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We conducted dilution depth-profile experiments in the eastern equatorial Pacific (EEP) to define regional characteristics of phytoplankton growth and microzooplankton grazing and to test the hypothesis that the process rates decouple in the deep euphotic zone where growth is negligible. We used an abbreviated 2-treatment dilution protocol to produce daily profiles at 8 depths of phytoplankton growth, microzooplankton grazing and cellular changes in chlorophyll a (chl a) content from surface waters to the 0.1% light depth. Experiments were conducted at 16 stations from 2°N to 4°S at 110°W and from 110° to 140°W along the equator. Results were surprisingly robust and coherent over this broad spatial area and showed a euphotic zone essentially divided into 3 equal depth intervals. Mean (±SD) growth rates (0.83 ± 0.16 d–1) exceeded grazing rates (0.42 ± 0.15 d–1) in the light-saturated upper third of the water column. Growth, and to a lesser extent grazing, declined with light in the middle third. Effective cell growth was negligible (0.02 ± 0.21 d–1) in the lower third (1 to 0.1% of surface irradiance), with grazing (0.18 ± 0.17 d–1) exceeding growth in this layer. The deep euphotic zone accounted for 25.4 ± 8.4% of the total euphotic zone chl a, 0.5 ± 7.8% of depth-integrated phytoplankton growth and 12.7 ± 7.2% of depth-integrated microzooplankton grazing on phytoplankton. The decoupling of growth and grazing processes under low light conditions at the base of the euphotic zone substantially affected our estimates of microzooplankton consumption of phytoplankton, which ranged from 51% of daily chlorophyll growth for experiments conducted in the upper euphotic zone to 69% for the depth-integrated euphotic zone. In addition, the excess of grazing over growth processes in the deepest stratum, which is typically overlooked in experimental studies, suggests that protistan grazers may have a much larger role in biogeochemical transformations of export fluxes than previously appreciated.

Decoupled phytoplankton growth and microzooplankton grazing in the deep euphotic zone of the eastern equatorial Pacific