The meiobenthos of the North Sea: density, biomass trends and distribution of copepod communities^*

Abstract

During a synoptic survey carried out in April-May 1986, 171 localities were sampled in the North Sea as delimited by the Straits of Dover in the south and approximately by the 100 m isobath in the north. Meiobenthos included Nematoda, Copepoda, Turbellaria, Gastrotricha, Polychaeta, Oligochaeta, Priapulida, Kinorhyncha, Ostracoda, Halacarida, Isopoda, Tanaidacea, Bryozoa, Cnidaria, Sipunculida, Echiurida, Nemertini and Tardigrada. Nematodes were the dominant group in virtually all stations, their densities ranging from 61 to 4167 individuals. 10 cm⁻². Only in the Southern Bight, where nematode numbers were low, did harpacticoids sometimes represent the dominant meiobenthic taxon. There was a tendency for nematode (and total meiobenthos) density to increase towards the north. A total of 278 copepod species belonging to 105 genera and 22 families were identified. Over 40% of the species were new to science; new taxa were found particularly among the interstitial families which were most important in terms of species diversity. Copepod density decreased rapidly to the north and this trend was followed by diversity. Individual ash-free dry weight (AFDW) was determined for 98 species of copepod. Total biomass reached a peak in the south (low mean individual AFDW, high density) and in the north (high mean individual AFDW, low density), but was low in the Central North Sea where the copepod communities were impoverished both qualitatively and quantitatively. Using the classification technique TWINSPAN (two-way indicator species analysis), it was impossible to define meaningful clusters (TWIN groups) on the basis of the 18 major meiobenthic taxa. However, seven distinct communities could be recognized on the basis of the copepod composition: (I) TWIN A largely coincided with the Southern Bight and showed high densities of predominantly interstitial species (Cylindropsyllidae, Paramesochridae, Cyclopinidae) and a few characteristic taxa from coarse sediments; (2) TWIN B was found in the coastal zone of the Netherlands, Germany and Denmark, and in the Dogger Bank, and was dominated by large Ectinosomatidae and Ameiridae, and by interstitial Leptastacidae; (3) TWIN C represented an impoverished community north of the Dogger Bank and consisted of large mud-dwelling species belonging to the Diosaccidae, Laophontidae and Ameiridae; (4) between the Scottish coast and Norwegian Deeps and in the Silver Pits Zosimidae, Cletodidae and Idyanthidae were the most important families (TWIN D); (5) TWIN E grouped the Norwegian Deeps, Devil's Hole and Fame Deep and showed a typical deepwater fauna represented by Ancorabolidae, Cerviniidae, Stenocopiinae and bathyal cletodid genera. Two minor clusters (a, P) coincided with the Dutch Wadden Sea (1 station) and the river outlets (Thames, Wash, Meuse/Scheldt) whose meiobenthos is subject to pollutants. Canonical Correspondence Analysis (CCA) clearly separated the five major twin groups. TWIN A-C were significantly correlated with sediment and could be arranged along a gradient of decreasing median grain size and increasing silt/ clay content. TWIN D was clearly related to latitude whilst TWIN E showed a clear preference for depth.

Author notes

^* Contribution no. 500 of the Delta Institute for Hydrobiological Research, Yerseke.