Nematode assemblages from subtidal sandbanks in the Southern Bight of the North Sea: effect of small sedimentological differences
Introduction
The Belgian Continental Shelf (BCS), located in the Southern Bight of the North Sea, is characterised by the presence of extensive sandbank systems. These sandbanks differ considerably from the regular seabed, since strong hydrodynamic currents induce changes in their topography. These changes are irregular (e.g. migration direction of superimposed bed forms changes over relatively short time intervals) Trenteseaux, 1993, Houthuys et al., 1994, Lanckneus et al., 1994 and can be influenced by high wind speeds (Houthuys et al., 1994). These unique geological circumstances create an environment that is unique, especially for sediment-inhabiting animals. Moreover, these sandbanks act as internationally important areas for seabirds (Maes et al., 2000), indicating them as valuable potential marine protected areas. Alternatively, some of the sandbanks are concession areas for sand extraction (Maes et al., 2000), and the gullies between the sandbanks are important for fisheries, creating a conflict situation between human and natural interests.
Given the importance of the sandbanks for the functioning of the North Sea ecosystem on the Belgian Continental Shelf (e.g. nursery area for fish (Dewicke, 2001), feeding and wintering area for seabirds (Maes et al., 2000)), baseline data describing benthic life and explaining distribution patterns are needed in order to propose a sustainable management policy for these areas. Meiobenthic communities, and especially nematodes, are accepted to be very suitable for providing this kind of data. Nematodes show high abundances, a ubiquitous distribution, short generation times and are restricted to the sediment throughout their lives Heip et al., 1985, Kennedy and Jacoby, 1999, Schratzberger et al., 2000.
Vanaverbeke et al. (2000) published data concerning the composition of meiobenthos on taxon level of all sandbanks systems (Flemish Banks, Hinder Banks and Zeeland Banks). However, on nematode species level, only three studies have been published Willems et al., 1982, Vincx, 1990, Vincx et al., 1990. All of these focused on the same sandbank, Kwintebank, located in the Flemish Banks area. The nematode communities from this sandbank were clearly different from the communities sampled in the regular seabed, and diversity at the sandbank was significantly higher than at the adjacent area, but similar to the more offshore part of the BCS (Vincx, 1990).
This paper aims to fill the gap in knowledge of benthic life in highly dynamic environments in shallow coastal waters by (1) describing nematode communities from isolated sandbanks/sandbank systems (H0: Nematode communities originating from different sandbanks are not different in terms of diversity and assemblage structure), (2) relating nematode communities to sediment granulometry (H0: Differences in nematode communities are not related to differences in sediment composition), and (3) describing differences between nematode communities from the individual sandbanks and surrounding flat sea beds (H0: nematode communities from the sandbanks and the surrounding seabed do not differ in terms of diversity and assemblage structure).
Section snippets
Study area
Sandbanks on the BCS are grouped into three major sandbank systems: the Flemish Banks, the Zeeland Banks and the Hinder Banks (Fig. 1). Detailed information on depth and orientation of these sandbanks can be found in Vanaverbeke et al. (2000). The crests of the Flemish Banks are situated some 4 m below MLLWS, while the crests of the other sandbanks are deeper (Zeeland Banks: below the 10 m depth line; Hinder Banks: well below the 10 m depth line) (Maes et al., 2000). Of each system, at least
Nematode community composition and diversity
Both TWINSPAN and DCA indicated the existence of 6 different nematode communities (Fig. 2). TWIN 1 comprised all stations from the Noordhinder, while the Bligh Bank stations were grouped in TWIN 2. The Gootebank stations were put together in a third group (TWIN 3). Kwintebank stations were divided into three groups: TWIN 4 comprised stations located in the northern part of the sandbank (plus channel station Kw12), while the southern stations were grouped in TWIN 5. TWIN 6 was a single station
Nematode community structure in relation to sediment granulometry
Both multivariate methods (TWINSPAN and DCA) discerned six different nematode assemblages on the sandbanks of the BCS. These groups were significant as revealed by MRPP, resulting in the rejection of the first null hypothesis (no difference in nematode communities from different sandbanks/sandbank systems).
However, a study of all the meiobenthic communities of the sandbanks on the BCS indicated that geographical position (e.g. sandbank/sandbank system) was not important in structuring the
Conclusions
This study shows that nematode assemblages from isolated sandbanks differ. Such differences were not found in investigations of spatial patterns on the same sandbanks using data on meiobenthic and macrobenthic taxon distribution. This emphasises the high spatial resolution obtained when nematode species data are used to describe possible differences in sediment ecology. Although all sediments belonged to a single sediment category (medium sand), it is shown that the median grain size and both
Acknowledgements
We would like to thank the master and crew of RV ‘Belgica’ for their skilful help during the sampling. Annick Van Kenhove and Wies Ghyselinck made many slides, Danielle Schram is acknowledged for sediment analysis. Dr. Ann Vanreusel, Bregje Beyst and three anonymous reviewers provided constructive comments on earlier versions of the manuscript. This research was funded by Ghent University (BOF, 1998–2003) and SSTC (Belgium-Sustainable Development-North Sea Project MN/DD/[40–41–42]).
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