Phytoplankton Species Richness Increases Marine Primary Productivity through Selection Effect

Despite half of global primary production being mediated by the activity of microscopic algae called phytoplankton, the effect of phytoplankton species richness on marine primary productivity lacks a mechanistic understanding. We show that phytoplankton species richness increases the productivity of marine ecosystems through selection effect. Using concurrent measurements of phytoplankton community structure, nitrate fluxes into the euphotic zone and productivity (measured with the ¹⁴C-uptake technique) from a temperate coastal ecosystem, we failed to predict observed productivities from monoculture growth models based on extracellular nitrate concentrations, nitrate stores uniquely assigned to rare species or random physiological parameterizations. Observed productivities were best described by a model in which the dominant species of the community approached their maximum productivities at the expense of intracellular nitrate stores. We interpret these results as evidence of species’ selection in communities containing a vast repertory. This differs from the positive effect of species richness in terrestrial plants, which is mainly driven by complementarity in resource use among species. The prevalence of selection effect was supported by open ocean data that show an increase in community dominance across a gradient of nutrient availability. These results highlight fundamental differences in the way terrestrial plant communities and marine phytoplankton control resource use and sustain world food stocks. We suggest that the maintenance of phytoplankton species richness is essential to sustain marine primary productivity.