H abitat signature catalogue

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Belgian Part of the North Sea

mmU N I V E R S I T E I T

GENT RCMGM A R E B A S S E

f f lBELGIAN SCIENCE POLICY

MESH

Table of contents

IN T R O D U C T IO N

IN T E R P R E T A T IO N O F A C O U S T IC D A T A

H A B IT A T S IG N A T U R E S

M o rp h o lo g ica l features• Large to very-large dunes• Small to m edium dunes• Gravel and sand ribbons• Barchan dunes

S ed im en t typ es• Boulders• Gravel• Sand gradients• M uddy deposits• Clay layers

B io lo g ica l features• lu i nice conchilega reefs• Owenia fusiformis reefs

G eo lo g ica l features

A n th ro p o g en ic activ ities• Pipelines and telecommunication cables• D um pspots o f dredged material• D redge furrows• Trawling marks

B A C K G R O U N D IN F O R M A T IO N

INTRODUCTION

T he Habitat Signature catalogue has been set-up in accordance with a E uropean need for a better visibility o f existing seafloor imagery. W ith 'signatureany record is m eant that dem onstrates a particular characteristic o f a habitat feature, ideally with a biological connotation. Through the M ESH project CMapping European Seabed Habitats), a compilation was made on the scale o f the N W European seas and its results can be consulted on-line (h ttp ://w w w .rebent.org /m esh/signatures) .

T he present publication comprises the Belgian contribution; however with additional examples and m ore site- specific inform ation. Focus is on subtidal acoustic imagery, acquired using very-high resolution side-scan sonar and multibeam technology.

W ith the catalogue, it is aimed at providing existing knowledge o f the seabed to a wider community: from the professional surveyor up to the public at large. Each signature comprises its geographic location, the type o f technique and, where possible, its interpretation in term s o f the physical and biological nature o f the seabed.

The catalogue does no t comprise imagery o f wrecks.

G hent University, Renard Centre o f Marine Geology acquired the acoustic imagery in the framework o f the MAREBASSE project (Van Lancker et al., 2007), M ESH (MESH Consortium , 2007) and the HABITAT project (Degraer et al., 2002; 2003). D ata prior to 2000 is related to Van Lancker (1999). Some o f the multibeam imagery originates from the Marine Sand Fund o f the FPS Economy, SME's, Self-Employed and Energy, Quality and Security, Quality and Innovation.

T h is p u b lica tio n c o n tr ib u te s to th e M E S H p ro je c t (M apping E u ro p e a n Seabed H ab ita ts , h t tp : / /www.searchmesh.net) that received European Regional D evelopm ent Funding through the IN T E R R E G III B Com m unity Initiative (http://w w w .nw europe.org) .

Reference:

Van Lancker, V., D u Four, L, Papili, S., Verfaillie, E., Schelfaut, K., M. Rabaut and Degraer, S., 2007. Habitat Signature Catalogue, Belgian part of the North Sea. Belgian Science Policy, SPSDII project MAREBASSE (Management, research and budgetingof aggregates in shelf seas related to end-users), 27pp.

INTERPRETATION OF ACOUSTIC DATA

The interpretation o f subtidal habitats is n o t always straightforward and will depend directly on the quality o f the signatures obtained. Generally, the higher the frequency used, the better resolution can be expected. However, the final quality o f the imagery may be ham pered by technical constraints, non-ideal survey design and above all weather conditions. In addition, imagery may vary from place to place and with bio-geographic conditions.

Mostly, images are interpreted in term s o f their reflectivity, texture and patterns, which can be done in a m ore or less unbiased way. Delineations (e.g. bedform s) can be drawn 'by eye', based on expert judgem ent {direct interpretation) and dimensions can be estimated.

However, one should be aware that the acoustic signal return is in decreasing order determ ined by (Blondel and M urtón, 1997): (a) the geom etry o f the sensor-target system (angle o f incidence o f each beam, local slope, etc.); (b) the physical characteristics o f the surface (micro-scale roughness, etc.); and (c) the intrinsic nature o f the surface (composition, density, relative im portance o f volume versus surface diffusion/scattering for the selected frequency).

