The role of physical-biological coupling in the benthic boundary layer under megatidal conditions: The case of the dominant species of the Abra alba community in the eastern Baie de Seine (English channel)

Recent evidence has shown that postlarvae and juveniles of the most dominan t species of the Abra alba community of the eastern Pale de Seine are able to re-enter the water column after larval settlement and undergo active and /or passive redistribution. However, few subtidal studies have been conduct ed to investigate and quantify the processes controlling the abundance of d rifting postlarvae and juveniles. The purpose of this study was to verify t he reproducibility or the variability of this phenomenon and to assess coup ling between the intensity of drifting and that of bed shear stress, which is dependent on tidal currents, wave characteristics, and sediment features . This study was conducted in June and July 1995 at two sites in the bay, o ne located at the center of the muddy fine-sediment community and the other at its external border with the fine clean sands of the Ophelia borealis c ommunity. At each site, suprabenthic sledge samples were taken during speci fic tidal conditions (low tide, slack water, and flood and ebb peaks) durin g a neap and a spring tide. Physical data (current velocity, turbidity, and suspended matter content) and meiobenthic samples were taken at the same t ime. Under megatidal conditions, drifting was a quantitatively important di spersal mechanism for at least 15 of the fine-sediment dominant species bel onging to various groups such as polychaetes, molluscs (gastropods and biva lves), and echinoderms. The fluctuation in abundance of drifters was relate d to tidal range, tidal period, location of the site, and the recruitment d ynamics. However, species differed strongly in their response to hydrodynam ical parameters. Three types of drifting species were observed: species tha t were only present in the water column ('undet. cardiids' and Mytilidae); species that were both drifting and present at the seabed (Abra alba, Phaxa s pellucidus, Eteone picta, Mysella bidentata, Ophiura texturata, Owenia fu siformis, Pholoe minuta, Phyllodoce lineata, Pectinaria koreni, and Spisula subtruncata); species that were present at the seabed but did not drift du ring the sampling period (Acrocnida brachiata, Eteone longa, Eumida sanguin ea, Glycinde normanni, Magelona mirabilis, Nemertean spl, Nephtys sp., Spio phanes bombyx, and Tubulanus polymorphus). Each drifting pattern is related to ecoethological species characteristics and may have very different cons equences on population dynamics.