Assessment of Climate Variability and Anthropogenic Stress on Zooplankton Populations in the Aquatic Ecosystems of Bhojpur District, Bihar, India, with Special Reference to Shahpur Block and Ara Town

Assessment of Climate Variability and Anthropogenic Stress on Zooplankton Populations in the Aquatic Ecosystems of Bhojpur District, Bihar, India, with Special Reference to Shahpur Block and Ara Town Sana Tasneem Dr. Bibha Verma Freshwater ecosystems of the Middle Gangetic Plains are increasingly exposed to the combined influence of climatic variability and anthropogenic disturbances. Changes in temperature and monsoonal precipitation can modify hydrological conditions, nutrient transport and oxygen availability, while agricultural runoff, domestic wastewater and urban drainage intensify eutrophication and organic pollution. Zooplankton respond rapidly to these environmental changes and therefore provide valuable biological indicators of aquatic ecosystem condition. The present study assessed seasonal and spatial variations in zooplankton communities in six aquatic ecosystems of Bhojpur District, Bihar, comprising three relatively less disturbed rural systems of Shahpur Block and three anthropogenically influenced urban systems of Ara Town. Monthly sampling was undertaken across four major seasons: pre-monsoon, monsoon, post-monsoon and winter. Physicochemical parameters, including dissolved oxygen (DO), biochemical oxygen demand (BOD), chemical oxygen demand (COD), electrical conductivity, total dissolved solids, turbidity, transparency, alkalinity, hardness, nitrate, phosphate and chlorophyll-a, were evaluated together with zooplankton abundance and diversity. The results demonstrated a pronounced rural–urban ecological gradient. Rural aquatic systems generally exhibited comparatively favourable oxygen conditions and greater zooplankton diversity, whereas urban sites were characterised by elevated organic pollution and nutrient enrichment. BOD and COD reached their highest levels at anthropogenically stressed urban sites, while nitrate and phosphate increased substantially during the monsoon, indicating enhanced nutrient delivery through surface runoff. Chlorophyll-a also increased under nutrient-enriched conditions, indicating eutrophic tendencies. Zooplankton assemblages were represented by Rotifera, Cladocera, Copepoda, Protozoa and Ostracoda, with rotifers becoming particularly prominent in urban, organically enriched waters. Pollution-tolerant taxa such as Brachionus calyciflorus, B. angularis and Keratella cochlearis were associated with stressed environments, whereas cladocerans and copepods were comparatively better represented in oxygen-rich rural systems. Zooplankton abundance showed a seasonal pattern, with maximum abundance during winter and minimum abundance during the pre-monsoon period. Diversity indices and Water Quality Index (WQI) further supported the distinction between relatively stable rural systems and degraded urban waters. Multivariate analyses, including PCA, RDA and hierarchical cluster analysis, demonstrated that organic pollution, nutrient enrichment, dissolved oxygen and water transparency were major environmental factors structuring zooplankton communities. The findings indicate that climatic variability interacts with local anthropogenic stress to influence zooplankton community organisation and reinforce the potential of zooplankton as cost-effective bioindicators for freshwater monitoring and climate-resilient management in the Gangetic Plains.