Seasonal Dynamics of Small and Large Zooplankton Communities and Stable Isotope (δ¹³C, δ¹⁵N) Signatures in Relation to Hydrobiological Parameters in Yerracheruvu, Siddipet District, Telangana, India

Abstract

The present study investigates the seasonal variations in hydrobiological parameters, zooplankton community structure, and stable isotope signatures (δ¹³C and δ¹⁵N) in Yerracheruvu, a freshwater lake located in Siddipet District, Telangana, India, from June 2024 to May 2025. Four sampling stations representing inlet, pelagic, littoral, and outlet zones were monitored across pre-monsoon, monsoon, post-monsoon, and winter seasons. Hydro chemical analysis revealed marked seasonal differences, with higher total phosphorus (0.12 ± 0.01 mg L⁻¹) and total nitrogen (1.3 ± 0.08 mg L⁻¹) during the monsoon due to agricultural runoff, while dissolved oxygen peaked in winter (8.0 ± 0.3 mg L⁻¹). Zooplankton abundance exhibited significant seasonal variation (p < 0.01); small taxa (rotifers and small cladocerans) dominated during the nutrient-enriched monsoon, whereas large taxa (copepods and large cladocerans) were more abundant in pre-monsoon and winter periods. Stable isotope analysis revealed seasonal shifts in δ¹³C and δ¹⁵N values of zooplankton and particulate organic matter (POM). Small zooplankton showed depleted δ¹³C (−28.5‰) during monsoon, reflecting increased terrestrial carbon input, while large zooplankton was consistently enriched in δ¹³C and δ¹⁵N, indicating higher trophic positioning. Principal Component Analysis (PCA) explained 69.9% of total variance, with PC1 driven by nutrient and small-zooplankton variables and PC2 by oxygen and large zooplankton associations. These results demonstrate that monsoon-driven nutrient enrichment strongly modulates trophic pathways and carbon flow, influencing both the structure and isotopic composition of zooplankton communities. The integration of hydrobiological and stable isotope approaches provides a comprehensive understanding of seasonal food-web dynamics in tropical freshwater ecosystems like Yerracheruvu.