Particle production by planktonic ciliates1

Favella sp. (a tintinnid) and Balanion sp. (a nonloricate ciliate) produce fecal aggregates which overlap in size with their dinoflagellate food. These aggregates have a higher C:N ratio than the dinoflagellates. As the aggregates disintegrate, smaller particles are produced. For Favella grazing on the thecate dinoflagellate, Heterocapsa triquetra, the volume of the fecal material produced is 22–23% of the volume of algae consumed. The similarity in size of microplanktonic algae and ciliate fecal aggregates makes the Coulter Counter inappropriate for use in determining clearance or ingestion rates in ciliate feeding experiments unless egestion rates can be estimated. Visual counts were compared with Coulter Counter data for an experiment in which Favella and Balanion grazed Heterocapsa. The Coulter Counter underestimated ingestion rates by 30% for Favella and by 18% for Balanion and confounded the results in respect to particle size selectivity. However, because electronic particle counting measures the net change in particle concentration (ingestion — egestion) it can be used to measure assimilation rates. The assimilation efficiencies for Favella and Balanion eating Heterocapsa were thus estimated to be 70 and 82%.