Comparison of Coulter Counter and 14C measurements of Acartia tonsa (Copepoda,Calanoida) grazing on Chlamydomonas sp. (Volvocales)

Both the Coulter Counter and ¹⁴C method were used to measure the grazing (clearance rates) of the marine calanoid copepod Acartia tonsa on different concentrations of a Chlamydomonas sp. culture. In most cases, clearance rates measured by the Coulter Counter method were higher than those measured by the ¹⁴C method by factors of 2 to 3. We explore several possibilities for the differences obtained between the two methods. We suggest that loss of radioactivity through grazer egestion might be the main reason for the discrepancy between methods. Food concentration did not affect the comparability of both methods' measurements.