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

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.