EFFECTS OF HIGH NATURAL SESTON CONCENTRATIONS ON THE FEEDING, SELECTION, AND ABSORPTION OF THE OYSTER CRASSOSTREA-GIGAS (THUNBERG)

Feeding, selection and absorption were determined for the Pacific oyst er Crassostrea gigas cultivated in the Bay of Marennes-Oleron, over a spring/neap tidal cycle. Physiological determinations were related to the highly variable food environment with continuous recordings of tur bidity and fluorescence. In this bay, resuspension processes have a ma jor influence on food availability and quality. Seston characteristics experienced by oysters can be summarized by high turbidity levels fro m 20 to 350 mg.l(-1) and a predominance of the detritic fraction among the organic fraction (mean C/N ratio = 16.57). Food is diluted by the fine resuspended sediment, and organic content of particulate matter in the water column decreases from 30% to 10% with increasing seston l oads. Significant differences (Ancova, P < 0.01), due to low retention efficiencies of the smaller particle size range, were recorded betwee n the food quality (estimated by the organic content and the total pig ment content) measured in the water column and the fraction retained b y the oyster's gill. Below seston concentrations of 90 mg.l(-1) ingest ion rate was regulated by pseudofaecal production. Above 90 mg.l(-1), a sharp reduction of filtration and rejection rates suggests physical constraints limiting food acquisition. The oyster selectively rejects inorganic from organic particles, enriching the ingested fraction. Amo ngst the potentially nutritive particles, significantly fewer particle s containing phytopigments were rejected relative to organic particles (non-linear regressions, P < 0.001). The negative influence, through food dilution, of high seston loads on net absorption efficiency was d etermined, This efficiency decreases with decreasing organic ingested fraction. Scope for growth calculations confirm the negative influence of seston loads, but show, supported by field growth measurements, th at resuspended organic particles play an important role in the oyster' s nutrition. (C) 1997 Elsevier Science B.V.