Plankton is the foundation of marine ecosystems and supports the survival of larger animals. The structure of plankton food webs shapes the energy transfer efficiency from primary producers to higher trophic levels, ultimately influencing fishery productivity. Two key aspects of food web structure are: horizontal trophic diversity, which reflects the diversity of primary producers consumed by zooplankton, and vertical trophic diversity, which reflects the extent to which zooplankton feed on prey occupying higher trophic levels and the overall length of the food chain. Together with predator–prey mass ratios (PPMR), these dimensions reflect the complexity of plankton food webs.
To better understand subtropical plankton food web structures, Assistant Professor Pei-Chi Ho and Distinguished Professor Chih-hao Hsieh of the Institute of Oceanography analyzed carbon and nitrogen stable isotope signals from different size classes of plankton collected between 2009 and 2018 in the East China Sea, around Dongsha, and the western Pacific off eastern Taiwan. These measurements were used to estimate horizontal and vertical trophic diversities and PPMR across these regions, and to investigate how these metrics relate to regional nutrient concentrations and the composition of autotrophic plankton communities.
The study found that nutrient enrichment enhanced horizontal trophic diversity by supporting greater abundance and diversity of primary producers, leading zooplankton to feed more directly on them. In contrast, increases in larger phytoplankton, such as diatoms, reduced vertical trophic diversity, as zooplankton tended to graze directly on these primary producers, resulting in shorter food chains. PPMR were negatively related to total primary producer biomass, suggesting that when primary production is higher, planktonic food webs tend to develop longer food chains.
These findings highlight how variations in nutrient supply and prey community composition shape the trophic structure of plankton food webs, offering new insights into the ecological processes that regulate energy flow in subtropical marine ecosystems.
The research has been published in Progress in Oceanography.
Source: Pei-Chi Ho, Chia-Ting Chen, Noboru Okuda, Pei-Ling Wang, Gwo-Ching Gong, Chih-hao Hsieh (2025) Nutrient supply, prey body size and biomass determine the trophic structures in marine plankton food webs. Progress in Oceanography, Volume 238,103557.
Figure 1. Schematics illustrating the effects of nutrient supply on the trophic structure of a plankton food web. Scheme MHMZ (microphytoplankton-herbivorous mesozooplankton dominant): increased nutrient supply supports high biomass of large autotrophic microplankton. Herbivorous and omnivorous zooplankton consume autotrophic plankton, shortening the food chain and raising the horizontal trophic diversity. Scheme PCMZ (picoplankton-carnivorous mesozooplankton dominant): increased nutrient supply significantly raises picoplankton biomass. The increased biomass of picoplankton supports higher heterotrophic nano- and microplankton, enhancing carnivory of mesozooplankton and a longer food chain, and thereafter increases vertical trophic diversity and lowers PPMR.
Figure 2. Relationship between horizontal trophic diversity (CR) versus inorganic nutrient concentrations (a), relationship between vertical trophioic diversity (NR) versus the cell size spectrum of prey (less negative NBSS indicating more large cells in prey community) (b), and relationship between predator-prey mass ratio (PPMR) versus phytoplankton biomass (c).
