Global relationship between phytoplankton diversity and productivity in the ocean

Abstract

The shape of the productivity-diversity relationship (PDR) for marine phytoplankton has been suggested to be unimodal, that is, diversity peaking at intermediate levels of productivity. However, there are few observations and there has been little attempt to understand the mechanisms that would lead to such a shape for planktonic organisms. Here we use a marine ecosystem model together with the community assembly theory to explain the shape of the unimodal PDR we obtain at the global scale. The positive slope from low to intermediate productivity is due to grazer control with selective feeding, which leads to the predator-mediated coexistence of prey. The negative slope at high productivity is due to seasonal blooms of opportunist species that occur before they are regulated by grazers. The negative side is only unveiled when the temporal scale of the observation captures the transient dynamics, which are especially relevant at highly seasonal latitudes. Thus selective predation explains the positive side while transient competitive exclusion explains the negative side of the unimodal PDR curve. The phytoplankton community composition of the positive and negative sides is mostly dominated by slow-growing nutrient specialists and fast-growing nutrient opportunist species, respectively.

Figure 1. Global PDR from ocean field data sets.

(a) Global PDR distribution using Gaussian random sampling of the raw data. (b) Global PDR curve using equally spaced log10 bins of biomass. Units: phytoplankton biomass (mg C m⁻³) and diversity (# species) (contributing >1% to total biomass).

Figure 2. Global ocean maps of annually averaged model outputs.

Primary production (mmol C m⁻³ d⁻¹); primary productivity (d⁻¹); photosynthetic active radiation (W m⁻²); zooplankton concentration (mmol C m⁻³); phytoplankton concentration (mmol C m⁻³); dissolved inorganic nitrogen (mmol N m⁻³); phytoplankton diversity (# species) (contributing >1% to total biomass); zooplankton/phytoplankton concentration ratio (n.d., no dimensions); subsistence nutrient concentration at equilibrium (R*) (mmol N m⁻³).

Figure 3. Hovmoller diagrams of model outputs (that is, time evolution of zonal averages).

Primary production (mmol C m⁻³ d⁻¹); primary productivity (d⁻¹); photosynthetic active radiation (W m⁻²); zooplankton concentration (mmol C m⁻³); phytoplankton concentration (mmol C m⁻³); dissolved inorganic nitrogen (mmol N m⁻³); phytoplankton diversity (# species) (contributing >1% to total biomass); zooplankton/phytoplankton concentration ratio no dimensions (n.d.); subsistence nutrient concentration at equilibrium (R*) (mmol N m⁻³).

Figure 4. Global ocean maps of annually averaged model outputs.

(a) Primary production (mmol C m⁻³ d⁻¹); (b) phytoplankton diversity (# species) (contributing >1% to total biomass).

Figure 5. Hovmoller diagrams of model outputs (that is, time evolution of zonal averages).

(a) Primary production (mmol C m⁻³ d⁻¹); (b) phytoplankton diversity (# species) (contributing >1% to total biomass).

Figure 6. Global PDR distribution of model outputs.

(a) Using annually averaged data; (b) using weekly averaged data. Units: primary production (mmol C m⁻³ d⁻¹); phytoplankton diversity (# species) (contributing >1% total biomass). Colour legend: the colourmap scale gives the relative (%) data density.

Figure 7. Global PDR curve of model outputs using equally spaced log10 bins of primary production.

(a) Using annually averaged data; (b) using weekly averaged data. Units: primary production (mmol C m⁻³ d⁻¹); phytoplankton diversity (# species) (contributing >1% total biomass). Colour legend: Prochlorococcus (blue line), Synechococcus (green line), flagellates (yellow line) and diatoms (red line).

Figure 8. Phytoplankton species traits that define their nutrient niches and competitive abilities.

(a) Species nutrient uptake curves as a function of dissolved inorganic nitrogen (mmol N m⁻³); (b) species subsistence nutrient concentration at equilibrium (R*) versus their nutrient uptake affinity (μmax/ks).

Figure 9. Community structure along an environmental gradient of increasing nutrient supply.

(a) Steady-state equilibrium; (b) transient non-equilibrium. Species biomasses are cumulative on the y axis. Units: phytoplankton (normalized) biomass no dimensions (n.d.). Colour legend: Prochlorococcus (blue line), Synechococcus (green line), flagellates (yellow line) and diatoms (red line).

Figure 10. KTW functional response.

Gj is the grazing rate upon prey species j (mmol m⁻³ d⁻¹); Q^j_switching gives the fraction of prey species j in the predators' diet (n.d.); Q_feeding gives the overall feeding probability of predators (n.d.); V_max gives the maximum ingestion rate (mmol m⁻³ d⁻¹); g_max is the maximum specific rate for ingestion (d⁻¹); k_sat is the half-saturation constant for ingestion (mmol m⁻³); Z is the concentration of predators (mmol m⁻³); Pj is the concentration of prey species j (mmol m⁻³); ρj is a constant prey preference (n.d.); β is the Hill coefficient that measures how the feeding rate varies with total prey density no dimensions (n.d.); and α is the KTW coefficient that gives the potential for selective predation: when α=1.0 there is no switching (for example, passive filter feeding); when α>1.0 there is prey switching (that is, active selective predation).