Herbivorous protist grazing balances phytoplankton growth in the North Pacific, leaving little primary production for export.

Grazing by protists can alter the abundance, size structure and species composition of phytoplankton available for coagulation, sinking, physical transport and consumption by mesozooplankton predators. While it is accepted that grazing plays a central role in regulating primary production, there is limited predictive understanding of the factors that influence grazing rates and their influence on export production. To build a mechanistic understanding of grazing by protists and to quantify the effects of predation on phytoplankton size and abundance in relation to carbon export, we measured phytoplankton growth and grazing rates during the North Pacific EXPORTS campaign centered near ocean station Papa. The immense interdisciplinary nature of the campaign enabled us to tease apart the environmental conditions that modulate protist grazing. Small phytoplankton (<5 µm) made up 74 ± 10% of the primary producer biomass, as measured through chlorophyll a concentration. Overall, phytoplankton growth and grazing rates were closely matched–average phytoplankton growth was 0.05 ± 0.32 d^‑1 and the average grazing rate was 0.10 ± 0.16 d^‑1, suggesting that any new phytoplankton biomass was consumed by protist grazers. Herbivorous protist grazing helped maintain the dominance of small cells. Grazing rates on nanoeukaryotes were two-fold higher than on picoeukaryotes and Synechococcus. Results from experiments that isolated the effect of light showed growth and grazing rates changed with light availability but not in a consistent manner. Good agreement between rates measured in bottle experiments and changes in phytoplankton abundance and community composition in situ indicates that shipboard experiments qualitatively and quantitatively captured the major loss processes of primary production. The tight linkage between primary production and herbivory suggests that herbivorous protists in the North Pacific play a foundational role in modulating primary production, plankton community composition, size spectra and potential for export production.