Diversity of benthic macrofauna in the eastern English Channel: comparison among and within communities

Abstract

The diversity of benthic macrofaunal assemblages in the eastern English Channel is described from 707 samples collected with a Rallier-du-Baty dredge during 1971-1975. Four assemblages were primarily defined by means of multivariate data analyses and clustering methods: the 'Abra alba com- munity', the 'Ophelia borealis community', the 'pebbles community' and a 'mixed assemblage' of the first three communities. Spatial heterogeneity of these communities is significantly correlated with sedi- mentary characteristics, although local variability appears to be controlled by both physical and biological processes. Ecological diversity of these communities was analysed considering species richness (S), the Shannon diversity index (H � ), and rank-frequency diagrammes (RFD). These analyses were performed at two spatial scales: for a single sample, and for a 'site' of 10 pooled samples. Thus, several sites were chosen in order to compare diversity patterns and species quantitative structure among and within the communities. The greatest species richness was recorded for the pebbles (57-69) and the A. alba (63- 79) communities. In the former, high values may be due to the complexity of microhabitats and the large flux of food related to strong currents. In the latter, both organic matter and terrestrial inputs associated with the mud favour the presence of deposit-feeder organisms. No clear trend was observed among and within the community sites in terms of the species quantitative structure. Thus, convex RFD shapes were observed in three assemblages: the 'mixed assemblage' (offshore site), the A. alba (North Sea), and the pebbles (Normandy coast). 'Sigmoid' shapes were observed in the pebbles (Dover Strait) and A. alba (English coast) communities. Presumably, these shapes can be caused by the combined action of phys- ical (strong currents, substrate stability, mud content in the sediments) and biological factors (co-occur- rence of species from different communities, strong species recruitment, relative dominance of carnivorous species).