Light-Dependent Grazing Drives Deep Chlorophyll Maximum Formation and Deepening in a Global Ocean Model

Deep chlorophyll maxima (DCMs) are subsurface peaks in the concentration of chlorophyll-a, a photosynthetic pigment used as a proxy for phytoplankton biomass. These oceanographic features are of interest because primary production at depth may be more likely to be exported via the biological pump, leading to carbon storage in the deep ocean. Further, because of their depth, accurate detection and quantification by satellite is challenging. At least three traditional hypotheses exist for the formation of DCMs: (1) Co-limitation: Phytoplankton experience co-limitation of light and nutrients, leading to biomass accumulation at the nutricline; (2) Photoacclimation: Phytoplankton increase pigment production at depth where light levels are lower; and (3) Behavioral Regulation: Phytoplankton exhibit behaviors, such as buoyancy regulation, leading to their accumulation at depth. While these hypotheses (and combinations thereof) explain DCM formation in many situations, they are not comprehensive. Here, we present evidence for a new hypothesis: that light-dependent grazing by microzooplankton suppresses surface phytoplankton populations, leading to a DCM formed by top-down control. Measurements of microzooplankton grazing rates show that phytoplankton consumption increases with increasing light availability, at least for some taxa. When we incorporate this light dependence into one-dimensional water column models, a DCM emerges, with depth varying seasonally with surface irradiance. This explains how DCMs may form even when surface nutrient availability is high. Furthermore, incorporation of light-dependent grazing into the global marine biogeochemistry model COBALT coupled to GFDL's Earth System Model ESM2M results in a 20 to 30-meter deepening of DCMs predicted in the subtropical gyres, improving the model’s congruence with existing measurements of the DCM in those regions. Thus, we suggest that incorporating key physiological traits, such as light-dependent grazing, may be critical to accurate estimation of primary production, food web structure, and carbon export in the surface ocean.