The present study tested the utilization of dead microbial biomass by two benthic deposit-feeders:
Abra alba (Wood) (Mollusca: Bivalvia) and
Eupolymnia nebulosa (Montagu) (Annelida: Polychaeta). Clams were collected in the Canet lagoon during spring 1989. Worms were collected in the Port-Vendres harbour during spring 1989. The
14C-labelled (glutamic acid, 24 h) sediment used during the study was sterilized with 1% chloroform, washed with sterile seawater, and dried (60°C; 48 h). This sterilisation procedure, called fumigation is the least harmful to the sediment (Novitsky 1986). Both clams and worms were incubated in the presence of the fumigated sediment for 5, 10, 20, and 50 h. At the end of each experiment we recorded the radioactivity in four compartments: (1) sediment, (2) dissolved organic matter (DOM), (3) CO
2, and (4) animals. The radioactivity of the sediment was subdivided into five fractions: (i) soluble in 2
N HCl, (ii) soluble in hot 5% trichloroacetic acid (TCA), (iii) soluble in 1
N NaOH, (iv) soluble in hot 6
N HCl, (v) residual (after combustion in a Leco carbon analyser). In the first set of experiments, after 20 h of incubation, 5.4 and 4.7% of the total radioactivity was taken up by clams and worms, respectively. However, a model revealed that this uptake could have been correlated with the release of radiolabelled DOM (33% of total radioactivity during the first 5 h). In order to test this assumption, we used the same protocol with three additional washes of the fumigated sediment. This resulted in a significantly lower uptake by the clams (1.9% of the total radioactivity by
t = 50 h), whereas the worms exhibited an uptake similar to that in the initial experiment (5.1% of total radioactivity by
t = 50 h). These results underline the importance of considering interactions with DOM when applying radiotracer techniques to the study of benthic food chains. The average ingestion rates of fumigated sediment by
A. alba and
E. nebulosa were 5.2 10
–2 mg sediment dry wt mg
–1 clam h
–1 and 3.5 10
–2 mg sediment dry wt mg
–1 worm h
–1, respectively, which is comparable to previous data reported for other deposit-feeding bivalves and polychaetes feeding on natural sediment or detritus. The low radioactivity recorded for CO
2 together with the similarity of the changes in the partitioning of the radioactivity within the sediment between control experiments and experiments carried out in the presence of clams or worms suggest low assimilation efficiencies. Therefore, the present study supports the fact that dead microbial biomass does not constitute an important food source for benthic deposit-feeders.
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