4 [µM]
1.6
10
1.2
0.8
0.4
0
16/Nov
17/Nov
18/Nov
0
11/Jun
19/Nov
12/Jun
13/Jun
14/Jun
21/Nov
22/Nov
23/Nov
CH4 [nM]
NOx [µM]
22
18
14
300
200
10
18/Nov
19/Nov
20/Nov
21/Nov
100
20/Nov
Fig. 3. Tidal dynamics of nutrients and CH4 at a time series station (Grunwald et al., 2007) close to Spiekeroog Island (East Frisian Wadden Sea) in 2007 and 2005, respectively.
Figures slightly modified after Grunwald et al. (2009, 2010). Grey lines indicate the tidal state.
M. Beck, H.-J. Brumsack / Ocean & Coastal Management 68 (2012) 102e113
107
5. Transport of nutrients and CH4 between Wadden Sea and
North Sea
3
Fig. 4. Seasonal variation of the nutrients Si(OH)4, NO
in the years
x , and PO4
2006e2010 determined in surface Wadden Sea waters at a time series station
(Grunwald et al., 2007) close to Spiekeroog Island (East Frisian Wadden Sea). Running
average concentrations are displayed. All nutrients strongly decrease during the spring
phytoplankton bloom. Nutrients like Si(OH)4 and PO3
4 , which are re-supplied by pore
water discharge, do hardly reach concentrations close to zero, whereas NO
x exhibits
very low concentrations from spring until autumn. PO43 is especially introduced into
the water column from surface sediments in summer when anoxic conditions in the
uppermost layer lead to reduction of Fe oxides and release of adsorbed PO3
4 .
open water column. In contrast, the freshwater contribution to the
CH4 budget of the back barrier water column is comparably small.
Although the watercourses draining the hinterland may exhibit CH4
concentrations up to 8 mM, which is more than one order of
magnitude higher than in Wadden Sea waters, the contribution was
estimated to be less than 10% (Grunwald et al., 2009). This
phenomenon is explained by the generally low freshwater
discharge into the backbarrier area, and the high CH4 concentrations in pore waters. Also, salt marshes may form a CH4 source to
the Wadden Sea, but few studies have focused on this topic so far
(see 6.3.3). In the water column, CH4 concentrations may diminish
due to degassing and oxidation. In the Wadden Sea area, degassing
of CH4 probably occurs when waters pass a shoal located off the
islands along an extension of the tidal inlets (Grunwald et al., 2009).
In general, this zone is characterized by enhanced wave activity and
turbulence. Methane oxidation is assumed to be of minor importance in Wadden Sea waters due to the short residence time within
the backbarrier area (Grunwald et al., 2009).
Along the East and West Frisian coastline, the boundary
between the North Sea and the Wadden Sea is formed by a chain
of barrier islands separated by tidal inlets. These tidal inlets
control the water exchange between the tidal flat area and the
North Sea. In contrast, the North Frisian Wadden Sea islands
located several kilometres off the coastline are separated by much
wider channel systems. The tidally induced water exchange leads
to import/export of biological and chemical compounds between
the Wadden Sea and coastal waters of the North Sea. Metabolic
end-members like nutrients and CH4, enriched in Wadden Sea
waters compared to the North Sea during most of the year, are
exported and influence the biogeochemistry of coastal North Sea
waters.
