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

Abstract

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 mm diameter in marine and freshwater systems. Moreover, the variation in C:N ratios is explained mainly 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 are an emerging result. Our model simulations reproduce the unimodal pattern with a C:N ratio of photoautotrophs ≤50 mm increasing with body size due to increase of photosynthetic carbon, whereas C:N ratios of organisms >150 μm decrease 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. © 2020 by The University of Chicago.