Understanding the response of phytoplankton to the cyclonic event Sitrang A case study in the Hooghly estuary of Sundarban Bay of Bengal region

Abstract

The study aims to investigate the impact of tropical cyclone Sitrang on the phytoplankton community and water quality parameters at estuarine zone of Hooghly River near Namkhana region of Sundarban. Focusing on the response of diatom and dinoflagellates to this post-monsoonal cyclone this study tries to address the questions regarding short-term ecological effects, including changes in water quality, shifts in the phytoplankton composition and abundance, and the potential factors driving these changes. Water samples were collected for consecutive three days before and six days after the occurrence of the Sitrang in October 2022. A substantial drop in pH, DO, and temperature after the immediate passage of the cyclone along with increased salinity, chlorophyll-a content, inorganic macronutrients, and TDS were observed. The cyclone led to a significant reduction in phytoplankton diversity, while concurrently causing a notable increase in their abundance. Prior to the cyclone 45 species of diatoms and 8 species of dinoflagellates were observed which was reduced to 25 and 6 species after the cyclone. Post-cyclonic period exhibited high predominance of dinoflagellates especially Tripos sp., and Noctiluca scintillans (4.3×103 cells/L). Canonical correspondence analysis (CCA) revealed nitrate and salinity are the major factors influencing phytoplankton abundance. The notable phenomenon of massive increase in the number of Tripos furca (2.04 ×104 cells/L) suggests their favourable growth condition triggered by the cyclone. Due to the rising intensity of cyclones in the Indian Sundarbans, close monitoring and continuing research is essential for a thorough understanding of phytoplankton dynamics.