POTAMOCORBULA-AMURENSIS - COMPARISON OF CLEARANCE RATES AND ASSIMILATION EFFICIENCIES FOR PHYTOPLANKTON AND BACTERIOPLANKTON

This study compared clearance and assimilation of natural bacterioplan kton (< 1.2 mum) and cultured phytoplankton by an Asian bivalve, Potam ocorbula amurensis. The average clearance rate for bacterioplankton wa s 45 ml h-1 clam-1 and was independent of the size (shell length, wet wt including shell, or dry tissue wt) of the clam. The clearance rate for phytoplankton is given by f = 162 + 166 x WW or f = - 40 + 199 x L where f, WW, and L are clearance rate (ml h-1), wet weight including shell (g), and shell length (cm). Bacteria were readily assimilated by P. amurensis. Gross assimilation was 73% after 49 h compared to 90% f or Isochrysis galbana. Net assimilation was 45 and 53% for bacteriopla nkton and L galbana, respectively. Bacterial carbon appeared to be res pired faster than algal carbon. As seen in other bivalves, feces produ ction increased and assimilation efficiency decreased at higher food c oncentrations. At the mean bacterioplankton and phytoplankton standing stocks found in northern San Francisco Bay, bacteria supplied approxi mately 13 and 16% of the sum of bacteria and phytoplankton C and N, re spectively, consumed by a 1-cm P. amurensis. We calculate that a 1-cm clam could double its C biomass in 221 d by feeding on bacterioplankto n and in 26 d by feeding on phytoplankton.