Use of Radioisotopes in Laboratory Energy Assimilation Investigations with Daphnia

Abstract

IN most zooplankton–phytoplankton grazing studies, assimilation by the grazers (which is the true measure of energy transfer in a food web) has been inferred indirectly from measurements of phytoplankton cell concentration and zooplankton filtration, egestion, growth, and respiration rates. A more rapid and critical method of assessing zooplankton assimilation is to measure it directly with a radioisotope tracer and translate radioactivity into energy units. Energy units (calories) are the logical choice for measurement of such rates, as all matter capable of being assimilated by an organism can be converted readily into a caloric ‘common denominator’ for easy comparison.