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 oyster 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 turbidity and fluorescence. In this bay, resuspension processes have a major influence on food availability and quality. Seston characteristics experienced by oysters can be summarized by high turbidity levels from 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 loads. Significant differences (Ancova, P < 0.01), due to low retention efficiencies of the smaller particle size range, were recorded between the food quality (estimated by the organic content and the total pigment content) measured in the water column and the fraction retained by the oyster's gill. Below seston concentrations of 90 mg.l(-1) ingestion 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. Amongst the potentially nutritive particles, significantly fewer particles 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 determined, 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, that resuspended organic particles play an important role in the oyster's nutrition. (C) 1997 Elsevier Science B.V.