Fig. 2

Light-dependent grazing drives deep chlorophyll maxima (DCM) formation in a one-dimensional model. a In the absence of light dependence of grazing (H_Z = 0), phytoplankton are roughly homogeneously distributed where sufficient light is available to support persistence. b However, when light dependence is introduced (H_Z = 50), a deep phytoplankton biomass maximum corresponding to a deep chlorophyll maximum emerges. c Phytoplankton accumulation in a DCM arises from two processes: elevated grazing near the water column’s surface, and depressed growth due to light limitation below the compensation depth. d Total biomass of both organisms (in g C m⁻²) is a function of surface input light. Once light availability has increased beyond the compensation irradiance for the phytoplankter, total phytoplankton biomass increases with increasing light until light levels are sufficiently high to sustain microzooplankton. At this point, an increase in total microzooplankton biomass slightly suppresses total phytoplankton biomass relative to lower and higher light levels, but at higher light levels the total biomass of both organisms is an increasing function of light. e A DCM forms only when both phytoplankton and microzooplankton are present in the water column. The depth of this DCM increases with increasing surface input light, but phytoplankton biomass concentration (g C m⁻³) at the DCM remains approximately constant. Other parameter values are k₀ = 0.001, k_P = 0.1, k_Z = 0.0005, p = 1, l = 0.5, g = 20, e = 0.1, m = 0.05, H_P = 0.5, H_Z = 50, H_A = 20, and D = 0.05