Fine-scale seabed mapping (Owenia fusiformis, and the razor clam Ensis directus) occur near bedload convergence zones resulting from a mutually evasive flood- and ebb-dominant channel system. Such zones are at the end of the channels, hence also fine grained sediments, food and larvae are trapped. The combination of the coarser-grained bedload with thedeposition of fines is indeed the optimum for a lot of suspension and detritus feeders. Still, highest abundances occur at the fringes of such a system where stress levels are intermediate. Hypotheses were successfully tested along the Dutch coastal zone. Those insights are important to assess changes in seafloor integrity and hydrographic conditions

Baeye, M.

Houziaux, J.S.

Rabaut, M.

Troost, K.

Van den Eynde, D.

van Dijk, T.A.G.P.

Van Lancker, V.

Vermaas, T.

Open Marine Archive

Marine and River Dune Dynamics – MARID IV – 15 & 16 April 2013 - Bruges, Belgium 
 
277 
Biogeomorphology in the field: bedforms and species, a 
mystic relationship 
V. Van Lancker(1), J.S Houziaux(1), M. Baeye(1), D. Van den Eynde(1), M. Rabaut(2), K. Troost(3), 
T. Vermaas(4) and T.A.G.P. van Dijk(4,5) 
 
1. Royal Belgian Institute of Natural Sciences. Management Unit of the North Sea 
Mathematical Models (MUMM). Gulledelle 100, B-1200 Brussels, Belgium  
E-mail: vera.vanlancker@mumm.ac.be  
2. Ghent University. Marine Biology Section, Belgium. *Present address: Cabinet Johan Van de 
Lanotte, Deputy Prime Minister and Minister of Economy, Consumer Affairs and North Sea 
3. IMARES, The Netherlands  
4. Deltares. Dept. of Applied Geology and Geophysics, The Netherlands 
5. University of Twente, Dept of Water Engineering & Management, The Netherlands 
 
 
Abstract 
Fine-scale seabed mapping (<5 m resolution) (e.g. surficial sediments, morphology and benthos) was conducted 
along the ebb-tidal delta of the Westerschelde estuary, Belgian and Dutch part of the North Sea. To understand 
variations in habitat and species distributions, mapping was combined with measurements of currents and turbidity 
throughout 13-hrs cycles and with sediment transport modelling results.  
 
Results showed that highest abundances of some ecosystem engineering species (e.g. the tubeworm Owenia 
fusiformis, and the razor clam Ensis directus) occur near bedload convergence zones resulting from a mutually 
evasive flood- and ebb-dominant channel system. Such zones are at the end of the channels, hence also fine-
grained sediments, food and larvae are trapped. The combination of the coarser-grained bedload with the 
deposition of fines is indeed the optimum for a lot of suspension and detritus feeders. Still, highest abundances 
occur at the fringes of such a system where stress levels are intermediate. Hypotheses were successfully tested 
along the Dutch coastal zone. Those insights are important to assess changes in seafloor integrity and hydrographic 
conditions, two descriptors to define Good Environmental Status within Europe’s Marine Strategy Framework 
Directive. 
1. INTRODUCTION 
Biogeomorphology is a field in which the 
understanding of benthic species in relation to their 
surrounding landscape is critical. In the marine 
environment, observations of both components 
remain relatively scarce, but increase steadily as 
multibeam technology allows capturing, 
visualizing and quantifying benthic and terrain 
variables on both small- and large-scales (e.g. Van 
Lancker et al. 2012). However, feedback 
mechanisms between geomorphology and benthos 
are not always clear. Complexity increases where 
also humans affect the benthos-landscape 
relationship. This is the case along the ebb-tidal 
delta of the Westerschelde estuary (‘Vlakte van de 
Raan’), where disposal of dredged material is 
regular practice for already 30 yrs. This paper 
discusses the relative occurrences of macrobenthic 
species in this area in relation to its driving forces. 
 
