Nutrient supply, prey body size and biomass determine the trophic structures in marine plankton food webs

Abstract

Trophic diversity and predator–prey mass ratio (PPMR) are key indicators of trophic transfer pathways and efficiency in marine ecosystems. Horizontal trophic diversity reflects the variety of resources at the base of the planktonic food web, whereas vertical trophic diversity represents the maximum food chain length. In size-structured marine food webs, community PPMR indicates the general prey size preference of zooplankton. In this study, we used size-fractionated stable isotope analyses (δ¹³C and δ¹⁵N) to assess horizontal and vertical trophic diversity and community PPMR, examining how nutrient supply influences the biomass and body size distribution of prey communities, and in turn affects size-structured trophic interactions. Our long-term monitoring across a nutrient gradient revealed that higher nutrient concentrations increased horizontal trophic diversity, suggesting that zooplankton exhibit more diverse prey preferences across size classes. Conversely, an increase in large autotrophic microplankton reduced vertical trophic diversity, indicating a shift in mesozooplankton feeding from heterotrophic microplankton to autotrophs, thereby shortening the food chain. In regions with low autotrophic microplankton biomass, community PPMR decreased with increasing total prey biomass, implying that mesozooplankton preferentially fed on heterotrophic microzooplankton in picoplankton-dominated, high-productivity systems. These findings highlight how nutrient availability shapes size-structured plankton food webs by altering the biomass and size composition of prey communities.