Drifting in post-larvae and juveniles in an Abra alba (wood) community of the eastern part of the Bay of Seine (English Channel)

Abstract

Recent evidence has shown that some intertidal benthic species may re-enter the water column after larval settlement and undergo active and/or passive redistribution but there are no comparable studies of subtidal benthoplanktonic species with an isolated distribution in a megatidal sea. The present study was therefore conducted in June 1992 at a site located in the Abra alba (Wood) community of the eastern part of the Bay of Seine. The purpose of this work was to determine the species involved in post-settlement transport, to assess the number and the developmental stages of the resuspended organisms and to investigate the processes controlling the abundance of drifting post-larvae or juveniles. Under megatidal conditions drifting was quantitatively important in at least eight fine-sediment dominant species. Their abundance fluctuations in the near-bottom water stratum correlated with the bed shear stress (τ) caused by the tidal current. Swell may induce a doubling of the intensity of τ. However, species differed in their responses to hydrodynamic variables. Some species were most abundant at flood tide ( Abra alba, Tellina fabula Gamelin and Natica alderi Forbes), ebb tide ( Cultellus pellucidus Pennant, Nassarius reticulatus L.) or both ( Mysella bidentata Montagu, Owenia fusiformis delle Chiaje, Pectinaria koreni Malmgren). The vertical distribution of the resuspended benthos varied with species, individual size and hydrodynamic conditions. The results are discussed in relation to the eco-ethological characteristics of the species and the consequences of such drifting mechanisms on population dynamics.