Southern North Sea

Southern North Sea

Location of the study area (hatched area, Dutch exclusive economic zone); doted contour, sea catchment area.

The study area is a subpart of the North Sea and encompasses 57 000 km².

The monitored habitats are distributed from a few meters to more than 60 m deep on a smooth bottom topography. The seabed sediment is mainly composed of fine and medium-fine sand, and the composition shift to very-fine sand to silt in the deepest areas. It originates from the last glacial period when the North Sea was a dry plain drained by rivers flowing to the south. The sea acquired its present state around 6000 years ago after the glacial period (Holtmann et al. 1996). Nowadays, the substrate characteristics are determined by tidal current velocity. Strong currents in the southern part maintain a gravel substrate whereas maximum current velocities decrease northward where mean grain size gradually decreases. Along the coast, the water temperature varies between 5 °C in winter to 18 °C in summer.

Human pressures on the North Sea comprise mechanical stress and disturbance of the seabed mainly through beam-trawling and sand-gravel extraction. Other potential stressors such as oil-spill, cable and pipeline density, consequent from ship traffic and drill cutting, are not excluded.

Southern North Sea

Bathymetry (left) and sediment granulometry (right) of the study area (from Holtmann et al. 1996)

Macrozoobenthos was chosen as a model to apprehend biodiversity given the multiple functions of macroinvertebrates and their acknowledged sensitivity to environmental disturbance. A regular benthos sampling covering the Dutch continental shelf has been ensured since 1991 (March-June). The available data totalise benthos occurrences in 1700 samples from 1991 to 2010.
The studied fauna comprises 17 phyla encompassing 428 species. Over the study period, the quality of taxonomic identification ensures a common taxonomic level for 360 taxa across the 1700 samples. The main representatives of the fauna are Annelida, Arthropoda, Mollusca and Echinodermata in terms of density and biomass. Over the twenty years, at the location scale, taxa richness varies from a few to 60 taxa. For more information, see Daan and Mulder (2005).The available dataset allows the exploration of spatiotemporal community dynamics over a long-term period. Current information concerning ship traffic, oil spill, sand-gravel extraction and other related descriptors are currently available (Noordzeeatlas). Temporal data on fishing pressure will be acquired and organism sensitivity will be explored at the species and community levels. An ongoing compilation of taxa biological traits will complete the necessary information in order to characterize community state based on the decomposition of functional diversity.