David Caron
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 water collected from the 50% light depth (5 - 15 meters) which was determined from the PAR sensor mounted on the TM rosette. Water collected in 30L GoFlo bottles on the trace metal clean rosette or in 10L Niskin bottles (coated springs) on the CTD rosette was drained into 50L carboys and used directly (wsw) or filtered through 200 micron acid washed nylon screen. Filtrate (<1.2 micron) for the dilutions was prepared using Gelman 1.2um cartridge filters which had been presoaked in acid to remove dissolved organics except for two experiments which used <0.2 micron diluent. All filtrations and dilution preparations were done at 1 degree Celsius in the ship's coldroom or the Baltic room. Incubation times which ranged from 24 - 72 hrs. were conducted in 1 liter polycarbonate bottles that were placed in 50% light attenuated flowing seawater on-deck incubators. Bottles used for grazing rate determinations were enriched with nutrients (10uM ammonium chloride, 1 uM sodium phosphate, 1 nM FeCl and 0.1 nM manganese sulfate). Some experiments were also amended with yeast extract ( 1 or 10 milligrams per liter final concentration). Samples were filtered onto 25mm GF/F filters and extracted for 24 hrs. at O degrees Celsius in the dark in 7ml of 90% 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. Incubated control bottles of diluent water had undetectable chlorophyll a concentrations at the beginning and end of each experiment. References

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

2. 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.