De Backer, A.

Hostens, K.

Vandendriessche, S.

Wittoeck, J.

Open Marine Archive

ICES CM 2010/Q:08 
Not to be cited without prior reference to the authors 
 
Weighing  natural variability and anthropogenic impacts: a case study of demersal fish and 
epibenthic communities in the Belgian Part of the North Sea. 
Annelies De Backer1*, Sofie Vandendriessche1, Jan Wittoeck1, Kris Hostens1 
1 Institute for Agricultural and Fisheries Research-Fisheries, Bio-environmental Research Group, 
Ankerstraat 1, B-8400 Oostende, Belgium 
*Email: annelies.debacker@ilvo.vlaanderen.be 
 
EXTENDED ABSTRACT 
Background 
Anthropogenic activities such as sand extraction, fisheries, shipping, the construction of 
pipelines or windmill farms, dredging and dumping of dredged material, have been shown to 
result in varying effects on the marine ecosystem (Köller et al. 2006, Wilhelmsson et al. 2006, 
Barrio Froján et al. 2008, La Porta et al. 2009, Ware et al. 2009). Numerous monitoring programs 
have been set up to evaluate the extent and nature of these effects. However, the effects of 
anthropogenic activities on the benthic life are often difficult to detect against a background of 
small and large scale natural variability resulting from differences in environmental variables, 
especially in the highly dynamic sandbank-dominated habitats in the Belgian Part of the North 
Sea (BPNS). 
The distribution of the macrobenthos in the BPNS is well studied, and four major species 
assemblages were defined (Van Hoey et al. 2004, Degraer et al. 2008). Each of these assemblages 
is determined by a number of indicator species and by typical density and diversity measures. 
The distributional patterns of the macrobenthic assemblages are mainly linked to sediment type 
(cf. average grain size and mud content). Based on that relation, a habitat suitability map for the 
four assemblages could be established for the BNPS (Degraer et al. 2009). 
On the other hand, distributional patterns of the epibenthos and demersal fish in the BPNS are 
not yet thoroughly presented. In the framework of an ecosystem approach, knowledge on 
epibenthos and demersal fish communities, in addition to macrobenthos data, is imperative, 
and will allow for sound ecosystem-based management. Furthermore, knowledge on the 
natural variability of these communities will underpin a sound interpretation of the detection of 
any ecological change in the area.  
 2 
 
A monitoring strategy based on medium-term data acquisition at fixed locations is used to 
define demersal fish and epibenthos communities and to evaluate their natural spatial and 
temporal variability in the BPNS. In total, 80 locations spread over the BPNS were sampled with 
an 8 m shrimp trawl during 1 to 9 (spring and autumn) campaigns between 2004 and 2009. Five 
tracks are located in sand extraction areas, five in dredge dumping sites and two in windmill 
areas, and could thus be defined as impact locations, while 68 tracks are regarded as reference 
locations. A number of environmental variables were used in the analysis of the spatial 
distribution of the encountered species. 
Results 
1) Characterisation of the species assemblages and distributional patterns 
The coastal - offshore transition is the dominant structuring factor on a regional scale (BPNS), 
which is reflected by a shift from a coastal system characterised by shrimp, ophiuroids and 
crabs to an offshore system with lesser weever, hermit crabs and dab (Fig. 1). The transition 
between coastal samples and offshore samples was consistent over the years and seasons. 
 
Figure 1: MDS plot with indication of the defined species assemblages (left) and characterisation of the species assemblages 
by average density, number of species and characteristic species (right)  
Within the coastal zone, three groups could be distinguished based on a cluster analysis and 
visualised in an MDS plot (Fig. 1), which mainly differed in the occurrence and densities of 
Ophiura ophiura, Ophiura albida, Crangon crangon, Liocarcinus holsatus and Nassarius reticulatus: 
Coastal 1 is found in the western part of the coastal area and a bit more offshore at the eastern 
part (Fig. 2). This assemblage, characterized by the species O. ophiura, C. crangon and L. holsatus, 
showed the highest densities (mean 2085 ind./1000 m²) and a high diversity (mean N° of species 
a
2b
 3 
 