Nutrient export from the Spiekeroog tidal flat area to the adjacent North Sea was estimated using a coupled EulereLangrangian
model as part of the ecological tidal model EcoTIM (Kohlmeier
and Ebenhöh, 2007, 2009). The model indicates that dissolved
6
1
inorganic Si(OH)4 (128 106 mol a1), PO34 (3 10 mol a ), and
6
1
NOx (29 10 mol a ) are exported from the tidal flat system to
the North Sea (Grunwald et al., 2010). A comparison of nutrient
levels and patterns of the Spiekeroog tidal flat area with data from
the North Frisian Wadden Sea shows similarities (Grunwald et al.,
2010; van Beusekom et al., 2009). Therefore, the model results of
the Spiekeroog area were extrapolated to the entire German
Wadden Sea. This extrapolation highlights an export of dissolved
nutrients from the Wadden Sea area in the same order of magnitude as the total nutrient input of the three German rivers Elbe,
Weser and Ems discharging into the North Sea (Grunwald et al.,
2010; Lenhart and Pätsch, 2001). The nutrient export by tidal
currents and a decreased turbidity in the open North Sea can lead to
high primary production in a belt of coastal waters seaward of the
barrier islands (Colijn and Cadee, 2003).
To estimate the net CH4 export from the Spiekeroog tidal flat
area to coastal waters of the North Sea, a similar EulereLangrangian
model approach was used (Grunwald et al., 2009). The model
shows that 3.3 106 mol CH4 are exported per year from the tidal
basin of Spiekeroog Island to the southern North Sea (Grunwald
et al., 2009). Assuming that the study area is representative for
the entire East Frisian Wadden Sea, a CH4 export to the North Sea of
7.8 106 mol CH4 per year was calculated, intermediate between
the methane exported by the rivers Rhine and Weser (Grunwald
et al., 2009; Upstill-Goddard et al., 2000). Overall, the estimates
highlight the importance of the tidal flat system for the nutrient
and CH4 budgets of the southern North Sea.
To balance the export of species such as dissolved inorganic
nutrients and CH4 from the tidal basins, it was hypothesized that
OM is imported from the North Sea to the tidal flat system (van
Beusekom and de Jonge, 2002). The proposed conceptual model
was based on three assumptions: (1) nitrogen limits the primary
production in the coastal zone, (2) a proportional part of the
primary produced OM is transported into the Wadden Sea and
(3) the imported organic matter is remineralized within the
Wadden Sea and supports the local productivity by nitrogen
turnover. To date, studies further verifying this conceptual
model are still missing. However, several studies indicate that
nutrients, CH4, dissolved inorganic carbon and metals are
exported from the tidal flat area to the adjacent North Sea
(Grunwald et al., 2009, 2010; Moore et al., 2011). Further, they
present first evidences that the export cannot be balanced by
the freshwater discharge to the tidal basins. Therefore, estuaries
and North Sea might be of importance to balance the budgets
as well.
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M. Beck, H.-J. Brumsack / Ocean & Coastal Management 68 (2012) 102e113
6. Open questions and future challenges
6.1. Extrapolation of results to the entire Wadden Sea
The geographies of the West and East Frisian Wadden Sea are
quite similar, whereas North Frisia differs by the lack of closelyspaced barrier islands, and by exhibiting wider tidal inlets. These
differences may influence the import/export of OM and metabolic
products between the Wadden Sea and the adjacent North Sea as
well as biogeochemical processes within the Wadden Sea area. Up
to date, few investigations have studied identical processes at the
same time in several tidal basins (Kraft et al., 2011), although such
studies are essential to differentiate between processes common to
the entire Wadden Sea, and dynamics induced by specific local
conditions. For example, Kowalski et al. (2011) compared seasonal
and tidal dynamics of the trace metal Mn in the East and North
Frisian Wadden Seas (Spiekeroog and Sylt-Rømø). The authors
observed similarities in seasonal Mn pattern, but significant
quantitative differences. Apparently, site-specific properties of the
different tidal basins have to be considered when establishing
budget calculations for the entire Wadden Sea.
