Comparison of microzooplankton community structure before and during Acromitus flagellatus swarm in the estuarine waters of northern Bay of Bengal

The scyphozoan jellyfish Acromitus flagellatus (Maas, 1903) formed massive aggregations in the River Saptamukhi channel, situated in Sundarbans, the world’s largest deltaic ecosystem. The present study in this mangrove-fringed estuary aims to find the variations in the community structure of microzooplankton (protists and metazoans of 20-200μm size class) before the onset of the swarm and during it. It also briefly documents the trophic cascade exerted by the swarming A. flagellatus on the lower trophic levels. The taxonomic groups representing the microzooplankton community from this study included ciliates, dinoflagellates (heterotrophic and mixotrophic), rotifers, foraminiferans, and metazoan larvae (copepod nauplii, bivalve larvae, gastropod larvae, polychaete larvae). Copepod nauplii dominated the abundance in all stations both before the swarm (BS) and during the swarm (DS). Regarding the species composition of protists, tintinnids dominated the ciliate community, while heterotrophic dinoflagellates dominated the dinoflagellate community, contributing 32% and 30% of protist composition in the BS survey and 37% and 23% of protist composition in the DS survey, respectively. The abundance of another protist group, the planktonic foraminiferans, was very low and was obtained only in the DS survey. In terms of diversity composition and abundance, the microzooplankton community when compared to the BS survey, showed little difference during the A. flagellatus swarming event (DS), except for a few exceptions. Notably, during the swarm period, microzooplankton abundance showed a positive relationship with A. flagellatus numbers, while the mesozooplankton abundance reduced significantly, exhibiting the top-down control by A. flagellatus in the pelagic food web. •Microzooplankton community, when compared between the period of jellyfish swarm and before swarm, showed slight variations.•The abundance of the microzooplankton population remained fairly stable even during the intense swarm period.•Microzooplankton in this area both before and during jellyfish swarm were mostly represented by ciliates and dinoflagellates.•A cascading effect was observed in the food web during swarm period, due to top-down control by high jellyfish abundance.