Environmental Regulation of the Nitrogen Supply, Mean Trophic Position, and Trophic Enrichment of Mesozooplankton in the Mekong River Plume and Southern South China Sea

The mean trophic position (TP) of mesozooplankton largely determines how much mass and energy is available for higher levels like fish. Unfortunately, the ratio herbivores to carnivores in difficult identify field samples. Here, we investigated changes TP a highly dynamic environment encompassing four distinct habitats southern South China Sea: Mekong River plume, coastal upwelling region, shelf waters, offshore oceanic waters. We used set variables derived from bulk amino acid nitrogen stable isotopes particulate organic matter size fractions source across these habitats. found clear indications shift N sources biological production nitrate near-coastal waters with shallow mixed layer depths toward an increase diazotroph-N inputs deep where diazotrophs shaped phytoplankton community. was accompanied by lengthening food chain (increase TP). This may provide further support connection between diazotrophy indirect routing through marine web. Our combined δ15N approach also allowed us estimate enrichment (TE) entire regional ecosystem. When put context literature values, high TE 5.1‰ suggested link ecosystem heterogeneity less efficient transfer levels.