Fig 1.
(a) Maximum phytoplankton growth rates and maximum zooplankton grazing rates as a function of cell size (volume and equivalent spherical diameter, ESD) and taxon. (b) Schematic representation of the ecosystem model simplified to show only grazing and predation fluxes for clarity (for a complete representation of the model, see Fig 2 in Ward et al. (2012) [24]). Phytoplankton functional types (Prochlorococcus (P), Synechococcus (S), small Eukaryotes (E), Diatoms (D)) are divided into different size classes (P1, P2, S1, S2, etc.). Phytoplankton and zooplankton size classes (Z1, Z2, Z3, etc.) are represented with a color pallet used in subsequent figures.
Fig 2.
(a) Grazing effort as a function of prey biomass for cases 1–4 from Table 1. Solid lines correspond to cases presented in the core paper (other cases are presented in supporting information). (b) Specification of an explicit refuge food concentration T where grazing is initiated for case 4 (size-independent threshold, green lines with four different parameter values) and case 5 (body size-dependent threshold T as a function of zooplankton biovolume and ESD, pink lines). Black and white dots correspond to low and high estimates from the literature, respectively; pink lines represent T fitted to these estimates with an allometric relationship T = aTVb, with aT-low = 0.012 mmol C m-3, aT-high 0.025 mmol C m-3, and b = 0.27 (cases 5, Iv-SDT). Green lines represent size-independent T (cases 4, Iv-SIT).
Table 1.
Various mathematical formulations of the grazing rate R.
Fig 3.
(column 1) Annual mean surface chlorophyll-a concentration (mg Chl-a m-3); the top panel corresponds to observations from SeaWiFS, while other panels correspond to model outputs for grazing cases 1 to 5b. (column 2) Vertical sections of annual mean chlorophyll-a concentration across CalCOFI line 70 (mg Chl-a m-3); the top panel corresponds to observations from CalCOFI, while other panels correspond to model outputs at the same location for cases 1 to 5b.
Fig 4.
Seasonal variation of (column 1) the 4 plankton functional types (mmol C m-3) and (column 2) detailed size classes of zooplankton (mmol C m-3).
Fig 5.
Simplified schematic representation of grazing and predation fluxes in the ecosystem model for grazing cases 1 to 5b.
For each case, biomasses (circles) and fluxes (arrows) are color-coded as follows: Dominant in red, intermediate in blue, and low in dark grey. Negligible fluxes are not shown, and negligible biomasses are represented in light grey. See S1 and S2 Tables for detailed biomasses and grazing fluxes, respectively.
Fig 6.
(column 1) Seasonal variation of total phytoplankton biomass (blue), total zooplankton biomass (red) and the associated Z:P ratio (black), in mmolC m-3. The top panel corresponds to observations from CalCOFI line 90, while other panels correspond to model outputs for grazing cases 1 to 5b. The number in the upper-left corner indicates the seasonal mean of the Z:P ratio. (column 2) Spatial distribution of the annual-mean Z:P ratio. The solid black line indicates Z:P = 1.
Fig 7.
(columns 1 and 2) Richness and (column 3 and 4) niches of phytoplankton (columns 1 and 3) and zooplankton (columns 2 and 4) for grazing in cases 1 to 5b.