Benthic remineralization rates measured in the Spiekeroog and
Sylt-Rømø tidal basins show a less converse behavior, e.g., sulfate
reduction contributes <20% to total C remineralization in both
areas (Billerbeck et al., 2006b; Werner et al., 2006). However, smallscale variations may be large, especially at tidal flat margins (AlRaei et al., 2009; Beck et al., 2009; Billerbeck et al., 2006b;
Werner et al., 2006). Therefore, a detailed understanding of sedimentological and hydrodynamic conditions is required before
results can be extrapolated to larger areas and finally to different
tidal basins.
compared to marine settings, and reduction of Fe oxides turns out
to be an important electron accepting process for OM remineralization (Jakobsen and Postma, 1999; Larsen et al., 2006). To our
knowledge, too few studies have been conducted so far to estimate
whether the discharge of groundwater from coastal aquifers into
the adjacent Wadden Sea environment is of importance for
biogeochemical processes. The influence of large-scale geological
structures as well as small-scale sedimentary heterogeneities on
groundwater flow, discharge and chemistry complicates an
extrapolation of available data sets to larger areas.
In the tidal flat area of Spiekeroog Island, two 20 m long sediment cores were retrieved to extend the knowledge on biogeochemistry, microbial abundance, and activity of sulfate reducers/
methanogens beyond the intensively studied 5 m depth interval.
One core were drilled through mainly Holocene sediments deposited in a paleo-channel. This core exhibits inverse sulfate and CH4
depth profiles similar to the results described above, however, the
maximum CH4 concentrations are found at greater depths (Fig. 2;
Beck et al., 2011). The highest CH4 concentrations are located in
clay-rich, diffusion-dominated layers. Whether the CH4 trapped in
these layers is transported vertically or horizontally, or even
released to the atmosphere or the open water column is still
unknown.
6.2.1. Subtidal areas
Most studies elucidating biogeochemical dynamics in Wadden
Sea sediments have focused on eulitoral areas. In contrast, little is
known about biogeochemical processes in subtidal sediments, and
their importance for OM and nutrient cycles. Böer et al. (2009a,b)
studied microbial activities and carbon turnover in subtidal sandy
Wadden Sea sediments (Sylt-Rømø Basin, North Frisian Wadden
Sea, 0.5e2.5 m water depth). Similar to eulitoral sandy sediments,
the rapid input of OM results in a very active bacterial community
with a biomass turnover of 2e18 days. Advective pore water
transport increases benthic exchange, for example of O2 (Cook et al.,
2007). Still, the deposits of the up to 15 m deep channels controlling
the water exchange between Wadden Sea and North Sea remain
a black box.
6.2.3. Sandy beaches
In the Wadden Sea area, most sandy beach ecosystems are
located on the North Sea-facing sides of the islands. Additional
beach sites are found on the western and eastern tips of the West
and East Frisian islands. Therefore, we hypothesize that processes
operating in beach ecosystems are not only coupled to the
biogeochemistry of coastal North Sea waters, but may influence
biogeochemical dynamics in the Wadden Sea as well. In contrast to
intertidal or estuarine locations, few studies have focused on these
systems, but the awareness is increasing that the impact of beach
ecosystems on biogeochemical cycles in coastal oceans is not well
constrained, and its importance may be underestimated (Dugan
et al., 2010).
Beach sediments have been described as “geochemical and
microbial deserts” (Boudrau et al., 2001) due to their generally low
levels of OM and other reactive substances. This is based on the
conception that the biogeochemical importance of sedimentary
environments is proportional to their own stocks of OM and reactants. However, recent studies have shown that beach sediments
may be regarded as biogeochemical reactors promoting or accelerating OM remineralization (Anschutz et al., 2009). Permeable
beach sands permit enhanced seawater infiltration by high wave
energy (Robinson et al., 2007), thereby supplying OM and electron
acceptors.
First studies conducted in beach sands of the western tip of
Spiekeroog Island (East Frisian Wadden Sea) show that down to
1.5 m depth, the predominant electron acceptors for OM remineralization are molecular oxygen, nitrate, and Fe/Mn oxides
(Schwichtenberg, 2010; Ungermann, 2009). In contrast to intertidal
sediments, sulfate is less important for microbial respiration and
OM degradation. This implies that OM oxidation in beach sands
may lead to the release of nutrient-rich pore waters, but the
discharge of CH4-rich waters is rather unlikely, or may only occur
below the 1.5 m depth range so far studied.
