A multispecies bloom caused by the centric diatoms, viz. Coscinodiscus radiatus, Chaetoceros lorenzianus and the pennate diatom Thalassiothrix frauenfeldii was investigated in the context of its impact on phytoplankton and microzooplankton (the loricate ciliate tintinnids) in the coastal regions of Sagar Island, the western part of Sundarban mangrove wetland, India. Both number (15-18 species) and cell densities (12.3×103cellsl-1 to 11.4×105cellsl-1) of phytoplankton species increased during peak bloom phase, exhibiting moderately high species diversity (H'=2.86), richness (R'=6.38) and evenness (E'=0.80). The diatom bloom, which existed for a week, had a negative impact on the tintinnid community in terms of drastic changes in species diversity index (1.09-0.004) and population density (582.5×103 to 50×103indm-3). The bloom is suggested to have been driven by the aquaculture activities and river effluents resulting high nutrient concentrations in this region. An attempt has been made to correlate the satellite remote sensing-derived information to the bloom conditions. MODIS-Aqua derived chlorophyll maps have been interpreted. © 2014 Elsevier Ltd.