Monsoon-Driven Environmental Gradients Structure Seasonal Mesozooplankton Community Turnover in the Moheshkhali Channel, a Subtropical Estuary in Bangladesh

Mesozooplankton play a key role in estuarine food webs, yet their seasonal community dynamics and environmental controls remain poorly resolved in monsoon-driven subtropical estuaries. This study investigated seasonal variations in mesozooplankton community composition and their relationships with environmental drivers in the Moheshkhali Channel, southeastern Bangladesh. Sampling was conducted at five stations across monsoon, winter, and pre-monsoon seasons, and zooplankton assemblages were analyzed using community indices and multivariate approaches. A total of 25 mesozooplankton taxa were recorded, with copepods dominating across all seasons. Community composition differed significantly among seasons, with monsoon assemblages characterized by higher contributions of larval and transient taxa associated with elevated turbidity and temperature, while winter and pre-monsoon communities overlapped under more stable hydrographic conditions. Multivariate analyses identified salinity, turbidity, and temperature as the primary environmental drivers shaping mesozooplankton assemblages, highlighting the role of freshwater–marine mixing in structuring seasonal community turnover. These results demonstrate that seasonal changes in mesozooplankton communities are driven more by species turnover than by changes in overall abundance or diversity. By emphasizing species-specific responses to environmental gradients, this study provides new ecological insight into estuarine mesozooplankton dynamics and establishes a baseline for long-term monitoring and management of subtropical estuaries in the Bay of Bengal.