David Caron
WHOI

Herbivory Methodology

Microzooplankton grazing on phytoplankton, herbivory, was measured
using the dilution technique (1) as modified by Landry et al (2). The
modifications of this method provide a means of determining
microzooplankton grazing rates of phytoplankton as well as
phytoplankton growth rates. Experiments were performed on shallow
(5-10m) and deep (35-80m) water samples at the "long" stations. Water
was collected in 30L GoFlo bottles on the trace metal clean rosette.
Filtrate (<0.2um) for the dilutions was prepared for each depth using
Gelman 0.2 um cartridge filters which had been presoaked in acid to
remove dissolved organic material. DOC analyses (courtesy of Dennis
Hansell, BBSR) and nutrient analyses (courtesy of Kathy Kogslund, UW)
indicated no measurable changes in nutrient concentrations or DOC due
to the filtration process. Incubations for 24 hrs. were conducted in 1
liter polycarbonate bottles which were placed in light attenuated
flowing seawater on-deck incubators.

Bottles used for grazing rate determinations were enriched with
nutrients (10 uM ammonium chloride, 1 uM sodium phosphate and 10 uM of
carbon as glucose). Samples were filtered onto 25mm GF/F filters and
extracted for 24 hrs. at 4 degrees C in 7ml of 100% acetone. After
extraction the filter was removed and the sample read in a fluorometer
(Turner Designs) calibrated against chlorophyll a. Net phytoplankton
growth rates were determined from the change in chlorophyll a
concentration in the unenriched whole seawater bottles over the length
of the incubation. Grazing rates on phytoplankton were calculated from
the regression relationship between the apparent phytoplankton growth
rates (change in chloropyll concentration) in the nutrient enriched
treatments and the fraction of unfiltered seawater in the various
treatments.

References

Landry, M.R. and R.P. Hasset. 1982. Estimating the grazing impact of
marine micro-zooplankton. Marine Biology 67:283-288.

Landry, M.R., J. Kirshtein and J. Constantinou. 1995. A refined
dilution technique for measuring the community grazing impact of
microzooplankton, with experimental test in the central equatorial
Pacific. Marine Ecology Progress Series 120:53-63.