Diurnal Variations in Zooplankton Community Structure and Abundance in Two Sites of the Negua Canal, West Bengal, India

Zooplankton community composition reflects environmental variability in estuarine systems, but within-day patterns in the Negua Canal have received limited site-specific assessment. This study investigates the spatial distribution and diurnal variations of zooplankton community structure and abundance at two contrasting sites along the Negua Diversional Canal in Purba Medinipur, West Bengal, from March to May 2026. Sampling was conducted at Site A (Nyaykali Bridge), a typical marine environment, and Site B (Essex Marine Ltd.), an area subjected to industrial wastewater discharge, across three distinct times of day: morning, noon, and evening. A total of 7–8 major zooplankton taxa were recorded, with Cyclops sp. emerging as the dominant species, followed by Diaptomus sp., Undinula vulgaris, crustacean larvae, and other copepod forms. Site A consistently supported a much higher abundance and diversity of zooplankton compared to the ecologically stressed Site B. This spatial disparity was associated with water quality parameters, as Site A maintained favorable conditions, including higher dissolved oxygen (4.8–6.4 mg L⁻¹) and lower carbon dioxide levels (12–16 mg L⁻¹), whereas Site B exhibited prominent organic and industrial pollution stress, characterized by severe oxygen depletion (2.4–3.2 mg L⁻¹) and elevated carbon dioxide concentrations (20–76 mg L⁻¹). Furthermore, a distinct diurnal pattern was observed across both locations, with zooplankton abundance peaking during the morning, dropping to its lowest levels during the afternoon, and showing a moderate recovery by evening. The independent and interactive effects of sampling site and time of day significantly influenced (p < 0.05) zooplankton density. This temporal fluctuation may reflect the behavioral phenomenon of diel vertical migration (DVM) as an adaptive response to avoid intense midday solar radiation, thermal stress, and visual predation. Ultimately, these findings underscore the high sensitivity of zooplankton to anthropogenic and environmental stressors, supporting their potential utility as reliable biological indicators for monitoring water quality and establishing crucial baseline data for the conservation and sustainable management of the Negua Canal ecosystem.