Particle‐size modification by two size classes of the estuarine copepod Acartia clausi1,2

Abstract

Two distinct size classes of Acartia clausi were allowed to graze on two different‐sized species of the chain‐forming diatom Thalassiosira. Both copepod size classes dismembered the larger T. gravida chains, producing a high frequency of smaller particles when T. gravida was in high concentrations. In low particle concentrations of T. gravida, the production of small particles was obscured by ingestion. Some particle dismembering might have occurred with the smaller T. nordenskioldii, but the effect also was masked by ingestion. Such particle modification might require a re‐examination of past grazing data. Particle volume ingestion was plotted as a function of particle volume concentration. At high particle densities, both large and small A. clausi could ingest greater volumes of the large T. gravida particles than the smaller T. nordenskioldii particles. At low particle volume densities, both size classes of copepod ingested similar volumes of both diatoms. Large A. clausi ingested greater food volume than small A. clausi, except at the very lowest food volume density (where ingestion was similar). Great variability in the ingestion data occurred as a result of copepod collections in different seasons of different years, and the use of single grazing coefficients is discussed in the light of this variability.