Microzooplankton grazing constrains pathways of carbon export in the subarctic North Pacific

To identify the effect of microzooplankton grazing on phytoplankton abundance and size structure, we quantified phytoplankton growth and herbivorous grazing rates throughout the euphotic zone and across a light gradient on the North Pacific EXport Processes in the Ocean from RemoTe Sensing (EXPORTS) cruise near Ocean Station Papa. During 30 days of continuous, Lagrangian observation in August and September of 2018, depth integrated chlorophyll a (Chl a) concentrations were stable and averaged 20 +/- 2 mgm(-2). Bottle-incubation experiments revealed that phytoplankton growth was balanced by microzooplankton grazing even when phytoplankton growth rates varied from 0 to 0.4 d(-1) in response to light manipulation. Microzooplankton grazing caused a decline in phytoplankton abundance that was balanced by increased phytoplankton cell size resulting in consistent phytoplankton biomass over time. Microzooplankton grazed phytoplankton at an average rate of 0.11 +/- 0.17 d(-1) which lead to an intrinsic phytoplankton growth rate of -0.07 +/- 0.26 d(-1). Predicted stocks from grazing experiments aligned closely (within 16%) with in situ Chl a dynamics and phytoplankton abundance, suggesting that the dominant loss process of phytoplankton was grazing by microzooplankton rather than physical mixing or sinking of phytoplankton. Consequently, microzooplankton played a critical role in regulating primary producer biomass and in transferring particulate organic carbon through the food web where a fraction could then be exported as byproducts of food web processes.

Automated summary

Microzooplankton grazing has a significant impact on phytoplankton growth and abundance, leading to a decrease in phytoplankton quantity balanced by an increase in phytoplankton cell size. The grazing rate of microzooplankton greatly affects the growth rate of phytoplankton, aligning closely with the observed dynamics of phytoplankton biomass.