A BIOASSAY APPROACH TO TEMPORAL VARIATION IN THE NUTRITIONAL-VALUE OF SEDIMENT TRAP MATERIAL

Gross sedimentation rates (GSR) are determined weekly over a 1 yr cycle at a shallow station (18 m) of the bay of Banyuls-sur-Mer. GSR range between 0.6 and 107.8 g . m(-2). d(-1). Spring and summer are characterized by relatively low and constant GSR whereas fall and winter are characterized by relatively high and highly variable GSR. The shape of the inverse relationship between GSR and organic content suggests the importance of resuspension in controlling GSR. At the studied station resuspension appears to be mainly cued by the wind. Ingestion rates and absorption efficiencies of the deposit-feeding bivalve Abra ovata fed on C-14-labelled sediment trap material collected on eight sampling dates characteristic of the main meteorological regimes are measured. When expressed in terms of organic matter, ingestion rates are similar (3.2 and 4.9 10(-3) mg OM . mg OM(-1). h(-1) for materials collected on May 11, 1993 and January 19, 1993, respectively). The relative consistancy of ingestion rates supports the compensatory intake theory. Absorption efficiencies (ranging from 6.4 to 21.0%) are highly affected by sampling dates. During fall and winter, absorption efficiencies are minimal when resuspension is maximal. During spring and summer, absorption efficiencies depend on the quality of materials sedimenting directly from the water column. Temporal differences in the quality of particulate organic matter available to benthic deposit-feeders should be included in the existing models of continental shelf food webs.