Stable Isotope Analysis of Trophic Structure and Food Web Dynamics in Lake Alau, Borno State, North East Nigeria

This study applies stable carbon and nitrogen isotope analysis to reconstruct the trophic structure and food web dynamics of Lake Alau, a shallow floodplain lake in Borno State, located in Nigeria's North East geopolitical zone. Lake Alau supports an artisanal fishery critical to food security in the Lake Chad Basin zone, but its food web structure has never been formally characterised, creating a gap in understanding energy flow and the trophic basis of fisheries productivity. Using a cross-sectional sampling design, samples of primary producers, macroinvertebrates, zooplankton, and fish were collected from three sites during wet and dry seasons. Stable isotope ratios of delta 13C and delta 15N were measured by continuous-flow isotope ratio mass spectrometry for all collected biota. Trophic positions were calculated using a two-source mixing model with Bayesian inference. Stable isotope biplots revealed a four-trophic-level structure including primary producers, primary consumers, omnivorous secondary consumers, and piscivorous apex predators. Seasonal shifts in isotopic baselines reflected changes in primary production sources between wet (allochthonous inputs dominant) and dry (phytoplankton dominant) seasons. Several small-bodied cichlid species occupied unexpectedly high trophic positions, revealing previously unrecognised insectivore roles. The study provides the first quantitative food web characterisation for Lake Alau and identifies pelagic primary production as the dominant energy pathway supporting commercial fish yields, with implications for nutrient management of the lake ecosystem. Keywords: stable isotope analysis, food web, Lake Alau, trophic structure, Borno State.