COMPARISON OF COULTER-COUNTER AND C-14 MEASUREMENTS OF ACARTIA-TONSA (COPEPODA, CALANOIDA) GRAZING ON CHLAMYDOMONAS SP (VOLVOCALES)

Both the Coulter Counter and C-14 method were used to measure the graz ing (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 t han those measured by the C-14 method by factors of 2 to 3. We explore several possibilities for the differences obtained between the two me thods. We suggest that loss of radioactivity through grazer egestion m ight be the main reason for the discrepancy between methods. Food conc entration did not affect the comparability of both methods' measuremen ts.