Main characteristics of the boundary layer macrofauna in the English Channel

Four hundred and thirty two suprabenthic hauls with a modified Macer-GIROQ sledge were taken over the period 1988-1996. Fifteen sites within the Engli sh Channel were sampled from the Roscoff and Plymouth waters to the Dover S trait, including the Bay of Morlaix, Bay of Saint Brieuc, the Bay of Seine and the Seine Estuary. The main characteristics of the Benthic Boundary Lay er (BBL) macrofauna in the English Channel were elucidated from these data. BBL macrofauna was found over the whole water column sampled (10-150 m). F irstly, the number of species collected was very high (up to 116 species fo r five hauls) except in the Seine estuary. Numerically, mysids were the dom inant group of organisms collected with the sledge, followed by amphipods, cumaceans, isopods and decapods. Abundance and biomass of the fauna were ve ry high in the Seine estuary, especially in the mesohaline and oligohaline zones (maximum >216 000 ind. 100 m(-3); >200 g AFDW 100 m(-3)) where the sp ecialised mysids Neomysis integer and Mesopodopsis slabberi occurred in ver y high abundance. At other sites, the abundance and biomass were moderate(1 00-10 000 ind. 100 m(-3); 15-2000 mg AFDW 100 m(-3)). Abundance reached its maximum when mysids or swarms of amphipods of the genus Apherusa were coll ected, and during the reproduction of the amphipods of the genus Ampelisca, as in June in the Bay of Morlaix. The number of species, abundance and bio mass showed seasonal changes in all sites with, in general, lowest values i n winter or spring, and highest values in autumn. For the open sea sites, t wo main geographical gradients were identified: i) in spring, a higher numb er of species in the western part of the Channel compared with the eastern part of the Channel and, in autumn, a higher number of species in the easte rn part of the Channel compared with the western part, and ii) an increasin g density gradient from west to east at spring and a decreasing gradient fr om west to east during the autumn. The suprabenthic fauna of the Channel ca n be considered as a unique community including three kinds of species: (i) good swimmer species which were common at all the time and all sites: e.g. Anchialina agilis, Gastrosaccus spp., Haplostylus spp. and Apherusa spp., (ii) species present at all sites only at night: e.g. Atylus vedlomensis, M egaluropus agilis, Melphipidella macra, Orchomenella nana, Pseudocuma longi cornis, Schistomysis ornata and Synchelidium maculatum, and iii) species co llected at one or a limited number of sites only at night: e.g. Ampelisca s pp, at Pierre Noire and Diastylis spp. at site F. The Seine estuary support s a typical suprabenthic estuarine community. Elements of the suprabenthic fauna also showed significant daily rhythms which took several forms depend ing on the species under consideration.