6.2.2. Coastal aquifers
A few studies focused on biogeochemical processes in coastal
aquifers hydrologically connected to the Wadden Sea (Andersen
et al., 2007; Hansen et al., 2001; Jakobsen and Postma, 1999;
Larsen et al., 2006). For instance, rates of sulfate reduction are
much lower in a shallow sandy aquifer of Rømø (Denmark)
6.2.4. Salt marshes
The importance of salt marshes with respect to their multiple
ecological values has been known for a long time. It remains
uncertain, however, whether the response of salt marshes to the
shifting climate will moderate or exacerbate warming, for example
by carbon sequestration or CH4 release (Bromberg Gedan et al.,
6.2. Understudied Wadden Sea ecosystems
Water column and eulitoral tidal flat sediments are the beststudied compartments of the Wadden Sea. Numerous studies
have identified microbial and biogeochemical processes and their
seasonal/tidal dynamics in both compartments. In contrast, other
Wadden Sea ecosystems have not been subject to extensive
research, e.g., subtidal areas, coastal aquifers, sandy beaches, and
salt marshes. A better understanding of these ecosystems is
essential to unravel OM, nutrient, and CH4 cycling in the Wadden
Sea as a whole.
M. Beck, H.-J. Brumsack / Ocean & Coastal Management 68 (2012) 102e113
2009). In general, the fresh water- dominated part of the salt marsh
system is know to be a relatively large source of CH4 (Chmura et al.,
2003). In contrast, the salt water part of the system releases much
less CH4 due to the presence of sulfate and AOM (Bridgham et al.,
2006). Therefore, Choi and Wang (2004) postulated that due to
higher rates of carbon sequestration and lower CH4 emissions,
coastal wetlands could be a more valuable carbon sink than other
ecosystem in a warmer world with a rising sea level.
The Wadden Sea salt marsh system comprises a fresh and a salt
water part separated by a transition zone. Biogeochemical studies
in the salt marshes of Langeoog Island (East Frisian Wadden Sea)
indicate that salt marsh sediments form an important iron source
for pore waters due to the presence of a high percentage of reactive
sedimentary iron (Kolditz et al., 2009). Only few indications of
sulfate reduction were found within this environment. The study
further shows the effects of flooding during a storm surge on
biogeochemical reactions, which may provide information on
possible effects of a rising sea level. Flooding will lead to a shortþ
term increase in pore water iron, manganese, PO3
4 , and NH4
concentrations.
Nevertheless, it is largely unknown how the Wadden Sea salt
marsh system will react to rising seawater levels. A possible
consequence might be the introduction of more sulfate into the
pore water system that could act as an electron acceptor for OM
oxidation. As a result, carbon would be converted into CO2 instead
of CH4, which in terms of radiative forcing contributes less to
climatic change. Sea level rise may therefore have a negative
feedback on the concentration of atmospheric greenhouse gases
through the suppression of microbial methanogenesis in salt marsh
systems.
6.3. Impact of climate change on biogeochemical cycles
Climate change may induce numerous environmental shifts
(Pernetta and Elder, 1992), which will probably influence biogeochemical processes. However, we can only describe some future
scenarios because few studies have focused on this topic so far.
Certain aspects of climate change have already been addressed in
recent publications focusing on the Wadden Sea and North Sea,
whereas others should be incorporated in future research plans.
Although climate change is a gradual process and difficult to study
in short-term research projects, scenarios for future changes may
be established based on the intensive studies conducted in the
Wadden Sea area, and on long-term time series such as Helgoland
Roads (Franke et al., 2004; Wiltshire et al., 2008; Wiltshire and
Manly, 2004) and Spiekeroog tidal inlet (Grunwald et al., 2010,
2007; Reuter et al., 2009). Furthermore, mild winters or storm
surges may serve as ‘proxy conditions’ for global warming and sea
level rise, respectively.
