Initiated by the Benthos Ecology Working Group of ICES a synoptic survey of the North Sea benthos was carried out in April -May 1986 (the North Sea Benthos Survey). During this survey 171 localities were sampled in the area delimited by the Strait of Dover in the south and approximately the 100 meter isobath in the north. A selection of 31 of these samples was analysed for nematode species composition. A total of 3262 nematodes were identified, resulting in 302 species in 154 genera. Relationships between nematode species composition and environmenta1 variables were analysed with canonical correspondence analysis. The variation in species composition was best explained by the median grain size and the silt+clay fraction of the sediment. There was also a trend in species composition with the latitude of the stations. Of all samples nematode density was measured and a series of diversity numbers and the maturity index were calculated. The variance in density numbers between the stations was best explained by the median grain size of the sediment. For the different diversity numbers, except for Noo, the variance was best explained by a combination of the depth of the stations and the clay+silt fraction of the sediment. The variance in the maturity index between the staions was best explained by a combination of the median grain size of the sediment, depth and latitude of the stations. Using the classification technique TWINSPAN four clusters of stations with relatively uniform species composition (TWIN groups) were identified, mainly along a gradient of median grain size and silt+clay content of the sediment. On the first dichotomy level in TWIN 1 +2 two genera were abundant (present in 50% of the samples or more with an average density of 1% or more) that differentiated this group from TWIN 3+4 and four genera were abundant in TWIN 3+4 differentiating it from TWIN 1 +2. The two groups differed significantly in the mean median grain size and silt+clay content of the sediment and in the mean depth of the stations. Also the mean density and individual dry weight of the nematodes were significantly different between the two groups. On the level of the second dichotomy TWIN 1 was characterised by five differential species, TWIN 2 by six , TWIN 3 by no differential species and TWIN 4 by eight. TWIN 3 is differentiated from TWIN 4 only by the absence of the differential species of TWIN 4. TWIN1 and TWIN 2 both consist of stations with very fine to silty sand; in TWIN 1 the mean silt+c1ay content is higher, TWIN 2 scored a higher mean maturity index. TWIN 3 and TWIN 4 both consist of fine to medium coarse sand stations with low si1t+clay contents. They differed significantly in none of the environmental variables but in TWIN 3 the mean nematode density was higher and the mean diversity was lower (for a11 diversity idices) than TWIN 4. No clear geographical boudaries could be established for the different TWIN groups, they were more or less evenly spread over the North Sea area. |