Interpretation in term s o f sediment nature and distribution remains difficult and requires good quality imagery. The m ost im portant parameters controlling the acoustic response o f marine sediments, ranked in order o f im portance are: (a) porosity; (b) density; (c) the overburden stress; (d) the degree and type o f lithification; and (e) the grain size and distribution. M ost probable, one can distinguish between soft and hard grounds, bu t going beyond that might require extensive ground-truthing. W hen the occurrence o f m acrobenthic species or communities correlates highly with sediment nature, it m ight be attem pted to interpret the imagery in term s o f sediment nature first and then to link up with the biology {indirect interpretation). In this case, it will be m ost crucial to estimate variations in porosity/density first. As such, highly reworked areas, such as sandbank tops, m ight have a lower signal return, although the grain size is often coarser than its surrounding environments. Additionally, sediments with even m inor silt-clay enrichm ent are m ore compacted and the area will have a higher signal return.

A n overview o f the strategies and processes involved in acoustic seabed m apping can be found in the M ESH Guidance Framework for Habitat M apping (www.searchmesh.net).

BACKGROUND INFORMATION

G eneralAllen, J.R.L., 1982. Sedimentary structures: their character and physical basis. Developm ents in sedimentology, 30A.

Elsevier Sc. Publ. Comp., A m sterdam -O xford-N ew York, 663 pp.Belderson, R.H., Kenyon, N.H., Stride, A.H. and Stubbs, A.R., 1972. Sonographs of the sea floor.; a Picture Atlas.

Elsevier Publ. Comp., A m sterdam -London-N ew York.Blondel, P. and M urtón, B., 1997. Handbook of seafloor sonar imagery. John Wiley & Sons, London, 336 pp.M ESH Consortium , 2007. Guidanceframeworkfor Habitat Mapping, (www.searchmesh.netlStride, A.H., 1982. Offshore tidal sands,processes and deposits. Chapm an and H aii,213pp.

B elg ia n part o f th e N o r th SeaBaeye, M., 2006. Sediment- en morfodynamische evaluatie van de "Puiten Ratel" sandbank in het perspectief van een duurgame

ontginningsstrategie.M.Sc.theü?,,\JiGent. 161 pp.Bellec, V., Van Lancker, V., Degrendele, K., Roche, M., Schotte, P. and Le Bot, S. (submitted). Geo-environm ental

characterisation o f the Kwinte Bank in relation to sand extraction. Journal o f CoastalResearch.Degraer, S., Van Lancker, V., M oerkerke, G., Van Hoey, G., Vincx, M., Jacobs, P. and Henriet, JP. 2002. Intensive

evaluation of the evolution of a protected benthic habitat: H 4 B IT A T . Pinal report. Federal Office for Scientific, Technical and Cultural Affairs (OSTC) - M inistry o f the Flemish Community, Environm ent and Infrastructure. D epartm ent Waterways and M arine Affairs Administration, Coastal Waterways, G hent University, G ent, 124 pp.

Degraer, S., Van Lancker, V., Moerkerke, G., Van Hoey, G., Vanstaen, K., Vincx, M. and Henriet, JP. 2003. E valuation of the ecological value of the foreshore: habitat-model and macrobenthic side-scan sonar interpretation: extension along the Belgian Coastal Zone. Final report. M inistry o f the Flemish Community, Environm ent and Infrastructure. D epartm ent. Waterways and Marine Affairs Administration, Coastal Waterways, 6 5 p.

Degrendele, K., Roche, M., Schotte, P., Van Lancker, V., Bellec, V. and Bonne, W (submitted). M orphological evolution o f the Kwinte Bank central depression before and after the cessation o f aggregate extraction. Journal of CoastalResearch.

Deleu, S., 2001. Zeebodemmobiliteitsstudie van de Hinderbanken regio. Unpublished Msc Thesis, Universiteit G ent, Renard Centre o f M arine Geology, G ent, 135 pp.

Deleu, S., 2002. Tide-topography interaction near the kink in the Westhinder sandbank (Southern North Sea). Thesis submitted to obtain the degree o f M aster in Science in Advanced Studies in M arine and Lacustrine Sciences Thesis, G hen t University, Renard Centre o f Marine Geology, G ent, 43 pp.

Delye, S., 2005. Aggregate resource evaluation in view of the sustainable management o f the new extraction gone at the western extremity of the Gootebank. Thesis submitted to obtain the degree o f M aster in Science in Advanced Studies in Marine and Lacustrine Sciences Thesis, G hent University, Renard Centre o f Marine Geology, G ent, 48 p.

D u Four, I. and Van Lancker, V. (submitted). Physical im pact o f dredged material disposal sites and their recovery after cessation o f dumping: a case study in the Belgian coastal zone. Marine Geology.

Le Bot, S., Van Lancker, V., Deleu, S., D e Batist, M. and Henriet, J., 2003. Tertiary and Quaternary geology of the Belgian continental shelf Scientific Support Plan for a Sustainable D evelopm ent Policy. SPS D II N orth Sea. Brussels, PPS Science policy publication D /2 0 0 3 /1 1 9 1 /1 2 ,75 pp.