6.3.1. Does vertical sediment growth keep pace with sea level rise?
How may this influence the biogeochemical cycles in the sediments?
The compilation of data from several stations along the Dutch
and German coast for the past 150 years indicates a mean sea level
rise of 3.6 mm year1 from 1971 to 2008, with higher rates in the
eastern part of the German Bight compared to the southern part
(Wahl et al., 2011). The threshold up to which sea level rise can be
compensated by sedimentation is still under debate, but it is
postulated to be lower for large tidal basins than for small ones
(Van Goor et al., 2003). The interaction between the rate of relative
sea level rise and the rate of sediment supply (from external
sources or the existing reservoir) defines the stratigraphic response
of the system. Three main types of stratigraphic responses have
been proposed: progradational, aggradational and transgressive
(Flemming, 2002).
109
If recent sedimentation rates are higher than local sea-level rise
evidencing vertical sediment growth, the biogeochemical reactor in
Wadden Sea sediments may remain unaffected by climate change.
However, if sedimentation cannot balance sea level rise, extended
inundation durations would lead to a decrease in pore water
discharge from creek bank sediments (Riedel et al., 2010). On the
one hand, this may reduce the amount of seawater cycling through
the sediment leading to less OM and electron acceptor resupply. On
the other hand, less nutrients would be released to the open water
column, thereby probably leading to a decrease in primary
productivity.
6.3.2. May changes in storm surge frequency/extremes influence
biogeochemical cycles?
In the Netherlands, an analysis of the number of storms with
a magnitude of >7 Bft does not show an increasing trend during
the last decades (Oost et al., 2009). In contrast, the yearly highest
water levels increased up to 8 mm per year since 1868 in the
Northern Wadden Sea (Hofstede, 2007). Apart from artificial
causes like dam building, this may be the result of a shift in storm
wind directions. Furthermore, storm surge extremes may increase
along the North Sea coast towards the end of this century (Woth
et al., 2006).
A first estimate of the impact of storm surges on nutrient
budgets in the Spiekeroog tidal flat area shows that losses in
inorganic Si(OH)4 and PO3
4 inventory during storm events account
for 3% and 10%, respectively, of the annual export of both species to
the North Sea (T. Riedel, unpublished). Autumn and winter storms
may therefore be important for exporting nutrients to offshore
waters. Consequently, an increase in storm surge frequency or
extremes may alter biogeochemical cycles in the Wadden Sea and
in coastal waters of the North Sea.
6.3.3. May global warming influence OM remineralization
pathways?
In the Wadden Sea area, seawater temperature depends on the
main wind direction and on global climate development. Longterm observations of the sea surface temperature from the
Western Wadden Sea (Marsdiep Inlet) indicate increasing
temperatures since 1980 leading to a warming of up to 1.5 C (van
Aken, 2008). The Helgoland Roads time series shows a warming
trend of 1.1 C since 1962 (Wiltshire and Manly, 2004).
Pore water temperatures show highest seasonal variations in
surface layers tightly coupled to changes in sea surface temperature (Reuter et al., 2009), but exhibit changes down to 2e3 m
depth in permeable sediments (Beck et al., 2008c). Only 5 m
below the sediment surface, temperatures remain rather constant
throughout the year. Several studies highlight the temperature
dependence of OM remineralization steps in Wadden Sea sediments (Al-Raei et al., 2009; Billerbeck et al., 2006b; Jansen et al.,
2009; Kristensen et al., 2000; Werner et al., 2006). For instance,
depth-integrated sulfate reduction rates of the top 15 cm increase
by a factor of about 10 from winter to summer (Al-Raei et al.,
2009; Arnosti et al., 1998; Kristensen et al., 2000). Although
temperature controls biogeochemical processes, other seasonal
factors such as OM availability simultaneously account for
increased microbial rates in summer.
