The benthic communities of the North Sea: a summary of the results of the North Sea benthos survey

The biota living near, on, or in the sea floor are collectively called the benthos. Distinction is made between plants (phytobenthos) and animals (zoobenthos), living within the sediment (infauna) or moving about on its surface (epifauna). The infauna is split into categories according to size: macrobenthos (animals larger than 1 mm), meiobenthos (animals between 50 µm and 1 mm), and microbenthos (protozoans, fungi, bacteria, smaller than 50 µm). Most frequently the three fractions are defined methodologically based on the mesh sizes of the sieves used to separate the specimens from the sediments (1 mm and 50 µm). The zoobenthos of the southern and central North Sea was sampled during the North Sea Benthos Survey executed in April-May 1986. Macro- and meiozoobenthos were sampled at 197 stations covering the ICES grid from 51°N to 58°N and from 2°30'W to 8°15'E (see Figure 1). At each of these stations preferably three box cores and one van Veen grab, but sometimes only Van Veen grabs, were taken. The complete list of replicates, dates, samples and stations has been reported to ICES (Anonymous, 1986) and is available from Dr C. Heip on request. Additional samples for epifauna were taken with a beam trawl or dredge. Before sampling, two intercalibration exercises had been performed. The first exercise aimed at intercomparison of the sampling gear and processing methods (sieving, washing, fixation, etc.). The results clearly showed that comparisons of macrofauna data are very difficult and that standardization of gear and processing, as well as taxonomic intercalibration, will be essential for future comparative studies. The second intercalibration exercise showed the necessity of a standardized method of estimating the ash-free dry weight of macrobenthic organisms. The results of both exercises have been presented to ICES (Heip el al. , 1985; Duineveld and Witte, 1987). Most of the stations were analysed for macrofauna bio- mass, density and species composition, for meiofauna density and copepod species composition, for epifauna composition, density and biomass, for sediment grain sizes, protein content, plant pigment content, organic matter and a series of heavy metals: Zn, Pb, Cd, Ni, Cr, Co, Cu, Mn and Fe.