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Body size, light intensity and nutrient supply determine plankton stoichiometry in mixotrophic plankton food webs


Ho, Pei-Chi et al. (2019), Body size, light intensity and nutrient supply determine plankton stoichiometry in mixotrophic plankton food webs, Dryad, Dataset,


Trophic strategy determines stoichiometry of plankton. In general, heterotrophic zooplankton have lower and more stable C:N and C:P ratios than photoautotrophic phytoplankton whereas mixotrophic protists, which consume prey and photosynthesize, have stoichiometry between zooplankton and phytoplankton. As trophic strategies change with cell size, body size may be a key trait influencing eukaryotic plankton stoichiometry. However, the relationship between body size and stoichiometry remains unclear. Here, we measured plankton size-fractionated C:N ratios under different intensities of light and nutrient supply in subtropical freshwater and marine systems. We found a unimodal body size-C:N ratio pattern with a maximum C:N ratio at ~50 μm diameter in marine and freshwater systems. Moreover, the variation in C:N ratios is mainly explained by body size, followed by light intensity and nutrient concentration. To investigate the mechanisms behind this unimodal pattern, we constructed a size-based plankton food web model in which the trophic strategy and C:N ratio is an emergent result. Our model simulations reproduce the unimodal pattern with C:N ratio of photoautotrophs ≤ 50 μm increasing with body size due to increase of photosynthetic carbon, whereas C:N ratios of organisms > 50 μm decreases with size due to decreasing photoautotrophic but increasing heterotrophic uptake. Based on our field observations and simulation, we extend the classic “light-nutrient” theory that determines plankton C:N ratio to include body size and trophic strategy dependency. We conclude that body size and size-dependent uptake of resources (light, nutrients and prey) determine plankton stoichiometry at various light and nutrient supplies.


The datasets of freshwater and marine size-fractionated plankton C:N ratio were collected by Norpac net of 50 and 200 um in Feitsui Reservoir and East China Sea. The plankton collected were seperated to several size classes (see Appendix A of the manuscript) and the C and N contents were measured by elemental analyzer (EA1108, Fisons, Italy, and FLASH 2000, Thermo SCIENTIFIC, USA). The molar C:N ratios were calculated according to the C and N contents of the plankton.

For model simulation, the original m-files can be found here. Parametere_AN.m includes the parameters that were kept constant in each simulation. ControlParam_AN.m includes parameters that could be changed in the simulation, such as the nutrient supply and light intensity. MixotrophWeb_AN.m is the main model ODEs.

Usage Notes

Missing data in Table S1 and S2 are labeled NA.


Gordon & Betty Moore Foundation, Award: 5479

Ministry of Science and Technology, Taiwan