Overall, global warming may lead to a decrease in oxygen
solubility and an increase in sulfate reduction rates, both resulting
in lower redox conditions in the sediments. To date, it is still under
debate whether this development may have positive or negative
effects on biogeochemical cycles and CH4 emissions. One may
speculate whether faster OM turnover may lead to less OM availability in sediment layers where methanogenesis occurs thereby
reducing CH4 emissions to the water column.
110
M. Beck, H.-J. Brumsack / Ocean & Coastal Management 68 (2012) 102e113
6.3.4. May climate change influence primary production and finally
biogeochemical cycles?
Phytoplankton plays a dominant role in carbon and nutrient
cycles of coastal areas by producing the major part of OM. The
extent of OM reaching the sea floor and being remineralized in
sediments depends on the water depth. In areas with shallow water
depth like the Wadden Sea about 50% of the total respiration takes
place in the sediment (van Beusekom et al., 1999), whereas the
percentage of benthic remineralization is lower in deeper parts of
the North Sea (Upton et al., 1993). Some studies addressed how
climate change may influence the coupling between the amount of
OM produced in the water column and the amount that reaches
bottom sediments. It is suggested that the impact of climate change
on local biogeochemistry largely depend on the extent of
phytoplanktonezooplankton interactions, which are influenced by
large-scale atmospheric and oceanographic changes (van
Beusekom and Diel-Christiansen, 2009; Wiltshire and Manly,
2004). The interaction between both factors determines how much
of the primary production reaches the benthic system. Additional
factors possibly influencing future phytoplankton dynamics are
global warming, nutrient inventory, alien species, and increased
river discharge and turbidity, both induced by an higher precipitation (Philippart and Epping, 2009).
7. Comparison with other temperate tidal flat systems
In our contribution, we so far focused on biogeochemical cycles
in the Wadden Sea. Here we would like to put the described
regional environmental dynamics into a broader, more global
context. The Wadden Sea represents an outstanding example of
a temperate-climate sandy barrier island coast. It differs from all
other comparable tidal flat and barrier island depositional systems
by its spatial scale and its diversity (Marencic, 2009). However,
similar depositional environments are found all around the world
in the temperate zone.
Examples of tidal flats are found along the Korean coast, the
Atlantic coasts of North Africa and North America, and the British
channel and North Sea coast. Typically, these environments exhibit
high primary production and organic matter remineralization rates.
Processes in the water column and the underlying sediments are
tightly coupled due to bidirectional exchange of nutrients and
organic material. For example, tidal flat systems along the Korean
coast exhibit recirculation patterns of seawater into and out of the
sediments similar to the Wadden Sea. Nutrients released by
submarine groundwater discharge stimulate primary production at
the seafloor and in the water column (e.g., Waska and Kim, 2010,
2011). However, the input of terrestrial material is of much
greater importance than in the Wadden Sea, as the transport of
nutrients to the coastal zone is strongly enhanced during the
monsoon season by the huge amounts of rain- and groundwater
flushed through the sediments (Waska and Kim, 2011). Furthermore, tidal water level fluctuations reach up to 10 m, compared to
a maximum of about 3 m along the southern North Sea coast. Both
monsoon and tidal processes may better facilitate pore water
recirculation in the Korean tidal flat area compared to the Wadden
Sea, however nutrient concentration in seeping pore waters are
partly several orders of magnitude lower in the Korean system (e.g.,
Kim et al., 2005; Moore et al., 2011; Riedel et al., 2011; Waska and
Kim, 2011).
