Impact of Noctiluca scintillans bloom and decay phases on coastal water quality and zooplankton dynamics in Indian coastal waters

Introduction Understanding of ecosystem changes induced by harmful algal blooms (HABs) and their impacts on coastal water quality, bacteria, and zooplankton communities remains limited. Non-toxic HABs, particularly Noctiluca scintillans , are increasingly reported in Indian coastal waters. Methods In the present study, a greenish bloom of N. scintillans was observed along the Gulf of Mannar and the Southeastern Arabian Sea during November 2022, with a consequent bloom crash (evidencing surface foam) observed along the southern Kerala coast. We collected samples from bloom, decay, and non-bloom zones to assess changes in hydrobiological conditions. Results and discussion Physicochemical data showed negligible variations in temperature (28.71–29.66 °C) and salinity (34.6–35.06), whereas dissolved oxygen levels varied from 6.06 to 8.47 mg/L. Nutrients exhibited strong fluctuations, particularly with the ammonium concentration (15.80 μmol/L) being higher during the bloom decay phase. Elevated chlorophyll-a and DOC confirmed high biomass and organic release, while bacterial pathogens proliferated under nutrient-rich conditions. Moreover, the concurrent rise in pathogenic bacteria, including Vibrio cholerae , suggests N. scintillans may serve as a potential vector for bacterial proliferation when it forms blooms. Zooplankton assemblages showed distinct responses during the bloom and decay phases of N. scintillans . Micro- and meso-zooplankton densities were elevated during the bloom. In contrast, the mesozooplankton was primarily dominated by calanoid copepods; later, these dominant taxa declined sharply during the decay phase and was substituted by cladocerans, appendicularians, and hydrozoans. These shifts in zooplankton communities reflect changes in hydrographic conditions associated with eutrophication and enhanced bacterial production during bloom collapse.