Nakas, G , 2007. Identifying hotspots o f biodiversity through classification of sonar data. Thesis submitted to obtain the degree o f M aster in Science in Advanced Studies in Marine and Lacustrine Sciences. G hen t University, Renard Centre o f M arine Geology and Marine Biology Section.

Roche, M., 2002. Utilisation dn sonar multifasceauxpour ¡a classification acoustique des sédiments et son application à ¡a cartographie de ¡a sgme de concession 2 de ¡a mer territoriale et duplateau continental belge. M émoire de Stage, Avril 2002 : 67pp.

Van Lancker, V. (1999). Sediment and morphodjnamics of a siliàclastic near coasta! area, in relation to hjdrodjnamical and meteorological conditions: Belgian continental shelf. Unpublished Ph.D. Thesis, Universiteit G ent, G ent, 194 pp

Van Lancker, V., D u Four, I., Verfaillie, E., Deleu, S., Schelfaut, K., Fettweis, M., Van den Eynde, D., Francken, F., Monbaliu, J., Giardino, A., Portilla, J., Lanckneus, J., Moerkerke, G. and Degraer, S., 2007. Management, research and budgetting of aggregates in shelf seas related to end-users (Marebasse). Final Sdentific Report. Belgian Science Policy, 125pp.

B io to p esConnor, D W , Allen, J.H., Golding, N., Howell, K.L., I.i eberknecht, L.M., N orthen , K. O. and Reker, J.B., 2004.

The national marine habitat classification fo r Britain and Ireland. Version 04.05. Jo in t N ature Conservation Committee, Peterborough, U K (Internet version: www.incc.gov.uk/M arineHabitatClassification).

Degraer, S., W ittoeck, J., Appeltans, W , Cooreman, K., Deprez, T., Hillewaert, H., Hostens, K., Mees, J., Vanden Berghe, W and Vincx, M., 2006. De macrobenthosatlas van het Belgisch deel van de Noordzee, D /2 0 0 5 /1 1 9 1 /5 . Federaal W etenschapsbeleid, Brussel, 164 pp.

Van Hoey, G , D egraer, S. and Vincx, M., 2004. M acrobenthic community structure o f soft-bottom sediments at the Belgian Continental Shelf. Estuarine, Coastal and Shelf Science 59(4): 599-613.

Morphological features

Large to very-large dunes

Habitat description: Large to very-large dunes have respectively a height between 0.75-3 m and more than 3 m. Depending on the regional conditions, small to medium dunes develop superimposed on the larger features. They can be symmetric or asymmetric in shape.

Biotope description: Dynamic, exposed sand habitats are occupied by opportunistic mobile fauna such as amphipods {Bathyporeia spp.), polychaetes (Nephtys cirrosa), sand eels {Ammodytes spp.) and mysids {Gastrosaccus spp.). These grade into the infralittoral sand communities as sediment disturbance decreases down the bank slopes as flow decreases and further into the mud communities (with Fabulina fabula and Magelona spp.) as the silt fraction increases in the least disturbed areas. This fauna typically classifies as the N. cirrosa or 0. Limacina community or their transitional species assemblages.

Zone: Goote Bank area Lat: 51° 24’ 46” Long: 2° 42’ 38”Acoustic technique: Multibeam - Kongsberg Simrad Em l002 (95 kfiz)Description: DTM, 2*2m Campaign name: St0426 Date: November 2004 Data ownership, acquisition & processing: Continental Shelf Department,Federal Public Service Economy, SMEs, Self- Employed and Energy, Quality and Security,Quality and Innovation

Comments: Large to very large dunes, with smaller dunes superimposed on their slopes.

Zone: Area north of the Vlakte van de Raan Lat: 51° 30’ 52” Long: 3° 7’ 8”Acoustic technique: Multibeam - Kongsberg Simrad Em l002 (95 kLE) Description: DTM, 2*2m Campaign name: ST0624 Date: 22-23 November 2006

Comments: Asymmetric large dunes with a height up to 2.5 m and an average wavelength of 150 m. Small dunes are superimposed on their slopes.

Small to m edium dunes

Habitat description: Small to medium dunes have respectively a height between 0.075-0.4 m and between 0.4- 0.75 m. Depending on the regional conditions, they develop superimposed on the larger features. They can be symmetric or asymmetric in shape.

Biotope description: See biotope description of large- to very-large dunes.

