American Society of Naturalists

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

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Pei-Chi Ho, Chun-Wei Chang, Fuh-Kwo Shiah, Pei-Ling Wang, Chih-hao Hsieh, and Ken H. Andersen (Apr 2020)

Our field observations and model extend the light-nutrient theory determining plankton C:N with body size dependency

Read the Article (Just Accepted)

Body size determines the variation of elemental ratio within plankton community in both marine and freshwater systems, in addition to light and nutrient availability

Plankton are tiny organisms that compose the base of aquatic food webs and critically support the growth of large animals such as fish and whales. Plankton are categorized into plant-like phytoplankton and animal-like zooplankton. Heterotrophic zooplankton usually have higher N and P body mass and their stoichiometry (C:N:P ratio) is less variable than photoautotrophic phytoplankton, and thus zooplankton face stoichiometric imbalance and decrease of assimilation efficiency consuming phytoplankton. Other than heterotrophic zooplankton and photoautotrophic phytoplankton, mixotrophs that can both photosynthesize and feed on prey may have stoichiometry between phytoplankton and zooplankton. In both freshwater and marine plankton communities, body size is the key trait that regulates trophic strategy and potentially stoichiometry. However, we lack the field studies that reveal the relationship between stoichiometry and plankton body size. Furthermore, the mechanisms determining the variation of stoichiometry with body size within a plankton community are yet to be explored. Ho et al. investigated the stoichiometry of subtropical freshwater and marine plankton and found that plankton C:N ratio is a unimodal function of body size, with the maximal C:N ratio occurring at 50 μm. Plankton C:N ratio also increases with light intensity and decreases with nutrient supply, which supports the light-nutrient hypothesis of ecological stoichiometry. To explain the unimodal pattern, they constructed a mixotrophic food web model in which the affinities of light harvest, inorganic nutrient uptake, and prey consumption depend on body size. Model simulations indicate that increasing C:N ratio of photoautotrophs smaller than 50 μm is due to the increase of photosynthetic C assimilation relative to inorganic N uptake as cell size increases. The C:N ratio of mixotrophs and heterotrophs larger than 50 μm decreases with body size, and it is mainly caused by low photosynthetic C assimilation and respiratory C loss. This study gives insights on how plankton stoichiometry is determined by body size, and extends the classical light-nutrient hypothesis to explain the variation of the stoichiometry of diverse plankton.


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 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 emerging 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.