<p>The feeding ecology of freshwater fish is strongly influenced by environmental variability, yet few studies have assessed how both seasonal and spatial changes in water quality and plankton dynamics affect the diet of native species in South Asian river–lake systems. This study explores seasonal and spatial variations in the diet composition of <i>Labeo gonius</i> in relation to physico-chemical parameters and plankton abundance in Chashma Lake, Pakistan. Monthly sampling from 2022 to 2023 was conducted at three ecologically distinct sites to assess water quality, phytoplankton, macrophytes, zooplankton, and fish gut content. Significant seasonal and spatial fluctuations in temperature, dissolved oxygen, turbidity, and nutrient levels influenced the availability of food resources. Spatially, variations in plankton and macrophyte abundance were associated with nutrient input, water depth, and habitat complexity. Phytoplankton diversity peaked in summer, dominated by Bacillariophyta, followed by Chlorophyta, Cyanophyta, and aquatic macrophytes such as <i>Chara</i> and <i>Hydrilla</i>. Zooplankton consumption was minimal, with <i>Daphnia</i> and <i>Cyclops</i> being the most frequently consumed taxa. Correlation analysis revealed that seasonal changes in temperature and nutrient availability significantly influenced the abundance of phytoplankton, macrophytes, and zooplankton, which in turn affected feeding behavior (<i>p</i> &lt; 0.05). Multivariate analyses showed that both fish size and site location significantly influenced diet composition, with larger individuals consuming more macrophytes (<i>p</i> &lt; 0.05). These findings highlight the flexible, environmentally responsive feeding behavior of <i>L. gonius</i> and improve understanding of its trophic ecology. This knowledge can guide sustainable fisheries management through optimized stocking schedules, habitat-specific conservation efforts, and nutrient regulation strategies.</p>