Plankton nutritional quality outweighs isotopic trophic position in predicting food chain length across oceanic gradients

Abstract

The Eastern Indian Ocean exhibits a gradient from eutrophic nearshore to oligotrophic offshore conditions. The trophic structure of the planktonic food web in this region was investigated in spring 2022 using stable isotopes (δ13C, δ15N), biomass, and piecewise structural equation models (PSEM). Regional comparisons based on grouped stations showed that the equatorial zone was highly oligotrophic with the shortest food‐chain length (FCL = 3.12), while the Bay of Bengal had the highest δ15N baseline and widest trophic range (0.10–4.0). The Sumatra upwelling and seamount area were nutrient‐rich, supporting high particulate organic matter (POM), phytoplankton biomass, and top predators with longer food chains (FCL = 4.26 and FCL = 4.73, respectively). Isotopic niches were broadest in nearshore and upwelling zones but most constrained in oligotrophic waters. Using station‐level data, the PSEM revealed that nitrite and chlorophyll a influenced isotopic patterns and FCL through POM carbon‐to‐nitrogen ratio (C : N), while plankton C : N played a significant role in extending the FCL. These findings emphasize how gradients shape food webs and the importance of upwelling and seamounts in supporting high‐trophic‐level species and carbon sequestration in the face of climate change.