Zone: Sierra Ventana area, dumping place of dredged material, Br&W SI Lat: 51° 26’ 42” Long: 3° 2’ 1” A coustic technique: Multibeam - Kongsberg Simrad E m l 002 (95kH 2) Description: shaded relief map, l* lm Campaign name: St0303 Date: 13 February 2003

Comments: Field of small dunes.

Zone: Middelkerke Bank area, southern partAcoustic technique: Side-scan sonar - Klein towflsh (500 kHz) and a digital sonar recorder model 595

Description: Uncorrected side-scan image Campaign name: St9711 Date: 5 May 1997

Comments:M edium dunes are superimposed on large to very large dunes. They have a different strike comparedto the larger dunes._______________________________________________

Gravel and sand ribbons

H abitat description: Highly linear ribbons o f alternating gravel and sand, orientated parallel to the direction o f the maximal tidal current.Sands ribbons have sharply defined margins. They can reach a length o f 3-4 km and a width up to 100 m (length:width ratio up to ~ 40:1). Their surface can be smooth or may be superimposed with small to large dunes.

Biotope description: Very patchy. Usually gravel patches are associated with typical gravel fauna (see page “gravel”) while an impoverished fauna belonging to the 0 . Umacina community characterizes the sand patches (typical of mobile sands).

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Comments:The aligned features, interpreted as gravel ribbons, are parallel to the prevailing current (more than lm /s ) (NE-SW). These bedforms are known to occur where the substrate is coarse and where sand availability is low. They are characterised by a high backscatter and a coarse texture. The ribbon pattern is due to the alternation o f high and low backscatter and the distance between the bands is 15-20 m. In this case, the length of the bands is at least 250-300 m long.

Zone: Hinder Banken area, swale west o f the WesthinderLat: 51° 27’ 6” Long: 2° 25’ 49”Acoustic technique: Multibeam - Kongsberg Simrad Em l002 (95 kHz)

Description: Multibeam backscatter image Campaign name: St0030 Date: 27 November 2000

Barchan dunes

Habitat description: Barchans occur when not enough transportable sediment is available for the formation o f fully developed ripples or dunes. This lack o f sediment is due to the immobility of part of the seabed sediment. The sediment underlying the bedforms, may be a stable armor and the exchange o f sediment between this armor and the bedforms may be small or non-existent. This habitat occurs in medium to high energy environments where tidal currents and storm- induced currents prevail. The current velocity for their formation falls within a range of 15-100 c m /s and controls their shape. The orientation o f their longitudinal axis reflects the current direction. A barchan dune can be inactive or active; the latter witnessed by small dunes on the stoss slopes. An amalgamation of different barchan dunes forms a barchanoid.

Zone: Buiten Ratel area Lat: 51° 19’ 10” Long: 2° 33’ 24” Acoustic technique: Multibeam Kongsberg Simrad Em l002 (95 kHZ)

Description: Shaded relief map Campaign name: St0305 Date: May 2002Data ownership, acquisition & processing:Continental Shelf Department, Federal Public Service Economy, SMEs, Self-Employed and Energy, Quality and Security, Quality and Innovation

Comments: Barchan dunes and barchanoids occur at the transition o f the Buiten Ratel swale to the sandbank itself. The horns of the dunes point in a N E direction, corresponding with the regional sand transport. The barchanoids have a wavy Crestline and a length up to 1 km. Their maximum height is 4.5 m. The wavelength ranges between 300 and 500 m. The substratum o f the Buiten Ratel swale consists of gravel.

Zone: Hinder Banken area, swale east of the OosthinderLat: 51° 27’ 55” Long: 2° 34’ 48”Acoustic technique: Multibeam - Kongsberg Simrad E m l 002 (95 kH Z)

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Description: Multibeam backscatter Campaign name: St0404 Date: 3-4-5 March 2004Comments: Typical shape of a submarine barchan dune. The orientation of the dune suggests a current direction from SSW to N N E. This barchan has a width o f around 545 m with a small asymmetry between the two horns.

Sediment types

Boulders

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Zone: Hinder Banken area, swale east o f the Oosthinder Lat: 51° 24’ 26” Long: 2° 30’ 17”Acoustic technique: Side-scan sonar - GeoAcoustics 159D (410 LH2) Description: Side-scan sonar images Campaign name: St0424 Date: 20-21 October 2004Comments: Details of side-scan sonar images showing blocks of 2 to 3 m in diameters.

Gravel

Habitat description: Gravel beds are exposed at the seabed surface. They are associated commonly with paleovalleys. In dynamic areas, the gravel beds may be overlain with sand/silt veneers of ~40 cm thick (causing local patchiness).

