Impact of a dinoflagellate bloom on the marine plankton community structure of Port Blair Bay, Andaman Island

An intense heterotrophic dinoflagellate bloom caused by Protoperidinium quinquecorne was observed at Port Blair Bay during May 2017. The bloom appeared during a period of significantly higher (p< 0.05) temperatures (35.85 ±1.66°C) and seawater salinity (33.21 ± 0.27 PSU) than the pre-bloom and post-bloom phase. Subsequently, we investigated the bloom impact on water quality and plankton community structure. Dissolved oxygen (DO) was reduced significantly (p< 0.05) during the bloom phase (4.41 ± 0.71 mg l⁻¹) as compared to the pre-bloom condition (5.80 ± 0.23 mg l⁻¹), suggesting an increase in respiration of heterotrophic bloom-forming dinoflagellate species. Further, a significant (p< 0.05) increase and decrease in silicate (8.36 ±1.27μmol l⁻¹) and chlorophyll-a (0.86 ± 0.7 mg m⁻³) concentrations, respectively were observed during the bloom phase, that coincided with an increase in P. quinquecorne grazing pressure on the diatom population. Ammonia enrichment was recorded post-bloom, which was associated with anaerobic decomposition of dead bloom biomass. P. quinquecorne population density increased from 20 cells l⁻¹ (pre-bloom) to 3.36x10⁶ cells l⁻¹ (bloom) and contributed up to 99.93% of the total phytoplankton community. P. quinquecorne grazed on micro-diatom and smaller tintinnids, resulting in a significant decrease in their abundance. The cyclopoid copepod Oithona spp. responded positively to the P. quinquecorne population and caused a considerable reduction in their abundance during post-bloom. This heterotrophic bloom was the result of high temperature and salinity conditions, coupled with the availability of preferred prey species (micro-diatom). Future monitoring studies are needed to predict harmful algal blooms (HABs) occurrence and their impact on the coastal fishery.