A system that has been studied in as much detail as the Spiekeroog tidal flat area is Waquoit Bay, located on the southern
shoreline of Cape Cod at the East coast of the United States. The bay
has served as field site for various oceanographic, hydrological,
geological, biological and geochemical studies (e.g., Charette and
Sholkovitz, 2002, 2006; Dulaiova et al., 2008; Rouxel et al., 2008;
Spiteri et al., 2008; Valiela et al., 1992). In contrast to most sites
in the Wadden Sea, Waquoit Bay is a shallow estuary with an
average depth of 1 m and a tidal range of about 1 m. Similarly to the
Janssand tidal flat, described in our manuscript as a typical
permeable sand flat of the Wadden Sea, the coarse-grained sediments/soils in Waquoit Bay permit submarine groundwater
discharge (Mulligan and Charette, 2006). In the Wadden Sea,
discharge is mainly driven by tides, while in Waquoit Bay saline
circulation due to tides and waves is confined to a shallow intertidal
zone. Instead, seasonal changes in water table elevation may
explain the observed large groundwater discharges (Michael et al.,
2005). The saline circulation through the coastal sediments was
estimated as 0.56 m3 m1 d1 (Mulligan and Charette, 2006), which
is in the same order of magnitude as the respective estimate for the
Wadden Sea (considering the relatively large uncertainties induced
by different methodologies; Riedel et al., 2010). In the seepage
zone, NHþ
4 concentrations are about two orders of magnitude
higher on the Janssand tidal flat compared to Waquoit Bay indicating that there discharging pore waters may exert a higher
impact on water-column biogeochemistry (Kroeger and Charette,
2008; Riedel et al., 2011).
As a conclusion, we can state that despite the global occurrence of temperate tidal flat systems comparable to the Wadden
Sea, these environments are strongly influenced by regional
climate conditions, tidal amplitudes, coastal currents, local
aquifers, weathering in the hinterland, and other environmental
factors. It will therefore require additional collaborative
research efforts to estimate the influence these tidal flat systems
exert on biogeochemical processes and element cycles on
a global scale.
8. Summary
Wadden Sea sediments function as a coastal filter system. OM
captured in this filter is degraded, and recycled nutrients are
released. Due to active biota and advective pore water flow, the
filter never clogs but is continuously renewed, sustaining the
potential for enhanced OM remineralization in this depositional
environment. Recirculation of seawater into and out of the
sediments leads to a tight coupling between benthic and pelagic dynamics, a prerequisite for continuously high primary
production.
Numerous studies have already focused on biogeochemical
dynamics in the Wadden Sea area. Nevertheless, various processes
and ecosystem compartments still need to be studied in more detail
to better estimate their contribution to the carbon, nutrient, trace
metal and trace gas cycles in the Wadden Sea. Only few studies
have addressed the coupling of processes occurring in the Wadden
Sea water or sediment column to other adjacent environmental
compartments, such as the open North Sea water column or the
atmosphere. This knowledge could, for example, help to determine
to which extent the Wadden Sea influences biogeochemical cycles
in the adjacent North Sea. An understanding of naturally occurring
processes, i.e., the environmental baseline, is also necessary to
predict whether climate change or other anthropogenic interventions (e.g., dike building, channel dredging) may have negative
effects on the ecosystem. Although the Wadden Sea area has been
under human influence since the Middle Ages, the rapid changes
induced by an increased land use and industrialization may be
a particular challenge for a more sustainable management of this
ecosystem.
All processes described in this contribution may be of importance in temperate tidal flat systems worldwide. However, regional
influences like climate conditions, tidal amplitudes or local aquifers
have to be taken into account when comparing different systems.
M. Beck, H.-J. Brumsack / Ocean & Coastal Management 68 (2012) 102e113
Acknowledgements
The authors would like to thank all colleagues involved in the
research group “BioGeoChemistry of Tidal Flats” for excellent
cooperation and numerous interesting discussions during eight
years of Wadden Sea research. Furthermore, we thank the guesteditor Maria A. van Leeuwe, two anonymous reviewers, and
Christian März for their comments, which greatly improved the
manuscript. Financial support was provided by Deutsche Forschungsgemeinschaft (DFG).
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