Biotope description: Attached epifauna can be sparse when gravel particles are small (~ 30 mm) and /o r mobile, but abundant in areas characterised by cobble/boulder (soft corals, hydroids, encrusting sponges and bryozoans). The sand patches in between the boulders are typically inhabited by the 0. Limacina community.

Zone: Hinder Banken areaAcoustic technique: Multibeam - Kongsberg Simrad Em l002 (95 kHz)

Lat: 51° 26’ 52” Long: 2° 37’ 42” Description: DTM of the swale, east o f the Oosthinder, 2*2m Campaign name: St0404 Date: 3-4-5 March 2004

Comments:The northern part of the swale between the Westhinder and Oosthinder, and the swale east of the Oosthinder are known to be characterized by coarse grained sediments. The multibeam images show the typical “hummocky” pattern, characteristic for gravel areas. The localised pits have a depth of 2 to 5 m and a spatial extension o f more than 1 km.

Lat: 51° 24’ 26” Long: 2° 30 ’17”Description: shaded relief map of the swale, east of the Westhinder, 2*2mCampaign name: St0424 Date: 20-21 October 2004

Zone: Hinder Banken area, swale east of the OosthinderAcoustic technique: video - Inspector color camera Description: video frames, with their location indicated on the multibeam image Campaign name: St0424

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Zone: Hinder Banken area, swale east of the Westhinder A coustic technique: video - Inspector color cameraDescription: video frames, with their location indicated on the multibeam image Campaign name: St0502 Date: 8-9 February 2005

Sand gradients

Zone: Trapegeer-Potje-Broersbank areaAcoustic technique: Side-scan sonar - GeoAcoustics 159D (410 kkfc) Description: Side-scan sonar mosaic, 0.01 *0.01 m Campaign name: Ha9909 Date: 10 September 1999

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Comments: A cross-shore transect with from left to right (distance of 500m): the top of a sandbank superimposed with medium dunes having a medium sand texture; a swale with fine sands with more than 5% silt-clay; and the extent of a very shallow sandbank, superimposed with wave ripples in coarse-grained sediments. The relatively low backscatter along the sandbank (left) is due to the high porosity o f the sediments, the higher backscatter in the swale due to compaction induced by the silt-clay fraction and the high backscatter along the second sandbank (right) due to the compacted coarse sediments. This example illustrates clearly the importance of compaction effects on the backscatter intensities o f acoustic imagery.

Zone: Smal Bank-WestdiepAcoustic technique: Side-scan sonar - GeoAcoustics 159D (410 khh)Description: Side-scan sonar mosaic (50 m slant range)Campaign name: HA9910 Date: 14 October 1999Comments: A cross-shore transect with from left to right (distance of + / - 900m): the steep slope of a sandbank superimposed with medium dunes having a medium sand texture (—250 pm); a swale with fine sands (—200 pm) with + / - 4% silt-clay; and the foot of the gentle slope of a sandbank with medium dunes in shell rich medium sands (—300 pm, 40% shells). High current velocities have created striations in the swale.

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M uddy deposits

Zone: PotjeA coustic technique: Side-scan sonar - GeoAcoustics 159D (410 kHz) Description: Side-scan sonar mosaic (75 m slant range)Campaign name: HA0003 Date: 1 -2 April 2000

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Comments:Detail o f a side-scan sonar mosaic. Differences in backscatter correspond with differences in sediment composition. The lightest reflectivity in the swale is correlated with the presence o f mud.

Zone: Potje areaAcoustic technique: Side-scan sonar, GeoAcoustics 159D (410 kHz) Description: Side-scan mosaic, 0.01 *0.01 m Campaign name: HA0003 Date: 1-2 April 2000

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Comments:Detail of a side-scan sonar mosaic of the southern branch of the Potje swale. Differences in backscatter correspond with differences in sediment composition. The lightest reflectivity in the swale is correlated with the presence of fluid mud. Trawl marks are visible clearly.

Clay layers

Habitat description: O n the Belgian part of the N orth Sea, mostly massive, very heavy and hard, green-grey clay occur. In swales, where the Quaternary cover may be less than 2.5 m, Tertiary outcrops are to be expected.

Biotope description: Clay layers are typically inhabited by the Bornea candida community, in which only the bivalve B. candida can be found in moderately high densities. This community is currently described only from one sample.