(S): 25; mean N1: 4.4) (Fig. 1). The spatial extent of this group was consistent over the years, and 
it coincides with the potential habitat of the diverse macrobenthic Abra alba community, which 
is characteristic for muddy fine sand (mean mud content 5.8% and mean median grain size 219 
µm) (Fig. 2). 
Coastal 2a is found between the harbour of Ostend and the Belgian-Dutch border (eastern 
coastal zone), extending offshore to the Vlakte van de Raan (Fig. 2). ‘Coastal 2a’ samples are 
predominantly spring samples exhibiting the lowest density values of the BPNS (mean density 
36 ind./1000 m²) and low diversity values (mean S: 14.7; mean N1: 3.7). The top characteristic 
species were C. crangon, Pomatoschistus sp. and O. ophiura (Fig. 1). 
Coastal 2b has a similar spatial distribution as ‘coastal 2a’, although contributing samples are 
most frequently found closer to the shore. The samples constituting this group were 
characterized by the species C. crangon, L. holsatus and O. ophiura. This is mainly an autumn 
assemblage exhibiting intermediate values of density and low diversity values, similar to the 
ones of ‘coastal 2a’ (mean density 353 ind./1000 m²; mean S: 19.1; mean N1: 3.6). Both ‘coastal 2’ 
assemblages overlap mainly with the potential habitat of the, likewise less diverse, Macoma 
balthica community occurring in muddy sediments (mean median grain size 95 µm) (Fig. 2). 
Within the offshore samples, two subgroups are defined based on varying densities of C. 
crangon, Echiichthys vipera and O. albida. The offshore assemblages coincide with the 
macrobenthic Nephtys cirrosa and Ophelia limacina communities, both characterized by low 
densities and low species richness and respectively occurring in fine to medium sands (mean 
median grain size: 274 µm) and medium to coarse sands (mean median: 409 µm). 
Offshore 1 samples are mostly found in the most remote parts of the BPNS (except in spring 
2006, during which even the most remote stations are characterized by the ‘offshore 2’ 
community). The samples belonging to ‘offshore 1’ all exhibit low densities (mean density 37 
ind./1000 m²), but a relatively high species number and evenness (mean S: 23.6, mean N1: 7.3). 
The top characteristic species were E. vipera, Pagurus bernhardus and O. albida.  
Offshore 2 is characterized by the species C. crangon, P. bernhardus and O. albida, and combines 
the higher densities found in coastal samples with the high diversity found in genuine offshore 
samples (mean density 111 ind./1000 m²; mean S: 27.2; mean N1: 8.3). Several samples in the 
spatial range of the ‘offshore 2’ zone are inconsistently placed in one of the offshore 
assemblages over years and seasons. Part of this inconsistency is induced by yearly varying 
water temperatures influencing the species distribution, and part by differences between 
sandbank tops and gullies within the sandbank systems of the offshore region. 
 
 4 
 
 
Figure 2: Overlay of demersal fish and epibenthos communities per sampling station on the macrobenthos habitat suitability 
map (from Degraer et al. 2009) of the BPNS for spring (left) and autumn (right). 
2) Main structuring variables 
The distLM results (based on BIC criterion and BEST selection) (Permanova +, Anderson et al. 
2008) showed that 40.5% of the biological variation could be explained by a combination of the 
variables distance to the coast, temperature, depth, mud percentage, median grain size of the 
sand fraction and salinity (Fig. 3). In other words, both site specific and temporal conditions 
were identified as important structuring factors. Distance was the most important variable, 
accounting for 25.7% of the explained variation. This again indicates the importance of the 
coastal - offshore transition on the regional (BPNS) scale, whilst defining epibenthos – demersal 
fish assemblages. Distance was followed by temperature explaining 5.3%, emphasizing the 
differences between spring and autumn samples, which are separated per community on the 
second axis in the dbRDA plot (Fig. 3). 
‘Impact’ was as well entered as a categorical variable, but was not retained as an explanatory 
variable for the distribution patterns of the epibenthos and fish assemblages of the whole BPNS. 
However, on small spatial and temporal scales, some differences between impact and reference 
zones were noted (Vandendriessche et al. in prep). These small scale differences need to be 
further investigated. 
 5 
 
 
Figure 3: Distance based RDA showing the environmental variables explaining part of the variation observed in the 
epibenthos and demersal fish communities (S: spring samples; A: autumn samples). 
 
Conclusion 
Knowledge of the natural spatio-temporal dynamics of the reference conditions is a major 
advantage for future quantification of anthropogenic impacts and also for the evaluation of the 
relevance of these impacts. This study shows that to optimize the impact assessment and filter 
out the natural variability in the BPNS, it is imperative that both control and impact samples 
belong to the same epibenthic and demersal fish assemblage. Moreover, to exclude the observed 
seasonal variation, impact and control samples should be taken in the same period of the year to 
minimize temporal dynamics. 
Furthermore, we find geographical evidence for close overlap between the macrobenthos and 
the epibenthos-demersal fish communities in the BPNS. To find out whether these ecosystem 
components react differently on the diverse human impacts, it is very valuable to assess the 
biological status of both ecosystem components and examine the impact of anthropogenic 
activities through different trophic levels of the ecosystem. 
On the spatial scale of the BPNS, the small scale impact of human activities on epibenthic and 
demersal fish assemblages fades away into the background of natural variability. This is 
actually a good sign, since the anthropogenic impacts do not push the ecosystem outside the 
natural limits. However, to unravel the long term and small scale impacts, more detailed 
studies are needed, taking into account the above mentioned requisites. 
 
 
 6 
 
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Weighing natural variability and anthropogenic impacts: a case study of demersal fish and epibenthic communities in the Belgian Part of the North Sea

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