The Biobrio 12(3&4), 2025

Quantification of anthropogenic nutrient enrichment effects on zooplankton secondary production and trophic dynamics in tropical reservoir of Jharkhand

Namat Nawaj, Nupur Lal, Madhumanti & Asha Mishra

ABSTRACT:

Anthropogenic nutrient enrichment poses a significant threat to the structural and functional integrity of tropical freshwater ecosystems. This study, conducted from 2024 to 2025 in the Getalsud Dam (Ranchi, Jharkhand), quantifies the impact of nutrient loading on zooplankton secondary production and trophic dynamics. Spatio-temporal sampling across a distinct nutrient gradient revealed severe eutrophication (TSI: 65.2) in the human-impacted inflow zone, characterized by elevated total phosphorus (85.4 μg/L) and nitrogen (2.4 mg/L). This enrichment fundamentally altered the zooplankton community, favoring rapid-cycling Rotifera (e.g., Brachionus calyciflorus) over large-bodied Cladocera, whose contribution to total biomass dropped sharply from 48% to 15%. While total zooplankton secondary production was paradoxically higher in the enriched zone (7.37 mg/m³/d) compared to the oligotrophic lacustrine zone (5.24 mg/m³/d), the increase was driven entirely by rotifer production (+260%). Crucially, this quantitative gain masked a significant qualitative degradation in trophic dynamics. A depressed Zooplankton:Phytoplankton biomass ratio (0.18) and a reduced seston C:P ratio (180:1) indicated severe stoichiometric imbalance and poor energy transfer efficiency. Consequently, trophic transfer efficiency (TTE) plummeted from 18% to approximately 8% in enriched waters. Our findings demonstrate that in tropical reservoirs like Getalsud, anthropogenic nutrients create an ecological paradox by increasing lower-quality production while disrupting the classical grazing food chain, ultimately compromising energy flow to higher trophic levels.

Keywords:    

Eutrophication, Zooplankton, Secondary Production, Trophic Dynamics, Getalsud Dam, Stoichiometry

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