Zone: Hinder Banken area, swale between the Westhinder and Oosthinder Lat: 51° 28’ 15” Long: 2° 33’ 43”Acoustic technique: Multibeam - Kongsberg Simrad Em l002 (95 kH2) Description: shaded relief map, 2*2m Campaign name: St0424 Date: 20-21 November 2004

Comments:The multibeam imagery shows a ridge-like feature along the axis of the swale, due probably to differential erosion of the clayey deposits of the Ypresian. In addition, SW-NE aligned features are observed; these are probably surface expressions of faulting phenomena in the Ypresian clay.

Zone: Westdiep areaAcoustic technique: Side-scan sonar - Klein towfish (500 kH2) and a digital sonar recorder model 595Description: Uncorrected side scan sonar image, 2*2m Campaign name: St9621 Date: 23-26 September 1996

Comments: Side-scan sonar imagery along the foot of the N ieuwpoort Bank, witnessing the sharp delineation between a sandbank-swale system. One channel shows small-scale bedforms, typical for the sandbank and the other channel shows the swale sediments. The lighter reflectivity facies is associated with muddy deposits; whilst the dark reflectivity corresponds with the outcrop of the compact clay. Trawling marks are also observed. Depths are around 10 m.

Biological features

Lanice conchilega reefs

Habitat description: In Belgian waters, dense I zima bioherms can contain up to 10000 protruding (up to 4 cm) polychaete tubes per m2. The tube consists of medium-sÍ2ed to large sand grains with a characteristic fan shape at the top. Measurements in the lower intertidal of Belgian sandy beaches revealed that ~Lanice bioherms had a patch si2e of 1 to 10 m2 and elevated 15 to 20 cm above the surrounding sediment. These high-density patches are scarcely spread throughout the lower intertidal and cover no more than 10 % o f the total area. Individual Lanice aggregations can thus be depicted as small-scale patches in a generally flat soft- sedimented environment. In the subtidal environment, high-density patches o f Lanice (from 1500 to 10000 ind. Lanice/m 2) are also found. Lanice conchilega is found in various types o f sediments, but displays a preference for fine to medium-grained sands (100 to 500 pm) with a relatively high mud content (10 to 40%). Conditions may be tide-swept, and the sediment may be mobile, but the biotope usually occurs in areas sheltered from strong wave action.

Biotope description: The sediment supports dense populations of the tube-building polychaete L. conchilega or sand mason. The baltic tellin Macoma balthica may be present. In Belgian waters, L. conchilega is the key-species o f the Abra alba, Mysella-bidentata community. The bioherm structures may further double to triple its b iod iversity estim ates (species richness: x 2 ; macrobenthic density: x 2-3) and often the term ‘Lanice reef’ or ‘sand reefs’ is used.

Location: Undisturbed reef under aquarium conditions. University of Ghent, Marine Biology (Marijn Rabaut).

Zone: Boulogne surM er (France)Description: overview of a typical Lanice reef habitat Date: 28June 2006Comments: Reefs occur in a patchy way; each reef having a different si2e, ranging from 1 m 2 to 10 m2; patches are visible because o f the increased sedimentation that is triggered by the L. conchilega specimens.

Comments: The fringes at the top o f the tube help to capture particles from the water column; the tentacles are visible between the fringes and bring the particles down to the mouth. Compared to sand flat areas without Lanice, it is clear from the picture that these organisms cause a high level o f habitat complexity.

Zone: Trapegeer, het Potje Lat: 51° 7 ’ Long: 2o 33’ Acoustic technique: Side-scan sonar, GeoAcoustics 159D (410 kHz)Description: Side-scan sonar mosaic, 1*1 mCampaign name: HA9910

Comments: Well distinct acoustic facies are distinguished in the high density area of L. conchilega. Mostly, circular to elongated patches are observed with a different reflectivity than its surroundings.

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Zone: Steep slope of the Baland Bank Lat: 51°12 ’ Long: 2° 47’Acoustic technique: Side-scan sonar, GeoAcoustics 159D (410 kHz)Description: Side-scan sonar mosaic, 0.1*0.1m Campaign name: HaOOll Date: 14 October 2000 Comments: High reflectivity patches

Zone: Sierra Ventana Lat: 51° 27’ 17” Long: 3° 2’ 45”Acoustic technique: Side-scan sonar, GeoAcoustics 159D (410 kHz)Description: Side-scan sonar mosaic, 1*1 m Campaign name: St0309 Date: 26-27 March 2003Comments: Remark the clear difference between the sand ripple field and the patches, formed by the tube-building polychaete worms, L. conchilega. They characterize a region on theside scan image with high backscatter values, a grainy texture and a mottled pattern.

