Role of filtering and biodeposition by Adamussium colbecki in circulation of organic matter in Terra Nova Bay (Ross Sea, Antarctica)

At Terra Nova Bay, the scallop Adamussium colbecki (Smith, 1902) characterises the soft and hard bottoms from 20 to 80 m depth, constituting large beds and reaching high values of density (50-60 individuals/m(2)) and biomass (120 g/m(2) DW soft tissues). To assess its role in the organic matter recycling in the coastal ecosystem, its filtering and biodeposition rates were evaluated in laboratory experiments during the austral summer 1993/94. Filtration rates, measured in a flow-through system, were calculated from the difference in particulate organic carbon (POC), nitrogen (PON) and chlorophyll-a (Chl-a) concentration in inflow and outflow water. Experiments were performed using natural sea water with POC, PON and Chl-a concentrations of about 450 mu g/l, 90 mu g/l and 2 mu g/l, respectively. The biodeposition rate and the biochemical composition of the biodeposits were studied in order to detect how the organic matter is transformed through feeding activity of A. colbecki. At + 1 degrees C temperature, the average filtering rate was about 11 h(-1) g(-1) (DW soft tissues) in specimens ranging in body mass from 2 to 3 g (DW soft tissues) and 6-7 cm long. The biodeposition rate in 3-8 cm long specimens, ranging from 0.4 to 5.7 g (DW soft tissues), was about 5.65 mg DW/g DW/day, leading to an estimate of Corg flux, through biodeposition by A. colbecki, of about 21 mg C m(-2) day(-1) at in situ conditions. Comparison between the biochemical composition of seston and biodeposits shows a decrease of the labile compounds, of the Chl-a/phaeopigments ratio in the biodeposits. The recorded C/N ratio decrease suggests a microbial colonisation in the biodeposits. This study suggests that Adamussium colbecki plays an important role in coupling the material fluxes from the water column to the sea bed, processing about 14% of total Carbon Bur from the water column to the sediments, with an assimilation efficiency of 36%.