2. STUDY AREA 
Measurements and observations have been made 
along the Belgian and Dutch part of the North Sea, 
a siliciclastic macro-tidal environment (tidal range 
of 4.5 m), comprising several groups of sandbanks 
(Fig. 1). Sediment transport is mainly driven by 
tidal currents (max. 1.2 m/s), though wind-induced 
currents may have a direct effect on sediment 
resuspension and bedform morphology. Along the 
Vlakte van de Raan sediment dynamics become 
more complex. Increased sediment fluxes arise 
from the disposal activities, but along its delta 
front, and more offshore, bedform patterns are 
very complex with varying rates and directions of 
migrating sand dunes (Fig. 1). Also on the disposal 
Marine and River Dune Dynamics – MARID IV – 15 & 16 April 2013 - Bruges, Belgium 
 
278 
ground itself a series of large sand dunes is present 
(Du Four & Van Lancker 2008) (Fig. 1). On this 
location, biodiversity is low, but regional 
observations of macrobenthos (species larger than 
1 mm) did show higher abundances in this area 
(Fig. 2).  
 
Figure 2. Macrobenthos densities and communities 
along the western part and delta front of the Vlakte van 
de Raan (Belgian part). Low species densities to no 
fauna (open circles) characterize the disposal ground; 
densities (sized circles) amount up to more than 25.000 
ind. m-2 with up to 43 species per 0.1 m² 
(Macrodat@UGent, Degraer et al. 2006). Along the 
delta front the Abra alba community thrives, being the 
most rich and diverse macrobenthic community on the 
Belgian part of the North Sea. Location see Fig. 1. 
 
Also, the razor clam Ensis directus, being the most 
important invasive species on the Belgian part of 
the North Sea thrives in this area (Houziaux et al. 
2011). This species is commercially exploited in 
the Dutch part of the North Sea. Yearly stock 
assessments take place, though the monitoring 
would benefit from a revised survey design based 
on the habitat preferences of the species (Troost et 
al. 2012).  
 
3. METHODOLOGY 
High resolution multibeam acoustic data were 
acquired (Kongsberg Simrad EM1002/99 kHz and 
EM3002/300 kHz) for seabed mapping and habitat 
characterization (yearly surveys in the period 
2006-2011). Depth and backscatter data were 
processed and data grids were produced at a 
resolution lower than 5 m. This allowed 
quantifying fine-scale sediment and terrain 
variation, as also bioturbation. Some species 
indeed alter the acoustic response of the seafloor 
(e.g. Degraer et al. 2008; Van Lancker et al. 2012), 
with dense colonies of tubeworms forming mounds 
of up to 40-50 cm. For the area, these were 
observed through fine-scale terrain analyses. For 
the time series analyses, vertical dynamics are 
quantified from the calculation of a linear trend in 
time over the available bed levels on a cell-by-cell 
basis (Van Dijk et al. 2011). Additionally, current 
and backscatter profiling (Acoustic Doppler 
Current Profiling, ADCP) was carried out along a 
transect perpendicular to the delta front, covering 
periods of 13 hrs tidal cycles. Sediment and 
macrobenthos samples (> 1 mm) were taken, both 
in the Belgian and Dutch sector. In this paper, 
multibeam results will be discussed in more detail.  
 
4. RESULTS 
In the area of highest macrobenthos abundances 
(Fig. 2), a complex of large to very large dunes 
was revealed (Fig. 3). Bathymetry data showed 
that the highest dunes occurred where the channel, 
separating the Vlakte van de Raan from the 
Akkaert Bank, gradually shallows. The ending of 
the channel and its higher relief is caused by the 
interaction of flood and ebb tidal currents, on a 
residual basis. This is typical in an area where the 
interaction between the tidal flow and the local or 
regional bottom topography leads to unequal 
strengths and duration of the flood and ebb tidal 
currents. Both currents evade from each other 
leading to bedload deposition in their interaction 
zone. This is called a mutually evasive sediment 
transport system, sensu Harris (1988). 
 
In the present case the flood is most dominant and 
has created a so-called flood channel. ADCP 
profiling showed strong and rectilinear currents 
along its axis (Fig. 6). Bedforms are clearly flood-
dominated, but become more symmetrical near the 
end of the channel. Analysis of the morphology, 
asymmetries and migration rates through time, 
showed that the northwestern side of the flood 
channel was merely characterized by ebb-
dominated dunes (Fig. 3). A similar evasive 
system occurs higher up the delta front, though the 
ebb-dominancy, as also the transport pathway is 
more difficult to evidence. In any case, currents are 
weaker here and the tidal ellipse is more rotary 
(Fig. 6). Generally, the dune area along this side of 
Marine and River Dune Dynamics – MARID IV – 15 & 16 April 2013 - Bruges, Belgium 
 
279 
the flood channel is more depositional. Figure 4 
shows the differences in vertical dynamics along 
the NW, Central and SE side of the axis of the 
flood-channel. Note that the dynamics are only 
high at the end of the flood-channel. 
 
