Rates of microzooplankton grazing were determined from serial dilution experiments as described in Landry et al. (2002). Seawater for most experiments was collected in 30-liter Go-Flo bottles, except for a few stations sampled with CTD Niskin bottles. Generally, two experiments were run at each station – one with water collected from the depth of penetration of ~25% of surface PAR, the other varying from 10 to 58% PAR depending on relative mixed-layer versus euphotic zone depths.
For each experiment, ten 2-liter polycarbonate bottles were used to establish a nutrient-enriched dilution series consisting of replicated bottles with 22, 45, 65, 86, and 100% natural (unfiltered) seawater with final concentrations of 0.5 micromolar (µM) ammonium, 0.03 µM phosphate, 1.0 nM FeSO4, and 0.1 nM MnSO4. Five additional bottles were filled with whole seawater with no nutrient enrichment. Two were used for initial samples, and the final three were incubated as natural seawater controls. All bottles were tightly capped after filling and incubated for ~48 hours in seawater-cooled incubators calibrated to the relative PAR light levels.
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