229 Netherlands Journal of Sea Research 30:229-238 (1992) IS THE 'MUDDY HETEROGENEOUS SEDIMENT ASSEMBLAGE' AN ECOTONE BETWEEN THE PEBBLES COMMUNITY AND THE ABRA ALBA COMMUNITY IN THE SOUTHERN BIGHT OF THE NORTH SEA? JEAN-MARIE DEWARUMEZ, DOMINIQUE DAVOULT, LAURA ELENA SANVICENTE ANORVE and SERGE FRONTIER Station Marine de Wimereux, URA 1363 du CNRS, BP 80, 62930 Wimereux, France ABSTRACT Three assemblages of the Southern Bight of the North Sea seem to constitute a special system of two adjoining communities separated by an ecotone. The specific trait of the middle assemblage, estab- lished on a muddy heterogeneous sediment, resembles the pebbles community in its western part and the Abra alba community in its eastern part. The pebbles community is dominated by epifauna (sessile and mobile species) and the Abra alba community is dominated by infauna. The bionomic and trophic structures of the 'muddy heterogeneous sediment assemblage' are a combination. This assemblage is composed of suspension feeders and deposit feeders, whereas the pebbles community is dominated by suspension feeders and the Abra alba community by deposit feeders. A comparison of the devel- opment of the Abra alba and the muddy heterogeneous assemblages during an eleven-year period shows that both assemblages have evolved in the same way. They show a bionomical stability during the same period, simultaneous bionomical changes, and recruitments of the same species at the same time. These parallel evolvements indicate that the muddy heterogeneous sediment assemblage can be considered an ecotone between the Abra alba and the pebbles communities. 1. INTRODUCTION fauna. The specific composition, the use of benthic In the Southern Bight of the North Sea, two gradients of reduced current strength run from the Dover Strait to the northeast and from the open sea to the coast (ANONYMOUS, 1968; PINGREE & MADDOCK, 1977). They create five macrobenthic assemblages organized and distributed according to the nature of the sediment: (i) the 'pebbles with a sessile epifauna community', (ii) the 'muddy heterogeneous sediment assemblage' (MHSA), (iii) the 'muddy fine sand with the Abra alba community', (iv) the 'clean medium sand with an Ophelia borealis community', and (v) the 'gravel with an Amphioxus lanceolatus community' (SOUPLET et al., 1980; DEWARUMEZ et aL, 1988; DAVOULT et aL, 1988, PRYGIEL et al., 1988). The first three, butt-jointed from the west to the east, seem to constitute a particular system. The MHSA resembles the pebbles commu- nity in its western part and the Abra alba community in its eastern part. Each community possesses a spe- cific trait but important species are shared by two adjoining communities. This paper examines whether the MHSA consti- tutes a true community or if it should be considered a transition (an ecotone) between the pebbles and the Abra alba communities. All the comparisons are based on mobile epifauna, infauna and countable epi- bionomical indices and a study of the diversity indi- cate the existence of a large-scale transition zone between two communities, while the comparison of simultaneous long-term series of the MHS and Abra alba assemblages could confirm the reality of such an ecotone. 2. MATERIAL AND METHODS The study area is located in the southern part of the North Sea between Cape Gris-Nez and Dunkirk (Fig. 1). The pebbles community takes up almost a third of the study area (PRYGIEL et aL, 1988). It was examined under two aspects: its typical facies in front of the Gris-Nez (Fig. 1, location '1') and a muddy coastal facies (Fig. 1, location '2') as defined by DAVOULT (1988). The MHSA occupies the channels between the sand banks, which appear only to the northeast of Calais (PRYGIEL et aL, 1988) (Fig. 1, location '3'). Abra alba community exists all along the coast beyond the Cape Blanc-Nez, at a depth no deeper than 10 m (DEWARUMEZ, 1979; PRYGIEL et al., 1988). It constitutes a biological continuum which runs to the Scheldt estuary and beyond (BELGRANO et aL, 1990) (Fig. 1, location '4'). Data on the pebbles community result from the