 
Figure 4. Trends of vertical dynamics based on 
multibeam time series, here in 2008 and 2009. Along 
the northern profile erosion and deposition alternate, 
because of alternating flood and ebb dominancy of the 
dunes; centrally the dunes are mostly erosive; to the 
southeast, up the delta front, mostly deposition occurs. 
 
Regarding the occurrence of macrobenthos, high 
resolution multibeam recordings showed extensive 
small-scale mounds in-between large to very large 
dunes in water depths around -20 m (Fig. 5). These 
were related to colonies (up to 6000 ind. m-²) of 
the tube-building polychaet O. fusiformis (Fig. 5). 
They occupy areas of up to 12 km².  
 
Figure 5. Detailed shaded relief map of an area with 
very large dunes; in the troughs extensive fields of O. 
fusiformis are present (rectangle on Fig. 3). The Owenia 
mounds are up to 40-50 cm high. 
However, higher up the delta front densities 
increased with a maximum of macrobenthos 
abundance along the upper slope, from -16 m 
onwards (Fig. 6). This is the fringe area, where 
flood and ebb residual currents mutually evade. Up 
to 11881 ind. m-² were found of O. fusiformis and 
more than 500 ind. m-² of E. directus. ADCP-
derived backscatter profiling showed that this zone 
is regularly affected by transient turbidity plumes 
descending the delta front during the ebbing phase 
of the tide. Dispersion modelling showed that the 
fine-grained material could originate from the 
disposal activities, though any fines, whether 
naturally-, or anthropogenically-induced, would be 
transported downwards. 
 
Hypotheses were tested along the Dutch Voordelta 
where it was aimed at predicting highest 
abundances of E. directus from multibeam 
bathymetry. From the observations described 
above, an area with a similar setting was chosen 
for time- and cost-efficient monitoring: from large-
scale bathymetry the end of a flood channel was 
selected (Fig. 1) and surveyed using high 
resolution multibeam technology. Again a complex 
of large to very large dunes was revealed with 
macrobenthos sampling results showing highest 
abundances (a so-called Ensis bed) at the southeast 
fringe of the dune system (Fig. 7).  
 
 
Figure 7. Distribution of biomasses of large Ensis 
(width > 16 mm), sampled by boxcores (IMARES 
monitoring Autumn 2012), projected on a multibeam-
derived digital terrain model. Note the ebb-shape of the 
dunes, regardless the overall flood-dominancy of the 
area. Location see Fig. 1. 
 
Marine and River Dune Dynamics – MARID IV – 15 & 16 April 2013 - Bruges, Belgium 
 
280 
Results are important in the framework of 
Europe’s Marine Strategy Framework Directive 
requiring Good Environmental Status (GES) of its 
marine waters by 2020. Related to physico-
chemical seabed attributes, descriptor 6 on seafloor 
integrity and descriptor 7 on hydrographic 
conditions are relevant here. GES for seafloor 
integrity refers to the structure and functions of the 
ecosystems that need safeguarding, without 
adversely affecting benthic ecosystems, whilst 
GES for hydrographic conditions means that 
permanent alteration of hydrographical conditions 
does not adversely affect marine ecosystems. 
Given the benthos-landscape relationship major 
changes to flood- and ebb tidal channel systems 
will undoubtedly affect the benthic ecosystem. 
Results also indicate that the deposition of fine-
grained material, e.g. as resulting from increased 
human activities, is not necessarily adverse to the 
structure and function of the ecosystem. Definition 
of allowable ranges is a challenge for all 
disciplines. 
 
