Coupling of ingestion and defecation as a function of diet in the calanoid copepod Acartia tonsa

The relationship between the rates of ingestion and defecation of the calanoid copepod Acartia tonsa was examined in laboratory experiments with different diets: the diatom Thalassiosira weissflogii, the autotrophic dinoflagellate Prorocentrum minimum, the heterotrophic dinoflagellate Oxyrrhis sp., the flagellate Dunaliella tertiolecta, and the bacterivorous scuticociliate Uronema sp. Experiments were run typically using between 6 and 8 concentrations ranging from 10 to 1150 mug C l(-1). Both diet and food concentration had significant effects on ingestion rate, pellet production rates and pellet volume. Ingestion and pellet production rates increased curvilinearly with food concentration. Copepods achieved the highest pellet production rates, the largest pellets and the shortest estimated gut passage times with the diatom diet. For all diets, gut passage time decreased with food concentration in the range 0 to ca. 250 mug C l(-1), but little thereafter. In addition, for all diets, but most obviously in the case of the diatom, pellet volume increased curvilinearly with food concentration. Maximum pellet volume was achieved at food concentrations ranging from 50 to 150 mug C l(-1). The defecation and ingestion rates were best correlated when the copepods fed on the diatom and the autotrophic dinoflagellate. Likewise, a decrease in estimated assimilation efficiency with increasing food concentration was apparent only for these 2 diets. This latter pattern is primarily due to an increase in pellet production rate, instead of pellet volume, relative to ingestion rate at the high food concentrations. We conclude that a diatom diet is the most likely one to result in attributes that enhance zooplankton-mediated export flux: high fecal pellet production rates, large pellet size and relatively low assimilation efficiency.