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The development of Sabellaria alveolata, a gregarious reef-building polychaete species, is maximal in Mont-Saint-Michel Bay (France), where trophic capacity is now threatened by increasing shellfish farming. As no data are available concerning the ecophysiological response of this species, the purpose of the present study was to obtain clearance rate and retention efficiency values to provide a first order of magnitude for the trophic role of this species. Data were obtained using a flow-through system with novel troughs suitable for 225 cm2 reef blocks containing a mean number of 940 +/- 102 (S.E.) individuals. The experimental diet used consisted of a mixture of two live microalgae, Skeletonema costatum (3800 cell ml-1) and Isochrysis galbana (23,700 cell ml-1), chosen to cover a broad size range (2 to 16 microns equivalent spherical diameter, ESD), as determined by a particle counter. On the basis of a mean clearance rate of 0.7 lh-1 obtained with reef blocks, the mean rate for an individual was estimated at 7.5 x 10(-4) L h-1. Particles larger than 6 microns ESD were cleared with 100% efficiency, but S. alveolata was unable to retain particles smaller than 2 microns ESD. The results are compared with data obtained for other polychaete species, and clearance rate values are extrapolated to an entire reef.

The aim of this work is to assess the use of (SPOT) multispectral visible infrared remote sensing to study microphytobentos assemblages in a shellfish ecosystem (Bay of Bourgneuf, France). SPOT satellite images (acquired at low tide in spring or autumn between 1986 and 1998) were calibrated using in situ radiometric data, and the normalised vegetation index (NDVI) obtained from these images showed microphytobenthos on bay mudflats. Proliferation was mainly along a north-south strip, essentially localised around the +2 m isobath and covering a surface area of 19 to 25% of the total mudflat area studied (420 to 550 ha). Three factors seem to be responsible for the spatial structure of the assemblages: bathymetry, nutrient input from the Falleron River and its channel, and the location of oyster-farming areas. Although spatial and spectral resolutions of multispectral remote sensing data have certain limitations, this approach opens up a new field of application for hyperspectral remote sensing, particularly for synoptic mapping of biomass distribution.