5. CONCLUSIONS 
Extensive fine-scale seabed mapping (e.g. surficial 
sediments, morphology and benthos), in 
combination with current and turbidity profiling, 
was carried out along the delta front of the ebb 
tidal delta of the Westerschelde, Belgian and 
Dutch part of the North Sea. It was shown that 
highest abundances of macrobenthos occurred in 
areas where fine-grained material naturally 
deposits on top of relatively coarser substrates. 
This combination is most favourable for 
suspension feeders (e.g. ecosystem-engineering 
tube worms and clams) that profit from the stable 
substrate, of which its coarseness is favourable for 
detritus trapping and for better oxygenation. In 
addition, the passing to deposition of fines 
provides ample opportunities for filtering food 
resources. Such zones are found: (1) Near bedload 
convergence zones, at the end of mutually evasive 
flood- and ebb-dominant channels. Higher 
dynamics results in the combination of relatively 
coarser sediments that are enriched with fine-
grained material that is, together with food and 
larvae, transported through the channel systems.  
Species that can withstand the overall higher 
dynamics will be sheltered in the troughs of the 
large to very large dunes. Though, highest 
abundances are found at intermediate stresses at 
the edge of such systems. (2) Up-slope delta fronts, 
abundances are higher since the edge zones are 
also enriched with fines that are deposited from 
turbidity plumes. Whether naturally or 
anthropogenically-driven (e.g. from disposal 
activities) these are transported downwards the 
slope during the ebbing phase of the tide. High 
abundances of macrobenthos were found in similar 
settings along the Dutch Voordelta area. 
 
6. ACKNOWLEDGMENT 
Data were acquired in the framework of the 
Belgian Science Policy projects QUEST4D 
(SD/NS/06B) and EnSIS (SD/NS/09A). Ghent 
University, Renard Centre of Marine Geology is 
thanked for the use of multibeam processing 
software Sonarscope (Ifremer). Ifremer granted the 
use of the CARAIBES software in the framework 
of the EU-FP7 Geo-Seas project. Officers and 
crew of RV Belgica are particularly 
acknowledged. 
 
7. REFERENCES 
Degraer, S., Moerkerke, G., Rabaut, M., Van Hoey, G., 
Du Four, I., Vincx, M., Henriet, J.P. & Van 
Lancker, V. 2008. Very high resolution side-scan 
sonar mapping of biogenic reefs of the tube-worm 
Lanice conchilega. Remote Sensing of Environment 
112 : 3323-3328. 
Degraer, S., Wittoeck, J., Appeltans, W., Cooreman, K., 
Deprez, T., Hillewaert, H., Hostens, K., Mees, J., 
Vanden Berghe, W. & Vincx, M. 2006. 
Macrobenthos Atlas of the Belgian part of the 
North Sea: 163 pp. Brussels: Belgian Science 
Policy. ISBN 90-810081-5-3.  
Du Four, I. & Van Lancker, V. 2008. Changes of 
sedimentological patterns and morphological 
features due to the disposal of dredge spoil and the 
regeneration after cessation of the disposal 
activities. Marine Geology 255(1-2): 15-29. 
Harris, P.T., 1988. Large scale bedforms as indicators 
of mutually evasive sand transport and the 
sequential infilling of wide-mouthed estuaries. 
Sedimentary Geology 57: 273–298. 
Houziaux, J.-S., Craeymeersch, J., Merckx, B., 
Kerckhof, F., Van Lancker, V., Courtens, W., 
Stienen, E., Rabaut, M., Perdon, J., Goudswaard, 
P.C., Van Hoey, G., Vigin, L., Hostens, K., Vincx, 
M. & Degraer, S. 2011. 'EnSIS' - Ecosystem 
Sensitivity to Invasive Species. Final Report: 100 
pp. Brussels: Belgian Science Policy, Research 
Programme Science for a Sustainable 
Development.  
Marine and River Dune Dynamics – MARID IV – 15 & 16 April 2013 - Bruges, Belgium 
 
