Potamocorbula amurensis: Comparison of clearance rates and assimilation efficiencies for phytoplankton and bacterioplankton

Abstract

This study compared clearance and assimilation of natural bacterioplankton (<1.2 µm) and cultured phytoplankton by an Asian bivalve, Potamocorbula amurensis. The average clearance rate for bacterioplankton was 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 × WW or f = −40 + 199 × 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% for Isochrysis galbana. Net assimilation was 45 and 53% for bacterioplankton and I. galbana, respectively. Bacterial carbon appeared to be respired faster than algal carbon. As seen in other bivalves, feces production increased and assimilation efficiency decreased at higher food concentrations.At the mean bacterioplankton and phytoplankton standing stocks found in northern San Francisco Bay, bacteria supplied ~13 and 16% of the sum of bacteria and phytoplankton C and N, respectively, 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 bacterioplankton and in 26 d by feeding on phytoplankton.