Grazing on chlorophyll by microzooplankton (<200 •um) and copepods was measured in the mixed layer of the high-latitude North Atlantic Ocean during May and August 1991. No significant grazing by microzøoplankton occurred in May during a spring bloom dominated by colonial Phaeocystis pouchettii and Nitzschia spp. As the bloom declined, the size distribution of chlorophyll shifted from dominance by the > 20 •um chlorophyll fraction to dominance by the <20 chlorophyll fraction. The impact of grazing by microzooplankton increased as the bloom declined, with microzooplankton consuming 100% of potential daily chlorophyll production following the bloom. In August,.when the phytoplankton was dominated by the <20 #m chlorophyll fraction, m•cr0zooplankton consumed 37-53% (mean = 41%_+11% s.d.)'of potential daily chlorophyll production. Averaged over all experiments, microzooplankton grazing accounted for 81% of daily chlorophyll production. The grazing impact of Calanus finmarchicus stages C4 and C5, which dominated mesozooplankton biomass in the upper euphotic zone in both spring and late summer, was concentrated on chlorophyll > 20/tin in both seasons; C. f in mar c h ic u s did not consume significant amounts of chlorophyll < 20 •um in either season. Compared to the microzooplankton, copepods did not consume a significant fraction of total chlorophyll in either season, accounting for only -1% of daily chlorophyll production. Introduction The objective of this study was to evaluate losses of chlorophyll due to grazing by microzooplankton and mesozooplankton in the mixed layer of the high-latitude North Atlantic Ocean under contrasting biological and hydrographic conditions during spring and summer. The research was conducted on RV Endeavor cruises EN224-3 and EN227 in spring and late summer 1991 under the aegis of the Marine Light Mixed Layers (MLML) Accelerated Research Initiative, a multidisciplinary effort focused on the biological, physical and optical dynamics of the mixed lay_cr. The MLML study site was occupied t'or 8 days •n May 1991 and 9 days in August/Scptcmber 1991. Paper number 94JC00983 0148-0277!95!94JC-00983505.00 tic Front, an area characterized by extreme seasonal forcing of biology and hydrography. Convective cooling of the water column during winter produces adccp mixed layer of at least several hundred meters, which provides new nutrients to the system [Robinson et al., 1979; Plueddemann et al., this issue]. Irradiance and phytoplankton standing stocks are low during winter when phytoplankton growth is believed to be very low. With the onset of stratification, typically in April, standing stocks of chlorophyll increase IWil!iams and Hopkins, 1974; Plueddemann et al., this issue; Stramska et al., this issue], and a strong diatom bloom reaching 2-5/•g L '•. Calanus finmarchicus and other copepod species appear in the euphotic zone at this time, but their grazing appears to have little impact on the bloom [Dam et al., 1993; Morales et al., 1991; Weeks et al., 1993]. Transition to the summer phytoplankto...