281 
Troost, K., Van Asch, M., Baeye, M., Brummelhuis, E., 
Davasuuren, N., Van den Ende, D., & Van 
Lancker, V. 2012. KBWOT 2012: the use of an 
acoustic technique in mapping beds of razor clams 
(Ensis sp.): 23 pp. Den Haag: Stichting DLO 
Centre for Fisheries Research (CVO). 
Commissioned by EL&I Directie Kennis). 
Van Dijk, T.A.G.P., van der Tak, C.,  de Boer, W.P. 
Kleuskens, M.H.P.,  Doornenbal, P.J., Noorlandt 
R.P. & Marges, V.C. 2011. The scientific 
validation of the hydrographic survey policy of the 
Netherlands Hydrographic Office, Royal 
Netherlands Navy. Delft: Deltares, report 
1201907-000. 
Van Lancker, V., Moerkerke, G., Du Four, I., Verfaillie, 
E., Rabaut, M. & Degraer, S. 2012. Fine-scale 
geomorphological mapping for the prediction of 
macrobenthic occurrences in shallow marine 
environments, Belgian part of the North Sea, in: 
Harris, P. and E. Baker (Eds.) 2012. Seafloor 
Geomorphology as Benthic Habitat: GeoHab Atlas 
of seafloor geomorphic features and benthic 
habitats: 251-260. Elsevier Insights. 
 
 
 
Figure 1. Deltas from the rivers Rhine, Meuse and Scheldt impacting on the Belgian-Dutch coastal zone. Note the 
complex offshore bathymetry with sandbanks and gullies. Bathymetric data compiled from Flemish Hydrography 
and Deltares 2011 (Nederlandse Hydrografische Dienst & Rijkswaterstaat Dienst Noordzee). Rectangles indicate the 
study areas of the Vlakte van de Raan (Belgium) and along the Dutch Voordelta (The Netherlands). 
 
 
 
 
Belgium The Netherlands 
Zeebrugge 
Rotterdam 
N 
0 10km 
Marine and River Dune Dynamics – MARID IV – 15 & 16 April 2013 - Bruges, Belgium 
 
282 
 
Figure 3. Detailed seabed morphology, as derived from multibeam seabed mapping. Areas with large dunes are 
observed south and north of the present disposal ground of dredged material (dashed circle). At the northeast 
extremity of the gully, a series of large complex dunes (up to 4 m) are present. In the axis of the gully and towards 
the slope their asymmetry is clearly flood-dominated; this gully is a flood-dominated channel. However, in the 
prolongation of the Akkaert Bank, dunes show an ebb-dominancy. The broad end of the flood channel is a bedload 
convergence zone, resulting from mutually evasive flood- and ebb-dominated transport zones. Here, colonies of the 
tube worm O. fusiformis occur in the troughs of the dunes (rectangle, see Fig. 6 for a detailed view), as also large 
quantities of E. directus juveniles, American razor blade and most important invasive species along the Belgian part 
of the North Sea. Higher up the delta front higher densities of O. fusiformis and E. directus adults occur (triangles). 
Four profiles (P1 to P4) are indicated along which bedform evolution was followed (see Fig. 5). The circles on 
profile P1 and P2 indicate where flood- and ebb-dominated dunes converge. The intersected line represents the 
maximum extent of the bedform area, as derived from broad-scale DTM’s. 
Marine and River Dune Dynamics – MARID IV – 15 & 16 April 2013 - Bruges, Belgium 
 
283 
 
Figure 6. Synthesis figure of sediment dynamics, as derived from acoustic measurements (multibeam and ADCP). 
Recently reworked sand dunes were observed (without major trawling traces, in contrast to earlier observations that 
mostly witnessed ripped-up seafloors). In this zone of active transport medium sands (mS) prevail. The dashed line 
delimits the zone of highest dynamics. Higher up the delta front sediments become finer (fS) and currents became 
more rotary and less in strength. The ebb current is here important. Highest densities and species richness occur in 
this upper slope area (e.g. O. fusiformis ~ 11.000 ind. m-2 E. directus (dark grey circles) > 500 ind. m-2). Light grey 
circles correspond with the occurrence of E. directus juveniles; they are most abundant near the end of the flood-
dominant channel. The upper slope area is most frequently subdued to transient turbidity plumes, naturally and 
anthropogenically-induced. Only, stress-resistant species can survive. 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Active bedforms 
Current and wave swept 
Flood 
dominated 
 channel 
Ebb dominated 
channel 
mS 
300-400 µm 
fS to mS 
<300µm More  
rotary currents 
rectilinear currents >1m/s 
 
 


Biogeomorphology in the field: bedforms and species, a mystic relationship

Biogeomorphology in the field: bedforms and species, a mystic relationship

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