Protist grazing contributes to microbial food web at the upper boundary of the twilight zone in the subarctic Pacific

Abstract

Grazing by herbivorous protists (microzooplankton) is a major loss pathway of primary production in the surface ocean, yet its impact below the well-lit surface ocean is largely unknown. The upper boundary of the twilight zone is critically important to understanding carbon cycling and is often the depth of highest attenuation of particulate carbon flux. Available measurements of primary production and grazing below the well-lit surface ocean suggest that the upper boundary of the twilight zone may harbor active but poorly constrained food web processes. Previous grazing rates from the base of the euphotic zone were measured in subtropical and tropical environments. Thus, the impact of protist grazing on prey populations remains unknown in colder conditions at higher latitudes. To advance understanding and provide mechanistic insight into processes occurring at the base of the euphotic zone (0.4-0.7% PAR), we measured predation rates on both phytoplankton and heterotrophic prokaryotes in the North Pacific, using a novel method that amplified the grazing signal by concentrating the predator community, enabling detection of grazing rates far below previous limits. Protists consumed 0.6% of the phytoplankton population daily and 12% of daily heterotrophic prokaryote growth. These conservative rate measurements document marginal removal of phytoplankton even in these colder regimes, implying flows of energy from single-cell primary producers and prokaryotes to single-cell protists at rates far below previous detection limits in this twilight region of a low-productivity system.