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Pei-Chi Ho and Chih-hao Hsieh

Institute of Oceanography, National Taiwan University

A study combining the observations in subtropical aquatic systems and theoretical modeling, led by Dr. Pei-Chi Ho, Prof. Chih-hao Hsieh and Pei-Ling Wang in IONTU and Prof. Ken H. Andersen in VKR Centre for Ocean Life, DTU, reveal the physiological mechanisms that explain the relationship between plankton body size and stoichiometry. This study is published online in the journal The American Naturalist.

The elemental composition (i.e. stoichiometry), of plankton is key to the energy transferring and community production in freshwater and marine pelagic food webs. Previous studies on the stoichiometric and biochemical composition of plankton have found that photoautotrophic phytoplankton is usually more fatty (more carbon-rich), than heterotrophic zooplankton. The light-nutrient hypothesis of ecological stoichiometry proposes that phytoplankton carbon increases with the ambient light to nutrient ratio: when light intensity is high, phytoplankton produce and accumulate carbon in the form of lipids. In contrast, zooplankton tend to keep stable stoichiometry and need to burn and lose extra carbon they uptake from phytoplankton. Though we know that the stoichiometry of phytoplankton and zooplankton is different, there is no research that investigates the general rules determining the stoichiometry of diverse plankton groups.

To fulfill the knowledge gap, this research team first examined the relationship between plankton stoichiometry and body size, which is the key trait that determines trophic strategies, in Feitsui Reservoir and the East China Sea. In the two distinct systems, plankton C:N ratio increased with body size and reaches the maximum at 50 μm, and then decreases with body size. To further elucidate the mechanisms that shape this unimodal pattern, a general food web model in which the trophic strategy of plankton is size-dependent was constructed. From simulations under different light to nutrient supply, we found a similar unimodal C:N ratio pattern in freshwater and marine systems. The model further revealed that the fast increase of photoautotrophic carbon production with phytoplankton body size causes the increasing trend of C:N ratio in small size plankton. On the contrary, the increase of heterotrophic feeding and respiration loss lower the C:N ratio in large size mixotrophs and heterotrophs. This research extends the classical “light-nutrient hypothesis” and explains the variation of stoichiometry within plankton community.

Reference:

Pei-Chi Ho, Chun-Wei Chang, Fuh-Kwo Shiah, Pei-Ling Wang, Chih-hao Hsieh, Ken H. Andersen.(2020) Body size, light intensity and nutrient supply determine plankton stoichiometry in mixotrophic plankton food webs. The American Naturalist doi/10.1086/707394.

Figure 1. Size-specific molar C/N ratio of plankton in (a) Feitsui Reservoir and (b) the East China Sea and near Dongsha Atoll.

Figure 2. Simulated encounter rates of light, nutrient, and prey (a), trophic strategy (b) and molar C:N ratio (c) of different plankton size groups.