Seasonal variation of mesozooplankton communities in relation to environmental variables from a subtropical estuary of Bangladesh

Mesozooplankton are essential trophic intermediaries and bioindicators in estuarine ecosystems. However, their seasonal response to environmental variability remains underexplored in the subtropical estuaries of Bangladesh. This study investigated the seasonal variation of mesozooplankton community composition and its environmental drivers in the Moheshkhali Channel, southeastern Bangladesh, based on data collected between 2023 and 2024. Mesozooplankton were sampled using conical net tows, and environmental variables were measured concurrently. Community structure was analysed using ecological indices and multivariate approaches. A total of 24 taxa across six phyla were recorded, with copepods as the dominant group. Mesozooplankton abundance peaked in winter (1845 ind/m³), while diversity ( H′ = 0.90), evenness ( J′ = 0.33), and richness ( D _mg = 2.33) were highest during the monsoon. Non-metric multidimensional scaling (NMDS) revealed distinct seasonal patterns, supported by analysis of similarities (ANOSIM) results indicating significant differences between monsoon and other seasons. Similarity Percentage analysis (SIMPER) identified Mysida , Gastropoda , Chaetognatha , and Copepoda as key contributors to inter-seasonal dissimilarity. The redundancy analysis (RDA) explained 70.7% of the constrained variation (RDA, R² = 0.862, adjusted R² = 0.613, p -value = 0.015), indicating that temperature, turbidity, and conductivity are the primary drivers. Conductivity emerged as the strongest individual predictor in the Biota–Environment (BIO-ENV) matching analysis (BIO-ENV, ρ  = 0.4045, p -value = 0.013). These findings highlight the influence of monsoon-driven hydrography on mesozooplankton communities, providing a valuable baseline for future ecological assessments under changing climatic and anthropogenic pressures.