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Owenia fusiformis reefs

Habitat description: In Belgian waters, dense Owenia fusiformis aggregations exists of up to 4000 tubes per m2. This tube worm is a thin, cylindrical, segmented worm o f maximum 10 cm long that builds a tube out o f shell fragments that are arranged like tiles in an overlapping manner. This makes the tube very robust, though flexible. The tube is slightly longer than the worm and the top two centimeters protrude from the surface. There is little known about the construction o f high density patches and their physical characteristics. The mechanisms of physical patch construction is certainly different compared with L. conchilega, as the tube is shorter and as the organisms has a longer lifespan. However, the appearance o f individual aggregations can for both species be depicted as small-scale patches in a generally flat soft-sedimented environment. 0 . fusiformis occurs in subtidal areas from near the shore up to 5000 m, but displays a preference for shallow soft- bottom areas with fine to medium-grained sands 100 to 500 pm). The biotope usually occurs in sheltered areas with a high load of organic matter.

Biotope description: In Belgium, the species is found in the Abra alba-Mysella bidentata community and in the English Channel in tha Abra alba-Pectinaria koreni community. Aggregations of 0. fusiformis have far reaching implications for the benthic community: There is a clear positive relationship between the occurrence of this tube builder, the species abundance and the species richness. Hence, 0. fusiformis can be considered as an ecosystem engineer which shapes the benthic community locally.

Comments: Detail o f the DTM revealing clearly the biogenic structures o f 0 . fusiformis in between the dunes. The patches have an average area of 391 m2 and elevates 18 to 40 cm above the surrounding sediment.

Zone: Area north of the Vlakte van de Raan Lat: 51° 30’ 36” Long: 2° 8’ 18”Acoustic technique: Multibeam - Kongsberg Simrad E m l002 (95 kHz) Description: DTM, 1*1 m Campaign name: ST0703b Date: 16 February 2007

Geological features

Habitat description:O n the Belgian part of the N orth Sea, deformations in the top Tertiary erosion surface occur mostly in 2 zones: the Noordhinder and the Goote-Raan deformation zones.

Zone: Thornton Bank area, swale east of the T horton bank Lat: 51° 28’ 18” Long: 2° 49’ 36”Acoustic technique: Multibeam - Kongsberg Simrad Em l002 (95 kHz)Description: DTM, 5x5mData ownership, acquisition & processing: Continental Shelf Department, Federal Public Service Economy, SMEs, Self-Employed and Energy, Quality and Security, Quality and Innovation

Comments: Occurence o f a synclinal fold in the top Tertiary erosion surface (Goote-Raan deformation zone).

Zone: Goote bank areaLat: 51° 26’ 27” Long: 2° 57’ 28”Acoustic technique: singlebeam Description: DTM, 80x80m Data ownership, acquisition & processing: Flemish Authorities, Agency for Maritime and Coastal Services, Maritime Access

Comments:Occurrence o f a synclinal fold in the top Tertiary erosion surface ( Goote- Raan deformation zone).

Zone: Hinder Banken Area, swale between the Westhinder and OosthinderLat: 51° 28’ 15” Long: 2° 33’ 43” Acoustic technique: multibeam - Kongsberg Simrad Em l002 (95 kHz)

Description: shaded relief map, 2*2m Campaign name: St0424 Date: 20-21 October 2004Comments: The DTM clearly reveals the scarp that traverses the Hinder Banken region, which probably corresponds to an old river terrace.

Anthropogenic activities

Pipelines and telecom m unication cables

Habitat description:Along the Belgian part o f the N orth Sea, there are 3 gas pipelines and 27 telecommunication cables (only 16 in use). The pipelines have a diameter of about 1 m. In sandbank areas, they are trenched usually into the seabed at a depth of 70 cm to 2 m. Where needed, they are covered with a protective gravel anc / or sand layer.

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Zone: Hinder Banken area, swale east o f the Oosthinder Lat: 51°28’41” Long: 2°37T2”Acoustic technique: Multibeam - Kongsberg Simrad Em l002 (95 kH2) Description: DTM, 2*2m Campaign name: St0404 Date: 3-4-5 March 2004

Zone: Hinder Banken area, swale between the Westhinder and OosthinderLat: 51°33’30” Long: 2°37’31” Acoustic technique: Multibeam - Kongsberg Simrad E m l 002 (95 kHZ)Description: DTM, 2*2m Campaign name: St0424 Date: 20-21 October 2004 Comments: The DTM shows a pipeline and a telecommunication cable.

D um pspots o f dredged material

Habitat description: Successive dumping of dredged material results in an irregular-shaped dump mound. On a smaller scale, depressions or topographic highs occur, resulting from individual disposal events.

