Keywords

1 Study Area

The BPNS is a siliciclastic macro-tidal environment (tidal range of 4.5 m) comprising several groups of sandbanks (Fig. 30.1). Depths range from 0 to −50 m Mean Lowest Low Water at Spring (MLLWS). Mean grain sizes range from fine to medium sands varying along a subtle gradient. Sediment transport is mainly driven by tidal currents (max. 1.5 m s−1), though wind-induced currents may have a direct effect on sediment resuspension and bedform morphology. Along the sandbanks, bedform patterns are simple to complex, with sand dunes on average 2–4 m high (100–200 m wavelength) having varying rates and directions of migration (on average 20 m year−1 and oscillating). The BPNS is one of world’s busiest continental shelf areas, with 30 years of disposal of dredged material and extraction of aggregates, as well as more than 150 years of bottom-disturbing fisheries. Human activities on the BPNS influence natural sediment fluxes, e.g., by dispersion and deposition of plumes originating from the disposal of dredged material, marine aggregate extraction, wind mill farms and fishing activities.

Fig. 30.1
figure 1

Sandbanks along the Belgian part of the North Sea, with the locations of the three study areas: A Shallow sandbank off the French-Belgian border. B Front of an ebb-tidal delta where a large dune field occurs at the end of a flood channel. B1 shows a biogenic reef (*) in the trough of large dunes inhabited mainly by the tubeworm O. fusiformis; Facies B2 is also indicative of dense aggregations of the latter, but high abundances of the razor clam E. directus were also observed; for B3 and B4, see Fig. 30.3. C Barchan dune area attached to a linear sandbank, with ecologically valuable gravel beds in their troughs. (Half) circles indicate the position and size of disposal grounds of dredged material. Arrows represent the main sand transport directions

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2 Methodology

Aggregations of ecosystem engineering species (e.g. tubeworms, clams) in bedform areas were derived from side-scan sonar (GeoAcoustics side-scan sonar; 410 kHz, Fig. 30.2) and multibeam imagery (Kongsberg Simrad EM1002/99 kHz (Fig. 30.3) and EM3002/300 kHz (Fig. 30.4). To understand the relationships between dense species aggregations and their physical habitat, current and backscatter profiling (acoustic Doppler current profiling) were also carried out. Sediment and biological samples were obtained for validation (Degraer et al. 2008; Rabaut et al. 2009; Houziaux et al. 2012). Here, only results from acoustic imagery are shown.

Fig. 30.2
figure 2

Side-scan sonar imagery showing occurrences of circular to elongated mounds (15–40 cm in height) (inset) corresponding to colonies of the tube worm and the ecosystem engineer L. conchilega along the lee side of topzones of shallow sandbanks (profile Fig. 30.1A). From west to east, mean grain sizes ranged from medium sands (280 μm) to fine sands (168 μm), with 5 % mud enrichment. See also Degraer et al. (2008) and Van Lancker et al. (2012)

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Fig. 30.3
figure 3

Bedform occurrences along the delta front of the Westerschelde ebb-tidal delta (Fig. 30.1B). Left Very large NW–SE oriented dunes (−22 m MLLWS) with superimposed smaller dunes. In the troughs of the very large dunes, circular to elongated mounds (*) (40–50 cm in height) occurred, corresponding to colonies of the tube worm and ecosystem engineer O. fusiformis. Right Sandy large dunes (−12 m MLLWS) overtopped with muddy sediments (*). The latter comprised dense aggregations of the tube worm O. fusiformis (>12000 ind. m−2) and the razor clam E. directus (>500 ind. m-2). Here, deposition occurred from along-delta sand fluxes that combined with a cross-delta flux of muddy sediments, both naturally and anthropogenically-induced (e.g. from disposal of dredged material, Fig. 30.1B). In this area, the habitat changed from sand to sandy mud (up to 40 % mud). See also Van Lancker et al. (2013)

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Fig. 30.4
figure 4

Bathymetry (left) and slope (right) map of barchan dunes, where the dune troughs host gravel beds overgrown with epifauna (−30 m MLLWS), and richer species compositions. The dunes are 6–7 m in height with wave lengths of around 200 m. The complex dune morphology protects the fauna from abrasion from fisheries, though it is hypothesized that the nature of biodiversity in the dune troughs is also related to acceleration and deceleration of the current over the steep dunes (~20°), resulting in trapping of muddy sediments near the lee side. The source of the fine-grained material may be natural, or anthropogenically-induced (e.g. aggregate extraction). In the longer term, depending on the nature of the flora and fauna, the addition of the fine-grained material may be beneficial (nutrient input) or adverse (smothering) for biodiversity. The slope variations in the troughs (up to 2°–2.5°) are indicative of the occurrence of gravel beds. The inset is a sampled gravel block (20 cm)

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3 Results

Three cases (A-B-C) are presented in which local higher relief structures (‘mounds’ sensu Van Lancker et al. 2012), as depicted by detailed terrain analyses of acoustic imagery, corresponded with local biodiversity increases. These zones were found (1) on the lee side of topzones of shallow sandbanks (Fig. 30.1, Location A; Fig. 30.2); (2) in dune areas with complex sediment transport pathways (Fig. 30.1, Location B; Fig. 30.3); and (3) near the lee side of complex dunes (Fig. 30.1, Location C; Fig. 30.4). Common explanatory denominators for the benthos occurrences were order, complexity, and also chaos.

Order relates to the fact that in the three cases a steeper morphological gradient associated with enhanced tide-topography effects was the first predictor of increased biodiversity. Acoustically, this translated into circular to elongated mounds 10–40 cm in diameter. These were seen mostly on the lee side of sandbanks (Figs. 30.1A and 30.2) or large dunes (Figs. 30.1B, C and 30.4), where species can shelter from the overall higher sediment dynamics (e.g., Fig. 30.1B1). However, some form of complexity was necessary for species to thrive. The dunes were mostly coarser-grained, and their morphology was more complex than that of their surroundings (e.g. 3D dunes, or very steep dunes), but above all fine-grained sediment flows were measured interacting with the coarser substrate. The transit and/or deposition of this flow, comprising also food and larvae, is attractive to suspension feeders (e.g. ecosystem engineering tube worms and clams). These profit from the robust substrate, whose coarseness is favourable for detritus trapping and for better oxygenation. Species aggregations were most dense where, in the longer term, the interaction of the finer-grained material with the coarser substrate led to chaotic or patchy sediment distributions (Fig. 30.3, right). This is most likely to occur in sand dune areas, where trapping of fine-grained material from sediment plumes, often generated by anthropogenic activities, is most effective (Fig. 30.3). This is not necessarily adverse for benthic communities, but a threshold likely exists up to which species can cope with the excess of fine sediments (i.e. living on the edge).

Figure 30.4 is another example in which steep dunes (‘complex’) with topography-induced sediment fluxes (‘order’), in combination with sedimentation from fine sediments (‘chaos’) resulted in higher biodiversity in their troughs. Here, the focus is on gravel beds that locally host rich epifauna communities.

Since the geological substrate, as well as sediment dynamics, are important predictors of biodiversity, these are now revisited systematically (Van Lancker et al. In Press) and will assist in assessing ‘Good Environmental Status’ within Europe’s Marine Strategy Framework Directive.