Phytoplankton nutritional content, rather than primary production, critically limits marine zooplankton production
Pei-Chi Ho and Chih-hao Hsieh
A study combining ecological stoichiometry and in situ zooplankton incubation experiments, led by Dr. Pei-Chi Ho and Prof. Chih-hao Hsieh in the IONTU, reveals that prey nutritional condition, especially carbon:nitrogen ratio (C:N), influences copepod production in subtropical marine systems. In contrast, primary production, although traditionally believed to determine zooplankton production, is found to have very weak influence. This is the first study that combines in situ measurements of primary and secondary production with stoichiometry in marine systems. This study is published in the journal Progress in Oceanography in July 2020.
Phytoplankton biomass availability is commonly considered the main factor limiting marine zooplankton production- zooplankton production should increase with primary production (PP). However, in addition to PP, the imbalance of elemental compositions between zooplankton and their prey is key to the energy transferring and zooplankton production in marine food webs. Laboratory manipulative experiments have revealed that zooplankton production decreases with high C:N and C:P ratio of prey. However, investigation of how prey stoichiometry affects zooplankton production is rare in natural marine systems, due to the difficulty of in situ measurements at sea.
To fulfill this knowledge gap, this research team conducted in situ incubations of copepod community to estimate the copepod production (CP) and measured PP and phytoplankton C:N and C:P ratios in the East China Sea and around the Dongsha Atoll in the South China Sea in 2009-2016. They found that CP decreases with high prey C:N and C:P ratio, and prey C:N ratio is the most important factor determining CP. Furthermore, CP is affected by taxonomic composition of copepods and phytoplankton. In contrast, PP is not the key factor explaining the variation of copepod community production. This research highlights the knowledge gained by simultaneously measuring phytoplankton and copepod production together with plankton elemental ratios for understanding stoichiometric effects on marine secondary production. These findings also cast doubt on the reliability of using primary production to infer fish production at regional scale.
Pei-Chi Ho, Esther Wong, Fan-Sian Lin, Akash R. Sastri, Carmen García-Comas, Noboru Okuda, Fuh-Kwo Shiah, Gwo-Ching Gong, Rita S.W. Yam, Chih-hao Hsieh. (2020) Prey stoichiometry and phytoplankton and zooplankton composition influence the production of marine crustacean zooplankton. Progress in Oceanography 186, 102369.
Figure 1. Copepod production decreases as phytoplankton C:N and C:P increases. In addition, phytoplankton and copepod taxonomic compositions also influence copepod production. Percentages of the relative contributions of the three main variables explaining the variation of copepod production are indicated. In contrast, primary production casts weak effect on copepod production.