Mr. Landry et al., A REFINED DILUTION TECHNIQUE FOR MEASURING THE COMMUNITY GRAZING IMPACT OF MICROZOOPLANKTON, WITH EXPERIMENTAL TESTS IN THE CENTRAL EQUATORIAL PACIFIC, Marine ecology. Progress series, 120(1-3), 1995, pp. 53-63
The standard dilution technique can provide unbiased estimates of phyt
oplankton growth and microzooplankton grazing rates only when certain
restrictive assumptions are met. The most important of these assumptio
ns - that grazing impact Varies in direct proportion to the dilution o
f grazer population density - can be easily violated when clearance ra
te of individual grazers and/or growth response of the grazer populati
on vary significantly with food concentration over the course of the i
ncubation. We have developed a modified protocol which now allows the
dilution technique to be applied unambiguously, even when its original
assumptions may be violated. The new protocol uses flow-cytometry mea
sured disappearance of fluorescently labeled tracer cells (FLB or FLA)
as an internal measure of 'relative grazing activity' in each dilutio
n treatment. Coefficients of phytoplankton growth and mortality rates
are determined from Model II regression analyses of 'net growth' versu
s 'relative grazing', rather than the usual Model I regressions of 'ne
t growth' versus 'dilution factor'. Tests of this hybrid experimental
design in the central equatorial Pacific during an EQPAC cruise in Aug
ust 1992 gave results essentially identical to the standard dilution i
nterpretation.