Biotope description: The dumping areas are generally density and diversity poor. Biological communities are characterised by a combined presence of the Abra alba — Mysella bidentata community mainly, the Nephtys cirrosa community and their transitional species associations. The Macon/a balthica community is found to a lesser extent. Quite often the samples are devoid o f macrobenthic fauna.

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Zone: Sierra Ventana, dumping place o f dredged material, Br&W SIComment: The former dumping site was closed in 1999 and is situated on a sandv shoal, whilst the present one is located in a tidal gully.

Lat: 51° 26’ 18” Long: 3° 3’ 28”Acoustic technique: Multibeam - Kongsberg Simrad Em l002 (95 kHz)Description: Shaded relief map, 1*1 m Campaign name: St0303 Date: 13 February 2003Comments: O n the former dumping site, closed in 1999, two types of small depressions occur in the troughs of the sand dunes. They are thought to be remnants of single dumping events. The elongated depressions have a length in the order o f 30 to 50 m, a width of 15 m and a depth o f 0.30 to 0.50 m. They occur in groups and are aligned N E direction. The circular depressions have a diameter o f 5 m and a depth of 0.25 m. They are not abundantly present.

Presen t dum ping site

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Lat: 51° 26’ 54” Long: 3° 1’ 10”Acoustic technique: Multibeam - Kongsberg Simrad Em 1002 (95 kHz)Description: Shaded relief map, l* lm Campaign name: St0309 Date: 26-27 March 2003Comments: Topographic highs and negative structures, are found on the present dumping ground. The negative structures are linear or curvilinear, aligned around the NW-SE axis and are 0.6 m maximally beneath the general topography. Their alignment corresponds with the transport pattern of the dredge vessels, discharging the dredged material. The topographic highs are less abundant and their height varies between 0.3 and 0.6 m.

san d p ile

Zone: Oostende, dumping place o f dredged material, Br&W Oostende Lat: 51° 16’ 34” Long: 3° 54’ 59”Comment: The former dumping site, which was closed in 2002 due to reaching the maximum dumping capacity, was relocated to the SWA coustic technique: Side-scan sonar - GeoAcoustics 159D (410 kHz)Description: Side-scan sonar mosaic, l* lm Campaign name: St0309 Date: 27-28 March 2003

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Lat: 51° 16’ 34” Long: 2° 54’ 59”Comments: Dredge spoils near the present dumping site.

Lat: 51° 18’ 8” Long: 2° 58’ 36”Comments: The high backscatter of the isolated dump spots is due to the dumping of bricks in the past.

Lat: 51° 17’ 59” Long: 2° 58’ 10”Comments: These linear spots are thought to be remnants of single dumping events. Their lineair nature can be explained by the way of dumping. The split hopper suction dredge vessels dump the dredged material by opening longitudinally their cargo while sailing.

Dredge furrows

Habitat description:The aggregate extraction activities leave marks on the seafloor, in the form of dredge furrows. Dredge furrows on sandbanks may dissappear within 2-3 tidal cycles whereas in areas with low sediment mobility, dredge furrows may be visible for up to a decade. Their depth varies from 10 cm to a maximum of 50 cm.

Zone: Kwinte Bank areaAcoustic technique: Multibeam - Kongsberg Simrad Em l002 (95 kEE)Description: DTM, 2*2m Campaign names: St0131 & St0203Data ownership, acquisition & processing: Continental Shelf Department, Federal Public Service Economy, SMEs, Self-Employed and Energy, Quality and Security, Quality and Innovation

Lat: 51° 17’ 15” Long: 2° 39’ 41” Lat: 51° 16’ 45” Long: 2° 39’ 42’

Trawling marks

Habitat description:Beam trawling activities leave marks on the seafloor. The depth o f the trenches varies between 5 and 10 cm and their width between 2 and 8 m.

Zone: Potje areaAcoustic technique: Side-scan sonar, GeoAcoustics 159D (410 kHz) Description: Side-scan sonar mosaic, 0.01*0.01m Campaign name: HA0003 Date: 1-2 April 2000

IComments: Trawl marks are visible clearly in the muddy deposits o f the southern branch of the Potje swale.

Zone: PotjeAcoustic technique: Side-scan sonar - GeoAcoustics 159D (410 kHz) Description: Side-scan sonar mosaic (75 m slant range)Campaign name: HA0003 Date: 1-2 April 2000

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Comments:Detail of a side-scan sonar mosaic showing a trawl mark in a muddy environment